key: cord-0040435-38woa0gq
authors: Garza, Jose M.; Cohen, Mitchell B.
title: Infectious Diarrhea
date: 2010-12-27
journal: Pediatric Gastrointestinal and Liver Disease
DOI: 10.1016/b978-1-4377-0774-8.10039-9
sha: 6dba5abd850a3eed2e6bcd1116d3959faf6e4c8c
doc_id: 40435
cord_uid: 38woa0gq

nan

In the United States, an estimated 21 to 37 million episodes of diarrhea occur annually in children younger than 5 years of age. 1 Ten percent of these children are seen by a physician, more than 200,000 are hospitalized, and between 300 and 400 die from the illness. Worldwide, the number of childhood deaths from diarrhea is higher than 4 million per year.

Knowledge of diarrheal disease has increased remarkably during the past few decades. 2 This increased understanding of pathogenic mechanisms has led to improvements in therapy. This chapter discusses the major viral and bacterial agents of infectious diarrhea, including their epidemiology, pathogenesis, clinical manifestations, diagnosis, and therapy.

Diarrheal disease caused by viral agents occurs far more frequently than does similar disease of bacterial origin. In fact, viral gastroenteritis is the second most common illness in the United States, after the common cold. 3 Despite the frequent occurrence of viral enteritides, the identification of a specific virus as causative agent is a relatively recent development.

Rotavirus and a number of other small round structured viruses have been identified as a major cause of nonbacterial gastroenteritis in children and adults. This discussion focuses on these established pathogens, then continues with a brief summary of several newer viral enteropathogens and the current status of several candidate pathogens.

Rotavirus was first identified as a specific viral pathogen in duodenal cells of children with diarrhea by Bishop and associates in 1973. Subsequent studies indicated that rotavirus is responsible not only for more cases of diarrheal disease in infants and children than any other single cause but also for a significant portion of deaths caused by diarrhea in both developed and developing countries throughout the world. 4 Rotavirus is responsible for 20 to 70% of hospitalizations for diarrhea among children worldwide. 5 Compared with other causes of gastroenteritis, rotavirus is more frequently associated with severe symptoms. 6 Before the initiation of the rotavirus vaccination program in 2006, nearly every child in the United States was infected with rotavirus by age 5 years. 7

The genus Rotavirus is classified as a member of the family Reoviridae of the RNA viruses. Rotaviruses are round particles 68 nm in diameter and are composed of two separate shells (capsids). The capsids surround a 38-nm icosahedral core structure, which in turn encloses the 11 double strands of RNA in the core. This structure gives the virus its characteristic appearance of a wide-rimmed wheel with spokes radiating from the hub, from which its name was derived (rota is Latin for "wheel"). 8 Rotaviruses are classified based on antigenic properties of various proteins found in the capsid structure. The VP6 protein on the inner capsid of the virus determines the rotavirus group. 9 Most viruses infecting humans are classified as group A, although rotaviruses from groups B and C have occasionally been associated with human diarrheal disease as well. The next level of classification is the subgroup, which is determined by other antigenic differences among the VP6 proteins. At least two subgroups are known to exist. 9 Subgroup typing has proved important in the study of patients who experience more than one episode of rotaviral infection. In these patients, recurrent infections usually but not necessarily involve agents of different subgroups, which suggests that subgroup antigens are not sufficient for inducing the production of protective antibodies. 10 Finally, the rotaviruses are classified into a variety of serotypes based on the antigenic differences of VP7 glycoprotein or the VP4 protease-sensitive hemagglutinin proteins that are found in the outer capsid. 11 VP4 is designated as the P antigenic protein because it is cleaved by the protease trypsin at the intestinal level, and VP7 is designated as the G antigenic protein because it is a glycosylated structure. There are at least 42 different G/P strains with different serotype combinations. However, five serotypes, G1P8, G2P4, GP8, G4P8, and G9P8, are the predominant circulation rotavirus G/P serotypes. 12 The prevalence of serotypes can fluctuate from year to year, 13 and although the five most common serotypes are responsible for approximately 95% of infections worldwide, there are substantial geographical differences. For example, in a recent global study, G1P8 was responsible for more than 70% of infections in North America, Australia, and Europe but less than 30% in South America, Asia, and Africa. 14 

Rotavirus infection appears to occur throughout the world. In temperate climates, a sharp increase in incidence of cases occurs during the winter months. 4 In the United States, the peak rotavirus season begins in November in the Southwest and ends in the Northeast in April. 4 In the tropics, year-round transmission occurs, with seasonal variation in some areas. 15 Transmission is primarily from person to person, through contact with feces or contaminated fomites. Respiratory transmission has been suggested but not proved. 16 Rotavirus is highly contagious because very few infectious virions are needed to cause disease in susceptible hosts. 17 Although the virus may affect all age groups, it most commonly produces disease in children between 6 and 24 months of age. Before vaccination, most children developed rotavirus antibodies by the age of 2 years, which helps to explain the observed decreased incidence of rotaviral infection in later

José M. Garza • Mitchell B. Cohen childhood. Rotavirus infection also occurs in adult populations with approximately half the frequency seen in children. Those adults whose children had rotavirus were more likely to be infected than were adults without infected children. 18 Most adults found to have rotavirus infection were asymptomatic; if symptoms were present, they were generally mild. This would seem to indicate that the antibody acquired earlier in life provides protective benefit.

The other age group that appears to have relative protection from rotavirus infection is the neonate. The virus can be found in stool samples from asymptomatic neonates. Neonatal epidemics of rotavirus excretion have been described in which approximately half of the nursery patients examined were found to have rotavirus. Many of these infants were asymptomatic, and those with disease had only mild symptoms. 19, 20 Breast-fed infants are less likely to be infected, and, when infected, these infants are apparently less likely than their bottle-fed counterparts to suffer symptoms of disease. This may reflect the protective effect of maternal antibodies in colostrum and breast milk. 21 Nosocomial spread of rotaviral illness among hospitalized infants has also been documented. 22 Factors associated with increased risk for hospitalization for rotavirus gastroenteritis among U.S. children include lack of breast-feeding, low birth weight, day-care attendance, the presence of another child younger than 24 months in the household, and having Medicaid or no medical insurance. 23 

Once a susceptible patient has come in contact with rotavirus, a 48-to 72-hour incubation period occurs before the onset of symptoms. 16 Illness typically begins with the sudden onset of diarrhea and vomiting, and fever is present in most patients. 16 The diarrhea is usually watery and rarely may be associated with gross or occult blood in the stool. 24 The fluid loss from diarrhea and vomiting may be severe enough to cause dehydration. Diarrhea caused by rotavirus usually lasts from 2 to 8 days. 25 Shedding of virus into the intestinal lumen begins about 3 days after infection and may persist for as long as 3 weeks. 26 A comparison of the characteristics of rotaviral infections with those of other enteric viruses is presented in Table 39 -1. In addition to gastrointestinal symptoms, patients with rotavirus often have respiratory tract symptoms. 16 Unlike fever and vomiting, none of the respiratory manifestations associated with rotavirus infection are helpful in the recognition of rotaviral disease. 27 The clinical symptoms of rotavirus infection are more severe in patients with underlying malnutrition. In the malnourished murine rotavirus model, a smaller inoculum is required for infection, less time is required for incubation, and the symptoms are more severe. 28 In addition, rotavirus replication can occur in the liver and kidney, at least in immunocompromised hosts. 29 Children and adults who are immunocompromised because of congenital immunodeficiency or because of bone marrow or solid organ transplantation sometimes experience severe or prolonged rotavirus gastroenteritis. 7 The severity of rotavirus disease among children infected with human immunodeficiency virus (HIV) is thought to be similar to that among children without HIV infection. 7

Rotavirus invades the villus intestinal epithelial cells and replicates, causing cell death and sloughing. Histologically, this is manifest as blunting of the intestinal villi, and in response to the loss of villus cells, there is crypt hypertrophy. The lytic infection of highly differentiated absorptive enterocytes and the sparing of undifferentiated crypt cells results in both a loss of absorptive capacity with "unopposed" crypt cell secretion (causing secretory diarrhea) and loss of brush border hydrolase activity (causing osmotic diarrhea).

Another possible mechanism for rotaviral diarrhea also has been demonstrated. The rotavirus nonstructural glycoprotein NSP4 has been shown to mediate age-dependent intestinal secretion in mice. 30 The relevance of this novel viral enterotoxin to human rotaviral infection is uncertain. Other models, including vasoactive inflammatory agents, have also been proposed; consistent with this, in rotavirus infection there may be an increase in the number of inflammatory cells in the lamina propria. Disease effects are apparently limited to the duodenum and the proximal jejunum, 16 because studies in patients with known rotavirus disease have yielded normal gastric and rectal biopsies. 31

Rotavirus was initially linked to acute gastroenteritis through electron-microscopic evidence of viral particles in biopsy specimens of affected patients. This technique continues to be used in rotavirus detection, especially in conjunction with monoclonal or polyclonal antibodies (immunoelectron microscopy). 25 The obvious drawback of this approach is the need for specialized personnel and equipment. Consequently, a variety of immunoassays have been developed for detecting group A rotavirus antigen in stool 31 ; most immunoassays have sensitivities and specificities in the range of 90%.

Currently, supportive care with oral or intravenous rehydration is the mainstay of therapy. 32 Although novel antisecretory therapies have been reported, 33 no antiviral agents effective against rotavirus have yet been developed. However, probiotic therapy has been shown to be effective in preventing and treating rotaviral infection. Treatment with Lactobacillus GG has been shown to shorten the course of rotaviral diarrhea by at least 1 day. [34] [35] [36] In addition, other probiotic agents (Bifidobacterium bifidum and Streptococcus thermophilus) have been shown to prevent diarrheal disease and shedding of rotavirus in a chronic hospital setting when given to formula-fed infants. 36 Oral administration of immunoglobulin has been shown to promote faster recovery 407 from rotaviral infection 37 ; this therapy should be reserved for severely affected hospitalized infants.

In infants, natural rotavirus infection confers protection against subsequent infection. This protection increases with each new infection and reduces the severity of diarrhea. 38 A rotavirus vaccine (Rotashield J, Wyeth-Ayerst, St. David's, PA) was approved for use in the United States and was placed on the American Academy of Pediatrics' recommended vaccination schedule. Although the vaccine was efficacious, an increased incidence of intussusception within 2 weeks of receiving the vaccine was identified by the Vaccine Adverse Event Reporting System (VAERS), leading to voluntary withdrawal by the manufacturer. 39 Two different rotavirus vaccine products are licensed and widely used in infants in the United States; they differ in composition and schedule administration. Safety and efficacy has been demonstrated for both vaccines; there is 85 to 98% protection against severe rotavirus disease and 74 to 87% protection against rotavirus disease of any severity through at least the first rotavirus season. 7 Neither vaccine was associated with intussusception, and the Advisory Committee on Immunization Practices (ACIP) does not express a preference for either one. 7

). Licensed in the United States in 2006, RotaTeq is a live, oral vaccine that contains five reassortant rotaviruses developed from human and bovine parent rotavirus strains (G1,G2,G3,G4, and P1A). The efficacy has been evaluated in two phase III trials among healthy infants. 40, 41 The vaccine is to be administered orally in a three-dose series at ages 2, 4, and 6 months with a minimum age for first dose at 6 weeks and maximum at 14 weeks and 6 days. The minimal interval between doses is 4 weeks and maximum age for last dose 8 months. 7

). Licensed in the United States in 2008, Rotarix is a live, oral vaccine that contains a human rotavirus strain (G1P1A). The efficacy has been evaluated in two phase III trials. 42, 43 The vaccine is to be administered orally in a two-dose series at ages 2 and 4 months with the same minimum and maximum age ranges and intervals as RotaTeq. 7 Early success from the vaccines has been documented; the National Respiratory and Enteric Virus Surveillance System (NREVSS) and the New Vaccine Surveillance Network (NVSN) indicated that the onset and peak of the 2008 rotavirus season were delayed by 15 and 8 weeks, respectively. as compared with the six previous consecutive seasons. 44 Further data indicate that the number of tests positive for rotavirus during the 2008 season decreased by more than two thirds as compared with the seven preceding rotavirus seasons. 7

Caliciviruses "Winter vomiting disease" was thought to be caused by nonbacterial gastroenteritis for decades before an etiologic agent was identified from an outbreak, in 1968, in Norwalk, Ohio. In this outbreak, only some of the patients had diarrhea; the predominant clinical manifestation was vomiting and nausea. Virus particles were visualized by immune electron microscopy on fecal material derived from the Norwalk outbreak. This represented the first definitive association between a specific virus (Norwalk virus) and acute gastroenteritis. Subsequently a number of similar etiologic agents were identified; before the cloning of the prototype Norwalk virus genome, 45 these viruses, which were a group of morphologically diverse, positive-stranded RNA viruses that caused acute gastroenteritis, were identified as Norwalk-like agents. These organisms were also named for the communities in which they were first isolated (e.g., Montgomery County, Hawaii, Snow Mountain, Taunton, Otofuke, and Sapporo viruses). Based on reverse transcription-polymerase chain reaction (RT-PCR), the sequence structure of these viruses has enabled their classification as human caliciviruses (HuCV). Human caliciviruses are now recognized as a leading cause of diarrhea worldwide among persons of all ages. 46 With the use of molecular tools, HuCV have now been preliminarily classified into four genotypes, represented by Norwalk virus, Snow Mountain agent, Sapporo virus, and hepatitis E virus. 47, 48 Recently the nomenclature of two genotypes has changed, renaming Norwalk virus as norovirus and Sapporo virus as sapovirus. 49 This HuCV classification system may allow the development of assays based on recombinant HuCV antigens or PCR products rather than the current cumbersome classification schemes that rely on human reagents (convalescent outbreak sera) of varying sensitivity and specificity. Molecular tools have already allowed the identification of HuCV as agents of both pediatric and adult viral gastroenteritis in foodborne outbreaks as well as outbreaks in nursing homes, hospitals, and a university setting. Despite the potential for future understanding of the contribution of individual HuCV to outbreaks of nonbacterial gastroenteritis, Norwalk virus still remains the prototypic agent of HuCV, and it is described in greater detail in the following section.

Epidemiology. Norovirus is worldwide in distribution. Of patients exposed to norovirus either naturally or experimentally, 50% develop clinical symptoms. 50 Studies evaluating the prevalence of anti-norovirus antibody among populations of various age groups initially demonstrated that the group from 3 months to 12 years of age had only a 5% antibody-positive rate. More recent epidemiologic studies, using baculovirusexpressed recombinant norovirus antigen in an enzyme-linked immunosorbent assay (ELISA), have demonstrated a serologic response in 49% of Finnish infants between 3 and 24 months of age. 51 These data contradict previous beliefs that norovirus most often caused disease in older children and adults.

Transmission of norovirus is most often fecal-oral. Unlike rotavirus, this usually involves the spread of infection to a large population through a common source rather than from direct, person-to-person contact. In one outbreak, an infected bakery employee transmitted the virus through food products to approximately 3000 people. 52 Outbreaks have also been related to ingestion of raw oysters and clams and to contaminated water supplies. Spread of this disease has been documented in closed-in populations such as those in long-term care facilities and cruise ships. 53 In addition to its fecal-oral spread, there is some evidence that norovirus is transmitted through a respiratory route in the form of aerosolized particles from vomitus. Contamination of environmental surfaces with norovirus has been documented during outbreaks. 54 Although previously referred to as "winter vomiting disease," norovirus produces outbreaks of disease that can occur throughout the year. 55 Several characteristics of norovirus facilitate their spread in epidemics: (1) low infectious dose (fewer than 10 viral particles), (2) prolonged viral shedding, (3) stability of the virus in relatively high concentrations of chlorine and a wide range of temperatures, and (4) the fact that repeated infections can occur with reexposure. 46 Pathophysiology. The histologic changes induced by norovirus in an infected host have been studied in small bowel biopsies from infected volunteers. Those volunteers who remained free of clinical symptoms had normal biopsy specimens, whereas those with symptoms exhibited marked, but not specific, changes, including focal areas of villous flattening and disorganization of epithelial cells. On electron microscopy, microvilli were shortened, and there was dilatation of the endoplasmic reticulum. These volunteers had repeat biopsies 2 weeks after the illness, and normal histology was again present. Other investigators have demonstrated the presence of normal gastric and rectal histology in patients affected by norovirus as is typical of viral gastroenteritis. Using norovirus virus-like particles (derived from capsid proteins) researchers have recently demonstrated that human histo-blood group antigens may act as receptors for norovirus infection 56, 57 and may explain the varying host susceptibility observed in outbreaks and volunteer studies. 58 Clinical Manifestations. The clinical manifestations of disease produced by the norovirus include nausea, vomiting, and cramping abdominal pain (see Table 39 -1). Diarrhea is said to be a less consistent feature of this illness. In the original outbreak, only 44% of patients experienced diarrhea, whereas 84% had vomiting. Other studies, however, have found that diarrhea occurs in most children and experimentally infected adult volunteers who become ill from this virus. Fever occurs in approximately one third of affected patients, but respiratory symptoms are not typically a part of this illness. An incubation period of approximately 24 to 48 hours has been noted before the onset of symptoms, 50 and symptoms persist for 12 to 48 hours. The typical symptoms of infection are in part also seen in premature infants but with a huge variety of clinical courses including abdominal distention, apnea, and sepsis-like appearance. 59 Diagnosis and Treatment. Norwalk virus could be detected in fecal samples for a median of 4 weeks and for up to 8 weeks after virus inoculation; peak virus titers are most commonly found in fecal samples collected after resolution of symptoms, and presymptomatic shedding was more common in persons who did not meet the definition of clinical gastroenteritis. 60 RT-PCR assays have been developed for detection of noroviruses in clinical and environmental specimens, such as water and food. 61,62 RT-PCR followed by nucleotide sequencing has been useful in epidemiologic studies, and also various commercial stool enzyme immunoassay (EIA) detection methods have been developed 46 ; the sensitivity is genotype dependent. 63 A rapid and accurate diagnostic assay is not widely available, but the presence of four epidemiologic features of norovirus disease can be useful in confirming norovirus as a cause of outbreaks: (1) vomiting in more than half of affected persons, (2) mean incubation period of 24 to 48 hours, (3) mean duration of illness of 12 to 60 hours, and (4) absence of bacterial pathogen in stool culture. 64 The treatment for norovirus is supportive; oral rehydration solutions are used if necessary. Significant dehydration is uncommon, and the need for hospitalization is rare. A number of candidate vaccines are currently being evaluated.

The enteric adenoviruses are among the more recently recognized viral pathogens that cause acute gastroenteritis. Adenoviruses are a large group of viruses long recognized for their role in the pathogenesis of respiratory infections and keratoconjunctivitis. Most of the 47 serotypes are known to be shed in the feces of infected patients. In patients with predominantly gastrointestinal symptoms, the organisms are detectable by electron microscopy of stool samples; however, they fail to grow in standard tissue culture conditions. Their unique cell culture requirements allow for the differentiation of nonenteric adenoviruses from the enteric serotypes (Ad40 and Ad41), which are recognized to be among the common causes of viral childhood gastroenteritis. 65 Infection with enteric adenoviruses apparently occurs throughout the year, with only slight seasonal variation. 66 This disease tends to affect predominantly younger children, with most patients being younger than 2 years of age. 66, 67 Enteric adenovirus is spread by the fecal-oral route. Transmission of the disease to family contacts is unusual.

Diarrhea is the most commonly reported symptom of enteric adenoviral infection. In contrast with diarrhea from other viral enteritides, diarrhea from enteric adenovirus typically persists for a prolonged period, sometimes as long as 14 days. Viruses may be excreted in the feces of infected patients for 1 to 2 weeks. Vomiting frequently occurs but is usually mild and of a much shorter duration than is the diarrhea. Dehydration has been seen in approximately half of affected patients, and hospitalization is sometimes necessary. The frequency of association of respiratory symptoms with enteric adenovirus infection is unclear. 67 The diagnosis of enteric adenovirus is best made by electron microscopy or immunoelectron microscopy of stool samples or from intestinal biopsy specimens. ELISA and PCR techniques have also been used successfully in enteric adenovirus diagnosis. Treatment is mainly supportive, and oral rehydration solutions are useful in cases of dehydration.

Astrovirus, similar to HuCV, is a single-stranded RNA virus grouped with the small round structured viruses. However, the recently derived sequence of the astrovirus RNA genome reveals that this agent is sufficiently different to be classified in its own family as Astroviridae. 68 Astrovirus is worldwide in distribution and tends to infect mainly children in the 1-to 3-year age group. In controlled studies in Thailand, astrovirus infection was the second most common cause of enteritis, after rotavirus infection, in symptomatic children. 69 Astrovirus infection occurred in 9% of children with diarrhea, compared with 2% of controls. Comparable findings have been reported in day-care centers in North America and Japan. Most children infected with astrovirus develop symptoms. Vomiting, diarrhea, abdominal pain, and fever all are commonly seen with infection by this agent, and symptoms typically last 1 to 4 days. Spread of the virus may occur via the fecal-oral route from person-to-person contact or through contaminated food or water. Asymptomatic shedding of astrovirus has also been reported.

A variety of other viruses are being studied to determine what role, if any, they may play in the pathogenesis of human enteric infections. With the exception of those viruses previously discussed in detail, insufficient data are available to ascertain clinical and epidemiologic differences, if any, among the various small round viruses.

Pestivirus, a single-stranded RNA virus of the togavirus family, has been found in the feces of 24% of children living on an American Indian reservation who had diarrhea attributable to no other infectious agent. 70 These children experienced only mild diarrhea but had more severe respiratory complaints.

Coronavirus is known to cause an upper respiratory illness in humans and has been shown to cause diarrhea in some animals. 71 The role of this agent in human diarrheal disease is unclear, and at least one study found coronavirus more commonly in children without diarrhea than in those who were ill. 72 Coronavirus was implicated in an outbreak of necrotizing enterocolitis. 73 Toroviruses are pleomorphic viruses recognized to cause enteric illness in a variety of animals. Members of this group, originally described in Berne, Switzerland, and Breda, Iowa, and named for those cities, have been seen in the feces of humans with diarrheal disease. 74 Because of the pleomorphic structure of toroviruses, electron microscopy was inadequate to prove an etiopathogenic role of these viruses in diarrheal disease. The more recent findings of torovirus-like particles by immunoassay, using validated anti-Breda virus antiserum, lends additional weight to the hypothesis that these are agents of human gastroenteritis. 75 Their causative role in human disease, however, remains unproved. Similarly, picobirnavirus is known to cause disease in animals and has been isolated from stools of humans with diarrheal illness. 76 Cytomegalovirus has been associated with enteritis and colitis. Except for Ménétrier's disease, caused by gastric cytomegalovirus infection, enteritis and colitis seem to occur almost exclusively among immunocompromised patients. In this population, cytomegalovirus causes viremia and is carried by the blood stream to a variety of sites, including organs of the gastrointestinal tract. Diagnosis may be made by virus detection in feces, by demonstration of typical cytomegalic inclusion cells, or by in situ hybridization. 66 

For an infecting bacterial agent to cause diarrhea, it must first overcome the following gastrointestinal tract defenses: (1) gastric acidity, (2) intestinal motility, (3) mucus secretion, (4) normal intestinal microflora, and (5) specific mucosal and systemic immune mechanisms. Gastric acidity is the first barrier encountered by infecting organisms. Many studies have demonstrated the bactericidal properties of gastric juice at pH less than 4. In patients with achlorhydria or decreased gastric acid secretion, the gastric pH is higher, and this bactericidal effect is diminished. Gastric acidity serves to decrease the number of viable bacteria that proceed to the small intestine.

Organisms surviving the gastric acidity barrier are trapped within the mucous layer of the small intestine, facilitating their movement through the intestine by peristalsis. If motility in the intestine is abnormal or absent, organisms are more readily able to initiate the infectious process. Some organisms can elaborate toxic substances that impair intestinal motility. Increased intestinal peristalsis, which occurs during some enteric infections, may be an attempt by the host to rid itself of infective organisms.

In addition to its role in conjunction with intestinal motility, mucus also serves to provide a nonspecific barrier to bacterial proliferation and mucosal colonization. This barrier has been shown to be effective in preventing toxins from exerting their effects. Exfoliated mucosal cells trapped in the mucous layer may trap invading microorganisms. Mucus also contains carbohydrate analogues of surface receptors, which may prevent invading organisms from binding to actual receptors.

The normal endogenous microflora of the gut serves as its next line of defense. Anaerobes, which are a large component of the normal flora, elaborate short-chain fatty acids and lactic acid, which are toxic to many potential pathogens. In breastfed infants, this line of defense is enhanced by the presence of anaerobic lactobacilli, which produce fermentative products that act as toxins to foreign bacteria. Further evidence in support of the importance of endogenous microflora is the increase in susceptibility to infection after one's normal flora has been reduced by antibiotic administration, as is seen with Clostridium difficile infection.

The most complex element in the host-defense armamentarium involves the mucosal and systemic immune systems. Both serum and secretory antibodies may exert their protective effects at the intestinal level, even though the serum components are produced outside the gut. An immune response may be specific to a particular infective agent or generalized to a common group of bacterial antigens.

Bacteria have developed a variety of virulence factors (Table  39 -2) to overcome host defense mechanisms: (1) invasion of the mucosa, followed by intraepithelial cell multiplication or invasion of the lamina propria; (2) production of cytotoxins, which disrupt cell function via direct alteration of the mucosal surface;

(3) production of enterotoxins, polypeptides that alter cellular salt and water balance yet leave cell morphology undisturbed; and (4) adherence to the mucosal surface with resultant flattening of the microvilli and disruption of normal cell functioning. Each of the bacterial virulence mechanisms acts on specific regions of the intestine. Enterotoxins are primarily effective in the small bowel but can affect the colon; the effects of cytotoxins and direct epithelial cell invasion occur predominantly in the colon. Enteroadhesive mechanisms appear to function in both the small intestine and colon.

Members of the species Salmonella are currently recognized as the most common cause of bacterial diarrhea among children in the United States. Surveillance data from the Centers for Disease Control and Prevention show that in 2008 the incidence of Salmonella was 16.2 per 100,000, and although there was an apparent increase in Salmonella infections, this rate has not changed significantly over the past 3 years. 77 Infection caused by Salmonella may result in several different clinical syndromes, including

(1) acute gastroenteritis; (2) focal, nonintestinal infections; (3) bacteremia; (4) asymptomatic carrier state; and (5) enteric fever (including typhoid fever). Each of these entities may be caused by any of the commonly recognized species of Salmonella.

Salmonella is a motile, gram-negative bacillus of the family Enterobacteriaceae. It can be identified on selective media because it does not ferment lactose. Three distinct species of Salmonella are recognized: Salmonella enteritidis, Salmonella choleraesuis, and Salmonella typhi. S. enteritidis is further subdivided into approximately 1700 serotypes. Each serotype is referred to by its genus and serotype names (e.g., Salmonella typhimurium) rather than the formally correct S. enteritidis, serotype typhimurium. S. choleraesuis and S. typhi are known to have only one serotype each. The most common serotypes in infants are Typhimurium, Newport, Javiana, Enteritidis, and Heidelberg. 78

Salmonella is estimated to cause 1 to 2 million gastrointestinal infections each year in the United States. 79 At Cincinnati Children's Hospital Medical Center, salmonellae are the most commonly isolated bacterial enteropathogens ( Figure 39 -1). The highest attack rate for salmonellosis is in infancy, with a lower incidence of symptomatic infection in patients older than 6 years of age. 79 Nontyphoidal Salmonella is usually spread via contaminated water supplies or foods, with meat, fresh produce, fowl, eggs, and raw milk frequently implicated.

A wide variety of foods have caused outbreaks of salmonella; a large outbreak involved contaminated alfalfa sprouts that were shipped worldwide. 80 Most of the egg-associated outbreaks have involved products such as mayonnaise, ice cream, 81 and cold desserts, in which salmonella can multiply profusely and which are eaten without cooking after the addition of, or contamination by, raw egg. Although "shell" eggs are frequently contaminated, the number of bacteria in infected eggs is often near or below the human infective dose. In contrast, with a generation time of 80 minutes at 20° C, one bacterium can become a billion in 40 hours, and with a generation time of 40 minutes at 25° C, it can do so in 20 hours.

Although any of these food sources may become contaminated through contact with an infected food handler, the farm animals themselves are often infected. Pets, notably cats, turtles, lizards, snakes, and chicks, may also harbor Salmonella. Personto-person spread of infection also occurs and is especially common in cases involving infants. A population-based case-control study was done in infants less than 1 year of age and identified the following risk factors: (1) travel outside the United States, (2) attending day care with a child with diarrhea, (3) riding in a shopping cart next to meat or poultry; and (4) exposure to reptiles. Breast-feeding was found to be protective. 82

Inocula of fewer than 10 3 salmonellae are probably sufficient to cause disease. 83 Patients in whom host defenses are diminished are more likely to develop clinical manifestations of the disease. This has been demonstrated in patients who have reduced levels of gastric acid. Patients with lymphoproliferative diseases and hemolytic diseases, especially sickle cell anemia, are more likely to experience severe disease and develop complications from Salmonella infection. The mechanisms for this increased susceptibility may involve altered macrophage function, defective complement activation, or damage to the bones from thromboses.

Having overcome host defenses, Salmonella produces disease through a process that begins with colonization of the ileum and the colon. The organisms next invade enterocytes and colonocytes and proliferate within epithelial cells and in the lamina propria ( Figure 39 -2). From the lamina propria, Salmonella may then move to the mesenteric lymph nodes and eventually to the systemic circulation, causing bacteremia. Because these organisms invade enterocytes and colonocytes, both enteritis, with watery diarrhea, and colitis, with bloody diarrhea, may result. This multistage infection of the host is directed by Salmonella-mediated delivery of an array of specialized effector proteins into the eukaryotic host cells via two distinct secretion systems. Additional secretion systems appear to be functional and contribute toward virulence but are not currently well characterized. 84 

After an incubation period of 12 to 72 hours, Salmonella usually produces a mild, self-limited illness characterized by fever and watery diarrhea. Blood, mucus, or both are commonly present in the stool. Bacteremia occurs in approximately 6% of Salmonella infections in children but much less frequently in adults. Patients may develop nonintestinal sequelae after Salmonella infection, including pneumonia, meningitis, and osteomyelitis. Even in those patients in whom no sequelae occur, excretion of the organisms may persist for several weeks. In patients younger than 5 years of age, the median time of excretion is 7 weeks, with 2.6% of patients continuing to shed organisms for 1 year or longer. 85 Studies have also shown a higher incidence of the carrier state among children with salmonellosis than is seen in adults. 85 Localization of Salmonella organisms in chronic carriers is often in the biliary tract and is frequently associated with cholelithiasis.

Diagnosis of Salmonella infection can be made through stool or blood culture. Use of enriched media and culture of material from freshly passed stools, rather than from rectal swab, increase the likelihood of recovering the organism. 85 Owing to the increased risk of developing the carrier state, antimicrobial treatment of uncomplicated cases of Salmonella gastroenteritis is not recommended. Treatment is recommended in patients at high risk for the development of disseminated disease, including those who are immunocompromised, those with hematologic disease, patients with artificial implants, those with severe colitis, and pregnant women. Treatment is also recommended for patients at any age who appear toxic.

Treatment of all children younger than 1 year of age with salmonellosis remains controversial because of the risk of bacteremia and secondary infections. Antimicrobial therapy is recommended for infants with Salmonella bacteremia. Parenteral antibiotics are recommended for any infant (younger than 3 months of age) with a stool culture that is positive for Salmonella. 86 Most Salmonella are sensitive to a wide variety of antibiotics, including ampicillin (35 mg/kg [maximum 1 g] per dose, given every 4 hours, intravenously, for 14 days), chloramphenicol (20 mg/kg [maximum 1 g] per dose, given every 6 hours, intravenously or orally, for 14 days), trimethoprim-sulfamethoxazole (trimethoprim, 5 mg/kg [maximum 160 mg], plus sulfamethoxazole, 25 mg/kg [maximum 800 mg] per dose, given every 12 hours, orally, for 14 days), and the third-generation cephalosporins. Resistance to ampicillin is increasing. 87 Ceftriaxone, cefotaxime, or a fluoroquinolone (not approved for use in children younger than 18 years of age) are often effective when resistance to other agents is demonstrated.

A follow-up stool culture usually is not warranted unless the patient is employed in the preparation of food. If evidence of a "cure" is necessary, two to three consecutive negative stool cultures, obtained 1 to 3 days apart, are sufficient.

Although uncommon in the United States, typhoid fever, caused by S. typhi, commonly affects children in developing countries. S. typhi differs from other salmonellae in that it requires a human host. The disease it causes also differs in severity from the typically mild gastroenteritis caused by other members of the genus; S. typhi infection also has a higher case-fatality rate.

Typhoid fever typically begins with a period of fever lasting approximately 1 week. Patients then complain of headache and abdominal pain. Diarrhea is not usually a manifestation of typhoid fever, and many patients experience constipation. Hepatomegaly and splenomegaly have also been frequently noted. The characteristic "rose spots" (palpable, erythematous lesions), typical in adult cases of typhoid fever, occur with far less frequency in pediatric patients. Patients may become chronic carriers.

Diagnosis of typhoid fever is made on the basis of positive blood cultures. S. typhi is usually sensitive to several antimicrobial agents, including ampicillin, chloramphenicol, trimethoprim-sulfamethoxazole, cefotaxime, and ceftriaxone. Drug choice is based on site of infection and susceptibility of the organism. A recent Cochrane review 88 showed that azithromycin appears to be better than fluoroquinolones in populations with drug-resistant strains and that it may also perform better than ceftriaxone.

Two typhoid vaccines are commercially available; a live, oral Ty21a and injectable Vi polysaccharide. They have been shown to be safe and efficacious and are licensed for people aged more than 2 years. 89, 90 Immunization of school-age or preschoolage children is recommended in areas where typhoid fever is shown to be a significant public health problem, particularly where antibiotic-resistant S. typhi is prevalent. Vaccination may be offered to travelers to destinations where the risk of typhoid fever is high, especially to those staying in endemic areas for longer than 1 month. 89 Other vaccines, such as a new modified, conjugated Vi vaccine called Vi-rEPA, are in development and may confer longer immunity. 90

Bacillary dysentery, an illness caused by Shigella, was described in ancient Greece. Osler, in 1892, referred to the disease as "one of the four great epidemic diseases of the world." He further stated: "In the tropics it destroys more lives than cholera, and it has been more fatal to armies than powder and shot." Despite our increased knowledge of the pathogenesis and treatment of shigellosis, this organism continues to be a significant cause of diarrheal disease.

Shigella is a gram-negative, nonmotile, non-lactose-fermenting aerobic bacillus, closely related to members of the genus Escherichia. The organisms are classified into four species or groups known as Shigella dysenteriae, Shigella flexneri, Shigella boydii, and Shigella sonnei (groups A, B, C, and D, respectively). Members of groups A, B, and C exist in numerous serotypes, but only one serotype of group D is known. S. sonnei is the most commonly recovered Shigella species in the developed world, accounting for 70% of isolates in the United States. S. dysenteriae and S. flexneri are the most commonly recovered species of Shigella in the developing world. 91

Shigella is worldwide in its distribution, and the incidence and severity of shigellosis span an equally broad range. In 2008, FoodNet calculated the incidence of Shigella infection in the United States to be 6.59 per 100,000. 77 Although Shigella occurs much less frequently in the developed world, in some studies it is the second most common pathogen identified in cases of bacterial diarrhea in children aged 6 months to 10 years. 92 It may also be the most common bacterial cause of outbreaks of diarrhea in day-care settings. Outbreaks of shigellosis have also been described in residential institutions and on cruise ships. This disease is endemic on American Indian reservations in the Southwest.

Shigella is predominantly spread via the fecal-oral route, with person-to-person contact the most likely method. Secondary spread to household contacts may occur. The infection may be spread through contamination of food and water, as often occurs in areas of poor sanitation and inadequate personal hygiene.

Risk exposures for cases include international travel in the week before symptom onset, attending or working in day care, contact with a child or household member with diarrheal illness, using untreated drinking water or recreational water, and sexual contact with someone with diarrhea. 78 It is important to know that shigellosis should still be considered in patients with watery diarrhea even without a contact history. 93

Patients infected with Shigella may experience a mild, selflimited, watery diarrhea that is clinically indistinguishable from gastroenteritis caused by a variety of other agents. The more classic form of shigellosis, however, is bacillary dysentery. This illness usually begins with fever and malaise, followed by watery diarrhea and cramping abdominal pain. By the second day of illness, blood and mucus are usually present in the stools, and tenesmus has become a prominent symptom. At this point, in approximately 50% of affected patients, the stool volume decreases, with only scant amounts of blood and mucus being passed. 91 This pattern of bloody, mucuscontaining stools is referred to as dysentery. Bacteremia is an uncommon feature of this illness, but several other complications have been reported, including seizures (in children), arthritis, purulent keratitis, and the hemolytic-uremic syndrome (HUS). Nonsuppurative arthritis is the most commonly occurring extraintestinal complication of shigellosis. Patients who carry the histocompatibility locus antigen HLA-B27 may be predisposed to the development of this complication as well as to the development of Reiter's syndrome. The association of seizures with shigellosis was earlier attributed to the neurotoxic effect of the Shigella toxin (Shiga toxin). It now seems likely, however, that the seizures may simply represent a subgroup of common febrile seizures and have no direct relation to the effects of Shiga toxin.

Shigella has been found to cause disease only in humans and in the higher apes. 91 The organisms are potent, with as few as 10 organisms being able to cause disease in a healthy adult. 91 Patients infected with Shigella may excrete 10 5 to 10 8 organisms per gram of feces. This high rate of excretion and the relatively low number of organisms required to produce disease make possible the widespread distribution of disease.

For Shigella to exert its pathologic effect on a host, the bacteria must first come into contact with the surface of an intestinal epithelial cell and induce cytoskeletal rearrangements resulting in phagocytosis. 94, 95 The bacteria then secrete enzymes that degrade the phagosomal membrane, releasing the bacteria into the host cytoplasm. Intracytoplasmic bacteria move rapidly, in association with a comet tail made up of host-cell actin filaments. When moving bacteria reach the cell margin, they push out long protrusions with the bacteria at the tips that are then taken up by neighboring cells, allowing the infection to spread from cell to cell (Figure 39-3) .

Shiga toxin is elaborated by all species, although in greater amounts by S. dysenteriae than by other species, 91 and may play a role in the pathogenesis of Shigella infection. The toxin has neurotoxic, enterotoxic, and cytotoxic effects. 91 Structurally, it is composed of an active, or A, subunit (molecular weight 32 kDa) surrounded by five binding, or B, subunits (77 kDa). 91 The B subunits bind to cell-specific receptors and are taken up by endocytosis. Within the cells, the B subunits are cleaved away, and the remaining A subunit is shortened by proteolysis. This molecule is thought then to bind to the 60S ribosome and inhibit protein synthesis, leading to cell death and sloughing. 96 This is the presumed mechanism for the cytotoxic effect. An enterotoxic effect of Shiga toxin in the ileum may account for the early watery diarrhea.

In patients with signs and symptoms of colitis, the diagnosis of shigellosis should be considered. Stool culture provides the only definitive means to differentiate this organism from other invasive pathogens. Shigella may be cultured from stool specimens or rectal swabs, especially if mucus is present, but there may be a delay of several days from the onset of symptoms to the recovery of organisms. Sigmoidoscopy or colonoscopy typically reveals a friable mucosa, possibly with discrete ulcers. Rectal biopsy may be useful to differentiate shigellosis from ulcerative colitis.

In addition to rehydration, antimicrobial therapy has been recommended for Shigella (1) to shorten the course of the disease, (2) to decrease the period of excretion of the organisms, and (3) to decrease the secondary attack rate, because humans provide the only reservoir for the organism. However, handwashing, rather than use of antimicrobials, is the most effective method to prevent person-to-person spread. Those clinicians who advise against the routine treatment of shigellosis with antibiotics argue that (1) the disease is most often self-limited and (2) the use of antibiotics may facilitate the development of resistant strains and may increase the likelihood of developing HUS.

We recommend antibiotic therapy only for patients who are severely ill at the time of diagnosis or who remain ill at the time of identification of Shigella in a stool culture. A wide range of antibiotics has been used to treat Shigella, necessitated by the development of resistant strains. Currently, the agent of choice is trimethoprim-sulfamethoxazole (trimethoprim, 5 mg/kg [maximum 160 mg], plus sulfamethoxazole, 25 mg/kg [maximum 800 mg] per dose, given every 12 hours, orally or intravenously, for 5 days). Ampicillin (25 mg/kg [maximum 500 mg] per dose, given every 6 hours, orally or intravenously, for 5 days) may be used if local strains are typically susceptible. 97 Amoxicillin is ineffective against Shigella. Nalidixic acid (55 mg/kg per day given every 6 hours for 5 days) has proved effective. Cefixime and ceftriaxone are alternative agents for resistant organisms. 91 Tetracycline, ciprofloxacin, and norfloxacin have been used successfully for the treatment of Shigella, but these agents are approved for use only in adult patients. Multidrug-resistant strains have occurred in Latin America, central Africa, and Southeast Asia. 98 Development of a vaccine for shigellosis continues to be a challenge. These efforts include vaccines using a modified Escherichia coli strain; one using a mutant strain of S. flexneri, which lacks the ability to proliferate intracellularly; and one based on a strain with mutations in its virulence genes. Vaccine development continues to be limited by the lack of a suitable animal model. 99

Campylobacter is a gram-negative, motile, curved or spiralshaped rod, exhibiting a "seagull" appearance when identified in stained stool smears. Multiple species of Campylobacter have been recognized, including Campylobacter jejuni, Campylobacter fetus, Campylobacter coli, and Campylobacter laridis, with C. jejuni being the one most commonly associated with disease in humans. Campylobacter upsaliensis has been reported as another member of this group that causes diarrhea, 100 and it seems probable that still others may be identified.

Campylobacter is recognized to be worldwide in distribution. In developing countries, Campylobacter is a significant bacterial cause of diarrhea in children younger than 2 years of age, yet it rarely occurs in developing nations in older children and adults. When infection does occur in the population older than 2 years of age, it tends to be asymptomatic. 101 It is likely that patients in these countries are infected with Campylobacter early in life and then develop immunity, thus making asymptomatic infection more typical in older children and adults.

In the industrialized world, most patients infected with Campylobacter develop symptoms. 101 The number of Campylobacter infections in these countries is now recognized to be quite high, with some studies finding this organism to be the most common cause of bacterial diarrhea. Campylobacter tends to infect people in two distinct age groups: children in the first year of life and young adults. Campylobacter spp. is the most common cause of bacterial enteric infections in the United States, causing an estimated 2 million infections annually. 102 Campylobacter may be spread by direct contact or through contaminated sources of food and water. Milk, meat, and eggs, especially if undercooked, have been implicated in outbreaks. These sources may be contaminated from human fecal shedding, or the organisms may be harbored in the asymptomatic farm animals. Campylobacter is commonly spread among populations of children in day-care centers. A population-based casecontrol study showed that risk factors for campylobacteriosis were drinking well water, eating fruits and vegetables prepared in the home, having a pet in the home with diarrhea, visiting or living on a farm, riding in a shopping cart next to meat or poultry, and traveling outside of the United States. Infants with campylobacteriosis were less likely to be breast-fed or to be in a household where hamburger was prepared. 103

The mechanisms through which Campylobacter produces disease are not fully understood but likely involve three potential mechanisms 104 the organisms penetrate the mucosa and replicate in the lamina propria and mesenteric lymph nodes. The variety of pathogenic mechanisms may account for the spectrum of disease caused by Campylobacter. It is also conceivable that different strains or serotypes of Campylobacter may demonstrate different pathogenic mechanisms, as is seen with diarrheagenic E. coli.

Campylobacter may cause disease ranging from mild diarrhea to frank dysentery. Typically, patients experience fever and malaise followed by diarrhea, nausea, and abdominal pain that may mimic appendicitis or inflammatory bowel disease. The symptoms usually resolve in less than 1 week. Bacteremia may rarely occur, with some species implicated more often than are others. Campylobacter is also known to cause meningitis, abscesses, septic abortions, pancreatitis, and pneumonia. Guillain-Barré syndrome and Reiter's syndrome are documented to occur as sequelae of Campylobacter infection. Increasing evidence has implicated C. jejuni as the most common antecedent of Guillain-Barré syndrome and the variant form, Miller-Fisher syndrome, a neuropathy associated with ataxia, areflexia, and ophthalmoplegia. 105, 106 Although evidence for molecular mimicry is still preliminary, it is likely that peripheral nerves share epitopes with C. jejuni; therefore, the immune response initially mounted to attack C. jejuni is misdirected to peripheral nerves. 106 After the resolution of symptoms, patients may continue to shed organisms for as long as 7 weeks.

Culture of the organisms, the gold standard for diagnosis, is routinely accomplished in most laboratories if selective media are used and cultures are incubated at 42° C. Because disease caused by Campylobacter is usually mild and self-limited, supportive treatment alone should suffice. In cases of severe disease, erythromycin (10 mg/kg [maximum 500 mg] per dose, given every 6 hours for 5 to 7 days) has been recommended. 97 The need for antibiotic therapy has been questioned, based in part on several studies demonstrating a decrease in the duration of excretion of Campylobacter after antibiotic treatment but no decrease in the duration of symptoms. In general, in these studies, antimicrobial therapy was begun late in the course of the illness. In a placebo-controlled, double-blind trial, Salazar-Lindo and colleagues 107 demonstrated a shortened duration of illness, from 4.2 to 2.5 days, in patients who received erythromycin by day 4 of their illness. For cases of Campylobacter septicemia, gentamicin (1.5 to 2.5 mg/kg per dose, intramuscularly or intravenously, given every 8 hours) is recommended, with chloramphenicol and erythromycin acceptable as alternatives. Tetracycline (250 to 500 mg per dose, intravenously, given every 6 to 12 hours) may be used in patients older than 8 years of age. 97 Ciprofloxacin is an effective alternative agent but is not approved for use in children younger than 18 years of age. Antibiotic treatment is recommended for outbreaks of Campylobacter in day-care settings, because treatment has been shown to eliminate fecal shedding of organisms within 48 hours. 104

The genus Yersinia includes the species Yersinia pestis, which causes plague; Yersinia pseudotuberculosis, known to cause pseudoappendicitis, mesenteric adenitis, and gastroenteritis; and Yersinia enterocolitica, recognized with increasing frequency as a cause of bacterial diarrhea. Yersinia is a gram-negative, coccoid bacillus that is facultatively anaerobic. It is nonlactose-fermenting and is observed to be motile at temperatures of 25° C but nonmotile at 37° C.

Yersinia was initially thought to occur with greater frequency in countries with cooler climates but is now recognized to be worldwide in distribution. Although the true incidence and prevalence of this organism are not known, in some areas yersiniosis occurs more frequently than does shigellosis. 108 Outbreaks due to Yersinia have been associated with spread through contaminated water and foods, including bean sprouts, tofu, and chocolate milk. 108 Pork has also been implicated as a source, as in the Fulton County, Georgia, outbreak in 1990, in which chitterlings were found to be the vehicle of infection. 109, 110 The organism tends to cause disease more frequently in young children, with 24 months the median age in one study. 111 Yersinia may also be spread among household contacts. In addition, there may be an increased incidence in the summer months. 111,108 A case control study from Sweden reported that risk factors for acquiring Y. enterocolitica in children less than 6 years of age were foods prepared from unprocessed raw pork products and treated sausages. Other factors were the use of pacifiers and contact with domestic animals. 112

Y. enterocolitica constitutes a heterogeneous group of serotypes with many identified virulence factors. 113 Y. enterocolitica produces disease in the intestine through an invasive route. After penetrating the mucosal epithelium, primarily in the ileum, organisms replicate in Peyer's patches and accumulate in the mesenteric lymph nodes. 108 Most serotypes produce an enterotoxin similar to the E. coli heat-stable toxin but only at temperatures lower than 30° C; therefore, this toxin may not have an important role in disease production by Yersinia in the human intestine. There is speculation on the role of preformed toxin in causing disease, because toxin may be produced when the organisms are present in refrigerated foods. 108 The virulence of Y. enterocolitica has been shown to be plasmid related. Different serotypes exhibit different degrees of virulence. Serotypes O:3, and O:9 are the ones most frequently associated with diarrheal disease in Europe and Japan, whereas a larger number of serotypes are seen in North America. 113 

The most frequent clinical syndrome caused by Y. enterocolitica is gastroenteritis, which typically affects young children. After an incubation period of 1 to 11 days, patients develop diarrhea, fever, and abdominal pain. 108 A marked increase in the leukocyte count is common. The symptoms usually resolve in 5 to 14 days but have been known to persist for several months. Excretion of organisms occurs for about 6 weeks. 111 Several complications, including appendicitis, have been documented after Y. enterocolitica infection. However, in older children and young adults, Yersinia is more likely to produce the pseudoappendicular syndrome, in which the signs and symptoms mimic appendicitis. 108 In this same age group, there has also been an association of Y. enterocolitica with nonspecific abdominal pain. Radiographic changes in the terminal ileum more often associated with Crohn's disease, namely mucosal thickening and aphthous ulcers, have been seen with yersiniosis in young adults.

Yersinia bacteremia occurs and, despite therapy with appropriate antibiotics, has a case-fatality rate of 34 to 50%. The finding of Yersinia in blood from asymptomatic donors, however, makes the possibility of transient bacteremia seem likely as well. 108 Sequelae of Yersinia infection include erythema nodosum and reactive arthropathy; however, these are more commonly seen in adults. 113 This arthropathy tends to involve the weightbearing joints of the lower extremities and has been noted to occur most often in Yersinia patients who carry the histocompatibility antigen HLA-B27.

Yersinia may be cultured with the use of selective media, preferably with "cold enrichment." Despite the best of methods, culture of Yersinia may require as long as 4 weeks. In addition to diagnosis by culture, Yersinia may also be detected serologically, through the use of agglutinin titers. These measurements appear to be useful only in conjunction with cultures, because agglutinin titers may be affected by a number of factors, including the patient's age, the underlying disease, and previous use of antibiotics and immunosuppressive agents. These titers may also be more useful in Europe and Japan, where infection is caused by a restricted number of serotypes.

Antibiotics have not been proved effective in alleviating symptoms of Yersinia or in shortening the period of its excretion. 108 Pai and associates 114 compared the efficacy of trimethoprim-sulfamethoxazole versus placebo in the treatment of Yersinia gastroenteritis and found no significant difference. It should be noted, however, that therapy was not begun until near the end of the course of the illness. In cases of severe disease and in patients with underlying illness, treatment is recommended. Trimethoprim-sulfamethoxazole, aminoglycosides, chloramphenicol, and third-generation cephalosporins are generally recommended. Tetracycline and quinolones are alternative choices for adult patients. 97 Gentamicin or chloramphenicol is recommended for treatment of septicemia. Because septicemia may be associated with an iron overload state, 115 cessation of iron therapy is also recommended during infection.

Although cholera is a disease rarely encountered in developed countries, it remains an important entity. 116, 117 Investigation of the pathogenesis of cholera led to the recognition and understanding of the mechanism of action of cholera toxin, which remains the prototype for bacterial enterotoxins. Cholera is also important, from a therapeutic perspective, in that initial efforts in the use of oral rehydration solutions were carried out in patients with cholera. However, most importantly, on a worldwide basis, cholera continues to be a major public health problem in almost all developing countries. 118 Cholera afflicts both children and adults, and cholera exists as an endemic disease in more than 100 countries. The death rate is highly dependent on the treatment facilities; the highest mortality rates are in Africa, where case-fatality rates have approximated 10%, especially during epidemic attacks. It is likely that cholera as an endemic infection causes 100,000 to 150,000 deaths annually.

Vibrio cholerae is a gram-negative, motile, curved bacillus that is free-living in bodies of salt water. V. cholerae is classified on the basis of lipopolysaccharide antigens. Until recently, all epidemic strains of V. cholerae were of the O1 serotype. Group O1 is further subdivided into two biotypes: classic and El Tor. Other serotypes were thought to cause sporadic cases of diarrhea but not epidemic disease. This dictum was discarded after the development of an ongoing epidemic in Asia and South America caused by a new serotype, O139, synonym Bengal. 119 Although the pathogenesis and clinical features of O139 cholera are identical to those of O1 cholera, persons having immunity to serotype O1 are not immune to the Bengal serotype. This lack of immunity is primarily a result of the unique O139 cell surface antigen.

V. cholerae is spread via contamination of food and water supplies. There is no evidence of an animal reservoir, but humans may serve as transient carriers. On rare occasions, humans may chronically carry the organism. Owing to the nature of its spread, persons living in areas with adequate sanitation are at minimal, if any, risk for encountering cholera. Cholera does occur in the United States, but usually as a result of imported food brought back by returning international travelers. Travelers from the United States to endemic areas are at low risk (incidence of 1 per 30,000 travelers). 120 Cholera has also been isolated from oysters in the Gulf Coast. 121 However, owing to the frequency of international travel, it is important for the clinician who encounters a patient with severe cholera symptoms (dehydration and rice-water stools) to suspect this infection even in nonendemic areas.

V. cholerae enters its potential host through the oral route, usually in contaminated food or water. Volunteer studies have shown that a relatively large number of organisms (approximately 10 11 ) must be ingested to produce symptoms. Similar to other ingested organisms, V. cholerae must survive the acidic gastric environment. The importance of gastric acidity as a host-protective factor is borne out by the increased occurrence of cholera in patients with absent or reduced gastric acidity.

The organisms travel to the small intestine, where they adhere to the epithelium. This process may be aided by production of mucinase. The intestinal epithelium remains intact with normal morphology. Vibrio species produce a toxin that is composed of a central subunit (A) surrounded by five B subunits; the latter bind to a ganglioside, GM 1 , which serves as the toxin receptor. This binding facilitates the transfer of the A subunit across the cell membrane, where it is cleaved into two components, denoted A 1 and A 2 . The disulfide linkage between A 1 and A 2 is reduced to liberate an active A 1 peptide, which acts as a catalyst to facilitate the transfer of adenosine diphosphate-ribose from nicotinamide adenine dinucleotide to a guanyl nucleotide-binding regulatory protein (G s ). G s then stimulates adenylate cyclase, located on the basolateral membrane, thereby increasing cyclic adenosine monophosphate. This result in turn leads to chloride secretion and a net flux of fluid into the intestinal lumen.

Although this mechanism of toxin action adequately explains the clinical symptoms of cholera, similar symptoms have been noted in patients infected with strains that do not produce the classic cholera toxin. This has led to the recognition that V. cholerae harbors additional virulence factors in the bacterial genome that may contribute to diarrheal disease and must be considered in the design of a nonreactigenic vaccine.

Newly recognized toxins produced by V. cholerae include zonula occludens toxin and the accessory cholera toxin. 122, 123 

After an incubation period, commonly 1 to 3 days, the symptoms of cholera usually begin abruptly with profuse, watery diarrhea and sometimes with vomiting. The stool soon becomes clear, with bits of mucus giving it the so-called rice-water appearance. Patients do not experience tenesmus but rather a sense of relief with defecation. Typically there is no fever. The rate of fluid loss with cholera can be remarkable in severe disease, with purging rates in excess of 1 L/hour reported in adult patients. Despite the dramatic presentation and health risk of "cholera gravis," most patients with cholera infection are asymptomatic or experience mild symptoms. In addition to people with reduced gastric acidity, people with blood group O are at increased risk for more severe disease. Other host factors that predispose to increased purging are less clear, but there is great variability in clinical symptoms after infection.

V. cholerae is identified by colonial morphology and pigmentation on selective agar (e.g., thiosulfate citrate bile salt-sucrose agar). Further identification depends on biochemical markers (e.g., positive oxidase reaction) and motility of the organism. Specific serotyping is used to confirm the identification.

The mainstay of cholera treatment is rehydration. In cases in which the disease is less severe and is recognized early, oral rehydration solutions are appropriate and effective. When purging is excessive (more than 10 mL/kg per hour), intravenous rehydration is required.

Antibiotics have been shown to cause a decrease in duration of the diarrhea, total amount of fluid lost, and length of time organisms are excreted. Tetracycline (250 to 500 mg per dose, given every 6 hours for 3 to 5 days) has been recommended as an appropriate antibiotic for adults, and furazolidone (1.25 mg/kg [maximum 100 mg] per dose, given every 6 hours for 10 days) has been suggested for children and pregnant patients. Ampicillin, chloramphenicol, trimethoprim-sulfamethoxazole, and doxycycline may also be used. Single-dose ciprofloxacin has also been shown to be effective in the treatment of V. cholerae O1 or O139, 124 although this drug is not approved for use in children. A recent randomized controlled trial showed that a single dose of azithromycin 20 mg/kg was superior to ciprofloxacin for treating cholera in children. 125 Despite much progress, an ideal cholera vaccine is not yet available. An ideal vaccine would provide a high level of long-term protection even to those at high risk for severe illness (e.g., people with blood group type O), and this protection would commence shortly after administration of a single oral dose. New oral vaccines have been developed for cholera, including both killed vaccines and live attenuated strains. 126, 127 CVD 103-HgR, a vaccine strain with a 94% deletion of the ctxA, proved efficacious against experimental challenge with V. cholerae El Tor Inaba 3 months after inoculation, suggesting it may be useful for travelers to endemic areas. 128 Unfortunately CVD 103-HgR was not effective in a field trial. 129 Peru-15, a nonmotile strain that colonizes better than CVD 103-HgR, has been shown to be highly effective in volunteer studies. 130 A reformulated bivalent (V. cholerae O1 and O139) killed whole cell oral vaccine was also found to be safe and immunogenic in a cholera-endemic area in India. 131 Other live attenuated O1 oral cholera vaccines are in earlier stages of development including VA1.3 vaccine from India, IEM 108 from China, 132 and an intranasal vaccine. 133

The noncholera vibrios, V. parahaemolyticus, V. pluvialis, V. mimicus, V. hollisae, V. furnissii, and V. vulnificans, have been shown to cause gastrointestinal illness, wound infections, and septicemia. 134 Although each organism has its own characteristics, most noncholera vibrios produce a protein toxin identical to the classic cholera toxin. Some species also produce a heat-stable toxin similar to E. coli heat-stable toxin. 135 Although these organisms produce a cholera-like illness, the stool may sometimes contain blood and leukocytes, and sepsis can occur. This has led to speculation that some members of this group, namely V. parahaemolyticus, may be capable of invasiveness as well as toxin production. 134 In the United States, gastroenteritis caused by these vibrios is most often associated with the ingestion of raw oysters. 136 Gastroenteritis caused by non-O1 vibrios tends to be far milder than that caused by V. cholerae. In severe cases of diarrhea or septicemia, antibiotics may be helpful, with the agents used for V. cholerae recommended.

E. coli constitutes a diverse group of organisms, including both nonpathogenic strains, which are among the most common bacteria in the normal flora of the human intestine, and pathogenic strains. Pathogenic E. coli strains that cause diarrheal illness have been recognized since the 1940s. 137 These diarrheagenic E. coli have been studied extensively and are currently classified, on the basis of serogrouping or pathogenic mechanisms, into six major groups: (1) enteropathogenic E. coli (EPEC), an important cause of diarrhea in infants in developing countries; (2) enterotoxigenic E. coli (ETEC), a cause of diarrhea in infants in developing areas of the world and a cause of traveler's diarrhea in adults; (3) enteroinvasive E. coli (EIEC), which cause either a watery ETEC-like illness or, less commonly, a dysentery-like illness; (4) Shiga toxin-producing E. coli (Stx-producing; formerly known as enterohemorrhagic E. coli), which cause hemorrhagic colitis and HUS; (5) enteroaggregative E. coli (EAggEC); and (6) diffusely adherent E. coli (DAEC), which along with EPEC have been implicated as causes of acute and persistent diarrhea. Each of these groups of E. coli has unique properties (Table 39-3) .

EPEC is a major cause of diarrhea in developing countries. As much as 30% to 40% of infant diarrhea, particularly in those less than 6 months of age, may be caused by EPEC, and in some studies EPEC infection exceeds that of rotavirus. [138] [139] [140] [141] In North America and the United Kingdom, EPEC infections were common during the 1940s through the 1960s; now they are most commonly associated with sporadic cases and nosocomial or daycare outbreaks. 142, 143 However, because of the general unavailability of serotyping, the true incidence of EPEC-associated diarrhea may be underestimated. A 1997 study in Seattle children with diarrhea, and a 2005 study in Cincinnati in which DNA probes were used to screen E. coli present in stool, found a high incidence of EPEC-like organisms (atypical EPEC) in this population. 144, 145 The hallmark of EPEC infection is the "attaching and effacing" lesion seen in the intestine. This lesion is characterized by destruction of microvilli and intimate adherence between the bacterium and the epithelial cell membrane. Directly beneath the surface of the adherent organism, there are marked cytoskeletal changes in the enterocyte, including accumulation of actin polymers. Often, the bacteria are raised on a pedestallike structure as a result of this actin accumulation. A number of steps are probably responsible for the development of this attaching and effacing lesion. As proposed by Donnenberg and Kaper, 146 EPEC pathogenesis consists of three phases: (1) localized adherence, which brings the bacteria in close contact with the enterocyte (e.g., docking); (2) signal transduction, including increases in intracellular calcium and protein phosphorylation; and (3) intimate adherence, a multigene process encoded in the bacterium by a locus of enterocyte effacement. 147, 148 The dramatic loss of absorptive microvilli in the intestine presumably leads to diarrhea via malabsorption. Although this is probably the predominant mechanism, some evidence suggests that a separate secretory mechanism is also involved.

Patients with symptomatic EPEC infection typically experience diarrhea, vomiting, malaise, and fever. The stool may contain mucus but does not usually contain blood. Symptoms with EPEC infection are more severe than with some other enteric infections and may persist for 2 weeks or longer. 137 In some patients, EPEC has caused protracted diarrhea with dehydration, malnutrition, and zinc deficiency as complications; treatment with parenteral hyperalimentation has been required. 143 EPEC can be detected by serotyping of isolated E. coli, 142 by demonstration of the presence of the enterocyte adherence factor or other virulence genes using molecular probes, 149 or by identification of the attaching and effacing phenotype using tissue culture cells. 150 These assays are not commonly used in the clinical microbiology laboratory. Diagnosis of EPEC may be made by demonstrating the presence of adherent organisms on small intestinal or rectal biopsy. 142, 143 Although controlled studies of antibiotic therapy for EPEC have been few, the significant morbidity associated with this agent argues for treatment with antibiotics in most cases. Trimethoprim-sulfamethoxazole (trimethoprim, 5 mg/kg [maximum 160 mg], plus sulfamethoxazole, 25 mg/kg [maximum 800 mg] per dose, given every 12 hours) has been used with some success, as have oral neomycin and gentamicin.

ETEC are recognized as an important cause of diarrhea in infants in developing areas of the world. In endemic areas, children in the first few years of life may be infected several times each year. It is an important cause of diarrhea in infants and in travelers from developed to undeveloped countries, especially in regions of poor sanitation. 151 In the United States, cases of ETEC among children are uncommon. ETEC is also a major cause of traveler's diarrhea in adults. Fecal-oral transmission and consumption of heavily contaminated food or water are the most common vehicles for ETEC infection. The prevention of the spread of ETEC depends on ensuring appropriate sanitary measures: handwashing and proper preparation of food, chlorination of water supplies, and appropriate sewage treatment and disposal. 151 The production of disease by ETEC begins with colonization of the small intestine. There the bacteria depend on fimbriae (also called pili) to facilitate attachment to the mucosal surface and overcome the forward motion of peristalsis. This attachment process causes no detectable structural changes in the architecture of the brush border membrane but does allow the bacteria to release their enterotoxins, heat-labile toxin (LT) and heat-stable toxin (ST), in close proximity to the enterocyte brush border membrane where toxin receptors are present. 152 These toxins in turn stimulate adenylate cyclase (in the case of LT) or guanylate cyclase (in the case of ST), and both ultimately result in a net fluid secretion from the intestine (see the reviews by Cohen and Giannella 153 and by Sears and Kaper 154 ). Two endogenous ligands for the ST receptor, guanylin and uroguanylin, have been identified. 155, 156 This discovery is consistent with the hypothesis that ST is a superagonist and exerts its diarrheal action by means of usurping a normal secretory mechanisms in the intestine (e.g., by molecular mimicry of these less potent endogenous ligands). Uroguanylin may also act as a hormone regulating salt and water excretion in the kidney in response to an oral salt load. 157 Clinically, ETEC infection causes nausea, abdominal pain, and watery diarrhea. Stools typically contain neither mucus nor leukocytes. ETEC can be diagnosed with the use of bioassays such as the suckling mouse assay, immunoassays, or gene probes specific for either ST or LT. PCR assays are also available. However, none of these assays is commonly used in the clinical microbiology laboratory. Supportive measures are sufficient therapy for most cases of ETEC diarrhea, with oral rehydration a mainstay of therapy. Antibiotics, including trimethoprimsulfamethoxazole, have been shown to decrease the duration of fecal excretion of the organisms. Quinolone antibiotics may be more effective, 158 but they are not recommended for use in children. Rifaximin was also shown to provide protection against and treatment for travelers' diarrhea. [159] [160] [161] Cholera toxin (CT) is more than 80% homologous to LT, and vaccination with CT-B subunit (CT-B) based vaccines elicits a protective immune response against LT-producing ETEC strains. Peru-15 (an oral live attenuated candidate cholera vaccine) has been engineered to express and secrete high levels of CT-B; this candidate vaccine Peru-15pCTB has promising characteristics of an oral, single-dose, bivalent cholera/ETEC vaccine 162 and is currently undergoing Phase 1 clinical trial.

EIEC share many common features, including virulence mechanisms, with Shigella. These organisms preferentially colonize the colon and invade and replicate within epithelial cells, where they cause cell death. 137 In addition, both organisms elaborate one or more secretory enterotoxins. Clinically, both Shigella and EIEC infections are characterized by a period of watery diarrhea that precedes the onset of dysentery (scanty stools containing mucus, pus, and blood). More commonly, in contrast to Shigella, only this first phase of watery diarrhea is seen in EIEC infection. This illness is clinically indistinguishable from other causes of bacterial diarrhea (e.g., ETEC) or nonbacterial infectious diarrhea. In a minority of patients with EIEC infections, the dysentery syndrome of characteristic stools, tenesmus, and fever is also seen. Bacteremia is not reported. Infection due to EIEC is uncommon, but foodborne outbreaks of disease have occurred in the United States and aboard cruise ships. Diagnosis is dependent on bioassay (the Sereny test), serotyping, ELISA, or DNA probe techniques. None of these tests is commonly available in the clinical laboratory. Treatment is currently limited to supportive measures, although ampicillin given intramuscularly has been associated with bacteriologic cure and clinical improvement.

Stx-producing E. coli are a distinct class of organisms that have been identified since 1983 as the cause of two recognizable syndromes: hemorrhagic colitis and HUS. 163, 164 Hemorrhagic colitis is an illness characterized by crampy abdominal pain, initial watery diarrhea, and subsequent development of grossly bloody diarrhea with little or no fever. Although there may be more than 100 serotypes in this class of diarrheagenic E. coli, in North America the E. coli serotype O157:H7 is the prototypic member of this family of organisms. E. coli O157:H7 is the most common cause of infectious bloody diarrhea in the United States. 165 Similarly, HUS, which is defined as the triad of acute renal failure, thrombocytopenia, and microangiopathic hemolytic anemia, is also highly associated with antecedent E. coli O157:H7 infection.

Stx-producing E. coli infections may occur in sporadic cases, but they have also been associated with outbreaks of disease in nursing homes, day-care centers, and other institutions; several reviews have been published. [166] [167] [168] [169] It is estimated that E. coli O157:H7 causes approximately 10,000 to 20,000 infections per year in the United States alone and may be responsible for 250 deaths annually. 170 Inadequately cooked hamburgers were most likely the source of the first outbreak and remain the most common vehicle of transmission. In 1993 there was a large epidemic in the western United States; inadequately cooked hamburgers were again implicated as the cause. Aside from ground beef, many other food vectors have been implicated. Epidemics have been attributed to apple juice or cider, and large-scale outbreaks in Japan have been associated with bean sprouts. Contaminated water has also been a source of infection. 171, 172 Common to all of these outbreaks is a reservoir of Stx-producing E. coli in the intestines of cattle and other animals that are asymptomatic. Infection is spread either by direct contact with intestinal contents or through droppings or water runoff from contaminated pastures. A low infectious dose for Stx-producing E. coli and the resistance of these organisms to gastric acid and to the food preserving process (high salt and drying) contribute to the high attack rate. The low infectious dose also contributes to frequent person-to-person transmission. [166] [167] [168] [169] Nonfoodborne outbreaks have been associated with attending child day care, 173 drinking contaminated water, 174 and swimming in unchlorinated water. 175 Both the very old and the very young appear to be at increased risk for Stx-producing E. coli infection and its complications. [166] [167] [168] [169] Clinical features and complications of E. coli O157:H7 infection include bloody diarrhea, nonbloody diarrhea, HUS, thrombotic thrombocytopenic purpura, and, uncommonly, asymptomatic infection. 166 Symptoms may persist for several days or, less commonly, for several weeks. Early reports suggested that carriage of the organism was brief and that prompt culture was necessary to recover these organisms. 176, 177 More recently, prolonged shedding has been observed. 173, 178 This has led to the recommendation that two negative stool cultures be obtained before a child is allowed to return to day care. 173 The identification of Stx-producing E. coli is made difficult because it is not possible to differentiate disease-producing E. coli from normal enteric flora on the basis of standard microbiologic techniques. There are currently six techniques for identification of Stx-producing E. coli: biochemical markers with serotyping (most commonly used), serum antibody tests, cytotoxin bioassays, DNA hybridization, PCR-based tests, and cytotoxin detection (including ELISAs). Some of these methods (e.g., toxin-based assays) detect the presence of cytotoxinproducing organisms, including non-O157 serotypes. It may be important to use both biochemical markers and toxinbased assays in clinical practice to identify organisms that are truly pathogenic. 179 The increased use of non-culture-based methods, such as Shiga toxin enzyme immunoassays, has resulted in a dramatic increase in reports of non-O157 Stxproducing E. coli.

Prevention of disease transmission is made difficult by the fact that these organisms colonize the intestine of healthy cattle and other food animals, including beef, pork, lamb, and poultry. Therefore, they can survive and multiply in the food chain. Proper cooking destroys these organisms; in hamburgers, an internal cooking temperature of 70° C (157° F) renders the meat safe. Practically, safe cooking most commonly results in a gray hamburger (not pink), with clear juices. Risk can be lowered by educating consumers about cross-contamination, use of warning labels now affixed to meat in the United States, and improvements in meat processing and microbial contamination detection.

At present there is no effective therapy to treat Stx-producing E. coli disease, so prevention is the most important strategy. Hemorrhagic colitis has been confused with a number of other conditions, including ischemic colitis, appendicitis, Crohn's disease, ulcerative colitis, cecal polyp, pseudomembranous colitis, and an acute abdomen (ileitis). Therefore, an important aspect of treatment of Stx-associated hemorrhagic colitis is making the correct diagnosis and avoiding unnecessary diagnostic studies such as angiography and laparotomy. The mainstay of therapy for hemorrhagic colitis is the management of dehydration, electrolyte abnormalities, and gastrointestinal blood loss. Antimicrobial agents may help by killing the bacterial pathogens, but they may also cause harm by increasing the release and subsequent absorption of Stx. 180 Trials of antibiotic treatment of Stx-producing E. coli infection are inconclusive. Although these organisms are uniformly sensitive to antimicrobials in vitro, at present there is no evidence that antimicrobial therapy is helpful in diminishing the severity of illness, shortening the duration of fecal excretion, or preventing HUS. 181 Of greater concern is a study suggesting an increased incidence of HUS in those treated with antimicrobials. 182 An attempt to assimilate findings of published series on the subject via a metaanalysis failed to identify an increased risk of HUS in those treated with antimicrobials. 183 Regardless, until more data are available on this topic, most experts would agree that treatment of Stx-producing E. coli with antimicrobials is not advisable. 184 Rifaximin does not cause replication of phage or strain lysis and therefore might not increase the risk of HUS, but it has not been studied in humans with O157 infection. 185, 186 A multicenter trial failed to demonstrate an improved clinical course in pediatric patients treated with Stx-binding resin. 223 Other toxin neutralizing therapies are currently under investigation, including the use of G3 receptor analogues and monoclonal antibodies. 186, 187 

EAggEc and DAEC were initially categorized as part of a larger group of enteroadherent E. coli. These strains differed from classical EPEC strains in that they did not show localized adherence in the Hep-2 cell assay. 188 The aggregative or "stacked brick" appearance of EAggEC in this bioassay permitted epidemiologic investigation, and EAggEc were found to be associated with persistent diarrhea in developing counties. There was uncertainty about EAggEc pathogenicity because these organisms are found in apparently healthy individuals and because some epidemiologic studies failed to show an association with disease. 189, 190 However, evidence from volunteer studies 191, 192 and outbreaks 193 has confirmed the pathogenicity of some EAggEC strains. Studies at Cincinnati Children's Hospital Medical Center have shown that EAggEC are an important unrecognized cause of acute infant diarrhea. 145 The mechanisms by which these organisms cause disease is thought to involve adherence to the intestinal mucosa including dispersin protein, a newly identified EAggEC virulence factor, followed by secretion of one or more enterotoxins and/or stimulation of IL-8 release by a flagellar protein. [194] [195] [196] DAEC are less well characterized but have also been associated with diarrheal disease. Both the HEp-2 cell assay and DNA probes have been used to identify these organisms, but these are not routinely available in the clinical microbiology laboratory.

C. difficile is a spore forming gram-positive anaerobic bacillus. Disease caused by this organism can manifest in a variety of ways, ranging from asymptomatic carriage to potentially lifethreatening pseudomembranous colitis. It is a frequent cause of antibiotic-associated diarrhea and a common nosocomial pathogen.

Of great interest in the study of C. difficile is the difference in the incidence of isolation of the organism and its toxin in various age groups. C. difficile toxin has been found in the feces of 10% of normal-term neonates and 55% of those in a neonatal intensive care unit. 197 Most infants found to have toxin in their stools are asymptomatic. A small group of toxin-positive infants have signs and symptoms of necrotizing enterocolitis, but no clear relation to C. difficile or its toxin has been demonstrated. The presence of C. difficile toxin in these asymptomatic infants may indicate the coexistence of some protective antitoxic substance 198 or may reflect a lack of appropriate toxin receptors in patients in this age group. 199 The incidence of C. difficile toxin positivity decreases beyond the neonatal period. The incidence of asymptomatic carriage in children older than 2 years of age approaches that in healthy adults (about 3%). Furthermore, not all of these organisms are toxin producers. Adults who develop disease from C. difficile infection are also more likely than children to experience severe colitis symptoms, although there are some reports of severe infection in infants, especially those with underlying intestinal pathology such as patients with Hirschsprung's disease or necrotizing enterocolitis. 200 Beginning in December 2002, outbreaks of an unexpectedly large number of C. difficile cases were reported in Quebec, Canada. 201 These outbreaks were characterized by a 4.5-fold increased prevalence over historical incidence rates, a 5-fold increase in mortality, and a 2.5-fold increase in the proportion of complicated cases. During the outbreaks in Canada, a new hypervirulent strain, identified as BI/NAP1/027, was found to be responsible for the increased prevalence and severity. 201 All pathogenic strains of C. difficile produce toxin A or B or both. This BI/NAP1/027 hypervirulent strain produces 16 times more toxin A and 23 times more toxin B than other strains and has now been found throughout the United States and in many parts of the world. Although rates of C. difficile infection are increasing coincidentally with this new strain, it does not appear that the increased prevalence is predominantly due to the emergence of the BI/NAP1/027 strain. Non-BI/NAP1/027 community-acquired strains appear to be more important to the overall increased disease burden. During 2005, nonhypervirulent strains caused severe disease in generally healthy persons in the community at a rate of 7.6 cases per 100,000 population without the usual risk factors of older age, exposure to health care facilities, or antimicrobial use. 202 A 5-year retrospective study revealed an increase in the number of patients seen in the emergency department with community-acquired C. difficile infection. 203 A recent prospective cohort study found C. difficile toxin in 6.7% of stool samples tested in children seen for diarrhea in a children's hospital emergency department in Seattle; this rate is may be an underestimate because in this study only toxin B positive strains were identified. 204 A similar incidence of community-acquired C. difficile associated diarrhea was found in Connecticut. 205 Pathogenesis C. difficile elaborates two important toxins responsible for the inflammation, fluid, and mucus secretion as well as damage to the intestinal mucosa. Toxin A, which is responsible for the activation and recruitment of inflammatory mediators, is a large protein (308 kDa) that binds to an enterocyte surface receptor and activates an intracellular G protein-dependent signal transduction mechanism. 206 Bound toxin results in altered permeability, inhibition of protein synthesis, and direct cytotoxicity. Toxin B demonstrates cytotoxic effects. Most strains produce both toxins, but there are some that elaborate only one or none. 207 A third "binary toxin" or cytodistending toxin (CDT) (actin-specific ADP-ribosyltransferase) is found in 1 to 16% of infected patients and may be associated with more severe diarrhea. Binary toxin has enterotoxic activity in vitro, but its role, if any, in the pathogenesis of C. difficile infection is not yet clear. It may act synergistically with toxins A and B in causing severe colitis. 208 The ability to form spores is thought to be a key feature in enabling the bacteria to persist in patients and the physical environment for long periods, thereby facilitating its transmission. C. difficile is transmitted through the fecal-oral route. Postulated risk factors include contact with a contaminated health care environment, contact with persons who are infected with and shedding C. difficile, and ingestion of contaminated food. 205 Some studies reveal increase risk in patients on gastric acidsuppressing medication. 207

Most patients experience mild, watery diarrhea; abdominal pain and/or tenderness may be present. Although symptoms often last only a few days and spontaneously resolve, in some patients, symptoms persist for weeks to months. There is a broad range of symptoms ranging from asymptomatic carrier, mild to moderate diarrhea with or without blood, colitis with mucopus, and, less frequently, pseudomembranous enterocolitis, where patients are often extremely ill, with high fever, leukocytosis, and hypoalbuminemia. Any mucosal disease, including inflammatory bowel disease, is thought to be a risk factor. In children, inflammatory bowel disease is associated with increased prevalence of C. difficile infection. 209 C. difficilerelated diarrhea frequently occurs in the setting of antimicrobial administration, and hospitalization is another major risk factor for the acquisition of infection.

C. difficile should be suspected in cases of colitis or mild diarrhea in which blood and leukocytes are noted in the stools. Concurrent or recent exposure (within several weeks) to antibiotics should increase the suspicion for C. difficile as the causative agent. The use of virtually any antibiotic may predispose to C. difficile disease.

The "gold standard" for diagnosis of C. difficile is the detection of toxin from fecal samples, using a cell cytotoxicity neutralization assay, and it is based on identifying C. difficile toxin B in cell culture. This assay has a high sensitivity and specificity, but it can take up to 48 hours. Stool culture requires specialized laboratory technique, and it will identify organisms that are not toxin producers, making interpreting a stool culture a challenge. Enzyme immunoassays can detect toxins A and/or B, are rapid, and are less expensive, with a turnaround time of a few hours. They have high specificity, but sensitivity is between 65 to 85% because of the high level of toxin that needs to be present. 207 Sigmoidoscopy in cases of pseudomembranous colitis typically reveals friable white exudate overlying multiple ulcerated areas. The histologic findings of such lesions are depicted in Figure 39 -4. Less commonly, pseudomembranes may not be present in the rectosigmoid but may be present in the more proximal colon.

Pending additional data, for now it seems prudent to restrict routine testing for C. difficile in children with appropriate symptoms who are younger than 12 months to those with unusual risk factors and to test children between 1 and 2 years of age with appropriate symptoms and antimicrobial exposure. Children older than 2 years of age should be evaluated in the same manner as older children and adults, and infection should be considered even in the absence of prior antimicrobial exposure. 200 In cases of mild diarrheal illness caused by C. difficile, discontinuation of any antibiotics the patient is receiving may be sufficient therapy. Although vancomycin is the only U.S. FDA-approved drug for treatment of C. difficile infection, oral metronidazole remains the first-line therapy for mild infection. Compared with vancomycin, metronidazole is much less expensive and has similar efficacy, but in severe infection vancomycin is more effective. 208 In cases of severe illness and especially in cases of pseudomembranous colitis, treatment should include oral vancomycin.

There is a fairly high rate of relapse of illness, generally 15 to 20%, after treatment of C. difficile. These relapses usually occur within 1 month of completion of therapy and sometimes but not always result from the activation of C. difficile spores remaining from the primary infection. 206 Most of these cases of B A relapse are responsive to a second course of metronidazole or vancomycin. The first relapse episode can be treated with the same agent that was used for the initial episode. For the second recurrence, vancomycin or a vancomycin taper or pulse therapy has been recommended. Recurrences can be multiple, and recurrent C. difficile treatment is a challenge. Other treatment options include alternative antibiotics such as rifaximin in conjunction with vancomycin 210 or nitazoxanide. 211 Lactobacillus GG and Saccharomyces boulardii have been beneficial for the prevention of antibiotic-associated diarrhea. 212 For treatment of C. difficile diarrhea recurrence, S. boulardii was found to be effective in adults but has not been well studied in children. Other therapies for recurrent infection include fecal transplantation 213 and intravenous immunoglobulin. 214 None of the toxin binding agents are currently proven to be effective. 215

Several organisms not previously recognized as enteric pathogens have been linked to diarrheal disease. This includes organisms of the genus Aeromonas and the closely related bacterium Plesiomonas shigelloides (previously classified as Aeromonas shigelloides). These organisms are gram-negative, facultatively anaerobic bacilli classified in the family Vibrionaceae. They are oxidase-positive, differentiating them from members of the Enterobacteriaceae. 216

Several members of the genus Aeromonas, including Aeromonas hydrophila, are common inhabitants of fresh and brackish water in the United States. These organisms were initially recognized as opportunistic pathogens in immunocompromised hosts, especially those with malignant hematologic diseases. The organisms also have been known to cause disease in patients with underlying hepatobiliary disease. 216 Aeromonas has been isolated from healthy persons as well and has therefore been thought to be part of the normal flora. Despite initial studies that yielded conflicting results, 217 it is now generally accepted that A. hydrophila is an enteric pathogen.

Studies in Australian children with diarrhea have found Aeromonas species present in 10% of patients. 218 Infection appears to occur most frequently in children younger than 2 years of age. 219 Of patients with Aeromonas isolated from stool cultures at Cincinnati Children's Hospital Medical Center, approximately 50% were younger than 3 months. Aeromonas infection is also seasonal, occurring more often in the summer months. 216 Not all Aeromonas species are pathogenic. In a prospective control study of children with diarrhea, Aeromonas was isolated only from control subjects. 145 The method of pathogenesis remains unclear. Both cytotoxic 219 and enterotoxic 216 properties have been observed, but neither these nor other pathogenic mechanisms are found consistently in strains isolated from patients with Aeromonas-associated disease. 217 Aeromonas caviae, a commonly isolated species, demonstrates both adherence and cytotoxin production. 220 Clinical symptoms attributed to Aeromonas can be grouped into three categories: (1) acute watery diarrhea, the most common syndrome; (2) dysentery, which usually is self-limited; and (3) persistent watery diarrhea. Cramping abdominal pain and vomiting may also occur. 219 Symptoms may occasionally be severe and, especially when dysentery is present, have been incorrectly diagnosed as ulcerative colitis. 218 In mild cases of Aeromonas infection, supportive treatment should suffice. In patients who are immunocompromised, are otherwise acutely ill, or have persistent illness, treatment with antibiotics is recommended. Trimethoprim-sulfamethoxazole is usually effective (trimethoprim, 5 mg/kg [maximum 160 mg], plus sulfamethoxazole, 25 mg/kg [maximum 800 mg] per dose, given every 12 hours for 14 days), as are tetracycline, chloramphenicol, and the aminoglycosides. 216 Most strains of Aeromonas are resistant to the penicillins, including ampicillin. 216 Plesiomonas P. shigelloides, like Aeromonas, is commonly found in the environment, 221 especially in bodies of water, including water from a home aquarium. 222 Unlike Aeromonas, however, Plesiomonas has been reported to occur in epidemics, with contaminated water often found to be the cause. 221 Plesiomonas is also known to be spread through improperly cooked seafood. 223 The pathogenesis of disease caused by P. shigelloides is not well understood. A cytotoxin has been found in some strains 221 but not in others. An invasive mechanism is also suspected, because of the colitis symptoms. 223 In addition to small-volume stools with leukocytes and possible blood, patients may also experience severe abdominal pain. Fever has been seen in approximately one third of patients. 223 In one group of adult patients, symptoms persisted longer than 2 weeks in 75% and longer than 4 weeks in 32%. 223 Diagnosis of P. shigelloides is made by stool culture. Although this illness is usually self-limited, treatment with antimicrobial agents has been shown to decrease the duration of symptoms, 223 with trimethoprim-sulfamethoxazole or aminoglycosides suggested as appropriate choices. There are no controlled trials of antimicrobial treatment of gastroenteritis caused by this organism.

Mycobacterium avium and Mycobacterium intracellulare, known collectively as Mycobacterium avium-intracellulare or Mycobacterium avium complex (MAC), are acid-fast bacilli that have been recognized primarily for their role in cases of atypical tuberculosis. These organisms are now recognized as causative agents of diarrheal symptoms as well. In a review of pediatric cases of atypical mycobacterial infections, Lincoln and Gilbert 224 described two immunocompetent patients whose clinical findings included diarrhea and colonic ulceration.

Of even greater significance than these sporadic cases of MAC infection in immunocompetent hosts is its occurrence among immunocompromised patients. In patients with the acquired immunodeficiency syndrome, MAC is among the most commonly isolated agents causing systemic bacterial infections. 225 These patients may also have chronic diarrhea and abdominal pain. 226, 227 MAC has also been noted to cause diarrhea in patients undergoing bone marrow transplantation 228 and in a patient with cystic fibrosis. 229 The MAC organisms may be cultured from gastric and duodenal aspirates obtained endoscopically and from the stool, the bone marrow, and the blood. 225 Endoscopic examination in patients with MAC may reveal findings similar to those seen in Whipple's disease, with minute superficial ulcerations in the small bowel. 227 Treatment of MAC infections with conventional antituberculosis agents usually is unsuccessful in eradicating the organisms or alleviating symptoms. 225 

Bacteroides fragilis is an anaerobic organism that is commonly isolated from normal stool flora. However, some investigators have identified a toxin-producing variant that is enteropathogenic. Enterotoxigenic Bacteroides fragilis (ETBF) organisms have been isolated from both healthy persons and those with diarrhea. 230 The only known virulence factor of ETBF is the B. fragilis toxin that stimulates secretion of the proinflammatory cytokine, IL-8. 231 Epidemiologic associations with diarrhea in children have been shown for ETBF in several studies 232-235 but not others. 236 A recent observational study in Bangladesh followed children more than 1 year of age and adults to identify individuals infected with B. fragilis. 231 A total of 1209 patients with diarrhea were screened, and 417 (34.5%) yielded B. fragilis, of which 86 (7%) were ETBF. The clinical presentation of infection included abdominal pain, tenesmus, and nocturnal diarrhea that lasted a median of 3 days and resulted in dehydration in 14% of individuals. Fecal leukocytes, lactoferrin, and proinflammatory cytokines increased in the ETBF-infected patients.

Intestinal spirochetosis, or the colonization of the large bowel by Brachyspira aalborgi and related spirochetes, has been implicated as a cause of diarrhea, 237 but its clinical relevance is still controversial. 238, 239 Some studies have shown an association between this organism and bloody diarrhea, 240 although asymptomatic colonization have also been reported. 241 A recent study assessed adult patients with chronic watery diarrhea; of 1174 patients, only 8 were positive for intestinal spirochetosis, it was not diagnosed in the controls (n = 104), and histological resolution of the infection with metronidazole paralleled clinical recovery in 6 patients. 242 The potential of this organism to cause diarrhea requires further evaluation.

This organism has been associated with diarrhea in sporadic cases and in at least one hospital outbreak. Although a causal relation between Hafnia alvei and diarrhea has not been clearly established, a subset of this organism may be enteropathogenic. Organisms isolated from patients with diarrhea typically demonstrate the attaching and effacing lesion seen with EPEC, whereas nonpathogenic isolates do not show this characteristic. 243

Invasive illness caused by Listeria is well known, but it was only recently that convincing evidence was obtained that Listeria can cause acute, self-limited, febrile gastroenteritis in healthy persons. At least seven outbreaks of foodborne gastroenteritis for which L. monocytogenes was the most likely etiology have been described. 244 Convincing evidence came from an outbreak of febrile gastroenteritis associated with the consumption of contaminated chocolate milk. 245 Commonly reported symptoms from outbreaks are fever, diarrhea, arthromyalgia, and headache. Diarrhea is typically nonbloody and watery. Fatigue and sleepiness is frequently reported after an incubation period of 24 hours or less. This gastrointestinal infection is typically self-limited without serious complications in healthy individuals with symptoms lasting 1 to 3 days. A wide variety of foods have been implicated including rice salad, corn-and-tuna salad, chocolate milk, cold smoked rainbow trout, corned beef, cheese, and cold cuts. 244 No data exist regarding the efficacy of treatment with antimicrobials in this illness, and it is not warranted in most instances.

Despite this chapter's extensive catalog of both bacterial and viral infectious agents, from 40 to 60% of cases of diarrhea are currently not attributable to any known cause. Undoubtedly, as techniques for identification and culture become more sophisticated, other causative agents will be identified and the percentage of diarrheal illnesses described as idiopathic or nonspecific will continue to decline. Advances in the widespread use of improved oral rehydration solutions have led to a decline in the morbidity and mortality associated with diarrhea. Future advances in preventive measures, including vaccines, may lead to a reduction of the incidence of diarrheal disease.

Lactobacillus GG administered in oral rehydration solution to children with acute diarrhea: a multicenter European trial

Safety and efficacy of a pentavalent human-bovine (WC3) reassortant rotavirus vaccine

Safety and efficacy of an attenuated vaccine against severe rotavirus gastroenteritis

Prevalence of diarrheagenic Escherichia coli in acute childhood enteritis: a prospective controlled study

Clostridium difficile -more difficult than ever

Estimates of morbidity and mortality rates for diarrheal diseases in American children

Clinical practice. Acute infectious diarrhea

Viral gastroenteritis

Rotavirus as a cause of diarrheal morbidity and mortality in the United States

Impact of acute rotavirus gastroenteritis on pediatric outpatient practices in the United States

Prevention of rotavirus gastroenteritis among infants and children: recommendations of the Advisory Committee on Immunization Practices (ACIP)

Efficacy of a quadrivalent rhesus rotavirus-based human rotavirus vaccine aimed at preventing severe rotavirus diarrhea in infants and young children

Visualization by immune electron microscopy of a 27-nm particle associated with acute infectious nonbacterial gastroenteritis

Sequential rotavirus diarrhoea caused by virus of same subgroup

Viral gastroenteritis

The ever-changing landscape of rotavirus serotypes

Assessment of the epidemic potential of a new strain of rotavirus associated with the novel G9 serotype which caused an outbreak in the United States for the first time in the 1995-1996 season

Global distribution of rotavirus serotypes/genotypes and its implication for the development and implementation of an effective rotavirus vaccine

Rotavirus: the first five years

Epidemiology of rotavirus diarrhea in a prospectively monitored American Indian population

Natural history of human rotavirus infection

Rotavirus infection in adults. Results of a prospective family study

Rotavirus infections in a maternity unit

An epidemic of diarrhoea in human neonates involving a reovirus-like agent and "enteropathogenic" serotypes of Escherichia coli

Asymptomatic endemic rotavirus infections in the newborn

Reovirus-like agent as a cause of nosocomial diarrhea in infants

A case-control study to determine risk factors for hospitalization for rotavirus gastroenteritis in U.S. children

A clinical study of rotavirus gastroenteritis

Rotavirus gastroenteritis

Improved detection of rotavirus shedding by polymerase chain reaction

Clinical features of acute gastroenteritis associated with human reovirus-like agent in infants and young children

Effect of age and malnutrition on rotavirus infection in mice

Extraintestinal rotavirus infections in children with immunodeficiency

Age-dependent diarrhea induced by a rotaviral nonstructural glycoprotein

Comparison of three enzyme immunoassays to tissue culture for the diagnosis of rotavirus gastroenteritis in infants and young children

The management of acute diarrhea in children: oral rehydration, maintenance, and nutritional therapy

Racecadotril in the treatment of acute watery diarrhea in children

A human Lactobacillus strain (Lactobacillus casei sp. strain GG) promotes recovery from acute diarrhea in children

Enhancement of the circulating antibody secreting cell response in human diarrhea by a human Lactobacillus strain

Lactobacillus GG administered in oral rehydration solution to children with acute diarrhea: a multicenter European trial

Oral immunoglobulins for treatment of acute rotaviral gastroenteritis

Rotavirus infections in infants as protection against subsequent infections

Intussusception among recipients of rotavirus vaccine -United States

Efficacy, immunogenicity, and safety of a pentavalent human-bovine (WC3) reassortant rotavirus vaccine at the end of shelf life

Safety and efficacy of a pentavalent human-bovine (WC3) reassortant rotavirus vaccine

Safety and efficacy of an attenuated vaccine against severe rotavirus gastroenteritis

Efficacy of human rotavirus vaccine against rotavirus gastroenteritis during the first 2 years of life in European infants: randomised, double-blind controlled study

Delayed onset and diminished magnitude of rotavirus activity -United States

Norwalk virus genome cloning and characterization

Noroviruses: a comprehensive review

Characterization of SRSVs using RT-PCR and a new antigen ELISA

Molecular characterization of a human calicivirus with sequence relationships closer to animal caliciviruses than other known human caliciviruses

Calicivirus infections in children

Norwalk virus infection of volunteers: new insights based on improved assays

Detection of Norwalk virus or Norwalk-like virus infections in Finnish infants and young children

Norwalk gastroenteritis: a community outbreak associated with bakery product consumption

Norovirus outbreak in an elementary school -District of Columbia

A summertime peak of "winter vomiting disease": surveillance of noroviruses in England and Wales

Norwalk virus-like particle hemagglutination by binding to h histo-blood group antigens

Human susceptibility and resistance to Norwalk virus infection

Norovirus and histo-blood group antigens: demonstration of a wide spectrum of strain specificities and classification of two major binding groups among multiple binding patterns

Norovirus infections in preterm infants: wide variety of clinical courses

Norwalk virus shedding after experimental human infection

International collaborative study to compare reverse transcriptase PCR assays for detection and genotyping of noroviruses

Detection and analysis of a small round-structured virus strain in oysters implicated in an outbreak of acute gastroenteritis

Diagnosis of Norovirus outbreaks by commercial ELISA or RT-PCR

Epidemiology of Norwalk gastroenteritis and the role of Norwalk virus in outbreaks of acute nonbacterial gastroenteritis

Enteric adenovirus infection and childhood diarrhea: an epidemiologic study in three clinical settings

Patients with enteric adenovirus gastroenteritis admitted to an Australian pediatric teaching hospital from 1981 to 1992

Adenovirus gastroenteritis

RNA sequence of astrovirus: distinctive genomic organization and a putative retrovirus-like ribosomal frameshifting signal that directs the viral replicase synthesis

Astroviruses as a cause of gastroenteritis in children

Infantile gastroenteritis associated with excretion of pestivirus antigens

Viral agents of gastroenteritis. Public health importance and outbreak management

The diagnostic gap in diarrhoeal aetiology

Isolation and propagation of a human enteric coronavirus

Toroviridae: a proposed new family of enveloped RNA viruses

Enzyme-linked immunosorbent assay reactivity of torovirus-like particles in fecal specimens from humans with diarrhea

Novel viruses in human faeces

Preliminary FoodNet Data on the incidence of infection with pathogens transmitted commonly through food -10 states

New information about pediatric foodborne infections: the view from FoodNet

Salmonella isolates from humans in the United States

An international outbreak of Salmonella infections caused by alfalfa sprouts grown from contaminated seeds

A national outbreak of Salmonella enteritidis infections from ice cream. The Investigation Team

A case-control study of the epidemiology of sporadic Salmonella infection in infants

A review of human salmonellosis: I. Infective dose

Salmonella takes control: effectordriven manipulation of the host

A review of human salmonellosis: II. Duration of excretion following infection with nontyphi Salmonella

Consensus: management of Salmonella infection in the first year of life

Increase in antimicrobial-resistant Salmonella infections in the United States, 1989-1990

Azithromycin for treating uncomplicated typhoid and paratyphoid fever (enteric fever)

Typhoid vaccines: WHO position paper

Vaccines for preventing typhoid fever

Infections of the Gastrointestinal Tract

Etiology and mechanisms of acute infectious diarrhea in infants in the United States

Acute non-outbreak shigellosis: ten years experience in southern Taiwan

The cell biology of infection by intracellular bacterial pathogens

Genetic basis of virulence in Shigella species

Shiga and Shiga-like toxins

Therapy for acute infectious diarrhea in children

Shigella infections

Shigellosis: the current status of vaccine development

Campylobacter upsaliensis" an unrecognised cause of human diarrhoea?

Campylobacter jejuni isolations from Mexican and Swedish patients, with repeated symptomatic and/ or asymptomatic diarrhoea episodes

Epidemiology of sporadic Campylobacter infection in the United States and declining trend in incidence, FoodNet 1996-1999

Sporadic Campylobacter infection in infants: a population-based surveillance case-control study. Pediatr

Campylobacter jejuni infection and Guillain-Barré syndrome

Miller-Fisher syndrome associated with Campylobacter jejuni bearing lipopolysaccharide molecules that mimic human ganglioside GD3

Early treatment with erythromycin of Campylobacter jejuni-associated dysentery in children

Yersinia enterocolitica

Yersinia enterocolitica O:3 infections in infants and children, associated with the household preparation of chitterlings

Yersinia enterocolitica O:3: an emerging cause of pediatric gastroenteritis in the United States. The Yersinia enterocolitica Collaborative Study Group

Yersinia enterocolitica gastroenteritis: a prospective study of clinical, bacteriologic, and epidemiologic features

Sources of sporadic Yersinia enterocolitica infection in children in Sweden, 2004: a casecontrol study

Yersinia enterocolitica and Yersinia pseudotuberculosis

Placebo-controlled doubleblind evaluation of trimethoprim-sulfamethoxazole treatment of Yersinia enterocolitica gastroenteritis

Antibiotic treatment of Yersinia enterocolitica septicemia: a retrospective review of 43 cases

The human, societal, and scientific legacy of cholera

Emergence of novel strain of Vibrio cholerae with epidemic potential in southern and eastern India

Cholera in the United States, 1965-1991. Risks at home and abroad

Isolation of Latin American epidemic strain of Vibrio cholerae O1 from

Vibrio cholerae produces a second enterotoxin, which affects intestinal tight junctions

Accessory cholera enterotoxin (Ace), the third toxin of a Vibrio cholerae virulence cassette

Randomised controlled comparison of single-dose ciprofloxacin and doxycycline for cholera caused by Vibrio cholerae 01 or 0139

Single dose azithromycin versus ciprofloxacin for cholera in children: a randomized controlled trial

Live oral vaccines against cholera: an update

Field trial of a locally produced, killed, oral cholera vaccine in Vietnam

Randomized, double-blind, placebo-controlled, multicentered trial of the efficacy of a single dose of live oral cholera vaccine CVD 103-HgR in preventing cholera following challenge with Vibrio cholerae O1 El tor inaba three months after vaccination

Efficacy trial of single-dose live oral cholera vaccine CVD 103-HgR in North Jakarta, Indonesia, a cholera-endemic area

Randomized, controlled human challenge study of the safety, immunogenicity, and protective efficacy of a single dose of Peru-15, a live attenuated oral cholera vaccine

A randomized, placebo-controlled trial of the bivalent killed, whole-cell, oral cholera vaccine in adults and children in a cholera endemic area in Kolkata

Urgent need of cholera vaccines in public healthcontrol programs

Intranasal immunization with recombinant toxin-coregulated pilus and cholera toxin B subunit protects rabbits against Vibrio cholerae O1 challenge

Noncholera" Vibrio species

Non-O1 Vibrio cholerae produces two newly identified toxins related to Vibrio parahaemolyticus haemolysin and Escherichia coli heat-stable enterotoxin

Non-O group 1 Vibrio cholerae gastroenteritis associated with eating raw oysters

Escherichia coli that cause diarrhea: enterotoxigenic, enteropathogenic, enteroinvasive, enterohemorrhagic, and enteroadherent

Prospective study of diarrhoeal disease in a cohort of rural Mexican children: incidence and isolated pathogens during the first two years of life

Prevalence of Escherichia coli strains with localized, diffuse, and aggregative adherence to HeLa cells in infants with diarrhea and matched controls

Enteropathogens associated with acute diarrheal disease in urban infants in Sao Paulo, Brazil

Summer diarrhoea in African infants and children

Adherence of bacteria to the intestine in sporadic cases of enteropathogenic Escherichia coli-associated diarrhea in infants and young children: a prospective study

A clinicopathologic study of enterocyte-adherent Escherichia coli: a cause of protracted diarrhea in infants

Genetic and phenotypic analysis of Escherichia coli with enteropathogenic characteristics isolated from Seattle children

Prevalence of diarrheagenic Escherichia coli in acute childhood enteritis: a prospective controlled study

Enteropathogenic Escherichia coli

A genetic locus of enterocyte effacement conserved among diverse enterobacterial pathogens

A cloned pathogenicity island from enteropathogenic Escherichia coli confers the attaching and effacing phenotype on E. coli K-12

Practical and economical method for using biotinylated DNA probes with bacterial colony blots to identify diarrhea-causing Escherichia coli

Actin accumulation at sites of bacterial adhesion to tissue culture cells: basis of a new diagnostic test for enteropathogenic and enterohemorrhagic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC): a recurring decimal in infants' and travelers' diarrhea

Age-related differences in receptors for Escherichia coli heat-stable enterotoxin in the small and large intestine of children

Enterotoxigenic Escherichia coli

Enteric bacterial toxins: mechanisms of action and linkage to intestinal secretion

Guanylin: an endogenous activator of intestinal guanylate cyclase

Characterization of human uroguanylin: a member of the guanylin peptide family

Uroguanylin knockout mice have increased blood pressure and impaired natriuretic response to enteral NaCl load

Prevention of travelers' diarrhea: ciprofloxacin versus trimethoprim/sulfamethoxazole in adult volunteers working in Latin America and the Caribbean

A randomized, double-blind, placebo-controlled trial of rifaximin to prevent travelers' diarrhea

Treatment of travelers' diarrhea: randomized trial comparing rifaximin, rifaximin plus loperamide, and loperamide alone

A randomized, doubleblind, multicenter study of rifaximin compared with placebo and with ciprofloxacin in the treatment of travelers' diarrhea

Construction and preclinical evaluation of recombinant Peru-15 expressing high levels of the cholera toxin B subunit as a vaccine against enterotoxigenic Escherichia coli

Hemorrhagic colitis associated with a rare Escherichia coli serotype

Sporadic cases of haemolyticuraemic syndrome associated with faecal cytotoxin and cytotoxin-producing Escherichia coli in stools

Escherichia coli O157:H7 diarrhea in the United States: clinical and epidemiologic features

Escherichia coli O157:H7 infections: a frequent cause of bloody diarrhea and the hemolytic-uremic syndrome

Hemorrhagic colitis associated with Escherichia coli O157:H7

Escherichia coli O157:H7-associated colitis. A clinical and histological study of 11 cases

Escherichia coli O157:H7: clinical, diagnostic, and epidemiological aspects of human infection

Escherichia coli O157:H7 and the hemolytic-uremic syndrome

A waterborne outbreak in Missouri of Escherichia coli O157:H7 associated with bloody diarrhea and death

A swimming-associated outbreak of hemorrhagic colitis caused by Escherichia coli O157:H7 and Shigella sonnei

Transmission of Escherichia coli O157:H7 infection in Minnesota child day-care facilities

A waterborne outbreak of Escherichia coli O157:H7 infections and hemolytic uremic syndrome: implications for rural water systems

Swimming-associated haemorrhagic colitis due to Escherichia coli O157:H7 infection: evidence of prolonged contamination of a fresh water lake

Haemolytic uraemic syndromes in the British Isles 1985-8: association with verocytotoxin producing Escherichia coli. Part 1: Clinical and epidemiological aspects

The role of Escherichia coli O 157 infections in the classical (enteropathic) haemolytic uraemic syndrome: results of a Central European, multicentre study

Long-term shedding and clonal turnover of enterohemorrhagic Escherichia coli O157 in diarrheal diseases

Shiga toxin-producing Escherichia coli in children with diarrhea: a prospective point-of-care study

Effect of subinhibitory concentrations of antibiotics on extracellular Shiga-like toxin I

controlled trial of antibiotic therapy for Escherichia coli O157:H7 enteritis

The risk of the hemolytic-uremic syndrome after antibiotic treatment of Escherichia coli O157:H7 infections

Risk of hemolytic uremic syndrome after antibiotic treatment of Escherichia coli O157:H7 enteritis: a meta-analysis

Shiga-toxin-producing Escherichia coli and haemolytic uraemic syndrome

Rifaximin does not induce toxin production or phage-mediated lysis of Shiga toxin-producing Escherichia coli

Escherichia coli O157: what every internist and gastroenterologist should know

Engineering an anti-Stx2 antibody to control severe infections of EHEC O157:H7

Comparison of two assay methods for patterns of adherence to HEp-2 cells of Escherichia coli from patients with diarrhea

Colonization by enteroaggregative Escherichia coli in travelers with and without diarrhea

Tissue cultureadherent Escherichia coli in infantile diarrhea

Pathogenicity of enteroadherent Escherichia coli in adult volunteers

Heterogeneity of enteroaggregative Escherichia coli virulence demonstrated in volunteers

Enteroaggregative Escherichia coli

Characterization of enteroadherent-aggregative Escherichia coli, a putative agent of diarrheal disease

Enteroaggregative Escherichia coli elaborate a heat-stable enterotoxin demonstrable in an in vitro rabbit intestinal model

Enteroaggregative Escherichia coli expresses a novel flagellin that causes IL-8 release from intestinal epithelial cells

Clostridium difficile toxin in asymptomatic neonates

Binding kinetics of Clostridium difficile toxins A and B to intestinal brush border membranes from infant and adult hamsters

Diminished Clostridium difficile toxin A sensitivity in newborn rabbit ileum is associated with decreased toxin A receptor

Clostridium difficile infections in children. Pediatr

Clostridium difficile-associated diarrhea in a region of Quebec from 1991 to 2003: a changing pattern of disease severity

Severe Clostridium difficile-associated disease in populations previously at low risk -four states

Changing epidemiology of Clostridium difficile-associated disease in children

Diarrhea etiology in a children' s hospital emergency department: a prospective cohort study

Surveillance for community-associated Clostridium difficile -Connecticut

Characterization of rabbit ileal receptors for Clostridium difficile toxin A. Evidence for a receptor-coupled G protein

Clostridium difficile -more difficult than ever

Impact of Clostridium difficile infection on pediatric inflammatory bowel disease

Interruption of recurrent Clostridium difficile-associated diarrhea episodes by serial therapy with vancomycin and rifaximin

Nitazoxanide versus vancomycin in Clostridium difficile infection: a randomized, double-blind study

Probiotics for the prevention of pediatric antibiotic-associated diarrhea

Recurrent Clostridium difficile colitis: case series involving 18 patients treated with donor stool administered via a nasogastric tube

Descriptive study of intravenous immunoglobulin for the treatment of recurrent Clostridium difficile diarrhoea

Toxin-binding treatment for Clostridium difficile: a review including reports of studies with tolevamer

Aeromonas hydrophila and Plesiomonas shigelloides as causes of intestinal infections

Aeromonas-associated diarrhea in children

Aeromonas-associated gastroenteritis

Clinical and microbiological features of Aeromonas hydrophila-associated diarrhea

Microbiologic and clinical evidence supporting the role of Aeromonas caviae as a pediatric enteric pathogen

Laboratory observations on Plesiomonas shigelloides strains isolated from children with diarrhea in Peru

Aquarium-associated Plesiomonas shigelloides infection -Missouri

Clinical features, epidemiology, and treatment of Plesiomonas shigelloides diarrhea

Disease in children due to mycobacteria other than Mycobacterium tuberculosis

Mycobacterium avium complex infection

Mycobacterium avium complex infections in patients with the acquired immunodeficiency syndrome

Disseminated Mycobacterium avium-intracellulare infection in acquired immunodeficiency syndrome mimicking Whipple' s disease

Mycobacterium avium-intracellulare infections after allogeneic bone marrow transplantation in children

Mycobacterium avium complex in a patient with cystic fibrosis: disease vs. colonization

Isolation of enterotoxigenic Bacteroides fragilis from humans with diarrhea

Association of enterotoxigenic Bacteroides fragilis infection with inflammatory diarrhea

Aetiology of diarrhoea in a birth cohort of children aged 0-2 year(s) in rural Mirzapur

Diarrhea caused by enterotoxigenic Bacteroides fragilis in children less than 5 years of age in Hanoi

Enterotoxigenic Bacteroides fragilis-associated diarrhea in children 0-2 years of age in rural Bangladesh

Enterotoxigenic Bacteroides fragilis: epidemiologic studies of its role as a human diarrhoeal pathogen

Detection of enterotoxigenic Bacteroides fragilis and its toxin in stool samples from adults and children in Italy

Genetic characterisation of intestinal spirochaetes and their association with disease

Human intestinal spirochaetosis: any clinical significance?

Intestinal spirochetes. Organisms in search of a disease?

Intestinal spirochaetosis in children

Intestinal spirochetosis in homosexual men

Intestinal spirochetosis and chronic watery diarrhea: clinical and histological response to treatment and long-term follow up

Heterogeneity in phenotypic and genotypic characteristics among strains of Hafnia alvei

Gastroenteritis due to Listeria monocytogenes

An outbreak of gastroenteritis and fever due to Listeria monocytogenes in milk