Encephalitis-Carrying Mosquitoes Return

A long-lived American cousin of West Nile virus is back in California, state health officials said Monday.

St. Louis encephalitis, which can cause illness or death in rare cases, has been detected in mosquitoes in rural Riverside County. No human cases have been reported.

“It’s important to emphasize that mosquitoes are not just a pest in California, but can potentially transmit diseases,” said Vicki Kramer, chief of the vector-borne disease section of the California Department of Health Services. “Although the public health risk is very low from St. Louis encephalitis, there have been documented human cases in California.”

The infected mosquitoes were found July 3 in the Coachella Valley, near the marshy Salton Sea. The heavily irrigated agricultural valley, a traditional entry point into California for migrating birds that can carry viruses in their bloodstreams, is often the first site where encephalitis is found.

Named after an epidemic that struck the Missouri city in 1933, St. Louis encephalitis is closely related to the fast-spreading West Nile virus, which arrived in New York City from overseas in 1999, and causes similar central nervous system disease.

Most healthy individuals bitten by an infected mosquito will have no symptoms. Others may experience headache, fever, fatigue or muscle ache. Severe, potentially fatal coma and other illness can affect the elderly and individuals with lowered immune systems, health officials said.

“It’s a persistent threat,” said Roy Campbell, medical epidemiologist with the Centers for Disease Control and Prevention in Fort Collins, Colo. On July 3, lab technicians with the Coachella Valley Mosquito and Vector Control District collected 50 adult females from a dry-ice bait site in a ranching area two miles south of Mecca, a farming community near the Salton Sea. The dry ice contains carbon dioxide, a lure for the pests.The mosquitoes were frozen and shipped to UC Davis for testing. On July 18, the results came back positive for the St. Louis strain. Cross-testing for the more aggressive West Nile form proved negative.

Branka Lothrop, who oversees the Coachella Valley district laboratory, said surveillance and pesticide applications were stepped up, and warnings were posted in the sparsely populated area.

St. Louis encephalitis is regularly found during the summer in California, particularly in heavily irrigated agricultural areas, so officials are not overly alarmed.

The last confirmed human case of St Louis encephalitis was in Los Angeles County in 1997. In a 1989 outbreak, 29 people became ill.

This summer, there has been one confirmed human case nationally in Mississippi. Researchers say two factors have reduced its spread in recent years: better flood control and increased indoor evening activity when mosquitoes are biting.

“People tend to sit inside in an air-conditioned house with screens in the windows watching CNN now. They’re not out on the front porch in the evenings like they used to be,” said the CDC’s Campbell. .

But that does not mean it is gone. Campbell said there was a major epidemic across two-thirds of the U.S. in 1975, for reasons that are still unknown.

Public health officials are also bracing for another onslaught of the aggressive West Nile strain, which in just three years has already nearly as many reported cases as the St. Louis strain in 36 years.

In 2002, there were more than 4,100 reported cases of West Nile virus and 300 deaths in 44 states. This year, 37 states have reported West Nile virus.

While only 26 cases among humans have been reported, that could parallel last summer, when a huge upswing began in August.

“We’re expecting to see it in California this summer,” said Ken August, a state health department spokesman.

Many doctors are not familiar with different encephalitis strains, experts said. They urge people, especially the elderly, to wear long sleeves and use insect repellent. State health officials asked anyone who sees dead birds to call a West Nile virus hotline at (877)-WNV-BIRD.