Secular trends in age at onset, sex ratio, and type index in leprosy observed during declining incidence rates.

Epidemiologic surveillance in Norway, the United States, Nigeria, Japan, Venezuela, India, and China, covering periods from 1851 to 1981, demonstrates a consistent decline in incidence rates of leprosy. At the same time, secular trends have been observed which imply an increasing age at onset, an increasing male excess, and an increasing fraction of new cases represented by multibacillary leprosy. Theoretically, an increasing age at onset may be caused by two mechanisms, namely postponement of infection to a later age and/or an increasing fraction of patients with long incubation periods. Cohort analyses have shown no increase in age at onset in subsequent birth cohorts, but rather have shown a decrease. The latter mechanism, the increasing importance of long incubation periods, is consistent with the shift toward multibacillary cases in which the incubation period is longer than that in paucibacillary cases. Apparently, this mechanism has also been present during the decline of tuberculosis. An increasing fraction of new patients with long incubation periods, resulting in an increasing age at onset, is proposed as a general principle to be expected in any disease in rapid decline which also has a long and varying incubation period. This theory offers a basis for assessment of secular trends.