St. Louis Encephalitis Virus Found in County : Health: The potentially deadly strain is detected in Thousand Oaks. The public is warned to avoid mosquito-infested areas.

A potentially fatal virus that is transmitted by mosquitoes has been detected in Ventura County for the first time, prompting a public health warning Friday to avoid mosquito-infested areas.

Officials at a state lab in Berkeley confirmed the presence of St. Louis encephalitis virus in a flock of “sentinel” chickens kept near a sewage treatment plant in Thousand Oaks, said Bob Gallagher, manager of the county environmental health division’s encephalitis virus program.

County health officials said they have never found the virus in domestic or wild birds since the county established its detection program in 1985.

No human cases have ever been reported in Ventura County. But county officials launched the detection program after an outbreak of the virus in 1984 infected 16 people in Los Angeles County, killing one person and contributing to the deaths of two others.

“Right now our primary interest is to let the public know that if they’re going to be out in the dusk and evening hours, they should avoid mosquito bites,” Gallagher said. “We now have a potential for mosquitoes in the county to have the encephalitis virus.”

Officials on Friday issued notices to local physicians and hospital emergency room workers to warn them about the virus and to report incidents of human cases, he said.

The county also notified Thousand Oaks’ Conejo Recreation and Park District of the potential risk to park visitors and workers at the sewage treatment plant.

The virus was detected in one of three sentinel flocks that serve as an early warning system for public health officials.

The flocks have been kept at three sites--the Strathearn Historical Park in Simi Valley, a state-owned fish hatchery in Fillmore and the sewage treatment plant in Thousand Oaks--since early summer.

Gallagher said tests of the two other flocks were negative, but county workers will set traps for mosquitoes to determine if the virus is spreading.

The encephalitis virus is one of several mosquito-borne illnesses found in the blood of wild and domestic fowl. It can be transmitted to humans through bites from a mosquito that has fed on an infected bird.

In most cases, because the virus attacks the brain and nervous system, infected humans will experience headaches, a fever and nausea, Gallagher said. But, he said, “in susceptible individuals, it can be a much more serious illness, and it can be fatal.” Older people, for example, are more likely to be severely affected.

Moorpark Mosquito Abatement District Manager Mark Westerline said workers conduct weekly inspections of areas known to be breeding grounds for mosquitoes.

Next week, the district is planning to step up a program to trap live mosquitoes, Westerline said.

“This is the first time we’ve gotten a positive reading in Ventura County that I know of,” Westerline said. “It’s definitely a reason to be concerned.”

Gallagher said the county has also issued an alert to homeowners to eliminate standing water around their houses, including emptying planters, old tires, cans and buckets.

The county has offered supplies of mosquito-eating minnows to residents who want to stock ornamental and fish ponds.

Residents should also avoid evening activities in areas where mosquitoes swarm unless they are wearing insect repellent or long-sleeved shirts and pants.

“There are degrees of alarm,” Gallagher said. In this case, he said, there’s no need to panic. “I don’t think people need to stay indoors once the sun goes down. They just need to be a little cautious.”