Variation in vulnerability to tuberculosis in America today: random, or legacies of different ancestral epidemics?

To test the hypothesis that vulnerability to infection by Mycobacterium tuberculosis is the norm for mankind but innate resistance to the infection is common in the USA today as a legacy of TB epidemics survived by one's ancestors.

A reaction of 10 mm or more to a tuberculin skin test (TST) was used to determine the prevalence of infection following community outbreaks of the disease. For further information, a survey was conducted of prevalence of tuberculin reactivity among health-care workers with frequent exposures to tuberculosis patients during bronchoscopy and sputum collection.

Sixty per cent of African Americans exposed in 26 community outbreaks were TST positive compared to only 40% of whites following comparable exposures. Similarly only 56 (41.2%) of 136 heavily exposed white health-care workers were TST positive.

When considered in conjunction with contrasting ancestral histories of exposure to TB, these observations suggest a difference in frequency of an innate ability to respond protectively to M. tuberculosis and to mount an effective mechanism to destroy it. This is best explained as a process of natural selection among largely separate heavily exposed ancestors. Several recent reports have identified genetically mediated mechanisms of immunity that could be involved with reduced vulnerability to tuberculosis. An understanding of these processes could aid in the development of immunomodulatory agents or vaccines.