First Confirmed Equine WNV Case in Montana for 2017

First Confirmed Equine WNV Case in Montana for 2017

West Nile is transmitted to horses via bites from infected mosquitoes.


The Montana Department of Livestock has confirmed the first equine case of West Nile virus (WNV) for the year in Yellowstone County. This follows detection of the virus in mosquito surveillance pools from Custer, Prairie, Blaine, Hill, and Phillips counties.

Montana typically sees WNV cases through late summer and into fall. In 2016 Montana confirmed eight equine WNV cases in seven counties, according to USDA Animal and Plant Health Inspection Service statistics.

“There is no direct treatment for the virus, but vaccination is highly effective in preventing disease,” said Tahnee Szymanski, DVM, assistant state veterinarian. “Horses that are vaccinated rarely die or are euthanized because of the disease.”

West Nile is transmitted to horses via bites from infected mosquitoes. Clinical signs of WNV include flulike signs, where the horse seems mildly anorexic and depressed; fine and coarse muscle and skin fasciculation; hyperesthesia (hypersensitivity to touch and sound); changes in mentation (mentality), when horses look like they are daydreaming or "just not with it"; occasional somnolence (drowsiness); propulsive walking (driving or pushing forward, often without control); and "spinal" signs, including asymmetrical weakness. Some horses show asymmetrical or symmetrical ataxia. Equine mortality rate can be as high as 30-40%.

Health Alert: West Nile Virus

Horses that have been vaccinated in past years will need an annual booster shot; in areas with a prolonged mosquito season, veterinarians might recommend two boosters annually—one in the spring and another in the fall. However, if an owner did not vaccinate their animal in previous years, the horse will need the two-shot vaccination series within a three- to six-week period.

“Vaccination is typically administered in the spring, but may offer some protection even this late in the season,” said Syzmanski. “Work with your veterinarian to determine if your horse could still benefit from a vaccination.”

In addition to vaccinations, horse owners also need to reduce the mosquito populations and their possible breeding areas. Recommendations include removing stagnant water sources, keeping animals inside during the bugs’ feeding times (typically early in the morning and evening), using fans to disrupt mosquitoes’ flight patterns, and applying mosquito repellents approved for equine use.

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