Health officials in Massachusetts say the threat from a mosquito-borne illness, Eastern Equine Encephalitis, has increased with the discovery of mosquitoes testing positive for the virus.

The mosquitoes collected in Bridgewater, about 25 miles south of Boston, are the first ones to have tested positive for the EEE virus in the state this year, the Massachusetts Department of Public Health reported.

In response, the risk level for Bridgewater has been raised to moderate, health officials said.

The disease, transmitted to humans by infected mosquitoes, can be fatal, and one elderly Massachusetts woman died from the disease last year, officials said.

However, human cases of EEE are rare, with an average of just six cases in the United States reported each year.

"EEE is an annual occurrence in Massachusetts," says DPH State Public Health Veterinarian Dr. Catherine Brown. "These were mammal-biting mosquitoes, and the findings should remind residents of the area to cover up and apply insect repellent when outdoors."

Symptoms of the disease include a fever that can reach 106 degrees, headache, a stiff neck and fatigue, with the onset occurring about three to 10 days following the bite of an infected insect. More serious cases can bring on convulsions, coma and eventual death.

The mosquitoes that carry EEE can also be carriers of West Nile virus, officials said.

Eastern equine encephalitis, found throughout the Western Hemisphere, was first identified in Massachusetts in 1831 when scores of horses died of the disease.

The first human cases were confirmed when as many as 30 children in the northeastern U.S. died of encephalitis in 1938 during outbreaks among horses in the region.

There is no known cure for EEE, but infections can be treated with steroid drugs and anticonvulsant medications.

A related but somewhat rarer virus causes Western equine encephalomyelitis, also spread by infected mosquitoes.

The disease is mostly found west of the Mississippi River in the United States and also occurs in South America.

It is considered less serious than EEE, with a much lower mortality rate in humans.

During periods of high risk people should take measures to reduce the risk of mosquito bites, officials said, including minimizing time outdoors at dawn or dusk when the insects are most active, keeping window and door screens in good repair, and use a good mosquito repellent following label instructions.

Those and similar measures will also be effective in protecting people from West Nile Virus also carried by some species of mosquitoes, health officials said.