Higher temperatures helped new strain of West Nile virus spread

Findings help explain spread of virus strain responsible for largest US epidemics

SANTA CRUZ, CA--Higher temperatures helped a new strain of West Nile virus invade and spread across North America, according to a study published in the June 27 issue of the journal PLoS Pathogens.

"The study shows that the warmer the temperature, the greater the advantage of the new strain. It also indicates that increases in temperatures due to global climate change would have major effects on transmission of the virus," said A. Marm Kilpatrick, first author of the paper and a senior research scientist for the Consortium for Conservation Medicine.

Kilpatrick, now an assistant professor of ecology and evolutionary biology at the University of California, Santa Cruz, joined with Laura Kramer and others at the New York State Department of Health's Wadsworth Center to conduct the study, which examined the effects of different temperatures on the transmission of two strains of West Nile virus.

The first occurrence of West Nile virus in North America was in New York City in 1999, when it caused a die-off of crows and other birds and 62 reported cases of human infections, including 7 deaths. In the two years after the introduction of the virus, the rate of transmission was relatively low. As it spread along the Atlantic seaboard, there were only 21 reported human cases in 2000 and 66 in 2001.

In 2002, however, a new strain of the virus emerged and rapidly spread throughout North America, completely displacing the old strain by 2005. Coincident with the spread of this new strain were two of the largest epidemics of West Nile virus recorded to date in North America, with 4,582 cases reported in 2002, 11,356 cases in 2003, and more than 270 deaths in both years. Since then, the number of reported cases per year has ranged from 2,500 to nearly 6,000, with more than 100 deaths each year.

Kilpatrick and Kramer set out to determine how the new strain of West Nile virus had displaced the first strain, and what effect temperature had on transmission by mosquitoes. They used laboratory tests to determine how soon mosquitoes are capable of transmitting the virus after feeding on infected blood. The results showed that the new strain is more efficiently transmitted than the older strain, and the advantage of the new strain increases with higher temperatures.