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West Nile Virus  What did we learn from last summer and how do we prepare for this summer?

Larissa Minicucci, DVM, Jeffrey Bender DVM, MS, DACVPM, Julia Wilson, DVM University of Minnesota, College of Veterinary Medicine

Lessons from Last Year

West Nile virus (WNV) caught many of us by surprise last year. There was an unprecedented impact on horse, people and birds in Minnesota. It strained our veterinary response infrastructure. Veterinarians quickly had to learn about the disease, transmission, and prospects for prevention. Our clients asked intelligent questions, many of which we did not have an answer for. As we approach the next season, here are a few lessons learned and some recommendations for the upcoming season.

Last year nearly 1000 horses were reported to the Minnesota Board of Animal Health and had serologic evidence of WNV. These diagnosed cases were based on serum IgG or tissue positive samples. We know that there were likely many more cases that were never seen by a veterinarian, tested, or reported. The actual number of cases is likely much higher. Across the nation and nearly 15,000 cases from 40 states were reported. The hardest hit areas included much of the Midwest from Texas to Minnesota and North Dakota.

Also there were over 300 confirmed cases in birds in Minnesota. Most of these WNV positive birds were crows and blue jays, but a number of other birds were affected including raptors. Furthermore, the Minnesota Department of Health reported 48 human cases with fortunately no deaths.

WNV was also found in a number of other species including dogs, reindeer, sheep, and squirrels across the nation. It still appears that most animals besides humans, birds of the Corvidae family, and horses are generally resistant to infection.

In reviewing the reported horse cases there are some interesting lessons. The predominant clinical signs included ataxia (50%), tremors (24%), depression (20%), fever (18%), and recumbancy (16%). The average age of horses affected was about 12 with about an equal distribution of cases among mares compared to geldings and stallions. Thirty-eight percent of cases died as a result of infection or were euthanized. The peak incidence of cases occurred in late August, with the earliest confirmed case in late July. Cases were diagnosed as late as early November. Most cases were not vaccinated or adequately vaccinated. Our initial findings support that vaccination was helpful in preventing severe illness and death.

Why were there so many cases last year? There are likely several factors. This includes favorable climatic conditions conducive for mosquito populations, a compentent vector, likely Culex tarsalis, and a susceptible equine population. As part of on-going efforts, the University Minnesota, the Minnesota Department of Health and the Minnesota Board of Animal Health will continue to work together to try an understand the ecology of disease transmission. Again, this summer the University will try to contact veterinarians and horse owners who have suspect cases. We are continuing to evaluate the long-term efforts on horses, better understand the likely mosquito vectors responsible for transmission, and possible adverse events from West Nile vaccine use. Your assistance is greatly appreciated. If you have questions about these activities, you can contact Jeff Bender DVM, MS at bende002@umn or (612) 625-6203.

Recommendations for the upcoming season

Currently, we are encourageing horse owners to work closely with their veterinarian regarding vaccination and other preventive measures to reduce the risk of WNV. Because rabies is also a risk, all horses should be evaluated. WNV testing is available at the Minnesota Veterinary Diagnostic Laboratory. The diagnostic laboratory can test serum for acute infection (IgM) and tissue from horses that have recently died. If you are trying to evaluate vaccine titers, serum should be sent to the National Veterinary Services Laboratory

Prevention measures include vaccination, elimination of mosquito-breeding habitat, and the use of approved insecticides. Mosquito-breeding habitat includes old tires, stagnant water, and clogged gutters. This may mean periodic changing of water in water buckets and containers (ideally every 5 to 7 days) or placing mosquito-eating fish in water containers. Also, if possible, house horses in screened areas during peak hours of mosquito activity (dusk and dawn). There are a number of insecticide products available. Some of these are variable in there effect and length of activity. Use according to label instructions. We strongly recommend vaccinating horse. As our on-going effort to understand the impact of WNV, we are also inquiring about vaccination status and any adverse reaction. Your help in these efforts are greatly appreciated.

For further and more detailed information, please consult the following web sites:

Local sources of information

- West Nile in Dogs and Cats
- College of Veterinary Medicine
- Centers for Disease Control and Prevention
- United States Department of Agriculture
- Minnesota Department of Health
- Minnesota Board of Animal Health

Updated: 6/16/03
Minnesota Veterinary Medical Association
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