Among the conditions of environment which aid the development of tuberculosis may be mentioned stabling with lack of ventilation, damp buildings, the keeping of many animals together, drafts of air which cause colds and catarrhs, and, in general, everything which prevents the animals from developing and maintaining the highest condition of health. None of these conditions of body or environment are sufficient to cause the disease, however, unless the animals are exposed to the Mycobacterium tuberculosis and it penetrates the tissues of their bodies.
The ways in which the tubercle bacilli find their way into the body may be considered under four heads: (1) By inhalation into the lungs; (2) by taking into the digestive tract in the milk of tuberculous cows or with other contaminated feed; (3) during coition when the sexual organs are tuberculous; (4) from the tuberculous mother to the fetus in the uterus. The bacilli can reach the lungs by inhalation only when the bacilli are thoroughly dried and pulverized and in condition to be carried by currents of air.
It is well known that the bacilli withstand drying for months before they lose their power of producing disease. They leave the bodies of diseased animals in several ways. There may be a little discharge occasionally coughed up as a spray from the diseased lungs, or this material may be swallowed and the bacilli carried off with the excrement, or milk may be spilt, or there may be a discharge from the vagina when the genital organs are tuberculous. There may also be ulcers of the intestines, from which many bacilli escape with the feces. The bacilli from these sources may become dried and pulverized and carried in the air of the stable and into the lungs of still healthy cattle, where the disease then develops.
The disease of the stomach, intestines, and mesenteric glands is very probably the result of feed infection. Tubercle bacilli may have been scattered upon the feed by diseased animals, but the most common source of such infection is the milk of tuberculous cows. Calves may become infected in this way. The disease may remain latent until the animal becomes older. The not-infrequent occur rence of tuberculosis of the uterus and ovaries makes it probable that the disease may be transmitted by a diseased bull or carried by a healthy one from a diseased cow to a number of healthy cows.
The source of infection is always some previous case of the disease, for the disease can never rise spontaneously; hence, in those stables in which there is frequent change of cattle the introduction of tuberculosis by cattle coming from other infected stables is the most frequent source of infection. Since the bacilli when dried can be carried by the air, it is not necessary that healthy animals come in direct contact with cases of disease to become infected. In general, the greatest number of cases occur in the immediate environment of cities, where there are not only abundant opportunities for infection, owing to the frequent introduction of new animals into herds, but where the sanitary conditions may be regarded as the poorest.
The bacillus of tuberculosis was discovered by Robert Koch in 1882. It is a slender, rodlike body (see [Pl. XXVIII], fig. 6) from one-third to two-thirds the diameter of a red blood corpuscle in length. As already explained, when the bacillus has become lodged in any organ or tissue it begins to multiply, and thereby causes an irritation in the tissue around it, which leads to the formation of the so-called tubercle. The tubercle, when it has reached its full growth, is a little nodule about the size of a millet seed. It is composed of several kinds of tissue cells. Soon a change takes place within the tubercle. Disintegration begins, and a soft, cheesy substance is formed in the center, which may contain particles of lime salts. When these tubercles continue to form in large numbers they run together, forming masses of various sizes. The disintegration which attacks them leads to the formation of large cheesy masses of a yellowish color, containing more or less of lime salts in the form of gritty particles. These large tuberculous masses are surrounded by or embedded in layers of fibrous tissue which in some cases becomes very dense and thick.
The disease is thus a development of these tubercles in one or more organs of the body. The distribution and number of the tubercles determine the course of the disease.
In a large number of cases the changes are limited to the lungs and the serous membranes[4] of the thorax and abdomen. Pathologists have been in the habit of calling the lung disease tuberculosis and the disease of the serous membranes "pearly disease." Statistics have shown that in about one-half of the cases both lungs and serous membranes are diseased, in one-third only the lungs, and in one-fifth only the serous membranes. At the same time the lymphatic glands near the diseased organs are usually involved. Other organs, such as the liver, not infrequently contain tubercles. Though the disease may remain restricted to a single organ, it now and then is found generalized, affecting all organs of the body.
In the lungs ([Pl. XXXIV]) the changes observed vary according to the age and intensity of the disease process. They usually begin with the appearance of very minute tubercles. These may appear in large numbers on the surface of the lungs or within the lung tissue. Later the contents become cheesy and partly calcified. When these tubercles are sufficiently numerous to become confluent, large masses may be formed, which undergo the same retrogressive changes of caseation and calcification. In addition to the formation of tubercles in the lung tissue, certain other changes take place. There is usually bronchitis with abundant catarrhal secretion; this plugs up the smaller air tubes, and the lung tissue supplied with air by the tubes collapses. Subsequently it becomes filled with yellowish, cheesy matter, which greatly distends the small air tubes and air vesicles (bronchopneumonia). The connective tissue between the lung lobules, around the tubercles, and around the air tubes becomes thickened and indurated. In the larynx and the bronchi tubercles may vegetate upon the mucous membrane, and ulcers may result from their breaking down. The inflammatory irritation which the growth of the tubercles on the surface of the lungs arouses gives rise to adhesion of the lungs to the ribs and diaphragm. This adhesion is sometimes so firm and extensive that the lungs appear grown to the chest wall.
When, therefore, the lungs in advanced stages of the disease are cut open we observe large yellowish masses, from one-quarter to three-quarters of an inch in diameter, of a cheesy texture, in which calcified, gritty particles are embedded and which are surrounded by very firm connective tissue. The neighboring lung tissue, when collapsed and involved in bronchopneumonia, has the color and consistency of pale-red flesh. The air tubes, large and small, stand out prominently on the cut surface. They are distended with a pasty, yellowish, cheesy mass, surrounded and enveloped in thick mucus, and their walls greatly thickened. The larger bronchi may be sacculated, owing to the distention produced by the cheesy contents.