Animal Diseases by L

Animal Diseases by L. T. GILTNER

VEETERINARIANS and farmers have waged an aggressive fight against animal plagues in recent years. They have kept malignant ones like rinderpest and foot-and-mouth disease, a constant threat from abroad, out of the United States. The worst native diseases, anthrax, anaplasmosis, brucellosis, mastitis, hog cholera, swine erysipelas, tuberculosis, rabies, and pullorum disease, they held in check by applying approved control plans, including sanitation, vaccination, and treatment with drugs. Some of the vaccines, notably those for preventing rabies and hog cholera, have been improved. All the biological products used in the control work have been maintained at a high standard of quality. Ex- perimental studies on the sulfa drugs have done much to show which infections are amenable to treatment with them. Sulfanilamide, sulfathiazole, and sulfadiazine are useful in treating some types of streptococcal infections, but ineffective against some other bacterial infections. Sulfathiazole, sulfaguanadine, and sulfadiazine have value against some forms of dysentery. Sulfathiazole alleviates symptoms of coryza in chickens, although the symptoms reappear when treatment is stopped. Sulfamerazine acts somewhat the same against infectious sinus- itis in turkeys. Sulfamerazine, although having a favorable influence against coccidiosis in chickens, is not effectve aganst lymphomatosis. Penicillin is beneficial, experimentally and in practice, in treating the type of mastitis that is caused by Streptococcus agalactiae. It has shown marked curative eflfect on experimentally induced swine erysipelas infection in turkeys. It was not effective in treating infectious equine anemia, a virus disease, and so far has had no value in preventing or treating other virus diseases. Of course, the sulfa drugs and penicillin, like all 703830°—47 7 81 82 YEARBOOK OF AGRICULTURE

new drugs, should be used only as prescribed by a veterinarian; their indiscriminate use may be wasteful and actually harmful to the animal patients. But all this is not to say that we have cured or are about to cure all diseases of animals. Much remains to be learned and done, as the following pages, in which I discuss some of the worst diseases, disclose.

Brucellosis of Cattle {Bang's Disease)

No disease of livestock has received greater attention from the stand- point of control measures than brucellosis of cattle, or. Bang's disease, as it is commonly called. Years of research have been spent in developing methods of control, and these have been applied in practical field studies. The studies have definitely established that no one system of control is applicable to all herds, but that the choice of method depends on conditions such as size of herd, degree of infection, environment, and whether the herd is for dairy or beef. As a consequence, the Department has approved the use of four methods of control in the Federal-State program for the control of Bang's disease, namely : Blood test and slaughter of all reacting animals ; blood test and slaughter, accompanied by the vaccination of calves; blood test to determine the amount of infection, but retention of reactors until vaccinated replacements are available; whole-herd vaccination. The first plan is the method of choice in small herds and in herds in which brucellosis has been of long standing, that is, where the storm of abortions has passed and the disease has taken on a chronic form. In herds from which certified milk is sold there is no alternative than test-and-slaughter, and this method also is used in the area-plan of control. The second method adds calf vaccination to the first. It has the extra advantage of building a herd that eventually will be more resistant to reinfection, should it occur. Calf vaccination has proved to be a most desirable practice in all herds in which infection is present. The third plan is a good one for herds in which such a large proportion of animals is shown on test to be reactors that their immediate removal, in the absence of suitable replacements, would work an extreme hardship on the owner. Calf vaccination is necessary in this plan and some owners prefer to vaccinate all animals up to breeding age. This plan applies especially to dairy herds in which the milk is pasteurized or sold to creameries, and in purebred herds where the preservation of valuable blood lines is involved. The retention of^ reactors should be considered only as a temporary expedient, to tide the owner over a period until vaccinated replacements can be raised and added to the herd. When replacements become available, the least desirable or profitable reactors ANIMAL DISEASES 83 should be removed from the herd first and this process continued until the herd is free of brucellosis. Three or four years should be enough to establish a negative herd. The fourth plan, whole-herd vaccination, is one of last resort. There are a few herds, called problem herds, in which the infection is of so severe a type, or the animals are so susceptible, that the removal of reactors following repeated tests cannot check the infection. In such herds, vaccination of the whole herd has had some success. In some herds the procedure has stopped abortions abruptly, but in others no improvement is evident for 6 to 8 months. Whole-herd vaccination has its main advantage in infected beef herds, where a calf crop is a prime requisite. There are several undesirable consequences of the vaccination of adult animals. The vaccinal blood titer, which cannot be told from that of actual infection, may persist for indefinite periods and thus interfere with sales or interstate shipment ; in cows more than 4 months advanced in pregnancy it may cause abortion; and if practiced in dairy cows, a marked drop in milk production may be expected from 10 to 14 days following vaccination. There is little reason to vaccinate all cows in a negative herd. The owner has little to gain, especially in view of the undesirable after-results. On the other hand, calf vaccination, preferably at 6 months of age, is good insurance in all types of herds, infected or negative, against the animal's subsequently becoming infected. The only drawback is that a small proportion of calves may be slow in losing the vaccinal blood titer, which may thus interfere with their interherd movement. The rapidity with which brucellosis in cattle may be eradicated in an infected herd under any of the four plans depends largely on herd management and sanitation.

Brucellosis in Swine

Brucellosis is spread and perpetuated in swine mainly through the infected boar and aborting sow. Recently it has been proved that the causal germ, Brucella suis, is sometimes eliminated in the urine of infected sow^s, and thus further contaminates the premises. Under such conditions the most practical method is to dispose of the entire herd, especially if it is small or not too carefully developed, and make a new start in clean quarters. Replacement animals should be most carefully selected from herds considered to be free from infection. All animals purchased should be negative in all dilutions to the blood test. If valuable blood lines are involved, or if the herd is large, success in developing a clean herd has resulted from separating weanling pigs negative to the blood test and rearing them in clean quarters as far away as possible from the infected 84 YEARBOOK OF AGRICULTURE

herd. This method should be followed in each farrowing season, with the extra precaution that the separated pigs are tested at intervals of several months until bred. When a clean herd has thus been established, the infected herd should be disposed of and the old premises thoroughly disinfected. Herd boars should be selected with utmost care to avoid reinfection of the herd. Other methods of control, including vaccination, have not proved practical. The infectiousness for man of the germ causing brucellosis in swine is another urgent reason for eradicating the disease.

Brucellosis in Goats

Except in some herds in the Southwest, goats are quite free from brucellosis. During the war, w^hen all sources of food supplies were being utilized, cheese made from goats' milk in the infected sections was believed to have been the cause of several cases of brucellosis in man. Because of the extreme infectiousness for man of the type of brucella affecting goats {Brucella melitensis) ^ all goats producing milk for human consumption should be blood tested. The immediate slaughter of all goats reacting to the test is the only method of control that should even be considered.

Mastitis of Cattle

Mastitis is undoubtedly the greatest scourge in the dairy industry. Each acute attack causes the loss of milk from the infected quarter until the condition improves, the graduated destruction of milk-secreting tissues and, thus, continued lower production during a lactating period until it no longer pays to keep the animal. Estimates are that at least a fourth of all dairy cattle in the United States have mastitis. Until recently there has been no recognized cure for chronic mastitis. Control measures were of a practical nature—disposing of animals with badly diseased udders, segregating infected cows that were still milking profitably, and preventing the spread of infection to clean cows through strict sanitation in milking and handling the herd. A few years ago drugs were developed that proved to be effective in curing many cases or a high percentage of cases of mastitis caused by Streptococcus agalactiae, the most common cause. The outstanding drugs are sulfanilamide, colloidal silver oxide, tyrothricin, and penicillin. They are infused into the udder through the teat canal. Penicillin, the least irritating of the drugs, will cure a large proportion of streptococcal infections if infused daily in proper dosage for 4 or 5 days. Unfortunately, the drugs are much less effective against other types of infection, and are not easy to use. Samples of milk from the ANIMAL DISEASES 85 quarters of each cow's udder should be submitted for laboratory examination to determine which animals are infected and the type of infection. Each quarter showing the presence of germs should be treated by infusing it with a suspension of the drug selected. After treatment, further checks on the milk should be made as a basis for the disposition of the animals. The control of mastitis calls for expert advice in herd management, sanitation, treatment, and disposition of affected animals, and should, therefore, be placed in the hands of a qualified practitioner. Drug therapy, even though effective, is only a helping factor in the control of mastitis; no dairyman should consider it as a substitute for proper sanitary practices and management. We suggest gentle, rapid milking, either by hand or machine; avoiding rough hand milking or improper regulation of the milking machine, which may cause irritation of the inner lining of the teat and afford a favorable atrium for infection; preventing injuries to teats and udder by providing clean, roomy, comfortable stalls; removing obstructions, such as high door sills and jagged stumps, which may injure teats and udders.

Tuberculosis of Cattle

All States are now in the modified-accredited status; that is, in no State is there an incidence of more than 0.5 percent of tuberculous infection in the cattle population. But some traces of infection remain in almost every State-—it is impossible to give the tuberculin test annually to every animal in the country. A way of discovering foci of infection has been worked out with the Federal Meat Inspection Division. When post- mortem examinations in abattoirs show animals to be tuberculous, their origin is traced, and the disease in the infected herd is eradicated by test and slaughter. The method, together with general testing, is slowly reduc- ing bovine tuberculosis to the vanishing point.

Anthrax

Anthrax, one of the oldest and most destructive diseases of animals and man, is still a problem in some parts of the United States. We obtained considerable necessary information on the history and incidence of anthrax in recent studies. Outbreaks of a disease believed to be anthrax occurred in Pennsylvania as early as 1834, in Louisiana in 1835, and in Mississippi in 1836. Outbreaks in New York were recorded in 1881, in Vermont and Massachusetts in 1887, and in California in 1888. Infected areas still exist in those States. Between 1915 and 1945 there was a gradual increase in the territory involved, with outbreaks of anthrax in livestock in 438 counties in 45 States. Large areas of recognized infection exist in South Dakota, Nebraska, Arkansas, Mississippi, 86 YEARBOOK OF AGRICULTURE

Louisiana, Texas, and California; small areas exist in Vermont, New Jersey, Delav/are, Utah, Ne\^ada, and Oregon. The disease in those areas can be largely controlled by annual preseasonal vaccination, strict quarantine, and prompt disposal of all infected material during outbreaks. Experimental studies, observations in the field, and reports from prac- ticing veterinarians indicate that anthrax spore vaccine of the proper strength administered between layers of skin is an effective type of pre- ventive vaccination. It is gaining in favor in many parts of the country, and is widely used in Texas, Louisiana, Nebraska, and South Dakota;

Anthrax stiU takes its toll of livestock in 43 States. Virulent outbreaks occur in South Dakota, Nebraska, Arkansas, Mississippi, Louisiana, Texas, and California.

in Nebraska, South Dakota, and small sections of Iowa and Minnesota, the number of herds quarantined for anthrax declined from 1,592 in 1937 to 20 in 1945. On the Indian reservations in South Dakota, where more than 45,000 cattle were vaccinated with intradermal spore vaccine from 1939 to 1945, only 16 animals were lost from anthrax. Considerable information w^as obtained from recent studies on the tenacity of B, anthracis and its spores. Dry anthrax spores were killed by dry heat at 149° to 150° C. in 60 minutes, but not in 30 minutes. Spore suspensions in distilled water were destroyed in 3 to 5 minutes by vigorous boiling, in 5 to 15 minutes at 100° to 101° C, and in 5 to 15 minutes in the autoclave at 15 pounds pressure (120° C), while several strains resisted heating at 90° to 91° G. for 60 minutes. Dry spores in dried ANIMAL DISEASES 87 blood on cotton swabs from field cases held in glass vials at 25° to 30'' C. were both viable and virulent after storage for 8 years. Suspensions of spores in normal saline resisted rapid shell freezing at —72° C. to — 78° C. and thawing at 37° C. for 45 successive times. Viable anthrax spores were recovered from the vapors of heated anthrax-spore suspensions and from the distillate of anthrax-spore suspensions. In the unopened carcasses of guinea pigs dead of anthrax, held at 28° to 30° C. for 80 hours, B, anthracis could not be isolated from the blood, but when held in the ice box at 5° to 10° C, B. anthracis could be recovered after 4 weeks in storage. The spores that formed from vegetative forms in bone marrow and muscle tissue from guinea pigs dead of anthrax were still viable after the decomposing tissues had been held 6 months at room temperature (28° to 30° C).

Newcastle Disease

Newcastle disease, a serious disease of poultry in many other countries, has been identified in several widely separated areas in the United States. Fortunately the disease has thus far caused relatively light losses here, although extremely heavy losses have been suffered in the Dutch East Indies, Korea, the Philippines, Ceylon, India, Kenya, the Middle Congo, parts of the European Continent, and Palestine. England and Australia have twice succeeded in eradicating the disease through rigid quarantines, prompt slaughtering of infected birds, and thorough cleaning of infected premises. In other countries the disease has spread alarmingly. In 1944, a comparatively mild disease in California that caused symptoms resembling infectious bronchitis and laryngotracheitis, but was frequently complicated by nervous symptoms such as paralysis, proved to be a form of Newcastle disease. It spread rapidly and caused the greatest mortality in chicks, but it affected birds of all ages, including turkeys. Egg production in previously thrifty laying flocks suddenly dropped to low levels. The disease, referred to earlier as a "respiratory nervous disease," or pneumoencephalitis, had apparently been present in California for several years. Its true nature remained unidentified until samples of the virus causing it were examined at Harvard University in 1944 by a group of specialists working under the direction of a War Department commission. Shortly thereafter the disease was identified in New Jersey and New York. It is known to have occurred in about 27 States, and there are strong suspicions of its existence elsewhere in the country. It cannot be said with certainty that any section of the country has escaped. The apparent spread of the disease, and its potential hazard to the poultry industry led to several regional conferences of animal-disease 88 YEARBOOK OF AGRICULTURE

specialists, and a conference of poultry pathologists, State livestock sanitary authorities, and poultrymen from all parts of the country with Department officials in 1946. We decided to collect all the information we could about the disease and methods for its control. Poultrymen, veterinarians, and regulatory officials were warned to be on the alert. Because of its similarity to other diseases, there may be considerable difficulty in making an exact diagnosis, which is possible only by technical examinations in the laboratory. Newcastle disease has certain aspects, however, that should arouse the poultryman's suspicions and lead him to report the facts promptly to a poultry pathologist and State veterinary authorities. The incubation period may vary up to 2 weeks and sometimes longer. The average is generally placed at 5 days. The onset of the disease is usually sudden, and it spreads rapidly in the flock. In rare instances, death may occur before symptoms are observed. In the United States, the mortality rate has been less than 20 percent in most outbreaks, but in some instances the death rate, especially among young birds, has been alarming. Respiratory symptoms, quite similar to those of infectious bronchitis and laryngotracheitis, are seen in Newcastle disease as it has occurred in the United States. Affected birds generally exhibit dullness, droopiness, weakness, slowness, lack of appetite, ruffled feathers, coughing, sneezing, rattling sounds in breathing, sleepiness, diarrhea, and fever. There may be a discharge of mucus from the nostrils; the accumulations lead to frequent swallowing and shaking of the head. A dark discoloration and swelling of the head may occur. Chicks usually emit a rapid, low, cheeping sound. Egg production is sharply reduced. Affected flocks that survive generally seem to improve markedly after a week or 10 days, but decreased egg production continues for 4 to 8 weeks or longer. It has been re- ported that during this interval the laying of abnormal eggs, including some with soft shells, inferior quality of albumin, or bubbly contents, is not uncommon. At the same time, the birds are apt to go into a heavy moult. Younger birds, even though they appear to have recovered from the disease, are apt to develop poorly. The nervous symptoms vary considerably. Sometimes they are absent. Weakness, progressing to paralysis, involves the legs and wings. Attacks like epilepsy may occur. Distortions of the neck, either downward, up- ward, or lateral, appear. Periodic shivering, twitching, incoordination, convulsive seizures, circling, backing up, and curling of the toes are common evidences of involvement of the nervous system. Such signs may develop abruptly or occur secondarily in birds that are apparently re- covering. Paralyzed birds may live for several weeks, but most of them eventually die. ANIMAL DISEASES 89

Post-mortem examination may reveal various changes, usually of little diagnostic significance. A catarrhal condition of the trachea, with accumulations of clear or yellowish mucus, or cheeselike material, occurs frequently. Cloudiness of the air sacs is common in chicks. Birds dying of the nervous type of the disease generally have no distinctive lesions. Years have elapsed since Newcastle disease was last identified in either England or Australia. The procedure followed was the radical one of destruction of all aflfected birds, followed by thorough cleaning and dis- infection of all infected premises before restocking was permitted. Rigid quarantines and control of all movements of chickens and other barnyard fowl were imposed. These means have also been used successfully in the United States in the eradication of European fowl plague. Such measures have been applied successfully with other highly contagious maladies when the diseases have been recognized early after their first appearance and before they have spread materially. The present situation is somewhat diflferent in that the infection has been prevalent for a number of years and has apparently become widespread. Sanitary authorities have been reluctant to adopt the slaughter program as a general procedure, although they agree it may be the best procedure in isolated outbreaks. Vaccination against Newcastle disease is still in the experimental stage, although rather extensive field trials have been carried out in California. Limited commercial production of vaccine was begun in 1945.

Some Methods of Control Although exact methods of controlling the disease have not been worked out in detail, the following may be considered as objectionable and dangerous procedures that should be avoided : Re-use of second-hand, unsterilized feed bags; admittance of unneces- sary visitors to poultry premises; admittance of chick sexers, blood testers, cullers, or any necessary service personnel without proper precautions; the use of unsteriHzed bird crates; careless disposal of dead birds; per- formance of post-mortem examinations of poultr>^ on the farm; shipment of sick birds in public conveyances to the laboratory; conducting egg- laying tests and poultry shows without authority from the livestock sanitary officials; failure to observe quarantine and other sanitary reg- ulations; obtaining eggs, chicks, or any birds from infected or exposed flocks; failure to report sick birds immediately to the proper authorities; failure to sterilize or properly dispose of oflfal and other waste materials from slaughtered birds. Additional information on the disease may be found in the Proceed- ings of the forty-sixth, forty-seventh, and forty-ninth Annual Meetings of the United States Livestock Sanitary Association in 1942, 1943, and 1945, the March 1946 issue of Nulaid News, the June 1946 issue of the 90 YEARBOOK OF AGRICULTURE

Journal of the American Veterinary Association, the July 1946 issue of the American Journal of Veterinary Research, and the April 1946 issue of the Cornell Veterinarian.

Hog Cholera

Hog cholera is still the most prevalent and devastating disease of swine. No herd in this country is absolutely safe from it, nor will be until the disease is wiped out. None of the methods that have been used successfully against some other diseases of livestock offers promise of success against hog cholera. Thus, the method used sometimes to eliminate foot-and-mouth disease—the slaughter of all infected and exposed animals—is not practical in the United States, although it is in Canada. The disease is too widespread in this country and too frequently is not recognized until it has spread throughout a community. Nor is the test- and-slaughter method, which promises ultimate eradication of bovine tuberculosis, applicable because no diagnostic test is known for the detection of hog cholera. Although a method of eradication is not available, the swine raiser is fortunate in possessing a dependable method of control. The serum- virus treatment, developed by the late Marion Dorset, affords complete and life-long protection against cholera to all except an insignificant few of the treated hogs. By its use, the great epidemics that once caused annual losses as high as 65 million dollars have been prevented, and any hog raiser willing to pay for the required treatment is relatively secure. But the method, since it involves the use of the live virus that causes the disease, offers no hope of eradicating hog cholera. Partly in recognition of that fact, scien- tists continued their search for a method of prevention that offered promise of being also a method of eradication. A result of the search was the development of hog cholera vaccines. One of them, also developed by Dr. Dorset, is known as crystal violet vaccine. It seems incapable of causing the disease, but nevertheless it affords a marked degree of protection., It is the virus of hog cholera so treated that it has lost its ability to produce the disease, but it retains its antigenic properties, or ability to confer protection against the disease. Field tests were begun soon after the vaccine was discovered. They have been continued on a graduated scale as confidence in its value has grown. These tests, although indicating the general dependability of the product, have also indicated the need of improvements. One of these involved the purity of the product. The early crystal violet vaccines frequently contained contaminants, micro-organisms that had been derived from the virus donor or that had gained access during preparation of the vaccine. It was possible by animal test to determine ANIMAL DISEASES 91 whether the contaminants present would be injurious to hogs treated with the product. But it was also possible for the contaminants by their growth to produce spoilage in the vaccine and to cause a loss of potency that could not be detected. As soon as the need for a sterile product was recognized, attempts were begun to supply that need. It was found that the use of a glycerol solution of the dye, crystal violet, used in preparing the vaccine, instead of the formerly used water solutions of dye and phos- phate, consistently afforded sterile vaccines. Under ordinary conditions of handhng they remain sterile. The new method of preparing the vaccine was patented by Dr. F. W. Tilley, of the Bureau of Animal Industry, in 1945, and the patent was assigned to the Secretary of Agriculture. The development of a sterile vaccine of satisfactory keeping qualities has been the chief recent advance in the product itself. A considerable advance also has been made in the recognition of the value of vaccines as a means of preventing hog cholera. As recognition grew, demand grew, and to supply the demand commercial establishments began preparing them. The use of commercially prepared vaccines was at first subject to certain restrictions imposed under the authority of the virus- serum-toxin act. By 1944, however, their merits had become so well demonstrated that the restrictions were withdrawn. The production and use of the vaccines is not limited to this country. As a result of an extensive series of studies conducted by the British Ministry of Agriculture, crystal violet vaccine is being made available to farmers in England. Research workers in Venezuela also confirmed the value of the vaccine. Indeed, reports from all countries except Canada have been favorable. The degree of protection afforded by the vaccine in Canada was far smaller than elsewhere. Whether the results of tests by Canadian authorities are due to a peculiarly of Canadian pigs or to some other cause remains to be determined.

Swine Erysipelas

Swine erysipelas is now recognized to be second only to hog cholera in economic importance to swine growers of the United States. It is not a consistent killer of swine, as is hog cholera, but causes losses by preventing normal rates of growth, altering tissues and organs to an extent that excludes their use for human food, lowering resistance of infected swine so that they readily fall prey to other diseases, and, in certain outbreaks, by actually killing. Prevention by immunization of swine, with the simultaneous administration of anti-swine-erysipelas serum and live-culture vaccine, has been a subject of experimental study by several of the State livestock sanitary authorities, the State experiment stations, and the Department. The study has been conducted on a cooperative basis under restricted conditions for 92 YEARBOOK OF AGRICULTURE

more than 8 years in 14 States where the disease is known to exist. Strict control over the production of the vaccine and serum has assured standard products of high quality for use in the work. This protective vaccination has been given nearly 7 million head of swine in 100,000 herds. The results have been satisfactory so far, but sufficient time to allow for complete evaluation of the sero-vaccination under all cyclic variations of the disease has not yet elapsed. The organism that causes swine erysipelas is also dangerous to man. In humans, the disease is known as erysipcloid, and occurs generally through wound infection. Although erysipeloid usually localizes, in some cases general infection and death have resulted. Care should be exercised, therefore, in handling infected swine and their products. The disease has also gained considerable economic importance for turkey growers. It manifests itself suddenly in turkeys around market time. The birds become droopy, sleeply, and depressed. Death usually occurs in about a day. There is a copious diarrhea in most cases. The wattles and caruncles become swollen and dark. On birds that recover, the caruncles may dry and drop off. Mortality ranges up to 50 percent, and the disease seems to affect young toms oftcner than young hen turkeys. Our preliminary investigations indicate that 10 cubic centimeters of anti-swine erysipelas serum (equine origin) introduced into the w^attles of artificially infected turkeys will cure about 50 percent of birds so treated, but all untreated birds die. By repeated administration of oil suspension of penicillin into the wattles of similarly infected turkeys, mortality was reduced to 10 percent. Rabies Rabies as a disease of livestock has never ranked in economic importance with the other well known infectious diseases, but it is exceedingly serious because it may affect all w^arm blooded species, including man.

Although dogs are the main victims of rabies, human beings and many farm animals also die from it. From 1938 to 1945 rabies caused 70,201 deaths

Cattle Horses Sheep Swine Cats Goats Miscel- Year Dogs laneous Man Total

1938 8,452 413 32 164 42 207 11 44 47 9,412 1939 7,386 358 36 17 38 269 10 172 30 8,316 1940 6,194 326 25 53 71 260 4 277 28 7,238 1941 6,648 418 39 68 159 294 9 212 30 7,877 1942 6,332 288 15 48 32 250 12 160 28 7, 165 1943 8,515 349 35 45 60 316 19 310 41 9, 690 1944 9, 067 561 32 40 43 419 14 311 53 10,540 1945 8,505 487 46 30 466 10 373 35 9, 963

Total . . . 61,099 3, 200 260 446 475 ¡2,481 89 |1,859 292 |70, 201 1 ANIMAL DISEASES 93

It is invariably fatal. Since 1938, the Department has gathered statis- tics on the incidence of the disease in the United States. In the 8 years from 1938 to 1945, a total of 70,201 cases was recorded, of which nearly 90 percent were in dogs and less than 7 percent in other farm animals. Incidence of the disease gradually declined from 1938 to 1942, with a sharp rise during the following 2 years and a small decline in 1945. Even a slight increase in the spread of rabies in a community gives rise to grave concern among veterinarians, public health officials, and cit- izens. Consequently aggressive programs have been inaugurated in the past few years in certain affected areas. Particular emphasis was given to vaccination as a very helpful prophylactic measure. Large-scale vaccinations of dogs in a number of States and communities were carried out promptly. In Washington, D. C, approximately 35,000 dogs were vaccinated in about 2 weeks. The results of vaccination have been gratifying; in many instances the number of cases dropped sharply upon the com- pletion of the vaccinations. Each lot of commercial rabies vaccine must be subjected to the Habel test, an accepted method for determining its power to immunize, before being released for sale. In the test, white mice are vaccinated with some of the rabies vaccine and these animals, together with an equal number . of unvaccinated mice, are inoculated with varying doses of rabies-fixed virus. The vaccine is not passed unless 50 percent of the vaccinated mice withstand at least a thousand fatal doses of the virus. Despite the high quality of present-day rabies vaccine and the good results obtained through its use in many dogs, the control and ultimate eradication of rabies must include other recognized measures. In late years the fox has played an important role in the spread of rabies in some localities. Should the disease become established in these animals, a program of systematic reduction of the fox population will become necessar)^

THE AUTHOR L. T. Giltner is assistant chief of the Division of Pathology, in the Bureau of Animal Industry, where his work has included research on all of the animal diseases discussed in this article.

FOR FURTHER READING Bunyea, Hubert, and Wehr, E. E.: Diseases and Parasites of Poultry, U. S. D. A. Farmers' Bulletin 1652, 1941.

Dorset, M., and Houck, U. G.: Hog Cholera, U. S. D. A. Farmers' Bulletin 834, 1939.

Eichhorn, A., and Crawford, A. B. : Brucellosis of Cattle, U. S. D. A. Farmers' Bulletin 1871, 1941.

Gochcnour, W. S.: Anthrax, U. S. D. A. Farmers' Bulletin 1736, 1934. 94 YEARBOOK OF AGRICULTURE

Grey, Charles G., and Dale, C. N. : Diseases of Swine, U. S. D. A. Farmers' Bulletin 1914, 1942. Schoening, H. W. : Rabies and Its Control, U. S. D. A. Yearbook, Keeping Livestock Healthy, pages 1109-1223, 1942.

Shahan, Maurice S., and Huiïman, Ward T. : Diseases of Sheep and Goats, U. S. D. A. Farmers' Bulletin 1943, 1943.

Stein, C. D. : Anthrax, U. S. D. A. Yearbook, Keeping Livestock Healthy, pages 250-262, 1942. Swett, W. W., Graves, R. R., Matthews, C. A., Cone, J. Frank., and Underwood, P. C: A Study of the Effectiveness of Sulfonamide Preparations in the Elimina- tion of Bovine Mastitis, V. S. D. A. Technical Bulletin 884, 1944.

ALSO, IN THIS BOOK Men Who Went Before, by Ernest G. Moore, page 1. Drugs to Control Parasites, by Benjamin Schwartz, page 71. Progress in Hog Production, by John H. Zeller, page 201. Developments in Sheep, by Damon A. Spencer, page 209. Keeping Poultry Healthy, by Theodore C. Byerly, page 231.