label suggests, outbreaks represent atypical rather than normal circumstances, at least in relatively low-prevalence regions such as the United States.
Thus, reports of outbreaks do not provide a solid basis for estimating the occupational risk of latent tuberculosis infection or active disease. Nonetheless, careful analyses of outbreaks can provide suggestive information about the potential risk of infection or disease among different types of workers. Particularly useful are studies that have compared workers with and without potential risk factors such as contact with patients at increased risk of tuberculosis (e.g., those on medical wards, HIV/AIDS units), work involving aerosol-generating procedures, and various demographic characteristics (e.g., income, race, place of birth, and place of residence). Although statistical analyses may find similar levels of risk in workplaces and workers’ community of residence, workplace investigations, including DNA analyses, make clear that workers do face a real risk of acquiring tuberculosis from patients, inmates, or others who they encounter on the job.
Garrett and colleagues searched the literature for published reports of tuberculosis outbreaks in health care settings (Garrett et al., 1999). They located reports on 28 outbreaks occurring between 1960 and 1996, mostly in hospitals. The more recent outbreaks (late 1980s and 1990s) differed from earlier outbreaks in that they more often involved serious cases of multidrug-resistant disease, affected relatively large numbers of patients and workers, and spread rapidly enough to be picked up by hospital and public health surveillance systems. A high percentage of the cases involved patients or workers who were seriously immunocompromised due to HIV infection or AIDS and who were thus at high risk of progressing quickly from tuberculosis infection to active disease. This made it easier for clinicians and others to recognize possible links to earlier hospital stays. In these more recent outbreaks, at least 20 health care workers developed multidrug-resistant tuberculosis, and 9 of them died. Skin test information for workers was often incomplete but pointed to additional workers who had become infected with M. tuberculosis without developing active disease. Garrett and colleagues described the epidemiological evidence for transmission of tuberculosis in the health care setting as “compelling” (Garrett et al., 1999, p. 489).
A review by Dooley and Tapper (1997) of 21 outbreaks in adult inpatient settings (many also reviewed by Garrett and colleagues ) reported that a single source of transmission was identified for 10 settings, whereas the others involved multiple sources. Some sources were discovered as a result of formal investigations, whereas others were discovered