NORTH COUNTY -- You've heard of a guard dog, but how about a sentinel chicken?

Four flocks of the cluckers, including one in Carlsbad, work for the county Department of Environmental Health. Their job is to let mosquitos bite them and people sample their blood for evidence of West Nile virus.

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West Nile virus, which mainly attacks birds, concerns health officials because it can be transmitted to people by mosquitoes that have fed on infected birds. In humans, it can cause flulike symptoms and, in rare cases, serious brain swelling and death.

The county has been using the chickens as part of its West Nile surveillance program since 2003, but this is the first year that multiple chickens have tested positive for the virus, according to county statistics.

So far this year, eight out of 10 birds in a coop near the Los Penasquitos Lagoon in Sorrento Valley have tested positive for the virus.

No birds in the county's three other 10-bird flocks have tested positive. The others live in Carlsbad, near the Buena Vista Lagoon; Santee; and San Diego, near the U.S.-Mexico border.

The flocks are strategically placed near mosquito-breeding grounds, county officials said. The positive tests in the Sorrento Valley helped the county focus its mosquito eradication and public information efforts in the area.

"We know the birds got it right there where they're located, and it lets us know there's an awful lot of the virus in that area," Chris Conlan, a spokesman for the county's Vector Control Department, said of the Sorrento Valley chickens.

Officials weren't surprised to find West Nile virus in its chickens this year: the number of dead birds that have tested positive for the virus is nearly at a record. So far this year, 154 birds have tested positive for the virus, with the peak of West Nile season not expected until late August or early September. The record for bird infections came in 2005, with 162.

A San Diego man who was hospitalized with the virus last week represents the county's first human case.

Even with so much evidence of the virus this year, the chickens remain a useful part of the detection program, Conlan said. A crow may fly miles from where it contracted the virus before dying, but the cooped chickens contract the virus from local mosquitos. The insects range less than a mile, he said.

Using chickens has several other advantages, Conlan said. The birds produce abundant antibodies but don't get sick from the virus. Also, unlike many birds, they don't transmit West Nile to noninfected mosquitos.

On Wednesday, it was the Carlsbad flock's turn to give blood. The birds live north of Westfield Plaza Camino Real mall, hidden by some bushes and accessible by a service road.

When county vector control technician Brandon Stidum pulled his truck in front of the coop, the 10 ruddy birds gathered at the front.

"Sometimes I let them out," the 27-year-old Stidum explained. "When you pull up the truck, they always come to the front."

The birds are tested for the virus every two weeks, he said. County employees visit the birds twice a week to feed them and clean their cages.

The manpower involved in maintaining the flocks is one reason the county only keeps four, officials said.

To prepare the chickens -- all hens -- to give blood, Stidum climbed in the coop and, one by one, picked them up, turned them on their backs, and bound their legs with a plastic tie. Afterward, they sat quietly on the coop's floor.

When all the hens were bound, Stidum took a blood sample from each. With a bird resting on the bed of his truck, Stidum jabbed its crimson crown with a lancet and let it bleed onto a test strip.

Afterward, he applied pressure to the bird's small wound for about 30 seconds, unbound it and let it peck and dig outside its coop.

The strips go to Sacramento for testing and come back in about a week, Stidum said. California tests the blood from more than 250 flocks throughout the state, according to a Department of Public Health spokesperson.

None of the hens by the Buena Vista Lagoon has ever tested positive for West Nile virus, Stidum said. This year's flock was installed in May and will reside there for the season, he said. They will be retired -- that is, given away -- before winter.

The eight birds in Sorrento Valley that tested positive for the virus have gone to live at a local church, Stidum said. Once the birds develop antibodies, they are no longer useful for future tests.

Stidum, whose work also involves testing mice for hantavirus and squirrels for plague, said he had little past experience with the chickens, but he has come to love them.

"They're as happy as can be," he said. "They have their own personalities."

Contact staff writer Sarah Gordon at (760) 740-3517 or sgordon@nctimes.com