Transmission of tuberculosis

Infection is spread almost exclusively by aerosolization of contaminated respiratory secretions. Patients with cavitary lung disease are particularly infectious because their sputum contains relatively greater numbers of bacilli and they cough frequently.

For infection to occur, bacilli must be delivered to the distal air spaces of the lung (the alveoli), where they are not subject to mucociliary clearance. To reach the alveoli, the bacilli must be suspended in very fine units that behave as the air itself. These units are the dehydrated residuals of finer particles generated by high-velocity exhalational maneuvers; cough-inducing procedures such as bronchoscopy or endotracheal intubation are particularly likely to generate infectious aerosols. These droplet nuclei are calculated to be 0.5 to 3 μm in diameter, may remain suspended in room air for many hours, and when inhaled, can traverse the airways to reach the alveoli.

Although patients with cavitary tuberculosis expectorate massive numbers of bacilli, the probability of generating infectious particles is relatively low. Household contacts of patients with extensive pulmonary disease who have had productive coughs for weeks or months before diagnosis have, on average, less than a 50% chance of being infected. The usual case of pulmonary tuberculosis has a low order of infectiousness when compared with an airborne disease such as measles. However, uncommon cases demonstrate extremely high rates of transmission; specific factors in these instances have not been clearly elucidated.

Transmission also occurs via aerosols generated by débridement or by dressing changes of skin or soft tissue abscesses caused by M. tuberculosis. Tissue agitation associated with autopsies and direct inoculation into soft tissues from contaminated instruments or bone fragments have also been reported. Fomites do not play a significant role in transmission.