Tests show the eight-year-old Belgian mare that died in Turtle Lake late last month had Eastern equine encephalitis. The state says the horse had been vaccinated two days earlier after another horse on the farm died. That horse was not tested for the virus.
Two years ago an equine encephalitis outbreak killed at least 42 horses.
Acting state veterinarian Robert Ehlenfeldt is urging horse owners to again vaccinate and re-vaccinate their animals.
The southeastern United States has been experiencing a major outbreak of the virus this summer with hundreds of horses dying and some human cases reported.