West Nile Virus in our Backyard

January 1, 2006 by Gregory Gough

Understanding disease-carrying mosquitoes' feeding behavior and preferences is the first step to controlling West Nile Virus, a disease first observed in New York in 1999. Since then, repeated epidemics in North America have infected more than a million people and killed more than 800.

The disease is primarily carried by mosquitoes from the genus Culex. Culex mosquitoes prefer to feed on birds, but occasionally bite mammals as well.

Smithsonian scientists, in conjuction with the Consortium of Conservation Medicine, have been studying the mosquitoes, and their victims, in urban and suburban areas in Baltimore and Washington, D.C. for the past few years. Over 80 percent of the mosquitoes have been Culex mosquitoes, and potential hosts of West Nile Virus.

They have trapped and taken blood samples from thousands of birds and mosquitoes. DNA has been analysed from the blood meals in the mosquitoes' gut to identify its host.

Preferred Prey

American robin

An interesting finding is that in late spring and early summer, over half of the birds bitten by mosquitoes were American robins although robins are only four percent of the bird population.

Unfortunately for us, the robins finish nesting in late summer and disperse from their breeding areas. The mosquitoes must find a new victim, and their second favorite food is human.

Graph: as robins decrease (black line) attacks on humans and WNV cases increase (red and blue lines). Fraction of Culex pipiens mosquito feedings on humans, robin abundance, and number of human WNV cases (2003-2005 averaged); all for Washington, DC and Maryland.

Humans are seven times more likely to be bitten by mosquitoes in late summer than in early summer and the actual cases of West Nile Virus spike at this time of year.

West Nile Virus is passed back and forth between the mosquitoes and their bird victims, but the virus cannot go from a mammal to a mosquito. If robins stayed in our backyards throughout the summer or if the mosquitoes fed on humans in the early part of the summer the disease would likely not be so widespread.

This article summarizes the information in this scientific paper:

Kilpatrick, A. M., Kramer, L. D., Jones, M. J., Marra, Peter P. and Daszak, P. 2006. West Nile virus epidemics in North America are driven by shifts in mosquito feeding behavior. PLOS Biology, 4: 606-610.

Download scientific paper