Horse News & Views - October 2002

Ministry of Agriculture, Food and Rural Affairs

Ontario Association of Equine Practitioners

• A number of clients are inquiring how to diagnose or confirm West Nile Virus (WNV) in their horse(s). The signs exhibited in horses with WNV are similar to those seen in horses suffering from a number of equine diseases, including rabies, eastern equine encephalitis and wobblers syndrome. Signs of illness may include ataxia, difficulty walking, knuckling over, head tilt, muscle twitches or tremors, inability to stand, circling, weakness or paralysis of limbs, apparent blindness, lip droop, grinding teeth and death. In a case where a horse dies or is euthanized, rabies must be ruled out. The Canadian Food Inspection Agency must be notified of a rabies suspect. Confirmation of a diagnosis of WNV in a live horse requires that two blood samples be taken (an acute and a convalescent) three weeks apart and submitted together to the laboratory. Confirmation of WNV in a dead horse requires that a sample of the brain stem, either chilled or frozen, be submitted to the laboratory.
• Equine arteritis virus (EAV), the causative agent of equine viral arteritis (EVA), is occasionally diagnosed in horses. EAV causes an influenza-like illness in adult horses, abortion in mares and interstitial pneumonia in foals. Severe edematous swelling of the legs and scrotum and an urticarial reaction of the skin are commonly seen. The virus is contagious by the aerosol route from foals, mares and geldings for less than 14 days. Stallions, however, persistently shed the virus in their semen. Persistently infected stallions are the critical reservoirs of the virus. (Hullinger et al. Journal of the American Veterinary Medical Association 2001; 219 (2): 946-952)