West Nile Virus Update

The devastating effects of West Nile Virus were greatly reduced this year, thanks in part to multiple factors. Active vaccination programs, mosquito control and a dry summer with lower mosquito numbers and the development of natural immunity have all played a positive role in keeping the number of infected horses lower this year.

In 2002 Minnesota had 992 reported cases and Wisconsin had 156. In 2003 Minnesota only reported 76 positive cases , 7 of those in Washington County. Wisconsin reported 52 positive cases in 2003. Our practice diagnosed 3 positive cases, all of which were either not vaccinated or overdue for their vaccinations.

Many of you may have heard recent stories that suggested the West Nile Virus vaccine may cause pregnant mares to abort or give birth to deformed foals. As a result, the USDA (not Fort Dodge Animal Health, the vaccine company) issued a statement that horse owners should be assured that the vaccine is safe and it should be used as protection against West Nile virus. Over 5 million doses of the vaccine have been used since its release. To date, there have been a very small number of reports regarding a possible association between the use of West Nile virus vaccine and reactions of any kind. The threat of your horse contracting West Nile virus is real, with 14,717 cases reported in 2002 and an estimated 40% of those cases ending in death. The vaccine is 95% protective with a reported reaction rate of 1:10,000 horses.

Our current vaccination recommendations remain the same. An initial vaccination should be given followed by a booster in 3-6 weeks and then revaccination every 4-6 months during the mosquito season. For horses previously vaccinated, a yearly booster before the onset of mosquito season and then revaccination in 4-6 months is recommended. Pregnant mare vaccination is not recommended until after day 35 of gestation. A booster should be given 4-6 weeks prior to foaling to pass the immunity on to the foal. Foal vaccination should start at 8 weeks of age.