For many mosquito-borne viruses, the classic transmission scenario involves ornithophilic mosquitoes vectoring the virus among avian hosts in a cycle of enzootic amplification. Mammals (dead-end hosts) become infected when the virus “spills over” into non-avian populations. The mechanism by spill over is not clearly understood, yet understanding spill over is critically important to understanding and managing enzootic outbreaks of these viruses. One hypothesis predicts that when a critical percentage of the bird population becomes infected, virus is introduced into non-avian hosts by mosquitoes with catholic feeding habits. A second hypothesis maintains that vector species shift from feeding on birds to feeding upon mammals in an annual pattern of host switching. As a consequence of this shift, infected mosquitoes transmit the virus to susceptible mammalian hosts. We tested these hypotheses by quantifying host use of mosquitoes at a wetland study site with documented arbovirus activity. Over a five-year period hosts of vector mosquitoes were determined by PCR amplification of DNA from vertebrate blood recovered from the guts of 893 field-collected mosquitoes. 

Results/Conclusions

In each year a shift in host preference was observed for Cx. erraticus, a mosquito which feeds on a wide variety of hosts including birds and mammals and is a likely vector of eastern equine encephalitis virus. Culex erraticus fed more or less equally on birds and mammals in March and April. In May and June, however, birds comprised the great majority of hosts (sometimes >90%). May and June is when most bird species have nestlings at our study site. In July or August of each year mosquitoes switched from feeding on birds to feeding on mammals (>90% in September of 2006 and 2007). The month in which host switching occurred usually coincided with virus infections in mammals (horses) reported by the Alabama Department of Public Health. Our work indicates that host-switching in vector mosquitoes is critically important in initiation of epizootic transmission of mosquito-borne viruses in nature. We hypothesize that the dramatic shifts in host use by mosquitoes is largely driven by host reproductive biology.