BENSON - A Minnesota veterinarian reported suspected West Nile virus (WNV) in a 25-year old mare in Swift county. Clinical signs of neurologic illness progressed very quickly, the Minnesota Department of Health reported on Monday.

The mare’s owners reported the horse fell ill in the evening and was unable to rise the next morning. The horse was euthanized by the attending veterinarian who submitted a blood sample to the University of Minnesota Veterinary Diagnostic Laboratory for West Nile Virus testing. Test results confirmed exposure to WNV (a Plaque Reduction Neutralization Test (PRNT) was positive). The horse had no documented history of vaccination against WNV. Three weeks prior to this case, another horse on the property presented with similar clinical signs and was euthanized but not tested for WNV. Three horses remaining on the premises appear healthy at this time and the owner plans to vaccinate them.

West Nile virus is regularly found in the United States, and birds serve as the primary host of the disease. Similar to EEE, this virus circulates between infected birds and mosquitoes. Once infected, mosquitoes can transmit the virus to horses or people. The virus can cause encephalitis, an inflammation of the brain and spinal cord. Infected horses may or may not show neurological symptoms and many recover completely, especially those who have a history of annual vaccination. While human WNV cases are reported in Minnesota every year, no human cases of WNV have been confirmed in the state in 2019 as of this release.

Vaccines for horses are widely available and have been proven to be effective in preventing infection. Steps can also be taken to reduce disease risk by reducing mosquitoes.

  • Change water in drinking troughs every week.
  • Mow long grass.
  • Drain stagnant water puddles.
  • Remove items mosquitoes use for breeding grounds, like old tires and tin cans.
  • Place and maintain screens over windows and stable doors.
  • Use mosquito repellents to protect horses and people from mosquito bites.

See annual maps of positive cases in horses on the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service website.

This is a high-risk time of year for West Nile virus transmission. People are at risk too, and the Minnesota Department of Health (MDH) advises wearing repellent, especially if you’re outside at dawn or dusk. Learn more about the virus on the MDH website.

Non-negative test results for West Nile virus disease, Eastern equine encephalitis and Equine infectious anemia must be reported to the Board of Animal Health (domestic animals) and Minnesota Department of Health (humans).