Infected mosquitoes

In North America, Vector transmission of WNV occurs primarily between May and November. It peaks from late August to mid-September.  This period coincides with the breeding season of mosquitoes. Breeding female mosquitoes feed on blood to obtain nutrients to produce eggs. Male mosquitoes are nectar feeders and present no direct threat.

The female mosquito uses sensors tuned to infrared and carbon dioxide to locate prey. It punctures the skin, injects anticoagulant saliva and then suck out a blood meal. Infected mosquitoes carry viral particles in their salivary glands. Click to view mosquito proboscis diagram

Evidence also supports the notion that infected females may lay infected eggs. Infected eggs yield infected adults. This is problematic because some infected gravid females can survive harsh winters by hibernating. Infected eggs may also survive the winter. In spring, as the temperature increases, the eggs hatch and hibernating female mosquitoes become active to infect birds, renewing the cycle of transmission. Because they are able to hibernate, infected mosquitoes or infected eggs can avoid suppression and control measures.

Birds are the preferred prey of many mosquitoes. A bird's high body temperature emits more infrared energy then other animals, making them more attractive.  The birds can sustain a high viral titer in the bloodstream for 1 to 4 days after exposure, spreading the virus to many mosquitoes. Humans and other animals are less attractive prey and are considered incidental hosts.  The viral load in incidental hosts is too low to serve as an effective reservoir. 

Mosquitoes of the genus Culex are the most common WNV vectors in the United States. But the virus has been isolated from many other mosquito species and blood feeding arthropods. 

WNV can infect and reside in many different hosts:

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Vector transmission of WNV occurs primarily between May and November and peaks from late August to mid-September.


Center for Disease Control and Prevention. Morbidity and Mortality. West Nile Virus Disease and Other Arboviral Diseases — United States, 2011. July, 2012. 61(27);510-514

Center for Disease Control and Prevention. Public Health Image Library (PHIL). Paul Howell. James Gathany. 2014

Washtenaw County Public Health. Fact Sheet: West Nile Virus (WNV). Retrieved 11/12/17.