West Nile virus-infected mosquitoes found in southeastern Calif.

EL CENTRO, Calif. (AP) The discovery of mosquitoes infected with West Nile virus along a key migratory route in southeastern California has health officials increasingly wary of the disease's spread.

The infected mosquitoes found at the southern end of the Salton Sea marked the disease's debut in the state's wilderness, health officials said.

The California Department of Health Services confirmed the mosquitoes tested positive for the virus, which is rarely fatal in humans. Chicken flocks in nearby Niland also tested positive for the disease, according to preliminary results.

The tests were conducted by researchers at UC Davis's Center for Vectorborne Diseases, which has been looking for West Nile virus in mosquitoes, birds and other animals for two years.

``We knew it was going to get here sooner or later,'' said John Edman, the center's director. ``Now that we know it's here, it's time to put in place preventive measures.''

The center plans to increase surveillance across the state, Edman said. Based on the pattern found in other states, he said he expects California will see a spread of human cases either this year or next year.

Infected mosquitoes carry the West Nile virus in their saliva and pass it on when they bite. Nearly 140 North American bird species can serve as hosts, according to the Centers for Disease Control and Prevention. Every year, millions of bird use the Pacific flyway which runs by the Salton Sea.

Mammals, including humans and horses, can catch the virus when they get bitten by an infected mosquito, but do not serve as hosts like birds do.

A single human case of the virus was reported in California this year. An Alameda County woman was bitten by an infected mosquito while visiting Colorado, and continues to suffer from severe muscle weakness in her legs, according to the state Department of Health Services.

Last year, California officials reported a lone case of the mosquito-borne virus, in a Los Angeles woman who recovered. It remains unclear how she was infected.

West Nile infected 4,156 people and killed 284 last year in the largest U.S. outbreak since the virus was first detected in the country in 1999. There have been at least 599 human cases this year, including 11 deaths, according to the CDC.

The virus has steadily pushed west and south since its arrival.

(Copyright 2003 by The Associated Press. All Rights Reserved.)