Crisler, O. S.

Elder, Cecil, 1893-

University of Missouri: MOspace

MARCH, 1943 BULLETIN 465 UNIVERSITY OF MISSOURI COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION M. F. MILLER, Director Hog Cholera C EC IL ELDER AND O. S. CRISLE R COLUMBIA, MISSOURI Hog Cholera CECIL ELDER AND O. S. CRISLER In recent years hog cholera has been very prevalent in Missouri. It is the most important disease of hogs and probably causes' heavier losses than all of the other diseases combined. The extent of the dis-ease is directly proportional to the hog population. Traffic or move-ment of swine increases the number of cases. It has been reported by the Bureau of Animal Industry that the losses average at least $20,000,000 a year in the United States, and in peak years may run even as high as $65,000,000. When hog population is increased and the physical equipment on a farm is crowded one may naturally expect an increase in the amount of disease. Unless proper precautions are taken losses from hog cholera infection may be very great. Al-though this disease usually reaches its greatest height during the fall months, it may occur at any time of the year. CAUSE The agent causing hog cholera is ultra-microscopic in size-meaning that it cannot be seen with high powered microscopes. It is usually designated as a filtrable virus because it passes through por-celain filters. This causative agent may be found in the blood of animals sick with the disease, as well as in the organs and body ex-cretions, particularly the urine. Infection gains entrance to a hog's body by way of the digestive tract with contaminated feed and water. The only place the virus will grow and multiply is in the body of the live hog. So far it has not been found practical to cuhivate it in the laboratory. Hogs of all breeds, ages, and both sexes, are susceptible to this disease. Young hogs are more susceptible than older hogs, and partic-ularly is this true in areas where hogs haV'e been raised continuously for a great many years. Although all breeds of hogs are susceptible, it appears that the better bred animals have the disease in a more severe form if they once become affected. Hog cholera virus is usually introduced into a herd through the purchase or addition of new animals. Infective material may be car-ried from one farm to another on the shoes of attendants, or on the wheels of farm vehicles, such as wagons and trucks. It may also be carried short distances by streams. Dogs and foxes have been charged BULLETIN 465 3 with carrying the infection from one farm to another. Trucks, ship-ping crates, railroad cars, etc., that have contained infected animals are particularly dangerous in the spread of the disease unless they have been properly cleaned and disinfected. Because of the practice of many hog raisers selling their hogs on the market if any in the herd show illness, most of the public yards are badly contaminated. Any hogs going through a public yard or loading place are very likely to be exposed to the disease. SYMPTOMS OF HOG CHOLERA The length of time between exposure to a disease and the appear-ance of the first symptom is known as the period of incubation. The period of incubation for hog cholera is usually from three to seven days, but may be somewhat longer than this. The increase in length of time may be due in some cases to the fact that the hogs did not pick up the infection at the time they were first subjected to ex-posure. In most outbreaks of hog cholera there is usually only one hog affected at first; this hog then exposes the balance of the herd, with the result that a large number of hogs sicken after a period of about two weeks has elapsed. Always the first symptom in this disease is an elevation of tem'-perature, but since this may occur from one to three days before other symptoms are noticed it is generally and most always overlooked by the hog owner. Temperatures of 104 0 to 106 0 F., or even higher are not uncommon. The first symptom usually noticed by the hog owner is an im-pairment in or loss of appetite. The hog may come to the feed trough as usual but is easily satisfied, or it may refuse to come to its feed unless urged to do so. There is a telJdency for the sick animals to hide in the litter or remain in darkened corners. When forced to move out of these places they move with reluctance and may stand with back arched and the head depressed after traveling but a short distance. The eyes are filled with a sticky discharge and sometimes the eyelids are glued together. In the first stages of the disease there is a tendency toward con-stipation but this is followed in a few days with diarrhea. The hogs lose weight rapidly and have a gaunt, tucked-up appearance. The thinner skinned parts of the body, such as the underside of the neck and abdomen, the inner ~ide of the front legs and thighs, may show areas which are dark red or even purplish in appearance. If the lungs are affected there will be a cough and difficulty in breathing. As the dis-ease progresses the animals develop a weak, wobbly gait, particularly 4 MISSOURI AGRICULTURAL EXPERIMENT STATION noticeable in the hind legs. They finally lie on the ground exhausted and usually die in convulsions. The symptoms of this disease are quite variable and it may be that not all of the symptoms described are found in anyone hog. When several hogs are affected all of the symp-toms described will be observed. The disease has been divided into the acute and chronic forms-depending upon the length of time the animals are sick. Generally the first cases that appear in an outbreak of hog cholera will be of the acute type, in which death takes place in seven days or less after the animal is first affected. The chronic cases last longer and the animals may be sick for two or three weeks, or even show evidence of disease and live for several weeks or months before they die. Not all hogs with cholera will die, but a few will make recoveries, while others make only incomplete recoveries. The death loss from cholera is quite variable but one may ex-pect the following to take place: On farms or in communities where cholera has not been present for the past few years and then breaks out in a herd, the mortality rate is likely to be very high-eyen ap-proaching in some cases 80 to 90 per cent. On farms or in communities where the disease has been more or less prevalent year after year the losses from cholera seldom run over 30 to 40 per cent, and may be considerably lower. CHANGES OBSERVED AFTER DEATH There are certain changes in body tissues observed on postmortem examination which aid in the diagnosis of hog cholera, but sometimes it is even difficult for trained veterinarians to diagnose the disease from the lesions or changes observed after death. It may be necessary to examine more than one animal 'to be sure of a diagnosis and it may be necessary to kill one of the visibly affected animals in order to arrive at a proper diagnosis. The most outstanding change observed after death is the ap-pearance of small hemorrhages, which vary from a pin point to a pin head in size, or even larger. These hemorrhages may be few or very numerous. They, as well as other typical lesions, may be entirely absent in the very acute cases of the disease. In other words, in some animals death may take place so quickly that there is little or no tissue change to be observed by the naked eye. Hemorrhages are particularly noticeable in the kidney, both under the capsule or connective tissue covering, and on the inner cut surface of the kidney structure. They may vary from ooe or a few BULLETIN 465 5 up to a number that makes the kidney have a turkey egg appearance. These small hemorrhages may also be found on the inner surface of the bladder, in the lungs, in the larynx, and in the mucous membrane of the stomach and intestinal tract. When the lungs have been affected one will also observe lesions of pneumonia. In cases of somewhat longer duration the so-called "button ulcers" may be observed in the stomach and intestinal tract. Since postmortem changes in this disease may be confused with changes occurring in other diseases, postmortem examination should be conducted by a graduate veterinarian. TREATMENT OF SICK ANIMALS Unfortunately, there is no drug treatment of any kind which is of significant value in treating animals already sick with hog cholera. Some benefit may be expected in the early stages of an outbreak when large doses of anti-hog-cholera serum are administered. It appears to be a waste of money and labor to attempt to treat hogs which are already showing well developed symptoms of the disease. As cura-tive treatment is of questionable value, it is best to place most of the emphasis upon preventive measures. PREVENTION OF HOG CHOLERA If hogs can be properly protected from exposure to hog cholera virus they could be raised free of this disease without resorting to vaccination. Since this is difficult on the average farm or in the feed lot, the best thing to do is to immunize the animals, which can be done by proper vaccination. One of the best ways to prevent heavy losses would be to have an accurate early diagnosis made by a veter-inarian when the first hog is noticed sick, rather than wait until large numbers of the animals have become exposed or are showing symptoms. If a diagnosis is made early, heavy losses can be prevented and most of the herd saved by vaccination. The use of anti-hog-cholera serum alone gives only a passive im-munity which may last for a short time, but which cannot absolutely be depended upon for much ,over two weeks. The injection of anti-hog-cholera serum, with potent hog cholera virus, but given in dif-ferent parts of the body (the so':'called double treatment) will develop an immunity in hogs usually sufficient to protect them against ordinary field exposure for the life of the animal. When the disease is present in a herd before vaccination is started larger doses of serum are absolutely essential. Many veterinarians increase the dosage under these conditions to 1 Yz or 2 times the normal dosage recommended . on the label of the container. Extra serum does no harm. Smaller doses 6 MISSOURI AGRICULTURAL EXPERIMENT STATION of serum can be used with safety only when the vaccination is done while the herd is stilI healthy and has not been subjected to any expos-ure to hog cholera infection. Strict sanitary measures should be taken when hogs are vaccinated and the animals kept in clean pens or turned on to pastures free from mud wallows, to avoid infection. Hogs may be vaccinated at any age but · the cost increases with increased age and weight. If for no other than economical reasons vaccination should be done while the hog is small, or about weaning age. When properly vaccinated at this age pigs will usually develop a lasting immunity-sufficient to carry them through life or against the usual field exposure. Some veterinarians prefer to vaccinate just before weaning time, while others recommend its use just following weaning. Both methods seem to develop satisfactory immunity. Pigs nursing immune sows are not always resistant to the disease but they generally carry some passive immunity which has been transmitted through the milk of the mother. This passive immunity rapidly dis-appears following weaning; therefore it is best not to wait too long after weaning before vaccinating. Occasionally when subjected to very virulent exposure nursing pigs will develop the disease even though the mother is immune. In outbreaks of this kind it would be advisable to have the pigs vaccinated at an early age. The question about the advisability of vaccinating pregnant sows has often been raised. If it is a matter of keeping the hog from dying with cholera she should be vaccinated whether pregnant or not. It is best to plan so it is not necessary to vaccinate pregnant animals, particularly in the last half of the gestation period. It is possible to use serum alone on pregnant sows during the last few weeks' of gestation and then give them the double treatment along with the pigs after the pigs are born. When herds are kept properly immunized it never becomes necessary to vaccinate pregnant animals. It is not safe to feed garbage or kitchen refuse to hogs unless the hogs have been immunized against cholera. This kind of feed is potentially dangerous, as it may contain scraps of uncooked meat, bacon rind, trimmings, etc., which are always considered questionable. CARE FOLLOWING VACCINATION Even though hog cholera vaccination develops a very satisfactory form of immunity, it should be kept in mind that such immunity may be broken down if drastic enough measures are used . Further, unless hogs are properly managed the maximum amount of immunity would not be obtained, even though vaccination had been properly carried BULLETIN 465 7 out. Sudden changes in feed should always be avoided whenever possible, and especially is this true within the first two or three weeks following vaccination by the so-called double method. It is best to reduce the grain feed at least one-half, particularly when the pigs are on heavy feed. This reduction in grain is advisable for at least two weeks, after which time the grain ration can gradually be increased -getting the animals back on full feed not less than three weeks fol-lowing vaccination. In their effort to fatten hogs as rapidly as pos-sible there is a tendency for most hog raisers to put their animals on full feed too soon after vaccination. All operations, such as castra-tion, ear notching, ringing, etc., should be avoided at the time hogs are vaccinated for cholera. It is best, if these operations cannot be done long enough before vaccination for the wounds to heal by the time vac-cination is given, that one wait two or three weeks after the admin-istration of hog cholera serum and virus. Some hog raisers claim to have vaccinated and castrated at the same time without any losses, but sooner or later they have generally experienced some bad results following this practice. When swine have been poorly managed or mis-managed following vaccination and bad results from the so-called "serum breaks" have occurred there is a tendency for the herd owner to blame the product used. Hog cholera serum and virus are produced under federal supervision and federal license, and the products on the market at the present time are dependable. Improper handling of serum and virus between the time it is produced and the time it is used on the hogs should be avoided . The virus should be potent, and along with the serum should be kept cool (ordinary refrigerator tem-perature) until it is used . Hogs that have received the double treatment or anti-hog-cholera serum and virus will eliminate virus for two to three weeks following vaccination. This virus is eliminated largely in the urine of the hogs. Pigs that are not immunized against cholera should not be put in the same pen or come in contact with pigs that have been recently vac-cinated. After a thirty-day period there is comparatively little danger of vaccinated hogs eliminating virus from their systems if they have remained healthy during this period. Hogs that are sick with cholera should be isolated to avoid further spread of disease and all hogs that have died from the disease should be destroyed by complete burning, or they should be covered with lime and buried deeply. It is not safe to put unvaccinated pigs in pens which have recently been occupied by hogs sick with cholera, unless the pens are thoroughly cleaned and disinfected. Pastures be-8 MISSOURI AGRICULTURAL EXPERIMENT STATION come relatively safe after a few months if hogs are not allowed access to them. The length of time it takes a pasture to clear up under natural conditions is somewhat variable, but this' will be much quicker during the summer months than in the winter months. Probably under aver-age conditions most pastures in Missouri become reasonably safe in from three to six months. Since livestock, other than hogs, are not susceptible to hog cholera infection, these animals may use such pastures with safety. In the control of hog cholera, cleanliness of hog pens and lots, good fencing, proper feeding practices, good housing, and the occa-sional disinfection of quarters are important factors to be considered. Newly purchased hogs should be held in isolation for a period of thirty days. This is always a good practice and safe-guards the home herd against disease, particularly cholera and other infectious diseases. OTHER DISEASES RESEMBLE HOG CHOLERA AND COMPLICATE CONTROL There are several other diseases which may resemble hog cholera or may even occur at the same time some of the hogs are sick with cholera. This tends to complicate the diagnosis . When other diseases are present in a herd at the time it becomes infected with hog cholera the losses from the latter disease are likely to be much higher. Prob-. ably one of the most serious dis~ases in hogs, besides cholera, is swine erysipelas. This disease resembles hog cholera very closely in both symptoms and lesions. It has only been within recent years that swine erysipelas has been important in this state but it is spreading at the present time. Most of the cases of swine erysipelas now are found in the northwest part of the state, or at least in the northern half, but unless proper precautions are taken this disease will gradually spread to other parts. It can also be controlled by vaccination. Vaccination for swine erysipelas should not be used, however, unless this disease has been definitely diagnosed in a herd of hogs. Since this latter disease is so easily confused with hog cholera, the services of a competent veter-inarian should be obtained in order to arrive at an early correct diag-nosis, and the herd treated accordingly. Other diseases, such as swine flu, anthrax, necrotic enteritis, pneumonia and pleurisy, enteritis, and the presence of worms have in some instances complicated the diagnosis of hog cholera. 

Hog cholera

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