In nature, vertebrate hosts of arboviruses often harbor parasites, including microfilariae. Laboratory studies have shown that concurrent ingestion of microfilariae and arboviruses by mosquitoes can result in significantly more efficient transmission of the arbovirus than when the same dose of arbovirus is ingested alone. That is because when microfilariae are ingested, they can penetrate the mosquito midgut and allow immediate dissemination of virus into the mosquito body cavity. This increases and accelerates viral infectivity of mosquitoes and can have 2 important epidemiological consequences. First, mosquito species that are normally refractory to viral infection because of midgut barriers to vira infectivity may now develop infections. This can increase the number and importance of secondary vector species involved in a virus transmission cycle. Second, microfilarial enhancement can accelerate viral development within the mosquito, shortening the time required for mosquitoes to become infectious (=extrinsic incubation period [EIP]). Small reductions in EIP can lead to large increases in vectorial capacity. This phenomenon is called "microfilarial enhancement of arboviral transmission" and has been validated using several artificially contrived model systems. However, microfilarial enhancement has never been tested using a naturally-occurring enzootic arboviral transmission system. That is what this proposal intends to do. In North America, songbirds harbor a diversity of filarial parasites, often at very high prevalence. Songbirds are also important for the enzootic maintenance of certain arboviruses, such as West Nile virus (WNV). The hypothesis of this proposal is that the microfilarial infections in songbirds may enhance WNV transmission by the enzootic mosquito vector, Culex pipiens. To test the hypothesis, infection outcomes will be compared between mosquitoes fed on dually infected birds (i.e., WNV plus microfilariae) versus singly infected birds (i.e., WNV only). The experiments will quantify the effect that dual infections have on the subsequent 1) viral dissemination rate within mosquitoes, 2) rate of WNV transmission by mosquitoes, 3) extrinsic incubation period of WNV within mosquitoes, and 4) daily survivorship of mosquitoes. The results of these studies will determine whether dually infected songbirds enhance virus transmission to mosquitoes compared with that of singly infected songbirds. If so, this opens the possibility that within a given population of WNV-competent bird species, those individuals with active microfilarial infections may constitute a subpopulation of potential "super transmitters". Significance. Without a fuller understanding of the potential role of microfilarial enhancement in naturally occurring systems, cryptic, potentially important amplifying hosts and vectors may be overlooked and our understanding of enzootic transmission of arboviruses may be incomplete.
Vertebrate reservoirs of mosquito-borne arboviruses are rarely pathogen-free. In nature, they are often chronically infected with microfilariae. In artifically contrived model systems, it has been shown that vertebrates concurrently infected with microfilariae and arbovirus usually infect mosquitoes with arboviruses much more efficiently than do vertebrates infected with only arbovirus. This project tests so-called 'microfilarial enhancement'for the first time in two naturally-occurring enzootic arboviral transmission systems. If enhancement is observed, then current paradigms of zoonotic arboviral transmission will need to re-evaluated to include the effect(s) of host co-infections.