Marin/Sonoma Mosquito & Vector Control District
800-231-3236 (toll free)
707-285-2200 (office)
595 Helman Lane, Cotati, CA 94931-9736

They are guardians of the public health, tireless and uncomplaining, except for the occasional ruffled feather.
    In lonely redoubts throughout California they stand between us and an outbreak of deadly, viral brain disease -- and, of course, they work for chicken feed.

Once pack at the lab, the strips will be tested to see if encephalitis is active in the area.

    These are California's "sentinel chickens." They protect and serve, and even have their own Web site.
    For 20 years they have been California's early warning system to detect outbreaks of encephalitis, the mosquito-borne virus that recently claimed three lives in New York City.
    County mosquito control biologists fan out twice a month to perform simple blood tests for encephalitis on 130 flocks of sentinel chickens throughout California.
    ``You can't help but laugh about it, especially when you are standing there with a chicken in your hands,'' said Ann Donohue, an entomologist with the Contra Costa County Mosquito and Vector Control District. She maintains three flocks of sentinel chickens, each with a dozen birds. ``It's somewhat comical, but it works.''
    At the core of this system are birds like Number 7107, a plucky, eight-month-old White Leghorn hen, employed by the San Mateo County Mosquito Abatement District.
    This week biologists Chindi Peavey and Sasha Porshnikoff performed the routine tests on 7107 and her 11 coopmates living just outside San Francisco International Airport, not far from a mosquito-infested salt marsh.
    ``This is the most efficient way of checking for the virus,'' said Peavey, a medical entomologist who received a doctorate from Berkeley.
    Peavey's birds live their lives seemingly oblivious to the crack of bats at a nearby ball field, the roar of freeway traffic, or the whine of their wide-bodied, aluminum-skinned cousins rising from and gliding to the runway.
    The science here is simple: If a mosquito carrying encephalitis bites one of the chickens, the bird's immune system will churn out antibodies to the virus. The biweekly tests can detect those antibodies on just a droplet of chicken blood. When a bird tests positive, Peavey will know that the virus is loose in the neighborhood.
    Encephalitis is harmless to infected chickens. In fact, the virus lurks naturally in wild fowl populations. For reasons as yet unknown, if the virus takes hold in horses and humans, it can be deadly.
    The trouble begins after a mosquito bites a bird carrying the virus. The virus multiplies in the mosquito's belly. When the mosquito takes her next meal she transmits fluids teeming with active virus into her dinner's bloodstream.
    ``Horses and humans are accidental hosts,'' Peavey said. The virus can cause high fever and swelling of the brain. Encephalitis has no effective medical treatment and is particularly deadly to children and the elderly.
    The good news is that unusually cool and dry weather has kept the California mosquito population down this year. Only a few of the sentinel chickens show signs of exposure to the virus. Every week the state updates a map of sentinel chicken sites. The state's $60 million mosquito control programs -- of which the sentinel chickens are a small but important part -- have helped to keep encephalitis a rare disease in California. According to Stan Husted, who directs mosquito disease control for the state health department, the most recent cases involved one victim of St. Louis encephalitis in Los Angeles in 1997, and another in Riverside County in 1994.

    This year no human cases have been recorded, but 13 sentinel chickens have tested positive in Tulare, Riverside and Imperial counties. Typically, more than 100 birds test positive annually. In 1997 there were 288.
    ``Fortunately it has been a quiet year for us in California,'' Husted said.
    Bleeding the chickens is a two-person job. Porshnikoff holds a hen by the feet and neck, while Peavey pricks the bird's floppy red comb with a lancet, and mops up a droplet of blood with a thin paper strip.

Once back in the lab, the strips will be tested to see if encephalitis is active in the area.

    The strips are dried and sent by mail to the Viral and Rickettsial Disease Lab in Berkeley, where they are tested for the presence of antibodies to two strains of encephalitis.
    St. Louis encephalitis, named after the city where it claimed 1,000 victims in the 1930s, is the same strain afflicting New York. Western Equine encephalitis is nasty to humans and horses. Until a horse vaccine was developed, it killed half of the horses it infected.
    The sentinel chickens of San Bruno have never tested positive -- good news for the densely populated region surrounding them. But when a bird ``seroconverts'' -- produces the antibodies to fight off the disease -- the local mosquito control districts spring to action. They step up spraying programs in wetlands to knock down the adult population of Culex tarsalis, the species of mosquito that carries encephalitis from wild birds in California.
    All of California's sentinel chickens are raised at a Modesto farm that keeps the birds in screened-in coops to keep out mosquitoes. At 18 months, they are ready for duty during encephalitis season, May to October.
    At the end of the hitch, there will be some good news for Number 7107. She's not headed for the deep-fat fryer. During the year at the airport, these sentinel hens also lay eggs, which makes them valuable to farmers after their duty is done.
    ``They get old and tough, and are better as laying hens,'' Peavey said. ``It's better to have eggs for a long time than just one chicken dinner.''

ENCEPHALITIS FACTS

St. Louis encephalitis and Western Equine encephalitis are viral strains that are found in wild birds and can be transmitted to humans by mosquitoes. A recent outbreak of St. Louis encephalitis killed three people in New York City.

Deadly mosquitoes
    1. The virus is carried into the region by migratory birds that pass through wetland areas inhabited by mosquitoes. The encephalitis virus does not make the birds ill. It can be found at low levels in robins, sparrows, finches and other common wild birds.
    2. Female mosquitoes feed on the infected birds prior to developing eggs. The virus multiplies in the mosquito's belly. During her next meal, she may transmit the virus to another bird, or to mammals.
    3. Humans are an ``accidental host'' for the virus. Encephalitis is not contagious and is only spread to humans by mosquitoes.

Culex tarsalis - "Encephalitis mosquito"
    --Bodies are brown with pale abdominal bands.
    --The mosquitoes tend to breed in wetlands and standing water such as in irrigation ditches.
    --Females can travel up to 30 miles in search of a blood meal.
Encephalitis
    --Encephalitis can cause an inflammation of the brain and damage to the central nervous system. Symptoms such as fever and headache appear within eight to 10 days of infection. An average of 130 cases are reported each year, and the virus is most lethal among infants and the elderly.