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.
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.