West Nile Contracted in Riverside County

California health authorities on Thursday reported the state’s first locally acquired case of West Nile virus this year, afflicting a 31-year-old Riverside County man who developed aseptic meningitis as a result.

Los Angeles reported one West Nile infection during the summer, but that case has generally been attributed to a mosquito that had been imported into the area on an airplane. The new victim, whose identity remained confidential, was apparently bitten by a local mosquito, according to Evelyn Tu, the state’s West Nile surveillance coordinator.

At least 15 other West Nile victims have been identified in California this year, but all were either Californians who contracted the virus while on vacation elsewhere or vacationers who contracted it elsewhere and fell ill here.

The Riverside man was hospitalized Sept. 30 but is recovering at home, according to state Health Director Diana M. Bonta. Laboratory tests confirmed the infection, she said.

The patient lives in the Moreno Valley area and works on drains, sewers and other infrastructure around the Santa Ana River, according to Dr. Gary Feldman, Riverside County’s public health officer. The man does not recall being bitten by a mosquito.

The Santa Ana River is considered a breeding ground for mosquitoes. Helicopters sprayed it last week with more than 3,000 pounds of bacterial pellets, which kill mosquitoes by dissolving their internal organs.

County officials zeroed in on wetlands, ponds and other stagnant water where the insects breed.

Despite such efforts, the virus continues to spread. On Thursday, three more dead birds infected with it were found in western Riverside and San Bernardino counties. A blackbird was found in Sun City, and crows were found in Rialto and Temecula. Infected dead birds are being discovered with increasing frequency, Feldman said. He urged residents to call (877) WNV-BIRD if they see a dead bird.

Experts had predicted that the first human infection would be in Riverside County because of a combination of climate and location, Feldman added.

Migrating “birds come through the Pacific Flyway, right through Riverside County,” he said. “We have a long mosquito season because we’re hot. Between the Salton Sea and all the irrigation that goes on out in the desert, there are plenty of breeding grounds for mosquitoes.

“We’ve got the right mosquitoes, the right climate and we’re in the right geographic place to receive the right birds,” Feldman said.

As of Thursday, 6,613 U.S. cases of West Nile infection had been reported to the Centers for Disease Control and Prevention, with 139 deaths. Last year, 4,156 cases were reported and 284 people died in what was then considered the largest outbreak of the disease in the Western Hemisphere.

The lower death total this year is generally attributed to earlier detection and prompt treatment.

Most people who are infected with the virus never develop a severe illness and don’t even realize they have acquired the virus. According to Dr. Lyle Petersen of the CDC, about 500,000 Americans will contract the virus this year. About 20% of those will develop symptoms similar to those of a three-day flu, and about one in every 150 will develop a serious illness such as meningitis.

Based on those numbers, more than 100 Californians have probably been infected by the virus without developing symptoms.

The first U.S. case of West Nile fever was reported in New York City in 1999. Since then, the virus has moved steadily west, carried by birds and transmitted from birds to humans by mosquitoes.

The Rocky Mountains proved to be a temporary barrier to the spread, but the virus seems to have leapt over them this year. Solitary infections have been reported in Utah, Arizona, Nevada and now California.

Petersen has predicted that the virus will hit the West Coast particularly hard next summer, especially in the San Joaquin Valley area. This year the virus turned up in the region in large numbers of birds and mosquitoes, and experience suggests that such an animal and insect infestation one year is generally followed by an explosion of cases in humans the next.

That was the case this year in Colorado, which has recorded 2,171 cases and 40 deaths so far, compared with 14 cases and no deaths in 2002.

Feldman said the first infection is a harbinger of what’s to come in the state. “It probably means L.A. and Orange County are next,” he said. “It is a matter of time. We certainly don’t have the tools to interrupt its spread.”

Feldman said it is unlikely there will be many more infections this year because the weather is already cooling and mosquito season is waning.

But some parts of the state, such as the desert, remain vulnerable.

“In the Coachella Valley, there is no end to the mosquito season,” Feldman said.

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Disease tally

The first human case of West Nile virus contracted in California was reported in Riverside County on Thursday afternoon. More than 6,600 cases and 139 deaths have been reported nationwide this year.

West Nile virus cases in 2003*

Human cases, as of Thursday morning

*--* State Cases Deaths Alabama 29 3 Arizona 1 Arkansas 11 Colorado 2,171 40 Connecticut 12 Delaware 13 1 Dist. of Columbia 3 Florida 32 Georgia 23 3 Illinois 30 1 Indiana 17 1 Iowa 108 4 Kansas 54 1 Kentucky 10 1 Louisiana 84 1 Maryland 22 3 Massachusetts 12 Michigan 1 1 Minnesota 124 4 Mississippi 53 1 Missouri 43 2 Montana 207 2 Nebraska 1,108 15 Nevada 1 New Hampshire 2 New Jersey 19 2 New Mexico 189 4 New York 49 6 North Carolina 19 North Dakota 293 4 Ohio 74 3 Oklahoma 48 Pennsylvania 151 5 Rhode Island 3 South Carolina 1 South Dakota 863 8 Tennessee 8 Texas 379 14 Utah 1 Vermont 1 Virginia 12 1 Wisconsin 11 Wyoming 320 8 Total 6,613 139

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*Human cases, including mild (West Nile fever) and severe (West Nile meningitis or encephalitis). About 65% of the cases were mild and 29% were severe. These numbers reflect cases that have been reported to the national electronic surveillance system established by the federal Centers for Disease Control and Prevention to assist states in tracking West Nile virus and other mosquito-borne viruses.

Source: Centers for Disease Control and Prevention

Maugh reported from Los Angeles and Mehta from Riverside.