These germs may be inhaled in the lungs with the air, admitted to the stomach and intestines with food and drink, or established in the flesh by inoculation through broken skin or mucous membrane.]
After getting into the body, tubercle bacteria multiply in the tissues to which they have been carried and produce the changes in them which we find on the examination of an animal suffering with tuberculosis. Tuberculosis, therefore, is simply the outcome of the growth of the tubercle bacteria in the organs.
=Where Tubercles Are to Be Found.=--Tuberculous areas may be found in almost any part of the infected animal, but the organs that are usually affected are the lymphatic glands, either in the throat, the bronchial glands or those about the intestines and on the liver; the lungs; the liver; the kidneys; intestines; udder and generative organs. The membrane covering the lungs (pleura), the heart (pericardium), and intestines (peritoneum), are frequently affected. It often happens that a large mass, or masses, of tuberculous tissue grow over one or more of these membranes. The most peculiar thing about bovine tuberculosis is the fact that frequently an animal will appear to be perfectly well, but when slaughtered will be found to have a large number of tuberculous areas or masses on the membranes or in its organs. The reason for this is that the diseased area is not at a vital point.
The organ or membrane affected depends upon the one to which the germ is carried. Usually animals are infected in but one organ in the beginning, and from this diseased area the germs spread through the blood vessels or lymph channels to other organs. When the diseased area is restricted to one organ or part, it is called “localized” tuberculosis, because it appears at the point where the seed or germ was first planted. When the germs spread through the circulation from this first or primary diseased area to other organs and set up new tuberculous growths, the condition is called “generalized” tuberculosis. When cattle are slaughtered for food, if they are found to be afflicted with localized tuberculosis, the flesh is considered to be fit for food, but if the disease is generalized the carcass is condemned.