Rains Are a Boost for West Nile Virus: It’s Here Earlier, Poses a Greater Risk

This winter’s record rains have created prime breeding conditions for mosquitoes, prompting health officials to warn of a potentially serious outbreak this year of the deadly West Nile virus that killed 27 people in the state last year.

One human case of the disease was confirmed in Los Angeles County this year, though just when the virus was contracted was unclear, health officials said. That victim is recovering.

In the past week, the virus has been found in two dead birds in Los Angeles County, a chicken in San Bernardino County and mosquito larvae from a pool of stagnant water in Orange County.

Those findings indicate that the heavy rains have started the spread of the virus earlier than last year, health officials said.

“We now know it’s in the mosquitoes and that’s the closest thing to getting it into the humans,” said Mike Hearst, an Orange County Vector Control District spokesman.

Health officials won’t speculate on the potential human toll this year, but they said the risk of infection increases with higher rainfall levels, because pools of standing water are mosquito breeding grounds.

“You have a better chance of being bitten this year because of the increased breeding conditions,” said Rosanna Westmoreland, a spokeswoman for the California Department of Health Services.

In 2004, the virus infected 829 people in California, killing 27, most of whom lived in Southern California. Most people who are infected do not develop symptoms, but a small percentage, particularly the elderly, can become severely ill.

Westmoreland said a small percentage of the population developed an immunity to the virus after being infected last year, but it was unclear whether that would lessen an outbreak this year.

In fact, the state could face a more severe virus season this year because a series of winter storms broke all-time rainfall records, dumping 39 inches in downtown Los Angeles and 29 inches in San Bernardino since July 1, state and local health officials said.

Mosquitoes contract the virus by feeding on infected birds and spread the disease to humans through bites.

This winter’s heavy rainfall and melting snow are expected to create hundreds of mosquito breeding pools in buckets, old tires, flower pots and clogged rain gutters throughout Southern California. Unattended swimming pools also act as prime mosquito breeding grounds.

In San Bernardino County, news of an infected chicken in Ontario -- the first sign of the disease’s return to the county this year -- prompted the Board of Supervisors on Tuesday to call for the creation of a task force to drain or spray stagnant pools and educate the public on ways to combat the spread of the virus.

The rains have “set the stage for a particularly bad season,” said County Supervisor Gary Ovitt, whose district includes Ontario.

James Felten, San Bernardino County’s Public Health Director, said the report of the infected chicken in Ontario last week “indicates that mosquitoes are active again ... the cycle has begun.”

The infected chicken was one of dozens kept by a local vector control district to gauge the spread of West Nile and other diseases. The chickens usually survive the infections.

Last year, the virus began to kill birds and infect chickens in early April. This year, the virus began to surface in February, signaling a longer and more dangerous virus season, said Robert Real, field director for the West Valley Mosquito and Vector Control District in San Bernardino County.

Last month in Orange County, officials reported two dead birds infected with the virus -- a snowy egret and an American crow. Last year, the West Nile virus was not found in birds in Orange County until March 31. Health officials also trapped a group of infected mosquitoes in January at the Riverview Golf Course on the Santa Ana River.

Hearst, the Orange County vector control spokesman, said the worst was still to come.

“The rains haven’t yet had their effect,” Hearst said. “The [variety of] mosquitoes most dependent on large sources of water are just now becoming active.”

Vector control officials said the heavy winter rains would cause the wetlands and marsh areas to be fuller than usual this year. But they said their biggest concerns were backyard pools of water and windows without screens to keep out the mosquitoes.

“A lot of people don’t like the way their yard looks through a window screen,” Hearst said. “But it looks a lot better through a screen than it would from a hospital bed.”

Public health officials say people can reduce the chances of being bitten by a mosquito and contracting West Nile Vvirus by staying indoors, using insect repellent, wearing pants and long-sleeved shirts, installing screen doors and emptying pools of standing water.

*

Times staff writers Susana Enriquez and Catherine Saillant contributed to this report.