News Release

Prediction: California will see 'a lot more' cases of West Nile Virus in 2004

Peer-Reviewed Publication

American Chemical Society

ANAHEIM, Calif., March 31 — An expert on mosquitoes and the spread of West Nile virus predicts the disease will break out of Southern California this summer and propagate northward, likely triggering "a lot more human cases."

"There's no shortage of mosquitoes here," despite efforts to control their populations, says John D. Edman, Ph.D., an entomologist at the University of California, Davis, and director of the school's Center for Vectorborne Diseases. He described efforts to track and contain the viral disease in North America today at the 227th national meeting of the American Chemical Society, the world's largest scientific society.

"By next fall I'll be surprised if it [West Nile virus] isn't north of Davis." The university town lies about 10 miles west of the capital Sacramento in North Central California.

California, the country's most populous state at more than 35 million residents, confirmed its first in-state infections of West Nile virus last year. Three patients in the Los Angeles area tested positive for the illness, which mysteriously appeared in the Western Hemisphere for the first time in 1999, in New York.

"If what's happened in other places is indicative, we'll see it first reappear in Southern California. And I think it'll make it over the Tehachapis this year," he says. "If that happens, we'll likely have a lot more human cases." The Tehachapi Mountains run east and west at the lower end of the San Joaquin Valley, which runs up the center of the state for more than half its length. At 5,000 to 8,000 feet, the Tehachapis form a natural barrier between Southern California, with its population centers of Los Angeles and San Diego, and the rest of the state.

Migrating birds, and especially crows, are the expected vehicles for carrying West Nile virus over the Tehachapis. The spread will likely pick up in early July, at the end of the nesting season, Edman says, explaining, "Fledglings move around a lot after leaving the nest." Wildlife experts are afraid that among them may be threatened or endangered species such as the California condor.

Edman predicts the spread of West Nile in California not just on its pattern of spread elsewhere in North America. Several characteristics unique to the state also factor in, from its agricultural methods to its distribution of populations, and in particular its type and numbers of mosquitoes and birds.

West Nile virus, like its American counterpart, the St. Louis encephalitis virus, appears to jump from infected birds to humans and other mammals via mosquito bites. According to the Centers for Disease Control and Prevention, roughly 20 percent of humans bitten by mosquitoes carrying the virus in their saliva develop flu-like symptoms. About 1 percent of those people develop more severe forms of the disease, such as West Nile encephalitis (inflammation of the brain), meningitis (inflammation of the membrane around the brain and spinal cord), or both. Mortality among patients with severe illness ranges from 3 to 15 percent, with the elderly the most vulnerable.

Mosquitoes thrive in agricultural regions that use irrigation and in urban areas where standing or polluted water is common. "Because California has so much irrigation agriculture compared to other states, the vector population [of mosquitoes] here will probably be more stable than elsewhere," says Edman. "California also has a widely varying geography and climate."

The diversity of environments probably means the spread of West Nile virus won't be as uniform as seen in states such as Nebraska and others in the Midwest, he added. However, it also means California has a wide variety of mosquito species and likely the largest population of Culex mosquitoes, which includes several species scientists believe are among the biggest culprits in transferring the virus to humans.

"California does have good mosquito control, but I'm not convinced mosquito control is so efficient that we won't have a problem anyway," the entomologist asserts. "The question is, what proportion of mosquitoes do you have to kill to prevent transmission? No one knows. You may kill three out of four, but maybe the one that gets away is the one that can transmit." Feeding behavior, not density, is likely the most important factor in the virus transmission, he says - and several species of Culex mosquitoes are unusual in that they feed on birds, humans and other mammals. Most mosquitoes are more selective in their feeding behavior, and thus would not spread the disease from one type of animal to another.

Working in California's favor is what Edman considers the country's most rigorous surveillance program. In addition to testing various groups of mosquitoes every week, examining dead birds and tracking reports of infected horses, most of California's 50 districts use sentinel chickens. This program monitors previously unexposed chickens and can pinpoint infectious areas much better than can tracking wild birds, which tend to roam.

Edman does not expect the West Nile virus to invade California's Bay area or the Sierra Nevada highlands, as both regions are neither warm enough nor have enough agriculture to attract the species of mosquitoes that can transmit the disease.

"But in general, if West Nile gets moved around the state by birds and really gets cooking—well, we have a tremendous number of crows and magpies in California and when a virus like this cooks, it can infect an awful lot of the bird population," Edman notes.

The National Institutes of Health and a state research-grant program funded Edman's research. Funds for surveillance come from the CDC in Atlanta.

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The paper on this research, AGRO 95, will be presented at 10:55 a.m., Wednesday, March 31, at the Hilton Anaheim, Capistrano A/B, during the symposium, "Agrochemical Issues in Urban Environments."

John D. Edman, Ph.D., is a professor in the Department of Entomology at the University of California, Davis, and director of the school's Center for Vectorborne Diseases. [EDITOR'S NOTE: Vectorborne spelling is correct in the title.]


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