Estimation of age-specific rates of acquisition and loss of Wuchereria bancrofti infection.

This study uses a reversible catalytic model to estimate the age-specific rates of gain and loss of Wuchereria bancrofti infection from data collected during a control programme in Pondicherry, South India. The data describe the infection status in 1981 and 1986 of two cohorts of individuals, one living in an area where vector reduction had been achieved, and the other in a comparable endemic area. The rate of loss of infection in the absence of reinfection is estimated for the cohort in the control area, and the rate of gain of infection by the cohort in the endemic area estimated by substitution in the model. The mean expected life span of patent infection is estimated to be 5.4 years. The instantaneous rate of loss of infection is independent of age, while the rate of gain of infection exhibits a convex age-profile, peaking in the 16-20 year age-class. The reduced rate of gain in adults is largely attributable to the increasing proportion of potentially resistant individuals with clinical disease. The results suggest that the age-distribution of bancroftian filariasis is primarily determined by age-dependency in the rate of acquisition of infection.