HELENA (AP) -- The Montana state veterinarian on Monday confirmed the season's first case of West Nile virus in a Montana horse.

Dr. Tom Linfield said the 4-year-old horse from Blaine County had not been vaccinated against the mosquito-borne disease and had to be euthanized.

Mosquitoes in Sheridan, Prairie, Custer and Valley counties have also been confirmed with the virus this season. Infection can cause encephalitis, affecting the nervous system, and can cause severe complications and death in horses.

Vaccination and mosquito control continue to be recommended as methods to help protect horses against the virus.

''There are USDA licensed vaccines available to help prevent equine cases of WNV encephalitis, and for horses not previously vaccinated, two initial doses given three to six weeks apart are recommended," Linfield said.

For horses that contract the virus, a special treatment is also available, he said.

So far this year, no humans have been confirmed with the virus, which can also cause serious illness and even death in humans.

Only six people were reported to have contracted West Nile last year. Three were hospitalized, but none died. In 2003, West Nile infected 228 people in Montana and killed four.

Montana had two human cases in 2002, 228 in 2003 and six cases last year.