Steve Quayle News Alerts

August 21, 2003
David Perlman, Chronicle Science Editor

The West Nile virus -- steadily spreading across the United States for four years -- has been found in California mosquitoes for the first time, state Health Director Diana Bonta said Wednesday.

The virus was found in mosquitoes collected from traps Aug. 4 near the Salton Sea in Imperial County. More infected mosquitoes probably will be detected in Southern California during the remaining summer months, said John Edman, director of the Center for Vectorborne Diseases at UC Davis.

It's all but certain that the virus will continue spreading through the state, borne mostly by migratory birds bitten by infected mosquitoes. The Salton Sea is an important stopover for birds on the Pacific Flyway.

Already, there is strong evidence that two flocks of "sentinel chickens" maintained in the Salton Sea area have been infected, Cynthia Cossen, a microbiologist at the state health department's viral disease laboratory in Richmond, said Wednesday.

In California, the first confirmed case of West Nile infection in a human was recorded in September 2002. The Los Angeles woman, who was briefly hospitalized, told authorities she had not traveled outside the Los Angeles basin.

Scientists, however, found no evidence at that time that mosquitoes in the state were carrying the virus. Epidemiologists believe the woman -- who lived near the Los Angeles International Airport -- may have been infected by a mosquito carried on an aircraft.

West Nile can sicken humans, horses and other mammals, but it is rarely fatal in people. So far this year, there have been 599 cases of West Nile infection in humans and 11 deaths nationwide, according to the Centers for Disease Control and Prevention. Last year, there were 4,156 cases and 284 deaths.

DISEASE HOT LINE

Because several species of birds are the most widespread sources of the virus, Bonta urged the public to report finding any recently dead crows, ravens, magpies, jays, sparrows or finches. The toll-free number to report suspicious finds is (877) WNV-BIRD.

State health officials have been bracing for the virus for months, watching it move across the country from the East Coast. In the West, Colorado has been hit the hardest so far.

"We've been expecting the virus to reach California this summer," Edman said, "because it has been moving westward all along. I'm sure we'll find more this summer, and this is just the first volley. But there's no way of knowing whether or when it will reach Northern California."

UC Davis researchers collected the first infected mosquitoes from illuminated traps baited with dry ice in the Wister Unit of the Imperial Wildlife Area near the tiny town of Niland, Edman said. Dry ice is solid carbon dioxide, and when it vaporizes, the mist simulates human breath and becomes attractive to the mosquitoes.

In Edman's laboratory the dead mosquitoes were pulverized and then subjected to three tests -- including one showing that the virus rapidly killed human cells in culture -- and all three clearly identified the virus as the one known as West Nile, he said.

Even more definitive analysis, in which the virus is grown in culture to determine the precise sequence of its genes, will be completed this week, Edman said.

All through the summer Edman's lab has tested more than 5,000 groups of one to 50 mosquitoes each as well as tissue samples from birds and other animals, he said.

ALL TESTS HAD BEEN NEGATIVE

All the tests were negative until last Tuesday, he said, when the first positive signs of West Nile were found. The public announcement was held off until more tests could be done.

Scientists from Edman's lab maintain a collaborative surveillance project with the Coachella Valley Mosquito and Vector Control District, and the two groups collected the mosquitoes for the Davis tests.

At the state's Richmond laboratory, Cossen said, her team regularly tests some 200 flocks of chickens maintained across the state as sentinels to signal the presence of three separate viruses: West Nile and St. Louis encephalitis, which are closely related, and the unrelated Western Equine encephalitis.

According to Cossen, blood tests for antibodies similar to the well-known "ELISA" test for HIV, the AIDS virus, showed that the blood of six chickens in two flocks from the southern Salton Sea area was most probably infected with West Nile.

A final test to determine whether the antibodies can neutralize the virus will be completed next week, and Cossen said she was confident the result would be definitive, showing the West Nile is indeed the culprit virus.

HUMAN SYMPTOMS

Despite widespread concern, most humans infected with West Nile virus develop only mild symptoms, such as headaches and nausea, and 8 of 10 experience no symptoms at all. People over age 50 are generally the most vulnerable to serious illness.

Only one illness from the virus has been reported in California this year, and that was in a 47-year-old Alameda County woman who had been bitten by an infected mosquito while visiting Colorado. She is suffering from severe muscle weakness in her legs, state health officials said.

"California's surveillance system is working," Health Director Bonta said Wednesday evening. "As West Nile virus has moved westward across the nation, we have been preparing for its arrival."

Officials urged people concerned about possible infections to take standard precautions against mosquito bites, including long-sleeved shirts and insecticides containing the active ingredient DEET.
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Prevention tips

Ways to avoid mosquitoes carrying West Nile virus:

-- Wear a mosquito repellent containing DEET; those without DEET aren't nearly as effective.

-- Stay indoors at dawn and dusk, when mosquitoes are most active.

-- Don't allow water on your property to stand for more than two days. Typical culprits are empty paint cans collecting water under decks, unused pools, blocked rain gutters, flower pots and forgotten buckets. Check items such as discarded tires and trash can lids for standing water.

-- Gadgets that catch bugs have not been proven to reduce the number of mosquito bites. -- The Mosquito Control Association also recommends frequently changing birdbath water, stocking ponds with mosquito-eating minnows and using larvicides in unavoidable standing water.

-- People can't get the West Nile virus from each other or by touching an infected animal.

-- Drill holes on the bottom of recycling containers that are left outside and in tire swings.

-- Clean and chlorinate swimming pools that are not being used. Mosquitoes may even breed in the water that collects on swimming pool covers.

Sources: Connecticut Department of Environmental Protection; New York State Department of Health; Centers for Disease Control and Prevention; Associated Press

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