Homelessness is a well-recognized risk factor for tuberculosis, although it is difficult to disassociate housing circumstances from a variety of other factors that overlap in this population (Zolopa et al 1994). Nevertheless, homeless persons probably constitute a 'core group' within the population that has epidemiological significance out of proportion to its absolute size (Barnes et al
In San Francisco there are an estimated 10 000 homeless or unstably housed persons, of whom 2774 were in a cohort that was evaluated prospectively by Moss et al (1998). The rates of tuberculosis were extremely high: 10 persons (0.36%) were found to have active tuberculosis at the time of initial evaluation, and there were an additional 25 incident cases during a median follow-up of 3.2 years (0.27% per person/year).
Molecular epidemiological analysis showed that 15 of the 20 incident cases for whom DNA fingerprinting was available were in 12 different clusters. Three of these 15 cases appeared to be the sources of three clusters.
The reasons for the high incidence of tuberculosis in the homeless and their epidemiological effect in causing new cases are multiple. In spite of a high rate of completion of chemotherapy overall in San Francisco, the highest rates of loss were among the homeless. At least some of these cases continue to be infectious. Many of the persons in this environment are vulnerable to tuberculous infection because of HIV infection or other factors that are more difficult to quantify, such as drug abuse, alcoholism, poor nutrition or generally poor health (Zolopa et al 1994).
In addition to the greater difficulty of completing treatment for tuberculosis among the homeless, traditional approaches to the administration of isoniazid preventive therapy are marginally effective at best. Without incentives, only 20% of homeless persons in the San Francisco cohort who were candidates for isoniazid preventive therapy completed six months of the drug. These findings concerning the homeless suggest that targeted screening programmes designed both to identify persons with active tuberculosis as well as candidates for preventive therapy are useful approaches to tuberculosis control in this high risk population (Schluger et al 1997). Considerable thought and individualization should go into determining the best ways of completing therapy. Directly observed therapy in itself is insufficient but must be combined with comprehensive case management and provision of a range of necessary services. Screening with the goal of identifying candidates for preventive therapy should not be undertaken unless there are provisions to ensure that evaluations are completed and preventive therapy can be given successfully. Given that such persons are at high individual risk and, if disease develops, pose a high community risk, directly observed preventive therapy should be used. As with treating active tuberculosis, individualized and imaginative ways of supervising drug administration must be applied. Because completion of preventive therapy should be the major goal of screening programmes, such programmes should not be undertaken unless the resources are available to follow administration of the preventive therapy to its successful completion.
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