On the inefficiency of transmission of Wuchereria bancrofti from mosquito to human host.

The high biting rate of Culex pipiens fatigans in Rangoon, combined with a low prevalence of microfilaraemia due to Wuchereria bancrofti, suggested a poor efficiency of transmission. Data obtained by the WHO Filariasis Research Unit in Rangoon were analysed, and the efficiency of the parasite from Stage III larva in the mosquito to the production of microfilariae was estimated as 6.04-6.71 x 10(-5), indicating that an average of around 15 500 bites by "infective" mosquitos is necessary to produce 1 case of microfilaraemia. This surprising result prompted a step-by-step analysis of the success of the parasite through departure from the mosquito, penetration of host tissues, survival to maturity, and encountering a mate in the human host. It was hoped that the second approach would identify some discrepancy in the original analysis, but when all sources of loss were combined, it was estimated that the expected efficiency was 4.78 x 10(-5). The two estimates are within observational error of each other. The degree to which the two approaches agree leads to the conclusion that survival of the parasite is reasonably well known at all stages, and the calculations indicate that a high proportion of the population of Rangoon must be carrying parasites that are either dead or immature. A complete quantitative statement of the epidemiology of W. bancrofti seems to be attainable.