  [Illustration: FIG. 88.—Koch's Bacillus tuberculosis.] Tuberculosis is a contagious an and domestic animals, affecting any the lymphatic glands and lungs, change in the tissues is the formation tubercle or nodule. HISTORY.—Tuberculosis is one of the oldest of known diseases of domestic animals and man. Its contagious character was proven by Villemin in 1865, who by experential infection transmitted tuberculosis from man to animals and from animal to animal. It was in 1882 that Dr. Robert Koch discovered and proved by inoculation experiments that the disease was caused by a specific germ (Fig. 88). Prior to the experiments by Villemin and Koch, the belief was that tuberculosis was due to heredity, unsanitary conditions and inbreeding. Following discovery of the specific germ and conditions favoring its development and spread, numerous scientifically conducted experiments were made. These resulted in practical methods of control and elimination of tuberculosis from herds having this disease. By carefully conducted experiments and other forms of educational work the infectious character of tuberculosis and the economic importance of preventative measures have been demonstrated. The average stockman is well informed regarding the character and economic importance of this disease, but there is no general application of this knowledge, and tuberculosis is increasing in dairy and breeding herds. The slow development of tuberculosis, and the absence of visible symptoms during the early stage of the disease, are responsible for this condition and the extensive infection of dairy and breeding herds. PREVALENCE OF THE DISEASE.—Tuberculosis is very prevalent among cattle and swine in all countries where intensive agriculture is practised. It is a rare disease among cattle of the steppes of eastern Europe and the cattle ranges of the western portion of the United States. In countries where dairying is an important industry, tuberculosis is a common disease of cattle and hogs. The abattoir reports of Europe and the United States show that tuberculosis is on the increase among domestic animals. The Bureau of Animal Industry of the United States Department of Agriculture reports that out of 400,008 cattle tested with tuberculin 9.25 per cent reacted. Melvin states that the annual loss from tuberculosis in the United States is about $23,000,000. In dairy herds in which the disease has existed for several years, it is not uncommon to find from 25 to 75 per cent tubercular. THE DIRECT CAUSE.—The direct cause of tuberculosis is Koch's Bacillus tuberculosis. This is a slender, rod-shaped microorganisms (Fig. 88) occurring in the diseased tissues, feces and milk of a tubercular animal. It belongs to that small group known as acid-fast bacteria. The tubercle bacillus is not really destroyed by external influences, and it may retain its virulence for several months in dried sputum if protected from the light. Its vitality enables it to resist high temperatures, changes in temperature, drying and putrefaction to a, greater degree than most non-spore-producing germs. Direct sunlight destroys the germ within a few hours, but it may live in poorly lighted, filthy stables for months. A temperature of 65\260 C. destroys it in a few minutes. Animals that, have advanced or open tuberculosis may disseminate the germ of the disease in the discharge from the mouth, nostrils, genital organs, in the intestinal excreta and milk. The germs discharged from the mouth and nostrils are coughed up from the lungs and may infect the feed. Milk is a common source of infection for calves and hogs. Allowing hogs to run after cattle is sure to result in infection of a large percentage of them, if there are any open cases of tuberculosis in the herd. PREDISPOSING CAUSES.—Any condition that may lessen the resistance of the body or enable the tubercle bacillus to survive the exposure outside the body favors the development of the disease and the infection of the healthy animals. Crowded, poorly ventilated, filthy stables lower the disease-resisting power of the animal, and favor the entrance of the germs into the body. Under such unsanitary conditions, tuberculosis spreads quickly among dairy cattle, and a large percentage of the animals develop the generalized form of the disease. Sanitary stables and yards do not prevent the spread of the disease among animals that live in close contact with one another. Fresh air and sanitary surroundings only check the spread and retard its progress. INTRODUCTION OF TUBERCULOSIS INTO THE HERD.—The common method of introducing tuberculosis into the herd is through the purchase of animals having the disease. Such animals may be in apparent good health at the time of purchase, and be affected with generalized or open tuberculosis. A source of infection is by unknowingly buying cows that have reacted to the tuberculin test. The indiscriminate use and sale of tuberculin are largely responsible for the large number of reacting animals that have been placed on the open market. This dishonest practice has resulted in the rapid spread of the disease in certain localities. For years a large percentage of the breeding herds have been infected, and the writer has met with several herds of dairy and beef cattle that became tubercular through the purchase of tubercular breeding animals. SYMPTOMS.—There is no one symptom by which we may recognize tuberculosis in cattle and hogs. None of the symptoms shown by a tubercular animal are characteristic, unless it is in the late stage of the disease. In a well-cared-for animal, the lymphatic glands in the different regions of the body, the lungs, liver and other organs, may be full of tubercles without causing noticeable symptoms of disease (Fig. 89). [Illustration: FIG. 89.—A tubercular cow. This cow was, to all appearances, in good health, but showed generalized tuberculosis on post-mortem examination.] Tuberculosis may attack any organ of the body, and in the different cases of the disease the symptoms may vary. Enlargement of the glands in the region of the throat, and noisy, difficult breathing are sometimes present. The udder frequently shows hard lumps scattered through the gland. Bloating may occur if a diseased gland in the chest cavity presses on the oesophagus and prevents the usual passage of gas from the paunch. Chronic diarrhoea may occur. If the disease involves the digestive tract, the animal is unthrifty and loses flesh rapidly. Coughing is not a characteristic symptom, and we should not place too much emphasis on it. If the lungs become tubercular the animal usually has a slight, harsh cough. The cough is first noticed when the cattle get up after lying down, when the stable is first opened in the morning and when the animals are driven. If the chest walls are thin, soreness from pressure on the ribs may be noted. By applying the ear to the chest wall and listening to the lung sounds, absence of respiratory murmurs and abnormal sounds may be distinguished, due to consolidation of the lung tissue, abscess cavities and pleural adhesions. In a well-advanced case the hair is rough, the skin becomes tight and the neck thin and lean. The animal may breathe through the mouth when it is exercised. Weakness may be a prominent symptom. [Illustration: FIG. 90.—Tubercular spleens.] Breeding animals that are well fed and cared for may live for several years before showing noticeable symptoms of tuberculosis. The disease progresses more rapidly in milch cows, especially if given poor care. Calves allowed to nurse a tubercular mother that is giving off tubercle bacilli frequently develop enlarged throat glands and the intestinal form of the disease. Hogs develop a generalized form of tuberculosis more quickly than cattle, but an unthrifty, emaciated condition is seldom noted in hogs under ten months old. POST-MORTEM LESIONS.—The effect of the tubercle bacillus on the body is to irritate and destroy the tissues. Lumps or tubercles form in the lymphatic glands, liver, lungs, spleen (Fig. 90), serous membranes, kidneys and other body organs (Figs. 91 and 92). The tubercles may be very small at first, but as the disease progresses they continue to enlarge until finally a tubercular mass the size of a base-ball, or larger, is formed (Figs. 93, 94, 95 and 96). Lymphatic glands may become several times larger than normal and the liver and lungs greatly enlarged. The pleura and peritoneum may be thickened and covered with tubercles about the size of a millet seed, or larger. Pleural and peritoneal adhesions to the organs within the body cavities are common. [Illustration: FIG. 91.—The carcass of a tubercular cow. Note the condition of the carcass, and the tubercular nodules on the chest wall, showing that the disease was well advanced.] [Illustration: FIG. 92.—A section of the chest wall of a tubercular cow showing a better view of the diseased tissue.] The tubercle usually undergoes a cheesy degeneration. Old tubercles may become hard and calcareous. Sometimes the capsule of the tubercle is filled with pus. A yellowish, cheesy material within the capsule of the tubercular nodule or mass is typical of the disease. THE TUBERCULIN TEST.—The only certain method of recognizing tuberculosis is by this test. There is no other method of recognizing this disease that is more accurate than the above test. The substance used in testing animals for tuberculosis is a laboratory product. It is a germ-free fluid prepared by growing the tubercle bacillus in culture medium (bouillon) until charged with the toxic products of their growth. The culture medium is then heated to a boiling temperature in order to destroy the germs. It is then passed through a porcelain filter that removes the dead germs. The remaining fluid is tuberculin. [Illustration: FIG. 93.—A very large tubercular gland that had broken down in the central portion.] There are two methods of applying the tuberculin test. The subcutaneous test consists in injecting a certain quantity of tuberculin beneath the skin, and keeping a record of the body temperature of the animal between the eighth and eighteenth hours following the injection. Tubercular animals show an elevation in temperature that comes on about the eighth or twelfth hour of the test. In the intradermal test, a small quantity of a special tuberculin is injected into the deeper layer of the skin. The seat of the injection in cattle is a fold of the skin on the under side of the base of the tail. In tubercular animals the injection is followed by a characteristic local swelling. [Illustration: FIG. 94.—A tubercular gland that is split open.] [Illustration: FIG. 95.—Caul showing tuberculosis.] [Illustration: FIG. 96.—Foot of hog showing tuberculosis of joint.] The control of tuberculosis is largely in the hands of the breeder and dairyman. This is a disease that requires the cooperation of stockmen and sanitary officers in the application of control measures. If there are several open cases of tuberculosis in a herd of cattle, the application of the tuberculin test, removal of the reacting animals and disinfection of the premises are not sufficient to eradicate the disease. It is necessary to repeat the tuberculin test within six months, and later at twelve-months intervals, until none of the animals that remain in the herd react. |
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