The Minnesota Board of Animal Health confirmed that a 25-year-old Swift County mare’s owners reported the horse ill in the evening and, by the following morning, she was unable to rise due to the quick progression of neurological signs. The attending veterinarian euthanized the mare due to her clinical signs. The horse had no documented history of West Nile virus (WNV) vaccination.
A blood sample submitted to the University of Minnesota Veterinary Diagnostic Laboratory (MVDL) by the veterinarian and tested using a Plaque Reduction Neutralization Test (PRNT) confirmed exposure to WNV.
Three weeks before this case, another horse on the same property showed similar clinical signs and was euthanized but not tested for WNV. Three horses still on the property appear healthy, and the owner indicated intent to vaccinate them.
Minnesota officials recently confirmed two other horses in the state with deadly insect-borne diseases: a 14-year-old Belgian mare in Otter Tail County (west-central Minnesota) with Eastern equine encephalitis (EEE) and a 10-year-old Pine County (east-central Minnesota) Quarter Horse gelding with equine infectious anemia (EIA).
WNV transmission occurs when infected mosquitoes feed on animals, as well as humans, after having fed on infected birds.
Clinical signs of WNV in horses include:
- Mild anorexia and depression
- Fine and coarse muscle and skin fasciculation;
- Hyperesthesia (hypersensitivity to touch and sound);
- Changes in mentation (mentality), when horses look like they’re daydreaming or “just not with it”;
- Occasional drowsiness;
- Propulsive walking (driving or pushing forward, often without control); and
- Spinal signs, including asymmetrical weakness; and
- Asymmetrical or symmetrical ataxia.
West Nile virus has no cure; however, some horses can recover with supportive care. Equine mortality rates can reach 30-40%. The American Association of Equine Practitioners includes WNV as one of the core diseases all horses should be vaccinated against at least annually.
