TUBERCULOSIS.
—Tuberculosis is a disease resulting from the growth of tubercle bacteria in the tissues of the animal. The bacteria, or germs, of tuberculosis, usually gain entrance to the organs of the body by being taken in with the food. Sometimes they penetrate through the membranes in the throat and get into the glands of the head. Sometimes they are taken into the digestive tract, where they pass through the walls of the intestines into the lymph channels and are carried through the large lymph vessel into the blood circulation. In some cases it would seem that the bacteria get into the lungs on particles of dust that are inhaled.
TUBERCULOSIS GERMS
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.
The Symptoms of Tuberculosis
vary according to the location of the disease. If it is in the glands of the throat it is suggested by their enlargement. If it is in a gland about the lungs, which, because of its enlargement, presses on the œsophagus (gullet), there might be bloating. If the disease is in the lung tissue there would be, after it is sufficiently advanced, coughing and perhaps difficult breathing. If the disease is in the liver, it cannot be readily distinguished until it is far advanced. If the disease is in the udder it manifests itself usually by the organ becoming firm or hard, and when the tissues are sufficiently broken down the milk from that quarter will be changed in appearance; sometimes it is thick, containing pus, sometimes thin and watery. It is very difficult to diagnose tuberculosis from the symptoms, as many other causes may give rise to similar manifestations.
As tuberculosis is caused by a specific germ, the disease is spread by the germs escaping from the diseased animals and getting into the bodies of healthy ones. The tubercle bacteria escape from the infected animal with some one or more of the natural discharges of the body. For example, if the cow has a bad tuberculous area in the lung, the bacteria may be discharged into one of the air tubes and coughed up into the mouth. Some of them will escape with the saliva and infect mangers or pastures. Some of them may be swallowed and escape from the body with the feces. If the disease is in the udder the germs will escape with the milk. There are some observations which indicate that sometimes the bacteria will escape with the milk where the udder is not affected. After the bacteria leave the diseased animal and are left in the manger, or in the pasture, or on the surface of water in the drinking trough, they can be readily taken up by healthy cattle that eat or drink after them. If they escape with the milk, calves and pigs that are fed with it readily become infected. After the germs get into the body of the healthy animal they will multiply and produce the disease, just as the seed of a noxious weed will, if blown into a new field, germinate and produce the weed there. Tuberculosis spreads from animal to animal on the same principle that weeds spread from one field to another.
In order to prevent the spread of tuberculosis it is simply necessary to prevent healthy animals from coming in contact with the diseased ones or eating or drinking after them.
As tuberculosis cannot be readily detected by a physical examination until the disease is far advanced in the organs affected, it is necessary, in order to determine which animals have the disease, to apply some test or to find the germs of the disease in their excretions. The simplest test that has thus far been discovered is the action of tuberculin. When tuberculin is injected under the skin of the animals affected with active tuberculosis the animals respond by a rise of temperature, which follows a somewhat definite curve. By means of this test it is possible to pick out the infected individuals so that they can be separated from the healthy ones. The test should be repeated in from six months to a year in order to detect any new cases which might have developed from latent or arrested ones. We cannot always get all of the infected animals with the first test any more than we can always remove every weed from the garden by one hoeing.
The Bang Method for the Control
of tuberculosis consists in separating the animals that are infected from the well ones and keeping them for breeding purposes. The calves are removed from their dams as soon as born and fed with the milk of healthy cows, or the pasteurized milk of the infected ones. It has been found that but a small percentage of calves that are raised under proper precautions from such animals have tuberculosis. By this means a sound herd of cattle may be developed from tuberculous animals. This method was introduced by Prof. Bang of Copenhagen, and it has been found to be very effective in Denmark and other countries in Europe. It has been applied with much success in a large number of individual herds in the United States. Its success depends entirely upon the care which is taken in keeping tubercle bacteria away from the calves.
In purchasing cattle for dairy or breeding purposes it is important that they should be taken from herds that are free from tuberculosis. The sound herd is the unit to be dealt with. Animals from such herds are far more reliable than non-reactors from tuberculous herds.