The overwhelming preponderance of tuberfulous infections affect the lungs and begin there. Pulmonary involvement is still the major cause of tuberculosis morbidity and mortality. The prevention and control of these pulmonary infections account for tuberculosis being a relatively uncommon cause of death today in the United States. A further grim aspect of th eresurgence of tuberculosis is the emergence of highly drug-resistant strains.

Primary pulmonary tuberculosis

Except for the rare intestinal (bovine) tuberculosis and the even more uncommon skin, oropharyngeal, and lymphoidal primary sites, the lungs are the usual location of primary infections. The initial focus of primary infection is the Ghon complex which consists of (1) a parenchymal subpleural lesion, often just above or just below the interlobar fissure between the upper and the lower lobes, and (2) enlarged caseous lymph nodes draining the parenchymal focus.

The course and fate of this initial infection are variable, but in most cases patients are asymptomatic and the lesions undergo fibrosis and calcification. Exceptionally, particularly in infants and children or immunodeficient elders, progressive spread with cavitation, tuberculous pneumonia, or military tuberculosis may follow a primary infection.

Secondary (Reactivation) Pulmonary Tuberculosis

Most cases of secondary pulmonary tuberculosis represent reactivation of an old, possibily subclinical infection. During primary infection, bacilli may disseminate without producing symptoms and establish themselves in sites with high oxygen tension, particularly the lung apices Reactivation in such sites occurs in no more than 5 to 10% of the cases of primary infection. Secondary tuberculosis, however, tends to produce more damage to the lungs than does primary tuberculosis.

The secondary pulmonary tuberculous lesion is located in the apex of one or both. It begins as a small focus of consolidation, usually less than 3 cm in diameter. Less commonly, initial lesions may be located in other regions of the lung, particularly about the hilus. In almost every case of reinfection, the regional nodes develop foci of similar tuberculous activity. In favorable case, the initial parenchymal focus develops a small area of caseation necrosis that does not cavitate because it fails to communicate with a bronchus or bronchiole. The usual course is one of progressive fibrous encapsulation, leaving only fibrocalcific scars that depress and pucker the pleural surface and cause focal pleural adhesions. Sometimes, these fibrocalcific scars become secondarily blackened by anthracotic pigment.

Histologically, coalescent granulomas are present, composed of epithelioid cells surrounded by a zone of fibroblasts and lymphocytes that usually contains Langhans giant cells. Some necrosis (caseation) is ually present in the centers of these tubercies , the amount being entirely dependent on the sensitization of the patient and the virulence of the organisms.

As the lesions progress more tubercles coalesce to create a confluent area of consolidation. In the favorable case, either the entire area is eventually converted to a fibrocalcific scar or the residual caseous debris becomes totally and heavily walled off by hyaline collagenous connective tissue. In these late lesions, the multinucleate giant cells tend to disappear.

In cases of suspected tuberculous tissue changes the diagnosis is confirmed by histologic staining, smears, and cultures of acid-fast organisms. Tubercle bacilli can be demonstrated in the early exudative and caseous phases, but it is usually impossible to find them in the late fibrocalcific stages. Lesions with sparse organisms can be highly infective one can estimate that finding a single acid fast bacillus in a routine histologic sample granuloma indicates that a total of atleast 2000 organisms are present within the granuloma. Hence it can not be assumed that their absence in histologic sections is tantamount to their total destruction because in many of these instances culture of the lesions or inoculation of this material into guinea pigs yields the organisms.