HELENA — The first equine case of West Nile virus of the year in Montana was confirmed in a horse in Yellowstone County, state veterinarian Tom Linfield said late Tuesday.

While the horse marks the first equine case of the virus, state health officials discovered the first case of West Nile virus this year on Aug. 4 when six pelicans nesting at a waterfowl refuge in northeastern Montana tested positive for the virus.

Since then, another seven pelicans at the refuge and a blackbird in Richland have tested positive for the virus. Mosquitoes have tested positive from samples taken in Custer County.

State officials say the virus may be here to stay. The disease surfaced in eastern Montana in August 2002.

‘‘We may always see a few cases of it because it's in the bird population,'' said Karen Cooper, spokeswoman for the Department of Livestock.

The horse in Yellowstone County, which is still alive and seems to be responding to treatment, had not been vaccinated against the virus, state officials said. Linfield said equine vaccines can protect healthy horses from the possibly fatal virus.

In 2002, a total of 134 Montana horses were diagnosed with the disease, and 38 of those horses died or were euthanized. In 2003, a total of 193 horses were diagnosed, and 70 of those horses died or were euthanized.

There were only two human cases of West Nile virus in Montana in 2002, but there were 228 human cases last season. Last year, four human deaths were attributed to West Nile virus.

Although the virus can cause serious illness in humans, about 80 percent of people who are infected notice no symptoms and about 20 percent experience mild flu-like symptoms. About 1 in 150 people infected with West Nile develops severe illness, and about one in 1,000 cases is fatal, state health officials said.

West Nile virus affects the nervous system and can cause severe complications and death in horses. The virus is spread through the bite of an infected mosquito, not from horse to horse, from horse to person, or from person to person.

Because the virus is spread by mosquito bites, the simplest form of prevention is to avoid mosquitoes by wearing insect repellant containing DEET. People are also advised to wear long-sleeved pants and shirts when outdoors, ensure doors and screens have no holes where mosquitoes can enter a structure and drain any standing water where mosquitoes can breed.