Health & Medicine

As West Nile season heats up, mosquito fight waged with fish, chickens – and spraying

Lab technician Paula Matney suits up before she enters a chicken coop at the Sacramento-Yolo Mosquito and Vector Control District facility in Elk Grove on Wednesday. Sacramento-Yolo mosquito officials keep coops of chickens that are regularly tested for West Nile virus. Lab technicians handle the birds, which are pricked for blood samples to see if they’ve developed West Nile antibodies. Similar chicken testing is done statewide.
Lab technician Paula Matney suits up before she enters a chicken coop at the Sacramento-Yolo Mosquito and Vector Control District facility in Elk Grove on Wednesday. Sacramento-Yolo mosquito officials keep coops of chickens that are regularly tested for West Nile virus. Lab technicians handle the birds, which are pricked for blood samples to see if they’ve developed West Nile antibodies. Similar chicken testing is done statewide.

With the deadly West Nile virus season heating up, state and local health officials are again battling the disease with an unusual arsenal of tiny fish, dead birds and plump chickens.

That trio, part of a years-long, multipronged approach to squashing West Nile, helps health officials identify the mosquito “hot spots” that need ground or aerial spraying.

This week, those efforts intensified, as local mosquito abatement officials increased their spraying after seeing a pronounced increase in West Nile activity, partly due to the triple-digit heat, which accelerates the proliferation of disease-carrying mosquitoes and dead birds.

“Recently, there seems to be higher than normal West Nile activity in the Sacramento Valley region and our Northern California counties,” said Dr. Vicki Kramer, chief of the vector-borne disease section of the state Department of Public Health. “There are several counties with increased infection rates at epidemic levels,” including Butte, Glenn, Yuba and Tulare.

Epidemic levels are reached when the number of infected mosquito samples hits a threshold compared with the total number tested.

As of Thursday, the state was reporting 35 counties with West Nile activity involving 254 dead birds, 756 mosquito samples and 43 cases of chickens testing positive for the disease. Last year at this time, the state reported far more cases of West Nile-infected mosquitoes and dead birds.

That on-the-ground knowledge is gleaned by from a mix of sources: testing chicken flocks for West Nile antibodies; checking dead crows, robins, magpies and other birds for the disease; and trapping, tallying and testing mosquito batches.

“We’re heading into a very busy August,” said Luz Rodriguez, spokeswoman for the Sacramento-Yolo Mosquito and Vector Control District, which has been conducting ground and aerial spraying in recent weeks. As of Thursday, 93 mosquito samples and 37 dead birds had tested positive for West Nile in Sacramento and Yolo counties. That compares with 310 mosquito samples and 184 dead birds at the same time last year.

Most people – 70 to 80 percent – who are infected with the West Nile virus never develop symptoms, but fatalities do occur.

Last week, Nevada County public health officials reported this year’s first West Nile death in the state. According to a report in The Union newspaper, the victim was a 79-year-old woman, an active community volunteer and philanthropist in Nevada City, whose family speculates she may have contracted the disease while on vacation in Southern California’s Coachella Valley area, which recently reported a high concentration of West Nile-infected mosquitoes.

Chickens as canaries

Like canaries in a coal mine, the state’s flocks of 158 sentinel chickens are an unusual, living defense against West Nile.

The chickens, which live in outdoor coops monitored by local agencies, are considered “mosquito magnets” – they readily attract mosquitoes but don’t get sick if bitten by a West Nile carrier. Instead, they develop West Nile antibodies; when those antibodies start showing up in blood tests of chicken flocks, mosquito abatement officials know they need to target an area for potential spraying.

At the district’s headquarters in Elk Grove, lab technician Paula Matney pulls on a white plastic jumpsuit and booties, plus goggles and gloves, before entering the outdoor chicken coop. She scoops up a plump, tawny-colored red leg chicken, cooing at the wriggling bird, part of a network of sentinel chicken flocks the district maintains in Rancho Murieta, Esparto, Knights Landing and Isleton.

Most of the state’s sentinel chickens are tested every two weeks.

“About the time these guys start showing positive, it’s an indication that human cases can be developing, too,” said Matney. The chickens, which are given new homes after their year of sentinel duty, supply an endless source of eggs, which are sold to district employees for 50 cents a dozen.

As of Thursday, two of the district’s 20 chickens tested positive for West Nile.

Statewide, sentinel-chicken testing has helped identify West Nile-carriers in the Sacramento Valley region, including 18 positive chicken identifications in Butte County, seven each in Sutter and Yuba counties and three in Glenn County.

Dead birds, often discovered by the public, are another alarm bell for West Nile. When mosquitoes feed on an infected bird, they can spread the disease to humans.

Mosquito-eating fish are also part of the West Nile battle. The Sacramento-Yolo district maintains 23 rock-lined ponds that serve as breeding centers for tiny, 2-inch fish, officially known as Gambusia affinis, a cousin to guppies. Last year, the district gave away 4,600 pounds of the mosquito-gobbling fish.

The fish are free to anyone – homeowners, farmers, pasture owners, rice growers – with a standing body of water they want to ensure against mosquito infestations.

State and local officials say they’re particularly concerned about untended residential swimming pools, which can be prime sources of mosquito breeding if they’re not kept chlorinated.

Like mosquitoes, the fish are prolific.

“We can drop in 40 or 50 fish in a neglected pool, then in a couple of weeks, there’ll be hundreds,” Rodriguez said. The mosquito-eaters have their work to do; over the course of a summer, a typical untended swimming pool can breed 1 million mosquitoes that can “fly anywhere and infect an entire neighborhood,” she said.

Between April and mid-October, Sacramento-Yolo crews also set dozens of mosquito traps around the district. Some are “stinky water” traps filled with smelly alfalfa or yeast-infested water. Pregnant mosquitoes, attracted to the smell, land on the water to lay their eggs, then are sucked in by fans set inside a fishing tackle box. Others are hanging traps, using light or CO2 to attract mosquitoes.

Typically, a single trap can collect between 100 and 500 mosquitoes in one night; at the Wildhorse Golf Club in Davis on Monday, one trap grabbed 1,047 mosquitoes overnight.

Back at the Elk Grove lab, the mosquitoes are counted and sorted, then carefully tabulated on color-coded counters. Using microscopes, a team of seven lab technicians pore over the trapped mosquitoes. Some are dark black, some are brown, some have distinctive stripes or fuzzy-looking antennae. On a high shelf, the lab stores giant plastic jars filled with mosquito carcasses from years past, as many as 475,000 per year.

Of 23 mosquito species in the two-county area, the Elk Grove lab is looking for the two that carry West Nile. The others? “They’re just nuisance mosquitoes,” Rodriguez said.

Other avenues of attack

The Sacramento-Yolo district also uses more conventional methods of attack, such as checking hundreds of catch basins and storm drains. In Sacramento city alone, Rodriguez said, there are more than 100,000 drains that can become breeding grounds. If mosquito larvae are found, pesticides are applied to kill off the immature mosquitoes.

The state also monitors reports from blood donor centers, which tests donated blood for certain infectious diseases, such as West Nile. In Butte County, for instance, a blood center recently reported two cases of West Nile-tainted blood.

Ultimately, despite all the tools, both animal and technological, mosquito abatement officials say they rely best on humans. They especially want the public to report sightings of dead birds, whether it’s a robin or a raven, that could signal where West Nile is developing.

“We depend on the public to be our eyes and ears,” said Rodriguez, “because we can’t be everywhere.”

This week, local mosquito control officials began aerial and ground spraying in Davis, Elk Grove, Lincoln, Orangevale, Roseville and Woodland, targeting locations ranging from parks and golf courses to agricultural and residential areas.

In Placer County, the Placer Mosquito and Vector Control District sprayed around 11 p.m. Wednesday in urban areas of western Roseville and northern Lincoln.

“We fly where the public health risk exists,” said Joel Buettner, general manager of the Placer district and president of the Mosquito and Vector Control Association of California. “Western Roseville is a hot spot for us every year, where residential areas are jutting into agricultural areas, so we wanted to make sure we hit those quickly.”

After last year’s record-breaking West Nile virus outbreak – with 801 reported cases and 31 deaths – this year’s season initially had a slow start, but is ramping up with the hotter weather.

“The heat means more mosquitoes are hatching,” Rodriguez said. “We’re finding more in the traps and seeing where the disease is spreading.”

How to avoid West Nile

The state Department of Public Health advises following the “three D’s”:

DEET – To prevent mosquitoes from biting, apply insect repellent containing DEET, picaradin, oil of lemon eucalyptus or IR3535.

Dawn/dusk – Mosquitoes bite in the early morning and evening, so cover arms and legs and wear repellent when outside. Make sure doors and windows have tight-fitting screens.

Drain – Mosquitoes lay their eggs on standing water. Eliminate all potential water sources by emptying flower pots, buckets, old car tires and other open containers, as well as stagnant swimming pools. Empty children’s wading pools when not in use.

More information:

The state’s public health website,, includes the latest county-by-county information on mosquito spraying, dead birds and other virus activity. To report dead birds, visit the state’s West Nile site or call its hotline: 877-968-2473 (877-WNV-BIRD).

To find spraying locations in your area, contact your local mosquito abatement district. For the Sacramento-Yolo Mosquito and Vector Control District, call 800-429-1022 or go to

West Nile at a glance

What it is: A mosquito-borne virus that typically shows up in California between mid-April and mid-October. Mosquitoes become West Nile carriers (or vectors) by feeding on infected birds, then spread it to other animals and humans.

Where it originated: Originally from Africa, West Nile disease was first detected in the U.S. in 1999 and now occurs in almost every state.

Why it’s dangerous: In most cases, West Nile virus goes undetected and symptom-free in people bitten by a virus-carrying mosquito.

About 20 percent develop flulike symptoms. But in less than 1 percent of cases, it develops into a neuro-invasive disease that can be fatal. Last year, 97 U.S. deaths were attributed to West Nile.

Source: Bee research