Household and dwelling contact as risk factors for leprosy in northern Malawi.

Data on household and dwelling contact with known leprosy cases were available on more than 80,000 initially disease-free individuals followed up during the 1980s in a rural district of northern Malawi. A total of 331 new cases of leprosy were diagnosed among them. Individuals recorded as living in household or dwelling contact with multibacillary patients at the start of follow-up were at approximately five- to eightfold increased risk of leprosy, respectively, compared with individuals not living in such households or dwellings. Individuals living in household or dwelling contact with paucibacillary cases were both at approximately twofold increased risk. The higher risk associated with multibacillary contact and the fact that dwelling contact entailed a greater risk than household contact if the association was with multibacillary, but not with paucibacillary, disease suggest that paucibacillary cases may not themselves be sources of transmission, but rather just markers that a household has had contact with some (outside) source of infection. When household contact was considered alone, the risks of disease were appreciably higher for younger than for older contacts and for male compared with female contacts. Despite the elevated risk of leprosy associated with household or dwelling contact, only 15% of all incidence cases arose among recognized household contacts. Given the dynamic nature of household membership and consequent misclassification of contact status, the true contribution to overall incidence of contact within household or dwelling settings is likely to be much higher than this, perhaps 30% or higher. Considering the predilection of males for infectious multibacillary forms of the disease, the transmission of Mycobacterium leprae at an early age, in particular to males, may be of particular importance for the persistence of leprosy in endemic communities. Although residential contact with a multibacillary case is the strongest known determinant of leprosy risk, the vast majority of such contacts never manifest disease, which indicates a crucial role for genetic and/or environmental factors in the transmission of M. leprae infection and/or the pathogenesis of clinical leprosy.