updated 10/31/2003 6:18:46 PM ET 2003-10-31T23:18:46

The worst outbreaks of West Nile virus seem to follow summer droughts preceded by mild winters, a pattern researchers are studying as a possible way to predict where the virus might hit hardest.

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“Drought is where this is focused,” says Paul R. Epstein of the Center for Health and the Global Environment at Harvard Medical School.

Though Epstein sounds convinced, other researchers say more work is needed.

“I suspect that it is going to be more complicated than just saying that West Nile virus transmission will increase or decrease based on periods of rainfall,” said Dr. Ned Hayes, an epidemiologist at the Centers for Disease Control and Prevention.

Epstein, one of several researchers studying factors in the spread of West Nile for a seven-year government study, points to these historical patterns:

The initial U.S. outbreak in New York in 1999 followed a three-month drought and three-week heat wave. Last year’s devastating spread followed a year of widespread warm winters and spring or summer droughts.

This year the disease has spread throughout the West with Colorado reporting more West Nile virus cases than any other state.

“There was no snowpack in the Rockies because of the warm winter,” Epstein said recently at a conference here. And, over the summer and fall, “the disease raced across 44 states and Washington, D.C., five Canadian provinces, and we started seeing outbreaks in the Caribbean and Central America and in Mexico.”

Colorado, Nebraska and other states with the worst outbreaks all have had droughts.

Outbreaks on the other side of the world in the 1990s — in Israel, Romania, and Russia — also were associated with drought, according to Epstein.

The Harvard researcher may be right, but he hasn’t proven it, said Dr. Bob Shope, a Texas professor who echoed the CDC official’s caution. There has been no controlled study and more data is needed, said Shope who teaches at the University of Texas Medical Branch in Galveston.

Still, Epstein explains how drought could intensify an outbreak, especially in urban areas.

Standing water collects in drains and catch basins, stagnating and attracting the female house mosquito for egg-laying. Ponds and creeks dry up, reducing the numbers of frogs, dragonflies and other predators.

Remaining sources of clean water become a gathering spot for birds making them an easier target for the insects. While house mosquitoes prefer birds to people, later in the summer, the species that find people just as tasty as birds become more pervasive.

“So the sequence of early drought allows it to really get spread among the bird populations, and a little bit of rain in the summer can spread it to humans,” Epstein said.

But Hayes, of the CDC said, “Many complicated interactions come into play that are often difficult to predict.”

He noted that although the mosquito thought most likely to be spreading the virus out West is in the same Culex genus as the house mosquitoes in the South and East, its breeding pattern is different.

Epstein’s research was sparked by similar patterns with St. Louis encephalitis, a closely related bug. That encephalitis strain first showed up in this country in 1933, three years into the Dust Bowl drought that started in the East and moved west.

At the CDC in the 1970s, Dr. Thomas Monath compared St. Louis encephalitis and weather. Ten of the first 12 big urban outbreaks, he found, came after two-month droughts; an 11th occurred after one month of drought.

The fact that the mosquitoes that spread St. Louis encephalitis also are thought to spread West Nile strengthens the argument, but three or four years is probably just not enough time to make any absolute conclusions, Monath said. “With St. Louis encephalitis, we were able to look back about 40 years over a series of multiple outbreaks.”

Going after larvae as much as possible is a good idea in areas prone to either virus, he said. “It’s much easier to control or prevent the disease by early measures than it is to try to prevent an outbreak once the virus is cranked up.”

“When we’ve got a little more data in, we’ll be able to say whether that hypothesis is supported by the data or not,” said Marm Kilpatrick, a research scientist at the Consortium for Conservation Medicine, which is coordinating the seven-year study.

Cornell University scientists are doing similar research, aimed at creating a Web-based calculator to advise public health officials on when conditions are friendly to mosquitoes.

The weather link is “a very promising idea,” Kilpatrick said.

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