The Washington Animal Disease Diagnostic Laboratory (WADDL) has confirmed 10 cases of West Nile fever in horses since Aug. 9.

The laboratory in Washington State University’s (WSU) College of Veterinary Medicine has tested 38 horses since Jan. 1. All 10 cases confirmed positive recently were in unvaccinated horses in Spokane, Lincoln, and Pend Oreille counties in Washington and Boundary County in northern Idaho. All 10 had neurologic disease that occurs with West Nile encephalitis characterized by inflammation of the central nervous system.

The cases are unrelated to the equine herpesvirus-1 case confirmed by WADDL  in a hospitalized horse at WSU’s Veterinary Teaching Hospital. The EHV-1 horse has since been humanely euthanized.

The reason for the spike in West Nile cases is unknown; however, the recent cases are located farther north than detections in previous years