A chicken in the Copperopolis area tested positive for West Nile Virus last month, the first sign of the disease in Calaveras County this year.

The Calaveras County Public Health Department announced the finding Thursday in a news release that stated the chicken was part of a sentinel flock that’s regularly tested for the disease as part of an early-detection program.

“It wasn’t like someone’s chicken at their private home or roaming the forest or something,” said Dr. Dean Kelaita, Calaveras County health officer. “It’s a specific flock of chickens maintained as part of a surveillance system for West Nile Virus and a couple of other viral illnesses that can live in bird species but infect humans as well.”

Such flocks are maintained throughout the state in cooperation with local mosquito abatement districts, because mosquitoes pass the illness from birds to humans and other animals, especially dogs, cats and horses.

West Nile can cause flu-like symptoms and, in rare instances, death.

“About 80 percent of persons infected with West Nile Virus will have no symptoms at all,” Kelaita said. “Close to 20 percent will get mild flu-like illness, and a small percent will experience the more severe neurologic symptoms.”

There is no known treatment or cure for the virus.

Kelaita said the Calaveras County residents should take steps to prevent being bitten by mosquitoes, which he described as the “three D’s of prevention”:

• DEET, the active ingredient in most mosquito repellants, which he encourages people to use before going outdoors;

• Don’t go outside at dawn and dusk when mosquitoes are most active;

• Drain standing water from around your home to prevent the insects from breeding.

There have been no human cases reported this year to public health departments in Calaveras and Tuolumne counties, according to Kelaita, who has also served as the interim Tuolumne County health officer since mid-August.

Kelaita said he’s not sure exactly how Tuolumne County handles its West Nile surveillance program because he’s just filling in until a new permanent health officer is hired.

The last time a serious case of the illness was reported in Tuolumne County was in 2005, when an outbreak led to 59 birds, three horses and at least one person becoming infected. That year was also a peak for Calaveras County, where 10 birds, six horses and two people were infected.

The first cases of the disease in bird populations in the United States appeared on the East Coast in 1999 and quickly worked their way westward.

Contact Alex MacLean amaclean@uniondemocrat.com or (209) 588-4530.