The impact of leprosy control on the transmission of M. leprae: is elimination being attained?

In 1991 the World Health Assembly decided to 'eliminate leprosy as a public health problem' by the year 2000. Elimination was defined as reducing the global prevalence of the disease to less than 1 case per 10,000. In 2000 the World Health Organization (WHO) announced that elimination was reached globally. Conventionally control of disease is defined as the reduction of disease burden to a locally acceptable level. Elimination of disease is defined as the reduction to zero of the incidence in a defined geographical area, and eradication is defined as the permanent reduction to zero of the worldwide incidence of infection caused by a specific agent. In leprosy however, WHO limited elimination to control instead of transmission, by using prevalence instead of incidence of disease. Leprosy statistics usually report on prevalence and new case detection. Prevalence is linked to length of treatment, which has changed over time. Trends in new case detection rates only reflect trends in incidence rates when no changes occur in case detection, but in the past 25 years case detection in leprosy has been determined strongly by operational factors. For the leprosy elimination strategy it was assumed that MDT would reduce transmission of M. leprae, but there is no convincing evidence for this. Data for evaluating the impact of MDT on transmission are not readily available because leprosy has a long incubation period. Also declines in case detection may have other causes, such as BCG vaccination. Mathematical modelling of the transmission and control of leprosy showed that the elimination strategy reduces transmission slowly, with a predicted annual decline in incidence ranging from 2% to 12%. Early case finding was the key factor to attain this decline. Future projections of the global leprosy burden indicated that 5 million new cases would arise between 2000 and 2020, and that in 2020 there would be 1 million people with WHO grade 2 disability. It is concluded that substantial progress has been made to control leprosy, but when elimination of disease is defined as the reduction to zero of the incidence, leprosy is definitely not eliminated. To attain elimination of leprosy it is necessary to find effective interventions to interrupt transmission of M. leprae and practical diagnostic tools to detect levels of infection that can lead to transmission. This requires extensive research in the areas of epidemiology and microbiology.