Leprosy and tuberculosis: The epidemiological consequences of cross- immunity

Abstract

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.