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

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Abstract
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. ( J Am Vet Med Assoc 2004;225:1084–1089)