Chapter XXX: 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).

_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.

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.

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.

_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.

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.

The most practical method of disposing of dairy cows that react to the tuberculin test is to slaughter them. Unless a large percentage of the herd is tubercular, it is not advisable to practise segregation and quarantine. This may be advisable if the reactor is a valuable breeding animal, unless visible symptoms are shown. The milk from reacting cows may be used if it is boiled or sterilized. Whenever a calf is born of a reactor, it should be separated from the mother and fed milk from a healthy cow.

The separation of the tubercular from the healthy cows must be complete. Separate buildings, yards and pastures that do not join the quarters where the healthy animals are kept should be provided. The person attending the reactors should not attend the healthy animals, and separate forks, shovels, pails and other utensils should be provided for the two herds.

The best method of controlling tuberculosis in hogs is to slaughter all reactors, disinfect yards and houses and move the herd. If the old quarters are free from filth and carefully disinfected, the hogs may be returned without danger of infection after six months. A retest of the herd should be made before returning them to the permanent quarters and the reactors slaughtered.

QUESTIONS

1. Give the history of the early experimental work in tuberculosis; give
the common methods of spreading the disease.

2. What are the symptoms and post-mortem lesions in tuberculosis?

3. Give the method of controlling tuberculosis.