Vector transmission heterogeneity and the population dynamics and control of lymphatic filariasis

A long-standing gap in lymphatic filariasis epidemiology is quantifying the potential effect that heterogeneous infection processes occurring in the major mosquito vector genera may have on parasite transmission and control. Although previous studies have focussed on examining the forms of the density dependent mechanisms regulating larval infection in various mosquito genera, there has been little work done thus far in investigating how such differential processes might interact with density-dependent processes occurring in other stages of the parasite life cycle to influence overall transmission dynamics between areas exposed to different transmitting vector populations. Here, we explore the impact that differences in vector genus-related larval infection dynamics may have on filariasis transmission and control using newly derived parasite transmission models incorporating the forms of the density-dependent processes regulating larval infection in the two major vectors transmitting filariasis, viz. culicine and anopheline mosquitoes. The key finding in this work is that filarial infection thresholds, system resilience, transmission dynamics and parasite response to control efforts, can all be influenced by the prevailing transmitting mosquito genus. In particular, we show that infection thresholds may be raised, system resilience to perturbations lowered and effects of repeated mass treatments in eliminating infection enhanced in anopheline filariasis compared to culicine filariasis, as a direct result of the occurrence and action of multiple positive density-dependent mechanisms influencing infection in this vector-parasite system, such as the "facilitation" function regulating larval infection dynamics in the vector and the inverse probability function governing adult worm mating in the host. These findings indicate that anopheline filariasis may be easier to eradicate than culicine filariasis for a given precontrol infection level, although the actual intensity of interventions required to achieve eradication may in fact be similar to that for culicine filariasis because of the higher infection levels generated as a result of the "facilitation" process in Anopheles transmission areas.