"The mosquitoes that bite humans most are actually not as important in transmission of West Nile virus to humans because they rarely bite birds and thus rarely get infected in the first place," Kilpatrick said.
"Instead, it's the species that feed mostly on birds and frequently get infected, but occasionally feed on people, that are most important."
Millions of birds have died from West Nile virus infection, with dramatic effects on the populations of some species. Crows, for example, are much less abundant than they were before the virus arrived.
The robin population, which had been growing rapidly, has leveled off.
"Robins were on a steady upward trajectory thought to be linked to human land use--they love lawns and agricultural fields," Kilpatrick said.
"Crow populations were growing even faster. Now crow populations have crashed downward and robins have leveled off, and we suspect that's due to West Nile virus."
The worst human outbreaks of West Nile virus in the United States occurred in 2002 and 2003.
According to Kilpatrick, it's not clear whether the reduction in human disease since then represents a long-term trend or short-term variability.
"It may be that with climatic conditions favorable for the virus we could again get very intense years of transmission," he said.
"We don't know yet how much of the year-to-year variation can be explained by climatic conditions or other factors, such as acquired immunity in birds or humans."
Many other diseases caused by mosquito-borne or tick-borne viruses could potentially be introduced to the United States from overseas, Kilpatrick said.
Understanding the ecology of these viruses may lead to strategies that could prevent a newly introduced pathogen from establishing itself as successfully as West Nile virus.
|Contact: Cheryl Dybas|
National Science Foundation