State public health officials have confirmed that a dead crow found in Tisbury last Tuesday has tested positive for West Nile virus, marking the first appearance of the potentially deadly disease on the Vineyard.

Tisbury health agent Tom Pachico received word from state officials Saturday that the crow was carrying West Nile, a virus that infected three Massachusetts residents last year, killing one.

Mr. Pachico said yesterday that he is planning to disseminate public health warnings about the virus, which is spread by mosquitoes.

"We got a hit," said Mr. Pachico. "But it's important for people not to stampede."

State officials also stressed that evidence of one bird testing positive for the virus is no reason to panic.

"Birds are certainly flying over there, but once a virus is introduced by a migrating bird, the next question is whether it takes hold in particular area," said Ralph Timperi, assistant commissioner of the bureau of laboratories at the state Department of Public Health (DPH). "You can't tell from a single bird whether it will become endemic on the Island."

So far this year, 31 dead birds found in the state have tested positive for West Nile. Most were in the Boston area.

State officials are quick to point out that it would take more than one infected crow found in one town to set off any kind of alarm. Cases last year in Tewksbury, Woburn and Acushnet were all preceded by higher counts of dead crows and blue jays testing positive.

In Tewksbury last year, the total of infected birds reached 37; in Acushnet the number was 15.

West Nile viral encephalitis is a rare disease caused by a virus. Birds, particularly crows and blue jays, are the most susceptible. Mosquitoes transmit the disease from bird to bird, but humans and horses bitten by an infected mosquito can also contract the virus.

According to the state public health advisory, symptoms of West Nile virus in humans can include fever, headache, swollen glands and a skin rash. More serious infections can be fatal and might include symptoms of high fever, neck stiffness, stupor, convulsions and paralysis.

There is no vaccine for the virus, and the best hedge against West Nile is to avoid mosquito bites.

"The message has to be to reduce mosquito exposure," said Dr. Alfred DeMaria, director of communicable diseases at DPH. "We don't know if most of the birds on the Island are infected. The bird could have flown over from the mainland, but it's safe to assume if they're not infected now, they will be infected."

Dr. DeMaria urged Island residents and visitors to use an insect repellent that contains the chemical DEET. The state fact sheet on West Nile also advises people to avoid outdoor activities between dusk and dawn, when mosquitoes are most active.

People should also rid their yards and homes of any stagnant water, the ideal breeding ground for mosquitoes.

So far, no mosquitoes collected on the Vineyard by state officials have tested positive for West Nile virus. Nor have officials resorted to any spraying to control mosquito populations.

West Nile virus was first identified in the United States in the summer of 1999 in New York. Since then, the disease has spread along the eastern half of the country as far west as Arkansas.

This marks the third summer that Massachusetts residents have had to contend with the presence of West Nile. But state health officials yesterday said they are hoping that the disease may be on the downturn.

"We haven't seen a level of high virus activity in one community," said Roseanne Pawelec, director of public affairs at DPH. "West Nile virus is cyclical, and this summer may prove out the theory that it's on a downward slope."

People who find dead birds should call their local board of health or DPH at 1-866-627-7968.