Limitation and facilitation in the vectors and other aspects of the dynamics of filarial transmission: the need for vector control against Anopheles-transmitted filariasis

Abstract

In certain filaria-mosquito combinations, the number of infective, third-stage larvae (L(3)) that develop in a mosquito is not proportional to the number of microfilariae (mff) ingested by that mosquito. As the number of mff ingested increases, the yield of L(3) per microfilaria may either increase (in a process known as 'facilitation') or decrease (in a process known as 'limitation'). Each ingested microfilaria that is successful (in terms of reaching the haemocoel) increases (facilitation) or decreases (limitation) the 'permeability' of the stomach wall for the next microfilaria. Limitation is seen in some culicine mosquitoes, especially the Aedes spp. that transmit Wuchereria bancrofti, which, in consequence, become relatively more efficient as vectors as they ingest fewer mff. This phenomenon makes the interruption of filarial transmission by Aedes spp. particularly difficult. As the survival of anopheline mosquitoes is adversely affected by filarial infection, the use of mass drug administrations (MDA) to reduce the prevalence and intensity of microfilaraemias may increase the mean lifespan of some of the local Anopheles species. If these same species also act as vectors of malarial parasites, effective, drug-based control of W. bancrofti may worsen the problem posed by malaria. Therefore, wherever malaria and bancroftian filariasis are co-endemic and caused by parasites transmitted by the same species of mosquito, MDA should be augmented by interventions (use of bednets or house-spraying) against adult Anopheles.