Unfortunately, clinical signs of West Nile virus (WNV) aren’t unique, making it difficult for practitioners to suspect WNV above other neurologic diseases. For this reason, researchers from the University of Florida set out to closely scrutinize the clinical signs of WNV to see if any are particularly unusual. Records of 46 horses admitted to the hospital in 2001 with confirmed cases of WNV encephalitis were reviewed to try to find features that set this disease apart.

The most common clinical signs were weakness and/or ataxia (100% of horses), fever (65%), and muscle fasciculations (twitching; 60%). Fourteen horses (35%) became recumbent from the illness, and 10 of these had to be euthanized. The overall mortality rate was 30%, compared to a rate of 38% across the United States in that year. Importantly, 18 of the 46 horses (39%) had previously received one WNV vaccination, and one horse had received two vaccinations. The most relevant clinical findings were fever, which differentiates WNV from EPM (equine protozoal myeloencephalitis) and cervical myelopathy, and the consistent findings of weakness and/or ataxia, coupled with muscle fasciculations.

Porter, M.B.; Long, M.T.; Getman, L.M.; et al. Journal of the American Veterinary Medical Association, 222 (9), 1241-1247, 2003