Murray Valley encephalomyelitis in a horse.

A 5-year-old Thoroughbred mare presented with signs of severe pain and was taken to exploratory laparotomy based on suspicion of an acute abdominal lesion. A mild gastrointestinal lesion was discovered, but was considered disproportional to the severity of signs displayed. The mare was later euthanased because of intractable pain. Comprehensive postmortem examination, including polymerase chain reaction testing of central nervous system tissue samples, allowed a definitive diagnosis of Murray Valley encephalomyelitis to be made. This case demonstrates the variability of clinical presentations in horses infected with Murray Valley encephalitis virus.