key: cord-0769209-m6rmc4my
authors: Yegani, M.; Korver, D.R.
title: Factors Affecting Intestinal Health in Poultry
date: 2008-10-01
journal: Poult Sci
DOI: 10.3382/ps.2008-00091
sha: 3488ad304271f991a7174eb99adfe2bfc4aaac46
doc_id: 769209
cord_uid: m6rmc4my

The gastrointestinal (GI) tract has the most extensive exposed surface in the body and is constantly exposed to a wide variety of potentially harmful substances. The GI tract acts as a selective barrier between the tissues of the bird and its luminal environment. This barrier is composed of physical, chemical, immunological, and microbiological components. A wide range of factors associated with diet and infectious disease agents can negatively affect the delicate balance among the components of the chicken gut and, as a result, affect health status and production performance of birds in commercial poultry operations. Phasing out of antibiotic growth promoters from poultry diets in Europe and recent moves toward reduction or removal of these compounds in other parts of the world including North America will likely change the microbial profile of the GI tract environment in commercial poultry. This paper reviews the GI tract from developmental, immunological, and microbial standpoints and then discusses factors that can affect health status of this system. Necrotic enteritis and coccidiosis and their interactions, and possible consequences of antibiotic growth promoter removal from poultry diets with respect to these diseases, are discussed in more detail.

Poultry meat and egg production have shown a considerable increase worldwide since 1970. The increase in the size of the poultry industry has been faster than other food-producing animal industries. The trade volume of poultry products has also increased parallel to the rapid growth of global poultry meat and egg production (Windhorst, 2006) . Available data indicate that the poultry meat industry has been more dynamic compared with the egg industry over these years (Windhorst, 2006) .

In the poultry industry, feed is the major component of the total cost of production for meat and egg production. Corn and soybean meal remain the main ingredients of choice for poultry diets worldwide. It seems that there are currently no globally applicable alternatives to corn and soybean meal, although inclusion of high levels of wheat in poultry diets is common in some parts of the world (Leeson, 2004) . Feed is probably the most important entity in the poultry industry that can expose the birds to a wide variety of factors through the gastrointestinal (GI) tract.

As incubation progresses, embryonic small intestinal weight increases at a much greater rate than body weight. During the last 3 d of incubation, the ratio of small intestinal weight to body weight increases from approximately 1% on d 17 of incubation to 3.5% at hatch (Uni et al., 2003) .

In the posthatch period, the small intestine continues to increase in weight more rapidly than the rest of body mass. Increases in intestinal weight and length are not identical in the duodenum, jejunum, and ileum. Intestinal development after hatch is also rapid with respect to enzymatic and absorptive activities ; see review by Sklan, 2001 ). The small intestine of the newly hatched chick is immature and undergoes morphological, biochemical, and molecular changes during the 2 wk posthatch, although the most dramatic changes occur within the first 24 h posthatch. The chicken small intestine matures in a manner similar to neonatal mammals .

Toward the end of incubation, the residual yolk sac is internalized into the abdominal cavity. Yolk provides much of the nutrition for the embryo during incubation directly through the circulation, whereas close to hatch and thereafter yolk also reaches the GI tract (Noy and Sklan, 1998) . It has been shown that during the initial 48 h posthatch, yolk contributes to small intestinal maintenance and development. During this period, the chick must make the transition from utilizing energy in the form of lipid supplied by the yolk to utilization of exogenous carbohydrate-rich feed .

Intake of exogenous feed is accompanied by rapid development of the GI tract and associated organs. The timing and form of nutrients available to chicks after hatch is critical for development of intestines. Early access to feed has been shown to stimulate growth and development of the intestinal tract and also enhance posthatch uptake of yolk by the small intestines Geyra et al., 2001; Potturi et al., 2005) . It has also been demonstrated that supplying nutrients to the growing embryo through in ovo feeding may enhance the development of the intestinal tract. Administration of exogenous nutrients into the amniotic fluid at 17 to 18 d of incubation enhanced intestinal development of chicks by increasing the size of the villi and by increasing the intestinal capacity to digest disaccharides. These observations indicated that small intestines of in ovo-fed chicks were functionally equivalent to conventionally fed 2-d-old chicks. Body weight of those chicks was greater than controls (Tako et al., 2004) . In another experiment with turkeys, it was shown that the in ovo-fed poults (injection of arginine and β-hydroxy-βmethyl-butyrate into amnion) hatched with a greater intestinal digestive and absorptive capacity than the conventional poults (Foye et al., 2007) .

Birds show slower intestinal development and depressed performance when access to feed is delayed (Corless and Sell, 1999; Vieira and Moran, 1999; Geyra et al., 2001; Bigot et al., 2003; Maiorka et al., 2003; Potturi et al., 2005) . Usual hatchery practices result in a 24 to 72 h transition between hatching and placing chicks on the farm (Uni, 1999) . The lack of access to feed during this time leads to a depression in intestinal function and bird performance, which may not be overcome at later stage in life Bigot et al., 2003; Potturi et al., 2005) .

The overall organization and mechanisms of immunity in birds are quite similar to those in mammals and this system is directly influenced by genetic, physiological, nutritional, and environmental factors (Qureshi et al., 1998; Sharma, 2003) . The immune system of birds is complex and is composed of several cells and soluble factors that must work together to produce a protective immune response. A properly functioning immune system is of special importance to poultry because most commercial flocks are raised under intensive rearing conditions. Under such conditions, the flocks are vul-nerable to rapid spread of infectious agents and disease outbreaks (Sharma, 2003) .

Lymphoid organs constitute the main structural category of the avian immune system. Among the lymphoid components, the bursa of Fabricius (a site of Blymphocyte development and differentiation) and the thymus (a site of T-lymphocyte development and differentiation) are considered to be primary lymphoid organs (Qureshi et al., 1998) . Functional immune cells leave the primary lymphoid organs and populate secondary lymphoid organs. The secondary lymphoid organs, characterized by aggregates of lymphocytes and antigen-presenting cells, are scattered throughout the body. Spleen, bone marrow, gland of Harder, bronchial-associated (BALT), and gut-associated (GALT) lymphoid tissues are the examples of secondary lymphoid organs (Sharma, 2003) . In chickens, GALT includes the bursa of Fabricius, cecal tonsils, Peyer's patches, and lymphoid aggregates in the urodeum and proctodeum (Befus et al., 1980) .

The gut lining forms the interface between foreign material such as feed and microflora, and the bird. Although most of these materials are not pathogenic, they are capable of stimulating an immune response (Hughes, 2005) . The form of antigen (solid or dissolved), its rate of degradation in the gut, rate of distribution along the intestinal tract, and the rate of absorption can each affect the immunological responses in chickens (Klipper et al., 2000) . Immune responses to GI antigenic stimulation could have negative impacts on feed efficiency, are energetically expensive, and divert nutrients away from production (Collett, 2004; Korver, 2006) .

Concomitant with the development of digestive structures and functions, a rapid development of the GALT occurs. The GALT represents a component of the mucosal-associated lymphoid tissue (MALT), which also includes bronchial, salivary, nasopharyngeal, and genitourinary lymphoid tissues. The MALT acts as the first line of defense on mucosal surfaces .

Lymphoid structures distributed throughout the intestinal tract represent the intestinal arm of the immune system (Qureshi et al., 1998) , but this distribution is not uniform across different segments of the intestine. The chicken's foregut is relatively poor in lymphoid follicles, but numerous follicles are present in the hindgut. These are especially abundant in the ceca ; see review by Sklan, 2005) .

Early feeding following hatch, as stated earlier, can influence both intestinal development and the matu-ration process of GALT. Delay in the onset of feeding causes delay in GALT function (Bar Shira et al., 2005) .

In chickens, the bursa of Fabricius plays a major role in antibody production. The development of the bursa and other GALT such as cecal tonsils and Peyer's patches starts during late embryogenesis (Kajiwara et al., 2003) . At the time of hatching, the bursal duct opens and simultaneously, transport of environmental antigens into the bursal lumen and further into the lymphoid follicles begins. Feed is not sterile and contains many antigens, so the earlier the feed passes through the GI tract, the sooner the proliferating stem cells will meet environmental antigens, which may help to create a wider antibody repertoire (Uni, 1998) .

Bar Shira et al. (2005) investigated the effects of short-term feed withholding (for the first 72 h after hatch) on development of GALT in newly hatched broiler chicks. The GALT activity was determined by antibody production (systemic and locally in the gut), distribution of B and T lymphocytes in the gut, expression of lymphocyte-specific genes, and distribution of B and T lymphocytes in the bursa. It was shown that, although development of GALT in the foregut (duodenum, jejunum, and ileum) was only slightly and temporarily impeded by feed withholding, GALT activity in the hindgut was significantly delayed during the first 2 wk of life. Systemic and intestinal antibody responses following rectal immunization to antigen were lower, colonization of the hindgut (cecum and colon) by T and B lymphocytes was delayed, as well as the expression of T-lymphocyte specific genes in the hindgut. It was also found that the increase of B and T cell population sizes in the bursa was delayed with time.

The GI tract microflora is a mixture of bacteria, fungi, and protozoa, but bacteria are the predominant microorganisms (Gabriel et al., 2006) . Because different bacterial species have different substrate preferences and growth requirements, the chemical composition of the digesta, to a large extent, determines the compositions of the microbial community in the GI tract . Broiler chicken diets containing corn, sorghum, barley, oats, or rye had various effects as corn-and sorghum-based diets increased numbers of Enterococcus, barley-based diet increased numbers of Lactobacillus, oats-based diet enhanced growth of Escherichia and Lactococcus, and rye-based diet increased the number of Streptococcus in broiler chickens (Apajalahti, 2004) .

There is also significant diversity in bacterial populations among different parts of the GI tract and population densities tend to increase from the proximal to distal GI tract . Each region of the GI tract develops its own unique microbial profile, and this community becomes more complex as chickens age (Gong et al., 2002a,b ; Van der Wielen et al., 2002; Guan et al., 2003; Lu et al., 2003; Amit-Romach et al., 2004) . Advances in ribosomal DNA-based molecular techniques have made it possible to obtain new information by identifying different bacterial populations in intestinal contents and mucosal samples as compared with routine culturing methods. These techniques are also helpful for monitoring the effect of diets and other variables on the microbial communities of the GI tract under commercial conditions (Apajalahti et al., 1998 (Apajalahti et al., , 2001 Gong et al., 2002b; Knarreborg et al., 2002; Van der Wielen et al., 2002; Amit-Romach et al., 2004) .

The early stage of the posthatch period is critical for the establishment of the gut microbial community. This process starts from a sterile gastrointestinal environment at the moment of hatching and continues toward establishing a relatively stable status as the animal ages Verstegen et al., 2005) . It has been shown that the composition of the microflora changes in relation to the age of the chickens, dietary factors, breed, and geographic location (Salanitro et al., 1974 (Salanitro et al., , 1978 Apajalahti et al., 2001; Knarreborg et al., 2002; Van der Wielen et al., 2002; Lu et al., 2003) . Apajalahti et al. (2002) have shown that one day after hatch, bacterial densities in the ileum and cecum of the broiler chicks reach 10 8 and 10 10 cells per gram of digesta, respectively. The numbers of microbes reach 10 9 per gram of ileal digesta and 10 11 per gram of cecal digesta during the first 3 d posthatch and remain relatively stable for the following 30 d. Provision of intestinal microflora of healthy adult birds to newly hatched chicks resulted in protection against intestinal infections including different types of Salmonella and also had positive impact on growth rate (Nurmi and Rantala, 1973; Goren et al., 1984 Goren et al., , 1988 .

Microbes of the GI tract can be generally divided into potentially pathogenic or beneficial groups. Harmful bacteria may be involved in localized or systemic infections, intestinal putrefaction, and toxin formation. Some intestinal organisms may have beneficial effects such as vitamin production, stimulation of the immune system through nonpathogenic mechanisms, and inhibition of the growth and establishment of harmful microbial groups (Jeurissen et al., 2002) .

The benefits of normal microbiota such as providing nutrients (Annison et al., 1968) and competitive exclusion (prevention of colonization by pathogenic bacteria) are costly for the host animal (Snoeyenbos et al., 1978; Soerjadi et al., 1982) . The cost associated with these benefits may include competition with the host for nutrients, stimulation of rapid turnover of absorptive epithelial cells, secretion of toxic compounds, and induction of an ongoing inflammatory response in the GI tract. All these effects occur at the expense of animal production performance (Drew et al., 2003; Richards et al., 2005; Verstegen et al., 2005) . As stated earlier, the intestinal microbial profile can be affected by the diet, and changes in dietary composition can alter the microbial community. The dietary composition and microflora and their interaction can affect the intestinal development, mucosal architecture, and the mucus composition of the gut .

Material ingested by a bird can contain nutrients, non-nutrients, and beneficial and potentially harmful organisms. In other words, the digestive tract of the chicken is a major site of potential exposure to pathogens. The lumen normally contains feed and its constituents, resident and transient microbial populations, endogenous nutrients, and secretions from the GI tract and its accessory organs such as the liver, gall bladder, and pancreas. The GI tract must selectively allow the nutrients to cross the intestinal wall into the body while preventing the deleterious components of the diet from crossing the intestinal barrier (Korver, 2006) .

The GI tract acts as a selective barrier between the tissues of the bird and its luminal environment. This barrier is composed of physical, chemical, immunological, and microbiological components. A wide range of factors associated with diet, infectious disease agents, environment, and management practices can negatively affect the delicate balance among the components of the chicken gut and subsequently impair growth rate and feed conversion efficiency (Hughes, 2005) . In this part of this review, some factors affecting intestinal health are briefly presented. Necrotic enteritis and coccidiosis and their interactions, and possible consequences of antibiotic growth promoter (AGP) removal from poultry diets with respect to these diseases, are discussed in more detail.

Nonstarch Polysaccharides. Although there is a wide range of anti-nutritional compounds present in various feed ingredients including cereals, the major group is the non-starch polysaccharides (NSP). All cereals used in poultry diets contain various levels of NSP such as β-glucans and arabinoxylans (Iji, 1999) . Common properties of the different NSP are their resistance to the animal's digestive enzymes and their tendency to create a viscous environment within the intestinal lumen, which result in excretion of sticky droppings (Choct and Annison, 1992a,b) .

High viscosity of the intestinal contents has been shown to cause digestive and health problems. NSP decrease digesta passage rate and availability of nutrients. Increased digesta retention time facilitates bacterial colonization and activity in the small intestine (Waldenstedt et al., 2000) . Barley, wheat, rye, and oats have high levels of NSP which are known to lead to increased digesta viscosity, decreased digeta pas-sage rate, digestive enzymatic activities and nutrient digestibility, depressed feed conversion efficiency, and growth rate of the birds (Bedford and Schulze, 1998) . Specific exogenous enzymes have the ability to break down NSP and reduce digesta viscosity, increase digesta passage rate, and improve bird performance (Almirall et al., 1995; Bustany, 1996; Choct et al., 1996 Choct et al., , 1999 Jorgensen et al., 1996; Leeson et al., 2000; Mathlouthi et al., 2003; Wu et al., 2004) .

Physical Texture and Form of Feed. The physical form of cereal components of feed may affect the morphological and physiological characteristics of the intestinal tract (Brunsgaard, 1998; Engberg et al., 2004) , although published reports in this area of research are inconsistent. Finely ground feed may increase mortality associated with necrotic enteritis compared with coarsely ground feed. Branton et al. (1987) observed that use of wheat ground with a hammer mill (finely ground) increased mortality to 28.9%, but a roller mill-ground wheat diet (coarsely ground) resulted in a mortality of 18.1%. Mortalities were associated with a combination of necrotic enteritis and coccidiosis.

Some studies have shown that dietary whole wheat may contribute to gut performance in broilers through the development of the GI tract, especially the gizzard and also increased absorption of dietary nutrients from the lower digestive tract Yasar, 2003; Engberg et al., 2004; Taylor and Jones, 2004) . Feeding whole wheat to broiler chickens reduced the numbers of Salmonella Typhimurium and Clostridium perfringens in the intestinal tract of the birds (Engberg et al., 2004; Bjerrum et al., 2005) . Inclusion of whole wheat into the diet increased feed conversion efficiency in some studies (Plavnik et al., 2002) , whereas others have not found any positive impact on feed efficiency. Svihus et al. (2004) found no significant effects of diets containing whole wheat on body weight gain and feed conversion efficiency, but results showed that nutrients were more efficiently digested and absorbed from these diets compared with diets with ground wheat. It was suggested that these improvements may result from increased pancreas and liver secretions. In another study on the effect of wheat form (ground or whole) on passage rate through the anterior GI tract, it was demonstrated that although the gizzard has a remarkably high capacity for processing diets with whole wheat, the average passage rate for a diet through the gizzard does not seem to be affected by the form of the wheat . Gabriel et al. (2003) observed that feeding of whole wheat to broiler chickens, experimentally infected with coccidiosis, enhanced development of Eimeria tenella in the ceca. This resulted in a significantly lower weight gain in whole wheat group compared with ground wheat-fed broilers. Banfield et al. (2002) found no effect of feeding whole wheat on performance of birds before infection with coccidiosis or during recovery from the infection. There were significant increases in activity and size of the gizzard and pancreas in whole wheat-fed birds. This increase in gizzard size is in response to the need to do more grinding to process the whole grains before digestion in the lower parts of the GI tract. Other studies (Banfield et al., 1999; Banfield and Forbes, 2001) have also demonstrated that the inclusion of whole wheat had no significant effect on control of coccidial infections.

Based on the results of the above-mentioned studies, it can be concluded that when the GI tract is healthy, inclusion of whole wheat into the diet may help to improve digestive tract function, but when the integrity of the intestinal tract is impaired, inclusion of whole wheat may decrease performance of the GI tract.

The intestinal tract provides the mechanisms by which the body derives nutrition from its environment while safeguarding the bird by various protective mechanisms. The etiology of an enteric disease is complex, as combinations of viruses, bacteria, and other infectious and noninfectious agents may be involved (Reynolds, 2003) . As stated earlier, the purpose of the GI tract is to break down feedstuffs into basic components for transport and absorption for use in maintenance, growth, and production. Physical, chemical, or biological disturbances of these processes can result in enteric disease (Dekich, 1998) .

Bacterial Infections. Low-grade damage to the intestinal tract by pathogenic bacteria may cause poor feed conversion efficiency and decreased rate of body weight gain in poultry flocks. More severe enteric damages by bacterial infections will result in overt disease and high mortality (Porter, 1998) . The lesions of necrotic enteritis (NE) can be among the most severe of any disease that occurs in the chicken intestine (Long et al., 1974) . This disease will be discussed later in this review.

Parasites. Among internal parasites infesting commercial poultry, protozoa are common and some cause moderate or severe diseases. Confinement rearing and high-density flocks have increased the exposure to parasitic diseases such as coccidiosis that have short and direct life cycles (no intermediate host is needed). In contrast, parasitic diseases that depend on an intermediate host for transmission, such as flukes, many cestodes, and some nematodes, have been practically eliminated (McDougald, 2003a) . There will be a further discussion on coccidiosis and its interaction with NE later in the text.

Histomoniasis or Blackhead, caused by Histomonas meleagridis, is a parasitic disorder of the ceca and liver of many gallinaceous birds, but the turkey is the most susceptible host (McDougald, 1998 (McDougald, , 2003c . This disease causes high mortality in turkeys, sometimes approaching 100% of a flock. In chickens, the mortality may be 10 to 20% with high morbidity, although many outbreaks pass unnoticed (McDougald, 2005) . The Blackhead disease organism is carried from host to host by eggs of the cecal worm Heterakis gallinarum (McDougald, 1998) . Lesions of histomoniasis were more severe in turkeys when C. perfringens was present as a monospecific contaminant than when Escherichia coli was present (McDougald, 1998 (McDougald, , 2003c .

Viruses. Many viral infections have been associated with enteric disease conditions (Reynolds, 2003) . These include rotaviruses (McNulty, 2003) , coronaviruses (Guy, 2003) , enteroviruses (McNulty and Guy, 2003) , adenoviruses (Pierson and Fitzgerald, 2003) , astroviruses (Reynolds and Schultz-Cherry, 2003) , and reoviruses (Rosenberger, 2003) . Typical impacts of viral infections on poultry are depressed daily weight gain, impaired feed efficiency, and decreased flock uniformity (Guy, 1998) . Enteric viral infections may occur in birds of all age groups but tend to predominate in young birds (Saif, 2003) . The outcome of these infections is usually dependent on other factors such as age and immune status of affected birds, virulence of the involved virus, other infectious agents, nutrition, management practices, and environmental factors (Guy, 1998) .

Hemorrhagic enteritis is an acute viral disease of turkeys older than 4 wk of age and is characterized by depression, bloody droppings, and death (Pierson and Fitzgerald, 2003) . Hemorrhagic enteritis is immunosuppressive in nature and may exacerbate other diseases (Sponenberg et al., 1985; Van den Hurk et al., 1994; Pierson and Fitzgerald, 2003) .

Avian reoviruses have been isolated from a variety of tissues in chickens affected by disease conditions such as viral arthritis-tenosynovitis, stunting syndrome, respiratory disease, enteric disease, immunosuppression, and malabsorption syndrome (Rosenberger, 2003) . In young broiler chickens, reoviruses are frequently associated with mortality, viral arthritis, and a general lack of performance including diminished weight gain, poor feed conversions, chronic feed passage problems, uneven growth rate, and reduced marketability (Hieronymus et al., 1983; Apple et al., 1991; Dobson and Glisson, 1992; Lenz et al., 1998) .

Although reoviruses have been isolated from flocks with enteric problems, the role of these viruses as primary agents has not been fully proven yet. It is, however, clear that reoviruses can interact with other infectious agents of chickens such as E. coli, infectious bursal disease virus, and Eimeria resulting in increased pathological effects and economic losses (Ruff and Rosenberger, 1985a,b; Rosenberger et al., 1985; Moradian et al., 1990) . Reoviruses may be one of several possible causative agents associated with malabsorption syndrome in chickens. The degree to which birds are affected with this disease is a result of differences in the virus-host interaction, virus pathotype, and age of the bird at time of infection (Sterner et al., 1989; Ali and Reynolds, 1997; Van Loon et al., 2001; Songserm et al., 2000 Songserm et al., , 2002 Songserm et al., , 2003 .

Malabsorption syndrome in broiler chicks presents a very confusing clinical picture for diagnosticians. In a series of trials conducted by Page et al. (1982) , an enlargement of the proventriculus and a reduction in the size of the gizzard were the most frequently encountered postmortem lesions seen. Decreased body weights were also observed in chicks to which reoviruses were orally administered.

Toxins. Feedborne toxins can cause enteric disease. Mycotoxins and biogenic amines are among the most common examples of feedborne toxins (Dekich, 1998) . The presence of mycotoxins in poultry feed has been identified as a widespread cause of economic losses due to impaired health status and reduced performance .

Adverse effects on the GI tract are probably the major cause of economic losses resulting from trichothecene mycotoxicoses (Schiefer and Beasley, 1989) . Mycotoxins such as T-2 toxin can cause caustic injury to the mucosa, destroying cells on the tips of villi, and affect rapidly dividing crypt epithelium. Histopathology of GI tract lesions due to acute intoxication by purified T-2 toxin is characterized by hemorrhage, necrosis, and inflammation of the intestinal epithelium, which occur before transient shortening of villi and reduction in the mitotic activity in crypt epithelium. Necrosis also occurs in the mucosa of the proventriculus and gizzard (Hoerr et al., 1981; Hoerr, 2003) . Mycotoxins can, in combination with other factors, predispose to or exacerbate outbreaks of diseases. For example, it has been shown that ochratoxin A and coccidial infections can interact to adversely affect broiler performance (Huff and Ruff, 1982) .

Biogenic amines including histamine, cadaverine, putrescine, spermine, and spermidine are present in animal protein products. It has been shown that biogenic amines are involved in the occurrence of malabsorption syndrome, which is characterized by decreased feed efficiency and enlargement of the proventriculus (Stuart et al., 1986) . Barnes et al. (2001) demonstrated that dietary histidine and cadaverine can cause the pathologies associated with proventriculitis. The action of histidine and cadaverine appeared to be additive or synergistic.

Necrotic enteritis, caused by Clostridium perfringens, has been reported in most areas of the world where poultry are produced (see review by McDevitt et al., 2006) . Clostridium perfringens is widespread and can contaminate the breeder farms, hatchery, growout houses, and processing plants (Craven et al., 2001 (Craven et al., , 2003 . The lesions of NE can be among the most severe of any disease that occurs in the chicken intestine (Long et al., 1974) . Toxins produced by this bacterium are responsible for intestinal mucosal necrosis, the characteristic lesion of necrotic enteritis (Al-Sheikhly and . The C. perfringens infections in poultry may be presented as acute clinical disease or subclinical disease (Shane et al., 1985; Kaldhusdal and Hofshagen, 1992) . The acute form is accompanied by increased mortality, whereas decreased digestion and absorption, reduced weight gain, increased feed conversion ratio are more commonly seen with the subclinical form of the disease (Kaldhusdal and Hofshagen, 1992; Lovland and Kaldhusdal, 2001; Hofacre et al., 2003) .

Clostridium perfringens is considered to be part of the normal gut flora of poultry, and predisposing factors must be present to produce clinical NE. Intestinal mucosal damage by coccidiosis in chickens is usually considered as one of the most important predisposing factors because coccidiosis is often seen to occur just before or concurrent with outbreaks of NE in the field (Long et al., 1974; Al-Sheikhly and Al-Saieg, 1980; Baba et al., 1992a Baba et al., , 1997 McDevitt et al., 2006) . In turkeys, this mucosal damage is usually caused by coccidiosis, ascaridiasis, or viral hemorrhagic enteritis (Gazdzinski and Julian, 1992; Norton et al., 1992; Droua1 et al., 1995) .

Evidence indicates that there is a strong relationship between certain feed ingredients and the incidence of NE. High levels of fishmeal , wheat (Branton et al., 1987; Riddell and Kong, 1992; Kaldhusdal and Skjerve, 1996; Branton et al., 1997; Annett et al., 2002) , barley (Kaldhusdal and Hofshagen, 1992; Riddell and Kong, 1992; Kaldhusdal and Skjerve, 1996; Annett et al., 2002) , or rye (Riddell and Kong, 1992) can exacerbate outbreaks of NE. It has been shown that inclusion of high levels of fishmeal may result in the overgrowth of C. prefringens. An increase in the concentration of particular amino acids (e.g., methionine) in the lower small intestine may be a triggering factor for the overgrowth of C. perfringens and clinical NE (Drew et al., 2004) . Presence of high levels of NSP in the above-mentioned cereals can increase viscosity of digetsa and decrease digesta passage rate; increased digesta retention time facilitates bacterial colonization and activity in the small intestine (Waldenstedt et al., 2000) . The increased incidence of NE due to these dietary factors might be, in part, related to an increase in clostridial proliferation (Annett et al., 2002) .

Coccidiosis has been studied for over a century and is a disease of major economic importance in the poultry industry (Williams, 2005) . Many species of coccidia are widespread in countries where poultry are produced on a commercial basis . Spread from bird to bird and from flock to flock depends on the survival of oocysts of the parasite in the bedding or soil (McDougald, 1998) .

The protozoan parasites of the genus Eimeria multiply in the intestinal tract and cause tissue damage, which results in mortality, interruption of digestive processes or nutrient absorption, reduced weight gain, and increased susceptibility to other disease agents. The severity of lesions depends mainly on the extent of exposure or number of oocysts ingested by the host (McDougald, 1998 (McDougald, , 2003b Williams, 2005) . Subclinical coccidiosis can be an important contributing factor in the development of necrotic enteritis in broiler chickens because mucosal damage facilitates the establishment and multiplication of C. perfringens Al-Sheikhly and Al-Saieg, 1980; Baba et al., 1992b; McDougald, 2003b) . It has also been shown that certain indigenous bacterial species such as Streptococcus fecalis, E. coli, Lactobacillus species, and Bacteroides species may play a role in pathology of cecal coccidiosis (Bradley and Radhakrishnan, 1973) .

Cecal coccidiosis (caused by E. tenella) may contribute to increased severity of histomoniasis (Blackhead disease) in chickens (McDougald and Hu, 2001) . It has been shown that immunosuppressive diseases or conditions may act in concert with Eimeria to produce a more severe coccidiosis. Marek's disease may interfere with development of immunity to coccidiosis (Rice and Reid, 1973) , and infectious bursal disease may exacerbate coccidiosis (Anderson et al., 1977; McDougald et al., 1979) .

Antibiotic growth promoters in agricultural animal production have been used since the mid-1940s (Dibner and . Evidence indicates that AGP have improved animal performance and health status (Coates et al., 1955; Miles et al., 2006) . Their removal from poultry diets will likely result in changes to the microbial composition of the intestinal tract, which may in turn have consequences for commercial poultry flocks (Knarreborg et al., 2002 ; see review by Bedford, 2000) . On the contrary, there is increasing evidence concerning economical feasibility of incorporation of antibiotics into poultry diets. A group of researchers at the Johns Hopkins University have recently investigated the economic effects of removing antibiotics used for growth promotion in commercial broiler chickens (Graham et al., 2007) . This economic analysis, utilizing large-scale empirical data collected by the US broiler industry, demonstrated that the use of AGP in poultry production was associated with economic losses to producers. It was concluded that the increased weight gains that resulted from AGP are not sufficient to offset the cost of the antibiotics.

The negative economic effects of antimicrobial growth promoter removal on poultry production in Denmark appear to be small and limited to decreased feed efficiency, which is offset, in part, by savings in the cost of AGP. There were no changes in weight gain or mortality related to the termination of AGP use (WHO, 2003) . Other evidence, however, suggests that removal of these antibiotics and subsequent increase in infections in animals, despite efforts to improve animal production management practices, has resulted in a substantial increase in the use of therapeutic antibiotics for food animals in Europe (Casewell et al., 2003) .

A review of the literature reveals that AGP have long been effective in prevention of NE in poultry flocks (Prescott, 1979; Elwinger et al., 1998; Vissiennon et al., 2000; Williams, 2005) . The incidence of NE in poultry flocks has increased in countries that have stopped using AGP (Van Immerseel et al., 2004) . The removal of these compounds from poultry diets in Europe, due to regulatory restrictions or consumer preferences, and recent moves toward reduction or termination of these compounds in North American poultry diets has put tremendous pressure on the poultry industry to look for viable alternatives to deal with NE-associated problems.

It is unlikely that a single economically viable replacement to AGP could be implemented (Dibner and Richards, 2005) . It has become increasingly clear that a multifactorial approach is needed to address the challenges specific to the on-farm situation. An effective alternative to antibiotics should have a significant and sustainable beneficial impact on animal production (e.g., feed efficiency, body weight gain) and health, be safe for both the animal and human population, be easy to apply and store, and provide a substantial return on investment (Collett, 2004) . Nonantibiotic feed additives, alone or in combination, can be effective in prevention of NE . It has also been demonstrated that treating day-old broiler chickens with normal intestinal flora from a healthy adult hen was associated with delayed intestinal proliferation of C. perfringens and reduced frequencies of NE, but regulatory restrictions make it impossible to use this approach at a farm level . It has been shown, however, that complex multispecies probiotics can have beneficial effects in reducing colonization of these bacteria (Schneitz, 2005; Klose et al., 2006) . Maternal immunization against NE can be another alternative measure for the control of NE in broilers (Heier et al., 2001; Lovland et al., 2004) . Although the European Union has banned use of AGP in poultry diets, coccidiostats are currently used in these countries. It is suggested that phasing out of coccidiostats is not realistic because it would have significant detrimental effects on poultry health and welfare (ACAF, 2007) .

The GI tract has the most extensive exposed surface in the body, and a wide variety of factors associated with diet and infectious disease agents can negatively affect the delicate balance among the components of the chicken gut. Disturbances in this balance may affect health status and production performance of birds in commercial poultry operations. Phasing out of AGP from poultry diets in Europe and recent moves toward reduction or removal of these compounds in other parts of the world including North America is a challenge to the poultry industry. It is suggested that phasing out of AGP will likely lead to changes in microbial profile of the intestinal tract. It is hoped that strategies such as infectious disease prevention programs and using nonantibiotic alternatives minimize possible negative consequences of this removal on poultry flocks.

European commission review of the regulation of coccidiostats and histomanostats as feed additives. 39th meeting of ACAF

Role of coccidia in the occurrence of necrotic enteritis of chickens

The interaction of Clostridium perfringens and its toxins in the production of necrotic enteritis of chickens

Stunting syndrome in turkey poults: Isolation and identification of the etiologic agent

The differences in intestinal viscosity produced by barley and β-glucanase alter digesta enzyme activities and ileal nutrient digestibilities more in broiler chicks than in cocks

Microflora ecology of the chicken intestine using 16S ribosomal DNA primers

Influence of infectious bursal disease on the development of immunity to Eimeria tenella

Necrotic enteritis: Effect of barley, wheat and corn diets on proliferation of Clostridium perfringens type A

Volatile fatty acids in the digestive tract of the fowl

Structure and dietary modulation of intestinal microbial communities

Percent G+C profiling accurately reveals diet-related differences in the gastrointestinal microbial community of broiler chickens

Culture-independent microbial community analysis reveals that inulin in the diet primarily affects previously unknown bacteria in the mouse cecum

Effective recovery of bacterial DNA and percent-guanine-pluscytosine-based analysis of community structure in the gastrointestinal tract of broiler chickens

Characteristics of the gastro-intestinal microbial communities with special reference to the chicken

Investigation of a chronic feed-passage problem on a broiler farm in northwest Arkansas

Effects of Eimeria brunetti infection and dietary zinc on experimental induction of necrotic enteritis in broiler chickens

Clostridial population and the intestinal lesions in chickens infected with Clostridium perfringens and Eimeria necatrix

Adhesion of bacteria to the cecal mucosal surface of conventional and germ-free chickens infected with Eimeria tenella

Effects of whole wheat dilution v. substitution on coccidiosis in broiler chickens

Effects of wheat structure and viscosity on coccidiosis in broiler chickens

Effects of whole wheat substitution in broiler diets and viscosity on a coccidial infection in broilers

Impaired immune responses in broiler hatchling hindgut following delayed access to feed

Effects of biogenic amines on growth and the incidence of proventricular lesions in broiler chickens

Removal of antibiotic growth promoters from poultry diets; implications and strategies to minimize subsequent problems

Exogenous enzymes for pigs and poultry

Gut-associated lymphoid tissue in the chicken. I. Morphology, ontogeny, and some functional characteristics of Peyer's patches

Effects of delayed feed intake on body, intestine, and muscle development in neonate broilers

The influence of whole wheat feeding on Salmonella infection and gut flora composition in broilers

Coccidiosis in chickens: Obligate relationship between Eimeria tenella and certain species of cecal microflora in the pathogenesis of the disease

The effect of added complex carbohydrates or added dietary fiber on necrotic enteritis lesions in broiler chickens

Influence of a wheat diet on mortality of broiler chickens associated with necrotic enteritis

Effects of cereal type and feed particle size on morphological characteristics, epithelial cell proliferation, and lectin binding patterns in the large intestine of pigs

The effect of pelleting an enzyme-supplemented barley-based broiler diet

The European ban on growth-promoting antibiotics and emerging consequences for human and animal health

Anti-nutritive effect of wheat pentosans in broiler chickens: Roles of viscosity and gut microflora

The inhibition of nutrient digestion by wheat pentosans

Effects of a xylanase on individual bird variation, starch digestion throughout the intestine, and ileal and caecal volatile fatty acid production in chickens fed wheat

Increased small intestinal fermentation is partly responsible for the anti-nutritive activity of non-starch polysaccharides in chickens

The effect of antibiotics on the intestine of the chick

Controlling gastrointestinal disease to improve absorptive membrane integrity and optimize digestion efficiency. Pages 77-91 in Interfacing Immunity, Gut Health and Performance

The effects of delayed access to feed and water on the physical and functional development of the digestive system of young turkeys

Incidence and tracking of Clostridium perfringens through an integrated broiler chicken operation

Incidence of Clostridium perfringens in broiler chickens and their environment during production and processing

Broiler industry strategies for control of respiratory and enteric diseases

Antibiotic growth promoters in agriculture: History and mode of action

Economic impact of a documented case of reovirus infection in broiler breeders

Effects of dietary protein source and level on intestinal populations of Clostridium perfringens in broiler chickens

Absorption of methionine and 2-hydroxy-4-methylthiobutoanic acid in conventional and germ-free chickens

Relationship of sex, age, and concurrent intestinal disease to necrotic enteritis in turkeys

Effect of antibiotic growth promoters and anticoccidials on growth of Clostridium perfringens in the caeca and on performance of broiler chickens

Influence of whole wheat and xylanase on broiler performance and microbial composition and activity in the digestive tract

The effects of in ovo feeding arginine, β-hydroxy-β-methyl-butyrate, and protein on jejunal digestive and absorptive activity in embryonic and neonatal turkey poults

Ontogeny of gut associated immune competence in the chick

Microflora of the digestive tract: Critical factors and consequences for poultry

Effects of whole wheat feeding on the development of coccidial infection in broiler chickens

Necrotic enteritis in turkeys

Enterocyte dynamics and mucosal development in the posthatch chick

Diversity and phylogenetic analysis of bacteria in the mucosa of chicken ceca and comparison with bacteria in the cecal lumen

Molecular analysis of bacterial populations in the ileum of broiler chickens and comparison with bacteria in the cecum

Reduction of Salmonella infection of broilers by spray application of intestinal microflora: a longitudinal study

Protection of chicks against Salmonella infection induced by spray application of intestinal microflora in the hatchery

Growth promoting antibiotics in food animal production: An economic analysis

Detection and identification of Lactobacillus species in crops of broilers of different ages by using PCR-denaturing gradient gel electrophoresis and amplified ribosomal DNA restriction analysis

Virus infections of the gastrointestinal tract of poultry

Turkey coronavirus enteritis. Pages 300-307 in Diseases of Poultry

A field study of naturally occurring specific antibodies against Clostridium perfringens alpha toxin in Norwegian broiler flocks

Effect of feeding whole cereals on performance, starch digestibility and duodenal particle size distribution in broiler chickens

Identification and serological differentiation of several reovirus strains isolated from chickens with suspected malabsorption syndrome

Mycotoxicoses. Pages 1103-1192 in Diseases of Poultry. Y. M. Saif

Mycotoxicosis caused by a single dose of T-2 toxin or diacetoxyscirpenol in broiler chickens

Using competitive exclusion, mannan-oligosaccharide and other intestinal products to control necrotic enteritis

Eimeria acervulina and Eimeria tenella infections in ochratoxin A-compromised broiler chickens

An integrated approach to understanding gut function and gut health of chickens

The impact of cereal non-starch polysaccharides on intestinal development and function in broiler chickens

Parameters and techniques to determine intestinal health of poultry as constituted by immunity, integrity, and functionality

The influence of dietary fiber source and level on the development of the gastrointestinal tract, digestibility and energy metabolism in broiler chickens

Development of Peyer's patch and cecal tonsil in gut-associated lymphoid tissues in the chicken embryo

Barley inclusion and avoparcin supplementation in broiler diets. 2. Clinical, pathological, and bacteriological findings in a mild form of necrotic enteritis

Reduced incidence of Clostridium perfringens-associated lesions and improved performance in broiler chickens treated with normal intestinal bacteria from adult fowl

Association between cereal contents in the diet and incidence of necrotic enteritis in broiler chickens in Norway

Immune responses of chickens to dietary protein antigens. I. Induction of systemic and intestinal immune responses following oral administration of soluble proteins in the absence of adjuvant

Development of a competitive exclusion product for poultry meeting the regulatory requirements for registration in the European Union

Effects of dietary fat source and subtherapeutic levels of antibiotic on the bacterial community in the ileum of broiler chickens at various ages

Overview of the immune dynamics of the digestive system

Future challenges in poultry meat production. Pages 1-7 in Interfacing immunity

Commercial enzymes and their influence on broilers fed wheat or barley

Gastrointestinal pathogenicity of adenoviruses and reoviruses isolated from broiler chickens in Alabama

Necrotic enteritis in broiler chickens. II. Pathology and proposed pathogenesis

Severely impaired performance in broiler flocks with high incidence of Clostridium perfringens-associated hepatitis

Maternal vaccination against subclinical necrotic enteritis in broilers

Diversity and succession of the intestinal bacterial community of the maturing broiler chicken

Posthatching water and feed deprivation affect the gastrointestinal tract and intestinal mucosa development of broiler chicks

Effect of xylanase and beta-glucanase supplementation of wheat-or wheat-and barley-based diets on the performance of male turkeys

Necrotic enteritis; a continuing challenge for the poultry industry

Intestinal protozoa important to poultry

Protozoal infections. Pages 973-974 in Diseases of Poultry. Y. M. Saif, ed. Iowa State University Press

Coccidiosis. Pages 974-991 in Diseases of Poultry. Y. M. Saif

Other protozoan diseases of the intestinal tract: Histomoniasis (Blackhead)

Blackhead disease (histomoniasis) in poultry: A critical review

A survey of coccidia on 43 poultry farms in Argentina

Blackhead disease (Histomonas meleagridis) aggravated in broiler chickens by concurrent infection with cecal coccidiosis

Interaction of infectious bursal disease and coccidiosis in layer replacement chickens

Rotavirus infections. Pages 308-320 in Diseases of Poultry

Avian enteroviruslike viruses. Pages 326-332 in Diseases of Poultry

Effect of antibiotic growth promoters on broiler performance, intestinal growth parameters, and quantitative morphology

Single and combined infections of specific-pathogen-free chickens with infectious bursal disease virus and an intestinal isolate of reovirus

High mortality of domestic turkeys associated with Ascaridia dissimilis

The effect of early feeding on growth and small intestinal development in the posthatch poult

Yolk utilization in the newly hatched poult

Energy utilization in newly hatched chicks

Yolk and exogenous feed utilization in the posthatch chick

New aspect of salmonella infection in broiler production

Malabsorption syndrome in broiler chickens

Hemorrhagic enteritis and related infections

Effect of feeding whole wheat on performance of broiler chickens

Bacterial enteritides of poultry

Effects of delayed placement on intestinal characteristics in turkey poults

The prevention of experimentally induced necrotic enteritis in chickens by avoparcin

Understanding immunology in disease development and control

Multicausal enteric diseases. Pages 1169-1171 in Diseases of Poultry

Astrovirus infections. Pages 320-326 in Diseases of Poultry

Coccidiosis immunity following early and late exposure to Marek's disease

The gastrointestinal microbiota and its role in monogastric nutrition and health with an emphasis on pigs: Current understanding, possible modulations, and new technologies for ecological studies

The influence of diet on necrotic enteritis in broiler chickens

Reovirus infections. Pages 283-284 in Diseases of Poultry. Y. M. Saif, ed. Iowa State University Press

In vitro and in vivo characterization of avian Escherichia coli. II. Factors associated with pathogenicity

Concurrent infections with reoviruses and coccidia in broilers

Interaction of lowpathogenicity reoviruses and low levels of infection with several coccidial species

Viral enteric infections: Introduction. Pages 299-300 in Diseases of Poultry

Bacteria isolated from the duodenum, ileum, and cecum of young chicks

Isolation, culture characteristics, and identification of anaerobic bacteria from the chicken cecum

Effects on the digestive system and energy metabolism. Pages 61-89 in Trichothecene Mycotoxicosis: Pathophysiologic Effects

Competitive exclusion in poultry-30 years of research. Food Contr

Snider. 1985. Etiology and pathogenesis of necrotic enteritis

The avian immune system

Development of the digestive tract of poultry

Development of defense mechanisms in the digestive tract of the chick

The effect of chronic feeding of diacetoxyscirpenol and T-2 toxin on performance, health, small intestinal physiology and antibody production in turkey poults

Protecting chicks and poults from salmonellae by oral administration of "normal" gut microflora

Weinack. 1982. Adherence of salmonellae and native gut microflora to the gastrointestinal mucosa of chicks

A comparative study of the pathogenesis of malabsorption syndrome in broilers

Enteropathogenicity of Dutch and German avian reoviruses in SPF White Leghorn chickens and broilers

Experimental reproduction of malabsorption syndrome with different combinations of reovirus, Escherichia coli, and treated homogenates obtained from broilers

Field outbreaks of colibacillosis of turkeys associated with hemorrhagic enteritis virus

In vitro and in vivo characterization of avian reoviruses. II. Clinical evaluation of chickens infected with two avian reovirus pathotypes

Proventricular hyperplasia (malabsorption syndrome) in broiler chickens

Passage rate through the anterior digestive tract of broiler chickens fed on diets with ground and whole wheat

Causes for improvement in nutritive value of broiler chicken diets with whole wheat instead of ground wheat

Effects of in ovo feeding of carbohydrates and beta-hydroxy-beta-methylbutyrate on the development of chicken intestine

The incorporation of whole grain into pelleted broiler chicken diets. II. Gastrointestinal and digesta characteristics

Reproduction and treatment of necrotic enteritis in broilers

Impact of early nutrition on poultry: Review of presentations

Functional development of the small intestine in domestic birds: cellular and molecular aspects

Posthatch development of mucosal function in the broiler small intestine

Posthatch development of small intestinal function in the poult

Morphological, molecular, and functional changes in the chicken small intestine of the late-term embryo

Effect of infection with hemorrhagic enteritis virus on susceptibility of turkeys to Escherichia coli

Spatial and temporal variation of the intestinal bacterial community in commercially raised broiler chickens during growth

Clostridium perfringens in poultry: An emerging threat for animal and public health

Isolation of a new serotype of avian reovirus associated with malabsorption syndrome in chickens

The role of the commensal gut microbial community in broiler chickens

Effects of delayed placement and used litter on broiler yields

Effect of avilamycin, tylosin and ionophore anticoccidials on Clostridium perfringens enterotoxaemia in chickens

Intestinal digesta viscosity decreases during coccidial infection in broilers

Intercurrent coccidiosis and necrotic enteritis of chickens: Rational, integrated disease management by maintenance of gut integrity

Changes in poultry production and trade worldwide

Impacts of antimicrobial growth promoter termination in Denmark

Influence of phytase and xylanase, individually or in combination, on performance, apparent metabolisable energy, digestive tract measurements and gut morphology in broilers fed wheat-based diets containing adequate level of phosphorus

Performance, gut size and ileal digesta viscosity of broiler chickens fed with a whole wheat added diet and the diets with different wheat particle sizes