Robins, Mosquitoes and West Nile Virus: A Perfect Trio
West Nile Virus (WNV), mosquitoes and American Robins appear to have a nice arrangement. While a number of bird species may carry the virus, the Robin in particular appears to be a key host. Recently fledged robins appear to be more important for the sustained transmission of the virus to mosquitoes than other birds, such as crows and jays that usually die shortly after they are infected.
WNV (Flavivirus) was first detected in North America in 1999, when it caused 62 reported human infections, including 7 deaths. American Crows suffered high mortality rates in the New York City area. Nearly a decade later outbreaks recur annually across much of North America.
It just so happens that the breeding season of both the mosquito, the transmitter, and the robin, the resistant carrier, coincide and create the perfect viral storm.
A recent analysis of blood DNA taken from the abdomens of 300 mosquitoes trapped in Connecticut found that 40 percent fed on American Robin blood and only 1 percent on crows. Not only are Robins a favored target of the mosquitoes, but many have WNV antibodies in their blood. Unlike Crows, they survived the infection.
Scientists have shown that in urban and residential areas the mosquito Culex pipiens feeds on American Robins until late summer when Robins disperse to other areas. This may be a contributing factor to both the spread of the virus and more severe human outbreaks of the disease later in the year. The mosquitoes may bite us more often when there are few robins available.
My friend Pete Marra, an ornithologist at the Smithsonian Migratory Bird Center, is studying songbirds across an urban to rural gradient to help understand and monitor WNV transmission.