community. Direct evidence about the source of transmission typically comes from investigations of possible workplace outbreaks of tuberculosis. In this context, an “outbreak” may be defined as transmission of M. tuberculosis that results in infection or active disease among workers, patients, and others exposed in a health care facility, prison, or other setting in which people with tuberculosis are treated, served, or detained (Garrett et al., 1999).2 Investigations of possible outbreaks include the careful questioning of affected workers not only about possible workplace exposures but also about possible community-related exposures involving family members and other close contacts outside of work.

Some outbreak investigations are supported by molecular epidemiology (DNA fingerprinting), which compares isolates of M. tuberculosis recovered from different individuals. Molecular analyses can help establish a chain of transmission that links workplace cases of active tuberculosis (but not infection) to source cases in the workplace or the community. As discussed further below, inferences about the source of infection or disease for a worker are still most often based on comparisons of occupational and demographic information for workers with and without occupational exposure to tuberculosis. In some cases, no clear source of infection or transmission—either work related or community based—is identified.

In analyses of tuberculosis risk in workplaces and communities, the term “community” or “community of residence” has no precise definition.3 Although a facility’s location may be identified as a particular city, county, or metropolitan area, the residences of workers in that facility may be widely spread across areas with very different rates of tuberculosis. For example, someone living in central Harlem can be expected to have a higher risk of community exposure to tuberculosis than someone from the Connecticut suburbs.

Few studies have matched detailed information on worker place of residence against equally detailed community data on tuberculosis cases. Information on a worker’s home zip code may improve on city or county as an indicator of community of residence, but a single zip code may still encompass an area with quite variable resident characteristics (e.g., incidence of tuberculosis and income levels). Also, collection of zip code data

2  

For tuberculosis infection, investigators focus on excess rates or clusters of skin test conversions rather than on single conversions. If an investigation does not indicate workplace transmission, then the presence of skin test conversions or cases of active tuberculosis does not constitute a workplace outbreak.

3  

Sometimes community is described in social rather than geographic terms. An example might be a close-knit community of recent immigrants. Recently, CDC reported an outbreak of tuberculosis in a “social network of transgender persons (i.e., persons who identify with or express a gender and/or sex different from their biological sex)” (Sterling et al., 2000a, p. 1).



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