An “imminent threat to public health” was declared Friday in Suffolk County by the state Department of Health after the discovery of mosquitoes carrying Eastern equine encephalitis virus, a rare but potentially fatal pathogen.

The virus — commonly called “triple E” — was discovered by the Suffolk County Department of Health Services on Aug. 16 in a mosquito sample from Manorville. The discovery came as part of the department’s routine mosquito surveillance.

Any finding of triple E in a mosquito sample poses a threat to public health, county and state health authorities said. State officials issue declarations of public health threats to help county jurisdictions respond quickly.

Dr. Scott Campbell, chief of the Arthropod-Borne Disease Laboratory in Yaphank, said the virus was found in a species of mosquito — Culiseta melanura — that typically does not bite humans. There is no evidence that triple E has spread to other mosquitoes that routinely draw blood from people.

Health officials are taking the discovery seriously because triple E can be fatal to humans and horses, Campbell said.

Encephalitis can lead to severe inflammation of the brain. Horse owners are being advised to take advantage of an available vaccine. There is no triple E inoculation for humans.

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Discovery of the sample with the triple E virus occurred as Campbell and a team of scientists at his facility tested mosquito specimens captured in strategically placed traps.

They use an algorithm to estimate the possible number of infected mosquitoes in the wild based upon the number of infected ones found in the sample.

Suffolk County health officials requested the health-threat declaration in a letter to the state Department of Health on Thursday, said Jill Montag, a spokeswoman for the state agency.

“The department issued the declaration today, which will allow Suffolk County to take additional steps to protect public health,” she said in an email Friday.

Campbell said the state’s action provides the necessary approval and funding for Suffolk health authorities to spray in ecologically sensitive red-maple swamp areas of Manorville. Spraying will help blunt the mosquito’s proliferation.

“It gives us a greater ability to spray in wetlands when we have a declaration like this,” Campbell said in an interview Friday.

The carrier, also known as the black-tailed mosquito, dines prolifically on birds, he said. And the more that infected mosquitoes feed on birds, the greater the virus becomes amplified in the bird population.

Other mosquito species — those that do bite humans — feed on the same birds and “are thought to be involved in transmission to people,” said Laura Harrington, program director of the Centers for Disease Control and Prevention’s Northeast Regional Center for Excellence in Vector Borne Diseases at Cornell University.

Campbell and Harrington defined the mosquitoes that bite humans as “bridge vectors,” because they transfer the pathogen from birds to humans.

These “bridge” mosquitoes “may include the Asian tiger mosquito, Aedes albopictus,” Harrington said. Asian tigers are prevalent on Long Island.

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“In other parts of the country the Asian tiger mosquito has been found infected with EEE, but it is unclear how important it might be in New York,” she said.

The last time a Suffolk mosquito sample was found to be positive for triple E was in 2008.