Rarely, M. tuberculosis may be transmitted by ingestion or skin inoculation. Determinants of infectivity 1. A patient's infectivity depends on the extent of pulmonary disease and the efficiency in which infective aerosols are generated. Extrapulmonary disease alone is rarely transmissible Untreated patients with smear-positive cavitary lung disease and productive cough are most infectious. Pathogenesis Sequence of events Inhaled tubercle bacilli, in the form of droplet nuclei, reach the terminal alveoli after avoiding entrapment by mucociliary clearance mechanisms.
Initial infection is most common in the lower lung segments where ventilation is greatest.
The organisms are ingested by alveolar macrophages, but continue to multiply. A mild local reaction with additional macrophages and lymphocytes develops. Within 2 - 4 weeks, infected macrophages carry the organism to hilar and mediastinal lymph nodes, where multiplication of the organism continues in presence of a minimal inflammatory response. Bacillemia, occurring at 4 - 6 weeks after inhalation of the organism, results from spread into the bloodstream from regional nodes. Both initial infection and bacillemia are usually asymptomatic or accompanied by mild self-limited systemic illness.