Tuberculosis is a specific infectious disease characterized by the formation of tubercles.

It is caused by the Bacillus tuberculosis, which is non-motile, non-sporogenous, aerobic, acid resisting, and purely parasitic. Occurs as a slender, rod-shaped, slightly curved body, usually with rounded ends, but sometimes showing distinct branches. It is about 1.5 to 3.5 long by 0.2 5. wide. It is found in sputa and in the lesion of tuberculosis. It is the cause of all forms of tuberculosis in man and may be transmitted to many of the lower animals. It is still unsettled whether the forms found in animals are capable of being pathogenic to man. The bovine bacillus, however, is apparently pathogenic in a small percentage of cases.

Staining is difficult, but after having once taken it up, the organism is with difficulty decolorized. Use Ziehl-Neelson method. Stains by Gram's.

Culture

Blood-serum, glycerin agar-agar, potato, and glycerin bouillon. It is difficult to cultivate, growth is slow, best at 370 C, none when below 290 C. or above 420 C. Growth is dry, lusterless, coarsely granular, wrinkled, and slightly yellowish.

Pathogenesis

Tuberculosis results from the successful invasion of the Bacillus tuberculosis. This may take place by means of: (1) the respiration; (2) the blood circulation; (3) lymphatic channels; (4) ingestion. After having gained entrance it may give metastases by and of the first three, by continuity of tissue, or by direct implantation.

The characteristic lesion is the miliary tubercle, which is gray in color as long as degeneration and caseation have not occurred; it then becomes yellow. It is rarely circumscribed by any definite boundary, and it tends to infiltrate and form tubercles in the adjacent tissues. It is a small area of inflammation and degeneration resulting from the action of the bacillus. The primary lesion does not necessarily occur at the point where the bacilli gained entrance. When the organism enters a suitable location, it undergoes multiplication. In a short time their number and the products of their metabolism bring about an increase in the number of fixed connective-tissue cells - epithelioid cells. These cells are the first to appear. A little later, through the chemotactic effect of the bacteria, lymphoid cells escape from the blood-vessels. According to which cell predominates, the tubercle may be either epithelioid or lymphoid.

Bacillus Tuberculosis In Sputum, Ziehl Gabbett. X 650 (Cornet and Meyer).

Fig. 28. - Bacillus Tuberculosis In Sputum, Ziehl-Gabbett. X 650 (Cornet and Meyer).

As the bacteria multiply, more nutrition is required, but this variety of inflammation is peculiar in that not only no new blood-vessels are formed, but the pre-existing ones are destroyed by endarteritis and thrombosis as the process advances. Consequently, the central area, the older portion, undergoes degeneration and coagulation necrosis.

The tubercle may be divided into three zones, according to its histologic characteristics: (I) A central zone containing bacteria and tissue cells that have undergone coagulation necrosis. (2) A median zone, in which are many epithelioid cells and frequently giant cells containing vesicular nuclei arranged peripherally and radially. (3) A peripheral zone, in which are found a few epithelioid, many lymphoid, and some plasma cells.

Subacute Tuberculosis of a Lymph gland. X 70 (Dürck).

Fig. 29. - Subacute Tuberculosis of a Lymph-gland. X 70 (Dürck).

1, Thickened capsule; 2, caseous centers of the tubercles. At the periphery of the gland the tubercles are still discrete, and between them lies lymphadenoid tissue. In the center of the gland the nodules have formed larger confluent areas. Numerous giant cells.

The giant cells as well as the epithelioid may come from the endothelium of the blood-vessels or lymph-vessels, from fibroblasts or from escaped leukocytes.

If the process has been rapid, the lymphoid cells usually predominate. If the individual's resistance is fairly good, some of the epithelioid cells may be converted into fibrous tissue. When resistance is marked, the tubercle may become encapsulated by fibrous tissue, and eventually become infiltrated by lime salts. This occurs only where the resisting power of the patient becomes greater than the destroying ability of the organism.

As, however, the bacilli keep continually multiplying, the tendency of the disease is to extend. This occurs by the organisms beings carried into the lymphatic channels either directly or by the action of phagocytes. The latter may carry and deposit them in a neighboring lymph-node, where secondary lesions will occur. Metastasis may also take place by the organisms gaining entrance into a vein, entering the general circulation, and setting up a more or less widely diffused general miliary infection.

Recovery from tuberculosis is more common than is generally believed. According to post-mortem examinations, 20 per cent, of the cases of tuberculosis recover. In such cases there is present the ability of the individual to resist the inroads of the process. The tubercle bacilli become encapsulated in a mass of connective tissue that prevents their further growth and extension. This new-formed tissue tends to contract and causes the broken-down portions to be absorbed, or else calcareous infiltration takes place.

These walled-off areas are, however, still a source of danger. Although tubercle bacilli do not form spores, yet infection may take place years after the connective-tissue growth, if for any reason the contents happen to escape.

When it remains quiet it is called "latent" tuberculosis.

The symptoms seen are probably due in a great part to the presence of associated pyogenic organisms. The night-sweats, fever, and loss of weight seen in cases of pulmonary tuberculosis are due to the associated bacteria. There is generally present some anemia, and many authors claim that there is an increase in the number of lymphocytes in the blood.

The liver frequently shows marked fatty infiltration and sometimes amyloid degeneration to a slight or a marked degree, depending upon the amount of suppuration.

The most common entrance for infection is the respiratory system. Sputum from tuberculous patients becomes dried and comminuted; it is then carried about by the currents of air and enters the body.

The intestines may become secondarily involved through infection brought about by swallowing the tuberculous sputum. Congenital tuberculosis may come from the paternal side from infection of the genitals; from the maternal side through infection of an ovum, or it may be transmitted through the placenta. Heredity is no longer thought to have much direct influence. It is now believed that what is inherited is nothing more than a weakened resisting power.