Most people don’t think twice about smashing a mosquito that has just inserted its proboscis into your skin. Fewer people think about where that proboscis has been before. Female mosquitoes need a blood meal from a vertebrate in order for her eggs to develop properly. And those vertebrates include everything from humans to birds to alligators and snakes. Granted some mosquito species prefer certain hosts over others, but it many areas of our country that mosquito sucking your blood likely was feeding on a bird before that. Birds, small mammals, and some reptiles are part of what scientists call the sylvatic (wildlife) cycle involving mosquitoes and viruses. Diseases with names like West Nile, LaCrosse Encephalitis, and Eastern Equine Encephalitis involve birds that become infected when a mosquito takes a blood meal. The virus circulates in the blood of the bird and can be picked up by another mosquito when it feeds. There needs to be enough virus particles in the blood for the mosquito to become a carrier or a vector. This is referred to as the threshold of infection. Therefore, transmission from bird to mosquito to bird and so on leads to amplification of the amount of virus in the ecosystem.
Do birds become sick? It all depends on the virus and how long the virus has been associated with the bird. For example, West Nile Virus (WNV) has been shown to infect over 385 species of birds. Some birds are more susceptible to virus than other birds. The American Crow is one of the most susceptible and is often used as a sentinel for WNV presence in the area. Dead crows in late summer may indicate WNV activity.
If a mosquito takes a blood meal when the number of virus particles (virions) is high, the mosquito becomes infected. Later, she is looking for another blood meal and heads toward your arm. When she injects her anti-coagulant, she also injects some WNV particles into your arm. There is a good chance that you will not become infected, but of those who do only about 1% will show any signs of the disease. The only way for the rest of us to know if we have been infected at some time is to look for antibodies to the virus in your blood.