The first case of West Nile virus in Montana for 2018 was confirmed Friday in a horse near Roundup.

Dr. Maryrose Beasley, with the Homestead Veterinary Service in Roundup, got a call Friday morning from the state veterinarian's office verifying that a horse she had treated was the first in the state this year to test positive for the virus. 

The horse, located several miles southwest of town, was weak and stumbling around, Beasley said, when the owner called her to examine him on Monday. 

"He was trying to come down a hill to the water trough and he almost took a nose dive," she said.

Beasley, who previously treated cases of West Nile in horses, recognized the symptoms and began treatment. She sent a blood sample into the state lab at the Department of Livestock.

Carried by mosquitoes, West Nile virus typically surfaces in Montana in August and September. Horses with the virus act a little drunk, Beasley said. They may seem like they've been kicked in a front leg or have suffered a break or a sprain.

"It affects the whole nervous system," she said, adding that when the virus is caught early, horses can be successfully treated.

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In humans, the virus can cause a mild fever, headaches and body aches, according to the U.S. Centers for Disease Control and Prevention. More rarely, it can cause neuroinvasive disease, with symptoms including convulsions and paralysis in extreme cases.

Severe illness can occur in anyone, but people older than 50 are at highest risk for severe infection. There is no human vaccine against the virus.

Horses can be vaccinated, and that should be done annually, Beasley said. 

"If (owners) haven't been keeping up on it, horses should get one and then in two weeks, booster it," Beasley said.

Mosquitoes carrying West Nile virus are more likely to be active at dawn and dusk. The CDC recommends wearing long-sleeve shirts and pants during those hours.

Draining sources of standing water, like planters, wading pools and bird baths is a good idea. Using insect repellent with DEET can also deter mosquitoes.

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