CONCORD, CALIFORNIA - - The Contra Costa Mosquito & Vector Control District announced today that another group of mosquitoes tested positive Friday for West Nile virus in Brentwood. In addition, two dead birds from Oakley and El Sobrante also tested positive for the virus. Fogging for adult mosquitoes was announced last Friday and will take place tonight, Monday, August 6 in Brentwood. The District fogged the same area in Brentwood twice this season, but two additional fogging events had to be canceled due to high winds.

District health officials are especially concerned because an unusually high proportion of mosquitoes from parts of Brentwood have tested positive for West Nile virus. According to the California Mosquito-Borne Virus Surveillance & Response Plan and the Centers for Disease Control, infection rates greater than five female mosquitoes per 1,000 are considered evidence of potential epidemic risk levels. For the 14-day period ending last Friday, District scientists estimated that 21 per 1,000 female mosquitoes in Brentwood were positive. It's important to note that many factors contribute to an overall assessment of risk in an area. Those factors include the type and number of mosquitoes in the area, the environmental conditions, whether or not and how many sentinel chickens or dead birds have tested positive for the virus, and whether or not people have tested positive for the virus.  

 "These infection rates mean there is an elevated risk of disease transmission in parts of Brentwood," said Steve Schutz, Ph.D., scientific program manager  for the District. "This is certainly cause for concern, but you have to look at the whole picture. The rest of our surveillance criteria have not yet reached that level, which means overall, the risk is elevated, but not alarming. I can't stress that enough-that you have to look at the whole picture".

District employees are concentrating their mosquito surveillance and control in a targeted area of Brentwood. Fogging is being conducted tonight and District employees will be personally visiting homes in the area to try to find mosquito sources and to educate residents about the importance of proper outdoor water management that does not produce mosquitoes.

"We are convinced, due to the type of mosquitoes found and our surveillance so far, that the mosquitoes sources are coming from underground or backyard sources," said Carlos Sanabria, operations manager for the District."

Residents are urged to wear mosquito repellent when outdoors, especially at dawn and dusk when mosquitoes are present.

West Nile virus symptoms of the mild form include fever, headache, tiredness, body aches, and swollen lymph glands. While the illness can be as short as a few days, even healthy people have reported being sick for several weeks. 

Less than one percent of individuals (about 1 in 150 people) infected with West Nile virus will develop severe illness. The severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological effects may be permanent. West Nile virus infection can be fatal.