September 9, 2014 / 9:55 PM / 6 years ago

West Nile virus outbreak hits California's Orange County

LOS ANGELES (Reuters) - An outbreak of the West Nile virus in California’s Orange County has infected more than 90 people so far this year, seven times as many as were diagnosed in all of 2013, and public health experts say the region’s prolonged drought may be a factor.

A female Aedes aegypti mosquito is shown in this 2006 Center for Disease Control (CDC) handout file photograph released to Reuters on October 30, 2013. REUTERS/James Gathany/CDC/Handout via Reuters/Files

Not only has the number of human infections reported in the Southern California county since January dwarfed the 12 cases documented for all of 2013, but the prevalence of West Nile in birds, which harbor the virus, and in the mosquitoes transmitting it has also spiked.

The 94 confirmed human cases in the county, three of them fatal, represent about half of the 180-plus documented statewide so far this year and nearly a quarter of the 400 cases reported nationally.

Local health officials say they face a tough challenge in educating the public about the threat and persuading people to safeguard themselves against insect bites.

“Getting people in Southern California, which is not a particularly buggy place, to think about covering up and using bug repellent is an uphill battle,” Deanne Thompson, a spokeswoman at the Orange County Health Care Agency, said on Tuesday.

But the reason why Orange County has been so hard hit is still a mystery, experts said. Los Angeles and Fresno counties each have reported about two dozen cases this year.


Mosquito populations in Southern California this season are below normal, owing in part to a persistent drought that has dried up much of the standing water where the insects breed.

But scientists believe those same dry conditions have driven more birds into populated areas in search of water, concentrating numbers of the virus’ preferred host and the mosquitoes that feed on them closer to people.

Outbreaks tend to flare up during warm weather, and the proportion of mosquitoes carrying West Nile this summer is much higher than usual.

The virus has turned up in 80 percent of mosquitoes tested in Orange County this year, the highest rate detected in California since the disease first surfaced there in 2004, said Jared Dever, spokesman for the county’s vector control agency.

The percentage normally testing positive for West Nile averages about 20 percent, he said.

Moreover, nearly 260 dead birds have tested positive so far this year, compared with 40 for all of 2013 and 109 during California’s last sizeable outbreak in 2012.

About 80 percent of people infected by West Nile show no signs of the disease and about 20 percent become ill, typically with flu-like symptoms such as fever, headache, nausea and swollen glands, according to the U.S. Centers for Disease Control and Prevention.

About one in 150 infected people develops severe illness, including a neuroinvasive form of the disease that can lead to tremors, convulsions, disorientation, coma and death, the CDC says. Neurological effects can also be permanent.

Most of the Orange County cases were diagnosed because patients became sick: 63 with neuroinvasive disease - including the three who died - and 21 who developed mild fever, Thompson said. Nine were identified through routine blood screening and were otherwise unaware they had been infected, she said.

Reporting by Steve Gorman; editing by Cynthia Johnston G Crosse

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