Epidemiologic trends of leprosy for the 21st century.

Major gaps still exist in the knowledge about leprosy, particularly with regard to how it spreads. Leprosy epidemiology remains complicated due to the specific characteristics of Mycobacterium leprae. To describe epidemiologic trends for the 21st century, the first part of this paper gives an overview of the epidemiology of leprosy, followed by past trends and the present situation of new-case detection as a proxy of the incidence. The third part, regarding predicted epidemiologic trends for the 21st century, elaborates on the main topic of this paper. With limited diagnostic tools to detect infection with M leprae, other methods are necessary to estimate trends in incidence and transmission. A computer program has been developed for modeling the transmission and control of leprosy (SIMLEP). The effect of failure to sustain early case detection beyond 2005 on leprosy incidence and case detection is shown. Important unanswered questions are whether the incubation period is contagious and how rapid close contacts of leprosy patients are infected. As long as such key questions remain unanswered, it will be difficult to estimate the impact of control strategies on the transmission of M leprae on resulting disease incidence. In the meantime we can expect that the global new-case detection trends will stay more or less stable or only decrease slightly for many years to come. There is a need of new preventive interventions to change this situation and reduce the incidence of leprosy in the 21st century.