Two birds in San Luis Obispo County tested positive for West Nile virus last month. The San Luis Obispo County Public Health Department reported that one bird was found in San Luis Obispo and the other was found in Nipomo.

Mosquitoes are vectors, meaning that they become infected with West Nile virus after feeding on infected birds and then may transmit the virus to animals and humans.

State health officials said that evidence of the virus in dead birds is often the first indication that the virus has been introduced to a new region, or that the risk of transmission is high.

Dr. Gregory Thomas, county public health officer, said in a statement that most people infected with West Nile virus do not show any symptoms.

“However, up to 20 percent of infected individuals can expect to experience symptoms such as fever, headache, body aches, and a small number may develop severe illness,” Thomas said.

Less than 1 percent of infected individuals develop severe illness. Symptoms include high fever, headache, coma, tremors, numbness and paralysis. These symptoms can last for several weeks, and neurological effects may be permanent. The virus infection can be fatal.

Symptoms develop from five to 15 days after the infected mosquito bite occurred.

Kurt Batson, director of environmental health at the health department, said that the county’s mosquito abatement efforts are particularly high at this time of year.

“We use a granular material made out of corn cobs and spread it with blowers or belly grinders. The material is essentially a bacteria that appeals specifically to mosquitoes,” Batson said. “The mosquito larvae eat the material which shuts down their development and eventually kills them.”

The California West Nile virus Web site said that 42 positive human cases of the virus have been reported in the state so far this year. Of the 42 cases, 15 were reported last week.

A total of 38 counties have reported virus activity.

Last year at this time, only 16 positive human cases of the virus had been reported and a total of 37 counties had reported virus activity.

The rise in positive human cases may be correlated to an increased number of infected birds. In 2006 at this time, 219 birds had tested positive for the virus. This year that number has almost doubled, with a reported 423 birds testing positive for the virus so far.

However, Batson said that West Nile virus is really a disease of birds, and that humans merely get in the way. He said that the increased number of positive human cases may also be weather or water dependent.

Batson recommended that people wear light colored clothing with long sleeves and long pants whenever possible to avoid being bitten by mosquitoes, and avoid spending time outside at dawn and dusk, when the insects are most active.

Other precautionary measures include draining areas of water where mosquitoes can breed, keeping windows and door screens in place and in good repair, and wearing mosquito repellant with DEET.

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