Instead of West Nile virus, public health officials have found a rarer form of encephalitis in Waldo, Kennebec and York counties.

Three people in Maine and one in Vermont have contracted Powassan encephalitis, an arbovirus carried by a common tick, which is fatal in 10 percent to 15 percent of the cases diagnosed, the Centers for Disease Control and Prevention in Atlanta reported Thursday. West Nile virus is an arbovirus transmitted by mosquitoes.

“Not only do we have arbovirus, but we think it’s a very uncommon [type],” said Dr. Kathleen Gensheimer, Maine’s state epidemiologist. “We don’t know how widespread Powassan is in New England, or even in the U.S.”

Public health officials found the virus while looking for West Nile virus. Discovered for the first time in North America in 1999, West Nile virus has spread from Florida to New Hampshire. Although West Nile virus has been detected in a community bordering southern Maine, it still hasn’t been found in this state.

“If we hadn’t been looking for West Nile virus we never would have found [Powassan],” Gensheimer said.

Powassan encephalitis first was discovered in 1958 in the Maritime Provinces of Canada. Only 27 cases have been confirmed since then in North America, according to the CDC. Most were in the Maritimes.

Powassan encephalitis shares many of the symptoms of West Nile virus: muscle weakness, headache, fever, dizziness and sleepiness. It is so similar that it is clinically indistinguishable from West Nile virus, Gensheimer said. But unlike West Nile virus, which is thought to be most dangerous to older people and to small children, Powassan is a significant threat to young and middle-aged adults.

The four individuals infected in Maine and Vermont ranged from 25 to 70 years of age.

The virus is spread by the bite of woodchuck ticks, which are small and similar in appearance to deer ticks. These ticks prefer to feed off the blood of mid-sized mammals like woodchucks and skunks. They also bite domestic animals and humans.

Gensheimer said there is little data on how long a tick must be attached for the virus to be transmitted. But she said everyone should use vigilance in trying to reduce the risk of tick bites by using insect repellent, tucking pants into socks and eliminating high grass around homes.

In the latest case, from June 2001, a 70-year-old man in Kennebec County who hadn’t left the state in 25 years contracted the virus. In the weeks before he was diagnosed, the man had been working on a boat hull, and frequently was lying on the ground. The yard was overgrown with bushes and family members reported seeing skunks, squirrels and woodchucks.

The man had a fever of nearly 105 degrees when admitted to a hospital with muscle weakness, diarrhea and sleepiness. Three months after admission, he is unable to move his left arm or leg.

In the first Maine case, a 25-year-old man in Waldo County sought medical help in July 2000 while suffering from a headache, vomiting and confusion. The CDC reported he suffered from muscle weakness and twitches and had difficulty answering simple questions.

Discharged from the hospital 44 days later, the patient needed help standing and performing daily activities, the CDC reported.

In September 2000 a 53-year-old woman in York County was admitted to a local hospital complaining of dizziness, visual impairment and a 103-degree fever. She required 21/2 months of rehabilitation but was walking and had regained strength nine months later.

The earliest of the four cases was that of a 66-year-old man in Washington County, Vermont. He displayed signs of the virus in September 1999. Eleven days after discharge from the hospital he still had memory lapses and cognitive problems, the CDC reported.

With Maine’s oldest case dating back to the summer of 2000, why didn’t the Bureau of Health warn Mainers earlier about the virus?

“With one case of anything you’re never quite sure what to make of it,” Gensheimer said. “When we had the second case, there was no doubt that this was something we couldn’t ignore.”

The Bureau of Health already has suggested steps to eliminate mosquito breeding sites and to avoid being bitten in anticipation of the arrival of West Nile virus.

Now Powassan is another emerging disease for people to think about and ample reason to avoid tick bites, some of which also spread Lyme disease, she said.