Elimination of leprosy FAQ
Why has a reduction of prevalence been selected as the yardstick for elimination?
The main thrust of the strategy to eliminate leprosy as a public health problem has been to reduce the prevalence of the diseaseto very low levels, through the widespread use of MDT. In the course of time, this can be expected to lead to a reduction in incidence, since the transmission of infection with M. leprae will have been interrupted.
In the past, a high proportion of the prevalence pool was taken up by the large number number of backlog cases, that is, cases that occurred over a considerable period of time and have remained active in the absence of effective treatment. In most endemic countries the implementation of MDT services has now cleared this backlog.
Now, as the expansion of MDT services is reaching previously uncovered or poorly covered areas, most of the new cases detected annually comprise cases who acquired the disease several years ago but remained undetected for various reasons; the true "incident" cases constitute only a very small proportion.
The long incubation period and lack of tools to measure infection and the disease status make it impossible to measure incidence nationally and, on a routine basis, as a yardstick for elimination. Reducing the prevalence is therefore an appropriate way of measuring progress towards the goal of elimination.
Will new cases of leprosy continue to occur after reaching elimination? If so, how can they be explained?
From the outset, WHO realised that new cases would continue to occur, albeit in much smaller numbers, in the post-elimination phase. This can be explained as a result of the disease making an appearance in individuals who acquired their infection several years earlier, even before the introduction of MDT. However, through the continued application of MDT, and through its expansion to previously uncovered areas, the number of new cases is expected to diminish steadily over a period of years.