There may be a correlation between summer droughts preceded by mild winters and outbreaks of West Nile virus, according to a researcher at Harvard Medical School.

“Drought is what amplifies the virus among birds and mosquitoes and makes for large outbreaks,” said Paul R. Epstein, associate director of the Center for Health and the Global Environment at Harvard Medical School. Epstein works with a team of researchers on the West Nile virus under the sponsorship of the Consortium for Conservation Medicine.

Despite West Nile’s short history in the United States—it first appeared in New York City in 1999, claiming seven lives—Epstein said he is able to point to an emerging pattern that tends to support his theory.

The New York City outbreak was preceded by a three-month drought, followed by a three-week heat wave in July. Major outbreaks over the past two summers—including those in 2002 in the Midwestern states of Illinois, Michigan and Ohio and in 2003 in Colorado and Nebraska—were all preceded by summer droughts and mild winters.

According to Epstein, “No one knows exactly how the virus came to New York. But we do have a good sense of the conditions that helped amplify it.”

Drought provides optimal conditions for the maturation of the virus in mosquitoes, namely in stagnant water, he said.

But despite Epstein’s assertions, some researchers say that more investigation into the connection between droughts and West Nile outbreaks is needed.

“There’s some validity to his theory but I’m not sure that it works in every case,” said Robert Tesh, a professor of pathology at the University of Texas Medical Branch in Galveston.

Tesh said that while droughts did precede outbreaks in New York and Colorado, among other places, they also occurred in Texas and Louisiana within the past two years, despite what he termed “relatively wet summers” in those areas.

West Nile, though new to the U.S., has a long history. First reported in Uganda in 1937, it has also affected areas of Europe and the Middle East.

Epstein claims that weather patterns surrounding outbreaks in Russia in 1999, Romania in 1996 and several in Israel dating back to 1951 support his theory.

“We looked back at some of the outbreaks of West Nile and it turned out that all of the big outbreaks in Europe in the nineties were related to drought,” he said.

Since its arrival in the United States, the virus has wreaked disastrous consequences. According to the Centers for Disease Control, since January 2002 there have been 8,822 human cases of infection reported, resulting in 372 deaths.

If Epstein’s theory proves to be correct, it could open the door to predicting where West Nile is likely to surface, and enable governments to preempt its deleterious effects.

Even some of those who do not completely agree with Epstein do not doubt this basic premise.

“During dry periods the water in sewers tend to stagnate, and that is a good breeding place for Culex,” the genus of mosquito responsible for the spread of West Nile, Tesh said.

Ultimately, the key to limiting the spread of West Nile lies in reversing the trend of global warming and climate change that the world has seen as of late, according to Epstein.

“We’ve underestimated the biological responses to warming and the associated intensification of weather extremes, and the costs associated with health,” he said.