OBJECTIVE: To determine signalment, clinical findings, results of diagnostic testing, outcome, and postmortem findings in horses with West Nile virus (WNV) encephalomyelitis. DESIGN: Retrospective study. ANIMALS: 46 horses with WNV encephalomyelitis. PROCEDURE: Clinical data were extracted from medical records of affected horses. RESULTS: On the basis of clinical signs and results of serologic testing, WNV encephalomyelitis was diagnosed in 46 of 56 horses with CNS signs. Significantly more males than females were affected. Increased rectal temperature, weakness or ataxia, and muscle fasciculations were the most common clinical signs. Paresis was more common than ataxia, although both could be asymmetrical and multifocal. Supportive treatment included anti-inflammatory medications, fluids, antimicrobials, and slinging of recumbent horses. Results of the IgM capture ELISA and the plaque reduction neutralization test provided a diagnosis in 43 horses, and only results of the plaque reduction neutralization test were positive in 3 horses. Mortality rate was 30%, and 71% of recumbent horses were euthanatized. One horse that had received 2 vaccinations for WNV developed the disease and was euthanatized. Follow-up communications with 19 owners revealed that most horses had residual deficits at 1 month after release from the hospital; abnormalities were resolved in all but 2 horses by 12 months after release. CONCLUSIONS AND CLINICAL RELEVANCE: Our findings were similar to those of previous WNV outbreaks in horses but provided additional clinical details from monitored hospitalized horses. Diagnostic testing is essential to diagnosis, treatment is supportive, and recovery rate of discharged ambulatory horses is < 100%.
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