973 15 »py 1 HOG CHOLERA TRANSMISSION THROUGH INFECTED PORK A THESIS Presented to the Faculty of the Graduate School op Cornell University for the degree of DOCTOR OF PHILOSOPHY BY RAYMOND RUSSELL BIRCH [Reprinted from the Journal of the American Veterinary Medical Association, Vol. LI. N. S. Vol. 4, No. 3, June, 1917.] HOG CHOLERA TRANSMISSION THROUGH INFECTED PORK A THESIS Presented to the Faculty op the Graduate School op Cornell University for the degree of DOCTOR OF PHILOSOPHY BY RAYMOND RUSSELL BIRCH [Reprinted from the Journal of the American Veterinary Medical Association, Vol. LI. N. S. Vol. 4, No. 3, June, 1917.] 1 n 3 In escwmgt. Cornell OnU. Library JUN 1 9 1917 HOG CHOLERA TRANSMISSION THROUGH IN- FECTED PORK* K. E. Birch, Ithaca, N. Y. There is no other acute infectious disease of animals which is so widespread as hog cholera. It occurs in almost, if not quite, all countries in which swine are raised, and in some countries there are few large areas entirely free from it. While it is most pre- valent near the more important shipping routes and in localities where large numbers of hogs are raised, it nevertheless appears frequently on remote farms and in localities far removed from busy traffic routes and centers. Its appearance in these seemingly well isolated places has been puzzling, for it is well known that it is caused by a specific virus, and that whenever, it appears in a herd, the virus has in some manner been transferred to the herd from other infected animals. Hog cholera virus, while it is not known to multiply outside the bodies of swine, is very tenacious and resists most natural de- structive influences for long periods of time. A very small quan- tity 1 of it will infect an animal, and it is, therefore, commonly sup- posed that such casual carriers as crows "buzzards, and also various domestic animals not themselves susceptible to hog cholera, are in a large measure responsible for the many seemingly mysterious appearances of the disease. While the facts at hand do not admit doubt concerning the possibility of hog cholera virus transmission by these carriers, there are good reasons to doubt whether they possess the degree of importance usually attributed to them. Circumstances seem to point to some important means of trans- mission less precarious than is furnished by such carriers. Hogs that are fed garbage very frequently contract cholera and garbage often contains pork trimmings. Since garbage feed ing is habitual both with farmers who feed only their own kitchen refuse and with men who make a business of removing and feeding city garbage, it seems reasonable to suppose that this practice may * Presented at the Meeting of the A.V.M.A. Section on Sanitary Science and Police, August 1916, Detroit, Mich. 1. King places the minimum fatal dose of hog cholera virus for a 50 lb. pig somewhere between 1/215 and 1/300 c.c. In his experiments the doses were administered intramuscularly. be responsible for many new herd infections. Further evidence supporting this belief is found in the facts that marketing the seemingly well animals in newly infected herds is a common prac- tice, and that hog cholera virus appears in the blood stream of in- fected animals quite early in the course of the disease. In the past, very little importance seems to have been at- tached to the transmission of hog cholera through infected bits of pork. Dr. James Law 2 mentions pork trimmings as a possible source of infection, but he lays special stress on dangers incident to feeding slaughter house refuse. Hutyra and Marek 3 make no mention of market pork as a possible means of hog cholera trans- mission, and neither do Friedburger and Frohner 4 . Dr. M. Dorset 5 in summarizing the various channels of inter-herd spread of the disease makes no mention of infected pork trimmings. So far as we know the first outbreak traced with any degree of accuracy to infected market pork was one in Canada which McGilvray G reported in 1912. Even that outbreak seems to have been regarded as an exception for very little has been done looking toward the preven- tion of this means of hog cholera transmission. Anti-hog cholera serum has removed one of the greatest ob- stacles in the way of hog cholera control. Not only does it pro- tect herds in which the disease is just starting and prevent its ap- pearance in other threatened herds, but it prevents, or should pre- vent, these herds from being shipped to market at times when they are in condition to infect other swine. It thus removes an almost un- bearable hardship to swine breeders that otherwise would accom- pany the enforcement of strict sanitary measures to prevent ship- ping cholera infected hogs. It has given good reasons to hope for the more complete control or eradication of hog cholera, and in so doing it has centered the efforts of a large number of veterinarians on a more thorough study of the disease itself, and on sanitary measures for its control. Since it cannot be effectively controlled as long as any one common means of transmission remains un- known or unheeded it has seemed desirable to procure exact ex- perimental data on the effects of feeding susceptible pigs bits ot pork such as might be found in garbage. The experiments have been conducted with three kinds of pork; fresh, refrigerated, and cured. Some of the specimens of each kind were taken from carcasses that would have passed in- spection, and others were taken from carcasses that would have been condemned. In all the experiments, before the specimens were removed for feeding, the hams were scalded and scraped as is done in butchering. Except as otherwise stated the material fed consisted of all or a part of the head of a femur together with adjacent parts. With one exception, experiment No. 1 in table No. 2, the hams all came from small shoats weighing less than one hundred pounds each, a fact which might have considerable in- fluence on results obtained from feeding cured pork. Large hams would naturally be expected to harbor virus in their depths with somewhat greater regularity than small ones when both are sub- jected to killing influences that work from without. The susceptible pigs to which the pork trimmings were fed were isolated with great care. In the earlier experiments small fly-tight pens were constructed of screen and matched lumber for this purpose. These were located on a hill several hundred feet from hog yards of any kind. When infection occurred in a pen it was immediately burned, and a new one was constructed on fresh soil for further experiments. The pigs fed in later experi- ments were placed in small individual fire brick pens so con- structed that the attendant could not touch the pigs within. Food and water were introduced through a joint of tile. After each experiment the pen used was cleaned out and a wood fire was kindled inside and allowed to burn for several hours. Thus, in all cases heat, rather than disinfectants, was used to destroy the virus. Most of the pigs were isolated a week or more before being fed and in no case did disease appear previous to feeding. In all cases the experimental pigs were selected from a herd of suscepti- ble animals, and, except as noted, disease did not appear in this herd subsequent to the time the animals were removed. These two facts practically exclude the possibility that any of the ex- perimental animals were infected prior to the time at which they were isolated. In judging the part played by meat inspection in removing cholera infected carcasses from the market, the federal meat in- spection regulations have been selected as a standard, because most of the meat inspected in this country is inspected by federal employees or by others who follow the federal regulations quite closely. Following are the paragraphs that govern antemortem and postmortem inspection in their relation to hog cholera: Regulation 9, section 2, paragraph 2. "All hogs plainly show- ing on ante-mortem inspection that they are affected with either hog cholera or swine plague shall be marked 'U. S. condemned' and disposed of in accordance with section 8 of this regulation." Regulation 9, section 2, paragraph 3. "If a hog has a tem- perature of 106°F. or higher and is of a lot in which there are symptoms of either hog cholera or swine plague, in case of doubt as to the cause of the high temperature, after being marked for identification, it may be held for a reasonable time under the su- pervision of an inspector, for further observation and taking of temperature. Any hog so held shall be reinspected on the day it is slaughtered. If upon such reinspection, or, when not held for further observation and taking of temperature, then on the ori- ginal inspection, the hog has a temperature of 106 °F. or higher it shall be condemned and disposed of in accordance with section 8 of this regulation." Regulation 9, section 2, paragraph 6. "All animals which, on ante-mortem inspection, do not plainly show, but are suspected of being affected with, any disease or condition that, under these regulations, may cause condemnation, in whole or in part, on post-mortem inspection, shall be so marked as to retain their iden- tity as suspects until final post-mortem inspection, when the car- casses shall be marked and disposed of as provided elsewhere in these regulations, or until disposed of in accordance with section 7 of this regulation." Regulation 9, section 4, paragraph 1. "All hogs, even though not themselves marked as suspects, which are of lots one or more of which have been condemned or marked as suspects under sec- tion 2 of this regulation for either hog cholera or swine plague, shall, so far as possible be slaughtered separately and apart from all other animals passed on ante-mortem inspection." Post-mortem inspection. Regulation 11, section 4, paragraph 1. "The carcasses of all hogs marked as suspects on ante-mortem inspection shall be given careful post-mortem inspection, and if it appears that they are affected with either acute hog cholera or swine plague, they shall be condemned." Regulation 11, section 4, paragraph 2. "Carcasses of hogs which show acute and characteristic lesions of either hog cholera or swine plague in any organ or tissue, other than the kidneys or lymph glands, shall be condemned. Inasmuch as lesions resem- bling lesions of hog cholera or swine plague occur in the kidneys and lymph glands of hogs not affected with hog cholera or swine plague, carcasses of hogs in the kidneys or lymph glands of which appear any lesions resembling lesions of hog cholera or swine plague shall be carefully further inspected for corroborative lesions. On such further inspection — "(a) If the carcass shows such lesions in the kidneys, or in the lymph glands or both, accompanied by characteristic lesions in some other organ or tissue, then all lesions shall be regarded as those of hog cholera or swine plague, and the carcass shall be con- demned. " (b) If the carcass shows in any organ or tissue, other than the kidneys or lymph glands, lesions of either hog cholera or swine plague which are slight and limited in extent, it shall be passed for sterilization in accordance with regulation 15. "(c) If the carcass shows no indication of either hog cholera or swine plague in any organ or tissue other than the kidneys or lymph glands, it shall be passed for food, unless some other pro- vision of these regulations requires a different disposal." Most of the virus used in the experiments was the same as was used in our routine work of serum production. It was of an exceedingly virulent strain obtained originally from Dr. W. B. iNiles of Ames, Iowa. Pigs inoculated with 2 c.c. of this virus were usually ready to kill for virulent blood in seven days. In the remainder of the experiments the virus used was obtained from Dr. A. D. Fitzgerald, Columbus, Ohio. This also was of a highly virulent strain. The method of securing carcasses that would pass inspection was to inject small shoats with 2 c.c. each of virulent blood and record temperatures every twenty-four hours subsequent to in- jections. When a decided elevation was recorded the pig was killed and autopsied ; then the ham was removed and scalded and a specimen secured for feeding. In each case the virus was in- jected into the right ham and the specimen fed was secured from the left ham. Complete data concerning these animals appears in table No. 1. Relative to the interpretation of results it should be stated that, except as noted, all the lesions produced were of the acute form of hog cholera, and all the animals that sickened displayed symptoms similar to those produced by that disease. The term "typical lesions of cholera" as used in all the tables in- dicates that the animals in reference revealed on autopsy pe- techiae in the kidneys, and in addition characteristic hemorrhage? (petechiae and ecchymoses) in one or more other organs. The animals that became infected were killed when severe symptoms developed in order that their blood might be used to hyper-immunize hogs in the routine of serum preparation. 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The material fed consisted of rind and subjacent fat. Portions were fed during a period of six days, and, especially in the later feedings, a decidedly rancid odor was present. It is possible that decomposition had something to do with the failure of such large quantities to produce infec- tion. The principal point to be noted is that most of the speci- mens fed produced hog cholera infection. Remarks on table No. 2, Section b. The experiments re- corded in this table were conducted to determine with what regu- larity fresh specimens from hogs killed while in the early stages of hog cholera, and the carcasses of which would pass inspection, would produce hog cholera when fed to susceptible pigs. Of the eight specimens fed, all produced the disease. Remarks on table No. 3, Section a. In this table, the meat referred to as frozen was hung in a rather open garret in an un- healed building from the time the animals were killed until sam- ples of their flesh were fed. The weather was such that the hams were frozen most of the time but in some cases there were perhaps a few days during which they thawed to some extent. The meat referred to as chilled was placed in an ordinary refrigerator dur- ing the time mentioned. It is very probable that experiment No. 17 would have proved negative had it been possible to obtain a subsequent check on the susceptibility of the pig fed. Litter mates of this animal were susceptible. Under the circumstances though the experiment was classed among those showing undetermined results. Experiments number twenty and twenty-two show interest- ing results. In Experiment No. 20 no visible symptoms appeared and no temperatures were taken. The pig subsequently proved to be immune in spite of the fact that it was a litter mate of seven others all of which were highly susceptible. Thus there is very little doubt that the animal was immunized by the material fed to it. Whether the immunizing effect was due to attenuation of the virus or to the small quantity of virus in the specimen is, of course, unknown. In Experiment No. 22 the pig fed showed moder- ate symptoms but recovered. At one time a temperature of 106°F. was recorded. There is little doubt that it also was im- munized in the same manner. Further, it is highly probable that had it been one of a herd of susceptible pigs others would have been infected by associating with it. — +3 p c co a CO o* gfl fc to £ w a CD ^2 bO = a P m 5 d a 02 S ED No. 21 killed when very weak. No. 22 probably infected from 21. Both showed cholera lesions. o © o '6 Eh M 58 CB * CD £ CO _a C £.2 , CO ^3 CD .2 ■"" ' 3 58 A CD CO a o "to CD 58 CD "o XI CD "58 CD 'E EH Pig found April 5 with prolapsed rec- tum and was killed. 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The outstanding fact -brought out in table number four is that the virus of hog cholera in pork is frequently but not always killed during the process of sugar curing. Just what makes the difference between those cases in which it is killed and those in which it is not killed? The three controllable factors involved in the destruction of viruses by chemicals are the kind of chemical used, its dilution, and the time during which it acts. Can any of these influences be so modified that they will destroy the virus in all cases? This is a question that still remains to be answered. As circumstances now appear there seem to be no chemicals that could well be substituted for salt and sugar as preservatives. The strength of the brine might be increased but there is a limit to an increase that would still leave the meat palatable. Increas- ing the time during which pork is in cure or increasing the time during which it is in the store house after being cured may offer •possibilities. The fact that the virus was killed in so many of the specimens might seem to indicate that the time limit during which it can survive the sugar curing process was being approached. As a matter of fact, however, there seems to be no definite relation between the time which the hams were in the store room and the certainty with which specimens from them would prove infec- tious. All the hams were in cure approximately six weeks. The ■time during which different ones were in the store room varied from two to eighty-four days. Specimens from the hams repre- senting these two extremes did not prove infectious. On the other hand specimens from two hams in the store room fifty-seven and eighty days respectively were found to contain living hog cholera virus. It thus appears that if time is to foe employed as a factor in destroying hog cholera virus in sugar cured pork, store room cost and interest on money invested must be considerations. It will be observed that although rind was fed in large quan- tities in individual cases, no infection was caused by it. It was fed in only three experiments though, and so few negative results cannot have much significance. In one instance, ham No. 307, feeding the rind did not produce infection and flesh and bone and, also bone marrow washings from the same ham produced hog cholera. In this one instance the virus was evidently killed in the rind when it survived in the deeper parts. Since rind is very likely to find its way into garbage it is a matter of interest and importance to determine how frequently it carries hog cholera (virus, and it is to be regretted that during the time these experi- ments were in progress scarcity of susceptible pigs prevented de- terminations of this kind. They are not, though, essential. The real problem is not to determine whether there are parts of a ham that do not contain hog cholera virus; it is rather to determine whether there are parts that do contain it. Bone and bits of iclinging flesh are frequently placed in garbage and danger is al- ways present in case they contain virus. It is simply present in a greater degree in case it is found that rind also produces in- fection. Besides hams, the parts most frequently sugar cured are ishoulders and bacon. There are no good reasons to doubt that shoulders carry hog cholera virus in about the same proportion of cases that hams carry it. It seems quite probable that cured bacon, because of its thinness and because of the relative lack of vascularity of its parts, is less likely to contain virus than are ihams and shoulders. This is a point that must be determined with certainty before carcasses showing slight lesions only can be disposed of in the most economical manner. Viewing the entire situation from the standpoint of biology a very interesting group of co-related facts is encountered. If ■the filterable virus were possessed of human intelligence it could iscarcely devise a more insidious and ingenious method of self preservation. It is known to multiply only in the bodies of swine and conditions favorable for its growth are therefore much re- istricted. Nevertheless, the difficulties met are overcome in a re- markable manner. The virus exists in the blood stream of the animals it infects and is thus distributed to all parts of the body ; at cannot at any time be detected with the microscope ; it is present an carcasses before gross examinations will detect it ; it does not infect human beings and thus escapes radical measures that would otherwise be taken for its destruction ; its presence in herds often drives them to market ; it secretes itself in pork where putrefaction, its most deadly natural enemy, is prevented or delayed by curing and low temperatures; then as a final link in a remarkable chain, the virus, in placing itself where possibilities for its distribution are practically limitless is at the same time placing itself in ma- terial which as a common practice is fed to hogs. ,? ft 02 -5 £ 60 S S © ® 08 2 ri i ■ > .2 * 5 -S - £^ gj rt 2 r £ O W5 " c3 'q 60 r/ , rO 0} tc rt s VI r* r- o r— > * s § g ^ & ° « e6 *> 3 2 ® B 03 CD O £ K o £~ | £^ * d3 S « o In general, the results shown in table No. 2, Section b„ should constantly be thought of in connection with those obtained in tables No. 3, Section a, and 4, Section a. The experiments re- corded in table No. 2, Section b, were conducted to determine whether hog cholera virus in sufficient quantities to infect swine is contained in hams taken from hogs killed while in the early stages of the disease. The experiments recorded in tables No. 3, (Section a) and 4, (Section a) were conducted to determine the effects of refrigeration and sugar curing on the life of hog cholera virus contained in hams. It seemed desirable in conducting the latter experiments to use hams from pigs known to be infected; otherwise it would not have been known whether negative results were due to absence of virus in the hams before they were treated, or to the fact that the virus was killed during the processes of re- frigeration and sugar curing. The experiments established two important facts; first, hog cholera virus in sufficient quantities to infect swine is quite con- stantly contained in fresh hams taken from hogs killed before symptoms, (other than rise in temperature) appear, and before lesions form; second, when specimens were taken from pigs show- ing lesions, 43% of the cured ones and 88% of the refrigerated ones proved infectious. Providing all originally contain virus in quantities sufficient to kill, there can, as far as we can see, be no conceivable difference between hams taken from pigs showing lesions and those taken from pigs that do not show lesions, as far as the effects of curing and refrigeration on the virus contained in them is concerned. However, in order to remove doubt concerning this point, experi- ments were conducted with two cured hams (table No. 4, Section b) and two refrigerated hams (table No. 3, Section b) taken from pigs showing no symptoms other than elevation of temperature and no lesions. One of the cured specimens and both of the re- frigerated ones produced infection. It therefore seems likely that had the hams referred to in table No. 2, Section b, been subjected to curing or refrigerating processes, the results would have been similar to those obtained from feeding specimens from virus pigs showing lesions. When the results of the experiments just described are ex- amined in their relation to practices observed in marketing, slaughtering, and inspecting swine, there are several phases of the situation that deserve consideration. Relative to marketing we are at once brought face to face with the fact that 40% of the pork consumed and 15% of that which is marketed in the country is not inspected. This is killed on farms, by local butchers, and by packing establishments that do not supply an inter-state trade. It is a well known fact that many herds are marketed as soon as hog cholera infection is dis- covered in them, and in places where there is no inspection prac- tically all hogs that appear well on foot are killed and sold for food. It is needless to add that large numbers of virus carrying carcasses must be included among those that reach our markets from these sources. Circumstances thus point to a need for ex- tension of both local and federal inspection. Turning now to the pork inspected under federal regulations let us examine the regulations themselves with a view to determin- ing how they operate to eliminate from the market carcasses that contain hog cholera virus. First, though, it should be stated that • the federal regulations compare favorably with those in force in other countries. The efficiency and thoroughness with which they fulfill their lawful purpose — the protection of human health and human life — is not questioned, but if they do not at the same time operate to protect the swine industry of the country, this fact. and the reasons for it should be known, the situation should be looked Bquarely in the face, and a remedy for it should be sought. Under existing conditions a consignment of cholera infected hogs reaches market and is first subjected to ante-mortem inspec- tion. With respect to hog cholera, it may contain five classes of hogs: first, dead hogs; these are condemned and tanked; second, hogs that show undoubted symptoms of cholera ; these are also condemned and tanked; third, hogs that show suspicious symptoms and temperatures below 106°F.; these are slaughtered; carcasses that show lesions of hog cholera are condemned or passed for pterilization according to the extent of the lesions; those that show no lesions are passed for food ; fourth, apparently normal hogs (and those showing suspicious symptoms) that have temperatures above 106 ; these are condemned or isolated for further tempera- ture records; in case further temperatures are taken the animals are condemned if their temperatures are still above 106° ; other- wise they fall into class three or class five; fifth, apparently nor- mal hogs that show temperatures below 106°F. ; these pass ante- mortem inspection and post-mortem as well if they do not show lesions of hog cholera in organs other than the kidneys or lymph glands. Briefly stated, the requirements in order that a given hog may pass inspection are that it shall not show undoubted symptoms of hog cholera, it shall not show suspicious symptoms plus any hog cholera lesions, it shall not show a temperature above 106°F., and regardless of ante-mortem findings the carcass shall not on post- mortem show hog cholera lesions in organs other than the kidneys or lymph glands. What are the chances for virus carrying car- casses to pass inspection? A consideration of symptoms, tempera- tures, and lesions in their relation to the time at which the flesh becomes infectious, will throw some light on this point. Relative to symptoms, it need only be stated that a hog will usually show elevation of temperature from one to three days be- fore any marked symptoms of hog cholera appear. The excite- ment to which hogs are subjected in shipping probably lengthens this time to some extent, because under such circumstances, a slight dullness and sometimes even graver symptoms cannot even by the closest scrutiny be detected. The temperature record, especially when the dividing point is placed as high as 106°F., offers a very uncertain standard upon which to separate infected animals from sound ones, but it consti- tutes a most valuable adjunct to other factors employed for the purpose. In the first place there is a wide variation in the normal temperatures of swine — from 101°F. to 104°F. In the second place weather conditions, excitement due to shipping, and other factors that cannot be controlled alter otherwise normal tempera- tures very materially. It is very probable that most of these in- fluences when they affect temperatures noticeably, operate to ele- vate rather than to lower them, and this probably is the reason why the dividing point — 106 °F. — has been placed so high. It is certain that some hogs may carry temperatures near 106° as a re- sult of excitement or exertion, and it is equally as certain that many others carry temperatures below 106° when they are suffering with hog cholera. Another important thing to recognize is the usual hog cholera curve. It rises quite rapidly as a rule, remains high for a few days, and then takes a decided drop, which, if death does not en- sue, is followed by a second elevation. The following, reproduced from Hutyra and Marek is intended to show a typical hog cholera curve. It appears originally in the centigrade scale, but it lias for the sake of convenience been changed to Fahrenheit. The temperature curves we record in young pigs usually rise above 106° for a short time, and as a rule they do not fall quite as low between the first and second elevations as did the above curve (Jan. 21). In other ways the curves we record correspond quite %^u<«^y L-ylts&t-txsnst*/ /e 7p 13 /* /sr /c /7 iv j