Leprosy transmission and mucosal immunity: towards eradication?

The declining prevalence of leprosy has not been matched by a declining incidence. Widespread adoption of multiple drug therapy (MDT) in closely monitored control programmes has not prevented transmission of Mycobacterium leprae. Despite the rarity of lepromatous patients, most of those living in endemic areas have immunological evidence of exposure to M. leprae. This paradox could be explained if, for many such individuals, infection was transient, did not result in disease development, but did allow the transmission of infection to other individuals. There is increasing evidence from nasal PCR studies that such sub-clinical transmission may exist and that mucosal immune responses to M. leprae may develop during resolution of initial infection. Sub-clinical infection appears to occur in clusters and may require close contact over a prolonged period for optimal transmission. Control of transmission may be feasible through identification and treatment of individuals within infection clusters, allowing progress towards the eradication of leprosy.