WNV in Northern Indiana

West Nile virus (WNV) was first detected in Indiana horses in 2001. In 2002, the state reported 704 cases, the fifth-largest outbreak in the United States. Michael Ward, BVSc, MSc, MPVM, PhD, from Purdue University's School of Veterinary Medicine, and colleagues investigated the 2002 outbreak of WNV in northern Indiana horses.

The purpose of the case series that was published was to describe the specific features of WNV in horses during the outbreak, vaccination status of affected horses, clinical signs, presence of anti-WNV antibody, case fatality rate, and local climate data. These data were then used to better describe conditions under which WNV strikes this region, and when horses should be vaccinated against WNV for maximal protection.

During the 2002 outbreak, 170 horses in three northern counties became ill with encephalitis-like signs. The first case occurred in early August and the last in mid-October, with the peak on Sept. 7. Of the 170 ill horses, eight had an incomplete vaccination history just prior to the outbreak and the remaining 162 were unvaccinated. WNV antibodies were isolated from the serum of 136 of the 162 non-vaccinated horses. Ward explains, "These are IgM antibodies. They are not protective. They only indicate exposure to the virus."

The most common clinical signs included ataxia (incoordination, 44%), hind limb paresis or severe weakness (35%), muscle fasciculations or tremors (35%), and recumbency (7.6%). Fever was an uncommon finding. By the end of the outbreak, 31of 136 unvaccinated or incompletely vaccinated horses died or were euthanized (22.8%).

Interestingly, a significant correlation was detected between mean daily minimum/maximum temperatures and number of WNV cases. Temperatures prior to the outbreak's peak were higher than after the peak. Ward explains, "The mosquito species that maintain WNV are those that feed on birds and horses. When migratory birds introduce WNV to an area and the climate is suitable for mosquito proliferation, and there is a population of susceptible horses, a WNV encephalitis outbreak can occur."

It was concluded that the peak risk period for WNV encephalitis in horses in northern Indiana is mid-August to mid-September. Because of the high case fatality rate, it was recommended that previously unvaccinated Indiana horses should be completely vaccinated against WNV to provide full protection four months prior to the anticipated peak of the WNV risk period (two vaccines are required, given three to six weeks apart, in unvaccinated horses). Previously vaccinated horses should be vaccinated two months prior to this peak.

Ward is currently working with researchers and state officials in Texas to use landscape characteristics and climate to forecast high-risk areas for equine WNV disease.

Ward, M.P.; Levy, M.; Thacker, H.L.; et al. Journal of the American Veterinary Medical Association, 225(1): 84-89, 2004.

About the Author

Susan Piscopo, DVM, PhD

Susan Piscopo, DVM, PhD, is a free-lance writer in the biomedical sciences. She practiced veterinary medicine in North Carolina before accepting a fellowship to pursue a PhD in physiology at North Carolina State University. She lives in northern New Jersey with her husband and two sons.

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