West Nile, which is spread to humans by mosquitoes, infected more than 4,000 people in 44 states and five Canadian provinces in 2002 and killed 284.
"It is entirely reasonable to expect that the North American impact of West Nile virus will be as significant in 2003 as it was in 2002," Dr. Paul Epstein, associate director of the Harvard Medical School Center for Health and the Global Environment, said during a teleconference from Boston.
"I think we are at the tip of looking at this both for the U.S. and Canada and Latin America," said Epstein, who was joined by Douglas Causey, a senior biologist at the Harvard University Museum of Comparative Zoology.
The Western United States could be particularly hard hit by the disease due to the drought conditions there, he said.
"Present and persistent drought projected in the Plains states, the West and Alaska could set the stage for amplification and spread of West Nile," Epstein said.
The warm winter and drought conditions create shallow pools of water filled with detritus that mosquito species known as Culex pipiens prefer as their breeding ground. Drought also decreases the numbers of animals that eat mosquitoes, such as frogs and fish, so there likely will be more mosquitoes in general, Epstein said.
However, he noted, this does not mean the Eastern and Southern United States will not see a large number of West Nile cases. The virus already is established in wildlife in those areas and heavy rains could increase other insects that transmit the virus to humans and birds and keep it in circulation.
Birds act as a reservoir for the virus. Mosquitoes contract it by feeding on infected birds and then transmitting it to humans when they bite.
Other experts also predict there could be an increase in West Nile in the Western United States.
"There are a lot of reasons why the Midwest and West might see an outbreak this year," Lois Levitan, program leader of the Environmental Risk Analysis Program at Cornell University, told United Press International.
"Given that the disease has now spread across the country to many different ecosystems ... you have many more places for which there can be the right conditions for mosquitoes, birds or drought or other conditions to induce disease," Levitan said.
In addition, fewer than 1 million people have been exposed based on current estimates, so most people have not yet acquired immunity to the disease. This indicates there likely will be more cases in which people develop neurological problems from the virus, she said.
Another reason there might be an increase in West Nile in the Western United States is "the key mosquito species transmitting (the virus) in the Midwest is a species that's a more aggressive biter of people and horses."
About 37 species of mosquitoes are known to carry West Nile, but it is transmitted to humans primarily by C. pipiens in the East because it generally prefers to feed on humans.
However, the Culex parsilis mosquito in the Midwest is more likely to bite people, which "would suggest there's more of a likelihood of (the virus) becoming a problem in people and horses" in the West, Levitan said.
The Harvard scientists said regions south of the United States -- including Mexico and Latin America -- are of particular concern because what happens there could have an impact on the United States.
West Nile could have a significant impact in Latin America where there are "many issues of malnutrition and diseases that make people immunocompromised" and thus vulnerable to West Nile, he said.
This in turn could affect the United States because "birds fly from the Arctic National Wildlife Refuge (in Alaska) down to Chile so whatever happens in Latin America is going to come back and forth," he said.
The researchers based their predictions on what has been seen with West Nile virus outbreaks in other regions of the world. Outbreaks of the disease in the 1990s in Europe, Israel and Russia "were all related to drought," Epstein said.
In addition, a similar disease known as St. Louis Encephalitis began in the dust bowl in St. Louis in 1933 and was amplified by drought, he said.
Causey said the virus is taking a toll on animal species as well, which in turn affects humans.
"Of great concern now is that West Nile virus has spread to 238 species of animals and probably much more," Causey said.
More than 130 species of birds are infected with the virus and West Nile "is a leading suspect in the disturbing tenfold drop in several bird species in Costa Rica over the past year," he said. Many of these are migratory songbird species that spend some of their time in the United States.
Bird deaths due to West Nile have increased fivefold since 2001. This is worrisome because if there is a decline in the numbers of birds of prey, such as hawks and eagles, there could be an increase in rodents, which spread disease, Causey said.
Greater surveillance of wildlife and response is needed to keep West Nile in check, Epstein and Causey said. Intensive statewide education and disease prevention programs are needed across the states, they said.
In this vein, the California Department of Health Services has already begun gearing up for the disease.
"We're really just getting extremely aggressive on West Nile preparation," Stan Husted, supervising public health biologist with the state's health department, told UPI.
California has increased its dead bird surveillance program to identify and screen dead birds for West Nile infection, Husted said. They have also stepped up public awareness of the disease and the governor declared the last week in April West Nile Virus Awareness Week.
The state health department has also begun putting out bacteria and hormones in mosquito breeding areas to keep down the levels of the insects. "We try to keep the mosquito populations down to such a low level that transmission is going to be less likely," Husted said.
Copyright 2003 by United Press International.
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