A mosquito caught on Monday in the town of Tisbury has tested positive for the West Nile virus, confirmed Thomas Pachico, health agent. It is the second Island appearance of the virus this year. The mosquito was caught in the southwest corner of the town, and was shipped with a pool of mosquitoes to the state laboratory in Jamaica Plain for testing. The town was notified Thursday evening.

The board of health recommends that the public cover the body with clothing, use insect repellent and avoid spending time outside during peak mosquito hours.

Last month a West Nile-infected mosquito was trapped near Mill Pond in West Tisbury. Subsequent tests of mosquitoes from each town, performed weekly during the summer, did not turn up any positives until this week.

West Nile is an infectious disease transmitted through mosquitoes, with birds acting as the most common host. Mosquitoes carrying West Nile have the potential to spread it to human beings.

The Islandwide mosquito trapping program is the last remnant of the county-led Integrated Pest Management Program, formerly directed by T.J. Hegarty beginning in 2001. The all-inclusive pest program lost town funding in late June, but the town boards of health agreed that they wanted the county to continue the surveillance of mosquitoes. Richard (Dick) Johnson, an Island biologist, took over the trapping responsibilities and spends 10 hours per week collecting samples from each town.

The trapping procedure relies heavily on Mr. Hegarty’s mosquito “tea mix,” an artificial breeding site which features an infusion of hay, oak leaves and a couple of gallons of water, a concoction the pests find irresistible. Attracted by the smell, they fly nearby and become sucked into the netting lining the top of the bucket. On Mondays, Mr. Johnson collects; on Tuesdays, he freezes and ships the specimens to a state lab to be tested for West Nile and eastern equine encephalitis virus (EEE).

Though Island officials have been trapping mosquitoes since 2009, this is the first summer that Dukes County has been officially recognized by the state as a mosquito district. State officials voted to recognize the project in order to comply with legislation enacted in 1957 designating a Dukes County Mosquito Control Project, and to meet board of health demand for effective surveillance.

“It’s actually not an honest-to-goodness mosquito control project,” said Lee Corte-Real, division director of the crop and pest services of the state Department of Agricultural Resources. He said surveillance is only one part of mosquito management, and recognition was granted somewhat reluctantly, so that mosquitoes could be sent directly to the state.

For the past four years, Mr. Hegarty sent mosquitoes to Barnstable County before they could be sent to the state laboratory in Jamaica Plain. Now the state accepts the insects directly from the Island, enhancing the probability that viable specimens will make it to Boston.

“We don’t have to piggyback and we don’t have to ask our neighbors that have been really good to us,” explained Mr. Powers.

Mr. Corte-Real raised concerns that though the Island is now able to better identify cases of EEE or West Nile, the project would be unable to respond adequately. “In the classic sense of a mosquito control project, it’s not one,” he said. “At some point in time if you get a positive, you need to have a response.”

In addition to surveillance, a comprehensive mosquito control project has the resources to spray adult mosquitoes and kill larva.

Mr. Powers said he doesn’t think the Island boards of health have ever discussed becoming a fully active control district, which would be overseen by a state commissioner. He said the Islands seem to be less at risk for mosquito-borne diseases. In contrast, “denser populations with inland swamps have had a much higher degree of hits than we have had,” he said.

While Mr. Corte-Real is less concerned about West Nile positives, because the virus is endemic to the state, EEE is a bigger concern. It “is a much more virulent disease,“ he said. He said Martha’s Vineyard is not equipped to respond to a positive EEE finding. “It’s sort of like going to the doctor and having him do a test and finding you have a disease, but having no insurance to cover the treatment.” His advice to Islanders would be to assemble a control project, hire state employees to manage it and acquire the resources to perform preventative work, like killing mosquito larva.

The state recognizes nine active mosquito control districts. The central part of the state, including Worcester, Hampden, Franklin and Hampshire counties, do not perform mosquito control.