Drought, warm weather increase concerns about West Nile virus


By Kate Daly | Special to the Almanac

Trapping mosquitoes, testing live chickens, vaccinating horses, unleashing bacteria. All of that is happening in San Mateo County right now in hopes of preventing a deadly outbreak of West Nile virus, the mosquito-borne virus that killed a record 31 people in California last year.

The fear is drought may compound the problem, says Chindi Peavey, manager of the San Mateo County Mosquito and Vector Control District.

The virus is known to start in birds and tree squirrels. Mosquitoes transmit it by biting infected birds or squirrels and then biting humans and other animals. Mild illness might result, but in some cases the virus leads to serious neurologic diseases and complications.

Mosquitoes develop faster in warm weather; they can become breeding adults in just three to five days, and like to lay their eggs in stagnant water.

In drought conditions, less water is flowing naturally and getting caught in pockets when creek beds go dry. Runoff from irrigation or car washing can puddle up or sit in storm drains. Fewer water supplies mean mosquitoes and animals are more likely to gather at the same place, increasing the chances of spreading the virus.

In June another dead bird tested positive for the virus in Menlo Park, making a total of three infected birds found in the county so far this year (two in Menlo Park and one in Redwood City). Last year there were a total of 21.

Ms. Peavey says her office set 20 traps around the positive bird and "so far we haven't detected any virus in any adult mosquitoes."

"Our approach to preventing is to keep the population (of mosquitoes) as low as possible," she says.

One biological control method the district uses is to release bacterial predators in water where mosquito larvae are present. Another is to place mosquitofish that feed on the larvae in closed water systems such as ponds.

The district will deliver the fish to the public for free.

Statewide activity

The level of West Nile virus activity is on the rise throughout California, according to the state's Department of Public Health. So far this year, 31 counties have reported virus activity, including seven in the nine-county Bay Area.

In addition to San Mateo County, Sonoma, Marin, Solano, Contra Costa, Alameda and Santa Clara counties have all had West Nile virus activity this year. The number of California counties with virus activity is 10 more than this time last year and also well above the five-year average of 18, health department officials said.

To date, 240 mosquito samples have tested positive for the virus, six more than last year at this time.


As for keeping horses safe, Ms. Peavey says she thinks most of the horses in the county are vaccinated with the possible exception of some "backyard horses."

Dr. Sinead Devine at Peninsula Equine Medical Center in Menlo Park says the only West Nile virus horse fatality she has seen was nine years ago. Most of the horses owned by clients of the center are vaccinated twice a year, in March or April, when the weather starts to turn, and then again in September.

"Because in this area there have always been (birds testing positive) and mosquito pools ... we're trying to control what we can control," Ms. Peavey says.

She says she thinks the vaccination is effective and cost-effective (approximately $40). The mortality rate for infected horses that haven't been vaccinated is reported to be as high as 33 percent.

The virus is not contagious from horse to horse or from horse to human. Both are considered dead-end hosts.

Dr. William St. Lawrence said he has never seen a case of West Nile virus in dogs or cats at his practice, Village Square Veterinary Hospital in Portola Valley. He says an infected dog could show flu-like symptoms, but "it's not a big deal."

The mosquito district maintains three flocks of sentinel chickens in the county to act as test subjects for the virus. They live in groups of 10 in the Jasper Ridge Biological Preserve in Woodside, in San Mateo and East Palo Alto.

The chickens get blood drawn on a regular basis to see if they have developed any antibodies to the virus, something that would happen if they were bitten by an infected mosquito. So far none has tested positive.

How to help

Chindi Peavey, manager of the San Mateo County Mosquito and Vector Control District, urges people to help the county watch for the virus by calling (877) 968-2473 to file a report within 24 hours of finding a dead bird, particularly a crow or blue jay.

Go to to file the report online.

She also suggests calling the district at (650) 344-8592 if mosquitoes are biting in the area.

Prevention measures people can take on their own include eliminating standing water; wearing mosquito repellent, long pants and long sleeves when outside; and staying indoors at dawn and dusk when mosquitoes are most active.

Bay City News Service contributed to this report.

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