HOWARD W. JOHNSON AND ALBERT F. RANNEY.
ROBERT KOCH discovered in 1882 the Mycobacterium the bacterium, or bacillus that causes tuberculosis. Thus the great German bacteriologist proved that tuberculosis is a specific, infectious disease, and he ended fears and uncertainties about it that men had had since the time of Moses.
As a country practitioner in Posen, he made many microscopic studies of bacteria. He devised a method of staining bacteria with aniline dyes and ways to grow colonies of bacteria. In 1890 he developed tuberculin a sterile liquid that contains the growth products of the tubercle bacillus as a test for tuberculosis. That discovery brought him a Nobel prize and marked the beginning of widespread efforts to end bovine tuberculosis, the type that attacks cattle.
The next big step in the conquest of tuberculosis was the discovery that human tubercle bacteria are not the same as bovine tubercle bacteria. That discovery was made by Theobald Smith, a pathologist in the Department of Agriculture whose other notable contributions included the discovery of the parasite that causes Texas cattle fever and its transmission by the cattle tick.
There followed some of the most fruitful scientific investigations that modern medicine has witnessed. The investigations gave us the definite knowledge of the nature of the disease that made it possible to eliminate tuberculosis from infected herds and to keep it out of healthy herds. They brought out a number of basic facts:
Practically every species of animals can be affected by tuberculosis. The disease in each species is similar in character and always ends in a slow wasting of the body tissues.
There are three types of tubercle bacillus. The kind found in man cannot produce progressive disease in many animals. The tubercle bacillus in cattle is different from the human type, although it can infect people. The third type exists in poultry. The three kinds of tubercle bacillus were classed as the human, bovine, and avian (bird) types.
THE TUBERCLE BACILLUS is a rod-shaped organism. It is 1.5 to 4 microns long-6,000 to 16,000 of them laid end to end measure 1 inch. The bacillus has a waxy capsule, which cannot be readily penetrated by the various stains used in bacteriological studies to render the organisms visible.
Because micro-organisms of the genus Mycobacterium are acidfast, they are hard to stain. Once they are stained, however, the stain cannot be removed by acid treatment, as it can from most other germs. Thus, if the micro-organisms in a sample are stained with a red dye and then treated with acid and counterstained with a blue dye, the tubercle bacilli will show red, but the other organisms, which are not acidfast, show blue.
THE TUBERCLE BACILLUS grows very slowly on culture media, which are foods for growing bacteria artificially. Bacteria are grown on media in order that they may be studied and closely related species may be differentiated. On the first culture of tubercle bacillus, it may be 1 month and sometimes 2 months before a colony of germs grows enough to be seen by the unaided eye.
Most other organisms show colony growth in 24 to 48 hours.
The three types of tubercle bacilli human, bovine, and avian produce colony growths different from one another on a culture medium. The growth of the human type appears thick and wrinkled. The bovine type looks sparse, rough, and dry. The avian type is heavy, smooth, moist, and glistening.
Tubercle bacilli usually enter the body by way of the mouth in contaminated food or water. Sometimes they are breathed directly into the lungs.
An animal spreads the bacilli when the tuberculous excretion from its lungs reaches the mouth and is washed into the water trough when it drinks. Then it subjects other cattle using the trough to the most dangerous kind of exposure. When they swallow the tuberculous sputum, the tubercle bacilli in it are passed unchanged through the intestines, and their dung is extremely infectious. Stagnant pools into which cows drop their manure thus offer means by which the disease may be spread, as tubercle bacilli in such pools may remain alive for a year or longer. Small streams may likewise become contaminated with tubercle bacilli. Around hayracks, dung may become mixed with particles of hay. One feeding of milk from a dam with tuberculosis localized in the udder may make it possible for a calf to pick up the disease.
TUBERCULOSIS exists in many forms according to the part of the body in which the organism becomes localized.
Human beings may have lupus, or tuberculosis of the skin; scrofula, or tuberculosis of the lymph nodes; bone and joint tuberculosis; tuberculosis of the linings of the brain, or tuberculous meningitis; and so on.
In animals, the organs most commonly affected are the lungs. In most species it is tuberculosis of the lungs on which depends the spread of the disease, from man to man or animal to animal.
In poultry, the liver and spleen are most commonly affected. Tuberculosis is spread from the birds' droppings.
The udder of cows may become infected when the disease is chronic, and large numbers of tubercle bacilli may be given off in the milk.
All organs and sections of the body have specialized glands, called lymph nodes, which filter out bacteria from the lymph stream and thus hinder the spread of infection to another part of the body. The lymph nodes are often the site of the first localization of tubercle bacilli. The nodes in the throat and neck have the first opportunity to become infected; next the mesenteric nodes, which drain the intestines; and next the lungs and their adjacent lymph nodes. Tubercles in other sections of the body are usually the result of the spread of infection from a primary center, or focus.
WHEN TUBERCLE BACILLI lodge in any part of the body, certain blood cells are attracted to the site and attempt to ingest them. The waxy capsule of the bacillus makes it resistant to destruction. Other blood cells congregate around the area and form a protective wall against the spread of the bacilli. Thus a tubercle, or morbid nodule, is formed. If the wall, or encapsulation, becomes dense on all sides, the tubercle remains stationary and is called an arrested lesion. Calcium salts may be deposited in the tubercle and transform it into a gritty, or calcified, lesion. If the lesion is very small, it might even be absorbed.
If the tubercle bacilli are not checked, the tubercle enlarges on the Outside, developing into what is known as a proliferative, or spreading, lesion. Bacilli may escape from this mass and cause the formation of new tubercles, either adjacent to the old tubercle or remote from it. During the growth of a tubercle in the lungs, a terminal branch of a bronchus, or air channel, may be surrounded. The poisons secreted by the bacilli tend to soften the inner parts of the tubercle, and they may be expelled into a bronchus, thence to the trachea, and then coughed up. A tubercle thus broken down is known as an exudative type of lesion.
Tubercles vary in number and kind in different individuals, according to the resistance offered. After localization of tubercle bacilli in the body, the resultant tubercle may disappear completely in a few weeks or months or it may cause the fulminative type of the disease, which spreads rapidly and results in death in a few weeks or months.
Usually the tubercle remains in the body as a walled-off, or encapsulated, tumor, and it may remain as such for the life of the host. At any time, however, such a tubercle may become active because of lowered resistance and be a focus for the spread of the disease to other parts of the body.
The time between the exposure to tubercle bacilli and the development of a tubercle is the period of incubation. The period varies in length. In the first place, tubercle bacilli multiply very slowly in the animal body, and infection develops only when the germs have multiplied to a certain number in the spot where the tubercle is formed. Depending on various conditions, it may be one to several months before the disease can be detected by the tuberculin test (to be explained later). When tubercle bacilli once gain a foothold, the disease is usually slowly progressive. It may be months or years before the general physical condition becomes noticeably impaired.
TUBERCULOSIS may be suspected when an animal shows a gradual loss of weight and condition. A chronic cough develops in cattle affected with tuberculosis of the lungs. It is remarkable, however, that cattle that appear to be in prime condition may be grossly tuberculous and may be spreading the disease.
Poultry may be suspected of being tuberculous when some of the flock show lameness, thinness, especially in the breast muscles, and paleness of combs and wattles.
In swine the disease may not be suspected because most of the animals are marketed during their first or second year of life and because the disease only rarely spreads from hog to hog. It is in older animals that the condition usually becomes apparent by a gradual loss in weight and condition or by enlargement of joints.
SOME SPECIES OF ANIMALS are susceptible to infection with only one type of the tubercle bacillus. Others may be susceptible to two types. Still others may be susceptible to all three.
Cattle are the chief hosts and likewise the chief disseminators of the bovine tubercle bacillus. Only rarely are they known to develop lesions or visible tubercles as a result of exposure to infected poultry or a tuberculous human being.
Horses and mules are resistant to all three types.
Chickens are susceptible only to the avian tubercle bacillus.
Sheep and goats as a rule are resistant to tuberculosis. A few flocks of goats and, rarely, a sheep or two have been found to be infected with the bovine organism. A few sheep that have been in close contact with infected poultry have shown lesions caused by the avian organism.
Swine may be infected by all three types of the tubercle bacillus. The bovine type causes the severest disease, but in the United States probably nine-tenths of the tuberculous lesions in swine are caused by the avian organism. Lesions in swine caused by the human tubercle bacillus are not of the progressive type and usually remain localized in lymph nodes of the head or intestine. The feeding habits of swine provide for ample exposure to all three types: To the bovine type in cattle-feeding lots where the swine eat cow dung; to the avian type on farms where there are tuberculous chickens, which may soil the ground, or where the farmer may throw his dead chickens to the hogs; and to the human type from uncooked garbage or from the sputum of a tuberculous attendant.