Leprosy and tuberculosis: the epidemiological consequences of cross-immunity.

OBJECTIVES: This study tested the hypothesis, first proposed by Chaussinand, that individual-level immunity acquired from exposure to tuberculosis may have contributed to the disappearance of leprosy from western Europe.

METHODS: The epidemiological consequences of cross-immunity were assessed by the formulation of a mathematical model of the transmission dynamics of tuberculosis and leprosy.

RESULTS: The conditions under which Mycobacterium tuberculosis could have eradicated Mycobacterium leprae were derived in terms of the basic reproductive rates of the two infections and the degree of cross-immunity.

CONCLUSIONS: If the degree of cross-immunity between two diseases within an individual is known, then the epidemiological consequences of this cross-immunity can be assessed with transmission modeling. The results of this analysis, in combination with previous estimates of the basic reproductive rate of tuberculosis and degree of cross-immunity, imply that tuberculosis could have contributed to the decline of leprosy if the basic reproductive rate of leprosy was low.