West Nile Virus confirmed in Polk County horse

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Saint Paul, Minn - West Nile Virus has been diagnosed in a 33 year old American Quarter Horse mare in Euclid, Minnesota. The attending veterinarian reported that on October 9 the horse presented with incoordination and exhibited weakness in both hind legs. The horse responded to supportive care to control pain and inflammation, and showed significant improvement in clinical signs within a week. Laboratory results confirmed West Nile Virus on October 23. The horse’s vaccine history is unknown prior to 2015 when the reporting veterinarian began vaccinating the mare annually, with the most recent vaccine being administered this spring.

“Vaccination is the primary method of reducing the risk of infection from West Nile Virus in horses,” said Dr. Courtney Wheeler, senior veterinarian in charge of the equine program at the Minnesota Board of Animal Health. “Although clinical disease may not be completely prevented, multiple studies indicate vaccination is effective, and veterinarians are encouraged to recommend it as a core vaccine for all horses.”

This is the first confirmed case in a Minnesota horse since August 2016. Positive test results for West Nile Virus must be reported to the Board of Animal Health. This year West Nile Virus has been confirmed in 246 horses in 37 states including Michigan, Iowa, Wisconsin and North Dakota and the Minnesota Department of Health has documented 23 human cases.

West Nile Virus is a zoonotic disease spread by mosquitoes, and thought to cycle between mosquitoes and birds. Mosquitoes contract the virus from birds and then spread it to mammals (and reptiles), most commonly humans and horses. Infected horses can become anorexic, depressed and show neurologic signs or behavior changes. The incubation period in horses is between three and 15 days. Many infected horses are asymptomatic. Clinical signs can be similar to those seen with other neurologic diseases including rabies.

Horse vaccines are available and have been used extensively, contributing to the decline in the number of affected horses. Additionally, disease risk can be reduced by limiting exposure to mosquitoes. This can be accomplished by changing horses’ drinking water regularly, mowing tall grass, draining stagnant water, maintaining screens and installing fans at horse stables, and using insect repellants on both horses and people. Also, mosquitoes primarily feed at dawn and dusk, and keeping horses indoors during these times can reduce risk.