Evaluation of an outbreak of West Nile virus infection in horses: 569 cases (2002)

OBJECTIVE: To characterize an outbreak of West Nile virus (WNV) infection in horses in North Dakota in 2002, evaluate vaccine effectiveness, and determine horse characteristics and clinical signs associated with infection. DESIGN: Retrospective study. ANIMALS: 569 horses. PROCEDURE: Data were obtained from veterinary laboratory records, and a questionnaire was mailed to veterinarians of affected horses. RESULTS: Affected horses were defined as horses with typical clinical signs and seroconversion or positive results of virus isolation; affected horses were detected in 52 of the 53 counties and concentrated in the eastern and northeastern regions of the state. Among affected horses, 27% (n = 152) were vaccinated against WNV, 54% (309) were not, and 19% (108) had unknown vaccination status; 61 % (345) recovered, 22% (126) died, and 17% (98) had unknown outcome. The odds of death among nonvaccinated horses were 3 and 16 times the odds among horses that received only 1 or 2 doses of vaccine and horses that were vaccinated according to manufacturer's recommendations, respectively. Horses with recumbency, caudal paresis, and age > 5 years had higher odds of death, whereas horses with incoordination had lower odds of death, compared with affected horses without these characteristics. CONCLUSIONS AND CLINICAL RELEVANCE: Vaccination appears to have beneficial effects regarding infection and death caused by WNV.