Genetic fingerprinting in the study of tuberculosis transmission.

The epidemiology of infectious diseases has traditionally relied on observed patterns of occurrence to infer transmission. In the case of a disease with a variable incubation period, such as tuberculosis (TB), a series of overlapping micro-epidemics can appear as a relatively steady notification rate at the population level. Recently, genetic fingerprinting has been used in epidemiologic studies to begin to assess such TB micro-epidemics. A crucial aspect of any TB control program is the ability to determine where transmission is occurring in order to prevent further spread of infection and prevent active disease by identifying newly infected people and providing them with preventive therapy. Genetic fingerprinting of Mycobacterium tuberculosis has vastly improved our ability to observe patterns of transmission in populations. It has helped to establish transmission links between individuals and to demonstrate instances in which related people were infected with unrelated strains. Since the 1980s the number of TB cases reported annually in Canada has been stable, at about 2000, translating into an incidence rate of about 7 cases per 100 000 annually. During this time, however, the groups at risk have changed. In 1980 the majority of cases of active disease occurred among elderly nonaboriginal people born in Canada whose disease was most likely the result of reactivation of remotely acquired infection. By 1994 the majority of cases involved foreign-born Canadian residents and aboriginal Canadians. The estimated incidence of TB among aboriginals is 80 per 100 000 annually, a rate 40 times that among nonaboriginal people born in Canada. Among foreign-born people the incidence rates vary greatly, usually reflecting rates in the country of origin. A substantial increase in TB risk among HIV-infected people has been well documented. Although the absolute number of HIV-related TB cases has been relatively modest in Canada, pockets of concurrent infection are increasingly observed in urban settings. Therefore, relatively stable overall rates of TB in Canada mask recent shifts in the epidemiology of the disease. Without an understanding of these changing trends it is unlikely that control programs will be able to address current and future challenges. In this article we describe the methods and review the progress, advantages and limitations of a powerful application of molecular biology to the investigation of an ancient human disease.