infectious diseases
Tuberculosis (TB)
Etiology of TB
Tuberculosis properly refers only to disease caused byMycobacterium tuberculosis(for which humans are the main reservoir). Similar disease occasionally results from the closely related mycobacteria,M. bovis,M. africanum, andM. microti. These three bacteria, together withM. tuberculosisand other less common mycobacteria, are known as theMycobacterium tuberculosiscomplex. TB results almost exclusively from inhalation of airborne particles (droplet nuclei) containingM. tuberculosis. They disperse primarily through coughing, singing, and other forced respiratory maneuvers by people who have active pulmonary or laryngeal TB and whose sputum contains a large number of organisms (about 10,000 organisms/mL, the limit of detection by fluorescent microscopy). People with pulmonary cavitary lesions are especially contagious because of the large number of bacteria contained within a lesion. Droplet nuclei (particles < 5 micrometers in diameter) containing tubercle bacilli may remain suspended in room air currents for several hours, increasing the chance of spread. However, once these droplets land on a surface, it is difficult to resuspend the organisms (eg, by sweeping the floor, shaking out bed linens) as respirable particles. Although such actions can resuspend dust particles containing tubercle bacilli, these particles are far too large to reach the alveolar surfaces necessary to initiate infection. Contact with fomites (eg, contaminated surfaces, food, personal respirators) do not appear to facilitate spread. are more contagious, and patients with Sputum examination, culture, and testing are more contagious than those with positive results only on culture. Patients with cavitary disease (which is closely associated with mycobacterial burden in sputum) are more contagious than those without. Respiratory secretions with lower viscosity are more easily aerosolized, and the effectiveness of cough and other respiratory maneuvers in generating aerosol varies greatly. Environmental factors also are important. Transmission is enhanced by frequent or prolonged exposure to untreated patients who are generating large numbers of tubercle bacilli in overcrowded, poorly ventilated, enclosed spaces; consequently, people living in poverty or in institutions are at particular risk. Health care practitioners who have close contact with active cases have increased risk. Thus, estimates of contagiousness vary widely. Some studies suggest that only 1 in 3 patients with untreated pulmonary TB infect any close contacts, but the World Health Organization (WHO) estimates that each untreated patient may infect 10 to 15 people per year. However, most of those who are infected do not develop active disease. Contagiousness decreases rapidly once effective treatment begins; cough decreases, and organisms are noninfectious even if they persist in sputum. Epidemiologic studies of household contacts suggest that transmission ends within 2 weeks of patients starting effective treatment, but more precise human-to-animal studies suggest that transmission ends within a few days of starting treatment. Much less commonly, contagion results from aerosolization of organisms after irrigation of infected wounds, in mycobacteriology laboratories, or by aerosol or direct puncture in autopsy rooms. TB of the tonsils, lymph nodes, abdominal organs, bones, and joints was once commonly caused by ingestion of milk or milk products (eg, cheese) contaminated withM. bovis, but this transmission route has been largely eradicated in countries where milk is pasteurized and cows that have a positive tuberculin skin test result are slaughtered. Tuberculosis due toM. bovisstill occurs in countries where bovine tuberculosis is endemic (eg, some Latin American countries) and in immigrants from those countries. The increasing popularity of cheese made from unpasteurized milk raises new concerns if the cheeses come from countries with a bovine TB problem (eg, Mexico, the United Kingdom). Bovine and human TB can be transmitted to other species such as badgers, deer, primates, and zoo animals. Slaughterhouses have been associated with zoonotic TB transmission. 