Wisconsin Horses Positive for EEE, WNV

A horse in Dodge County, Wisc., has died from Eastern equine encephalomyelitis (EEE), a mosquito-borne disease that is easily prevented by vaccination.

A yearling Quarter Horse died from EEE in Juneau July 28, one day after it showed neurologic signs consistent with the virus. The horse had not been vaccinated against the disease and the diagnosis was confirmed via laboratory testing.

There have also been two cases of West Nile virus, another mosquito-borne disease, in Wisconsin horses this summer. One case occurred in Chippewa Falls and one in Markesan (Green Lake County). Neither of those animals was vaccinated, and one died.

"Equine encephalomyelitis and West Nile virus infection are easily preventable," said Wisconsin State Veterinarian Robert Ehlenfeldt, DVM. "The vaccines are effective, and you should be vaccinating your animals every year--preferably in the spring before the first mosquito hatch.

"Even in areas where we don't normally have a lot of mosquitoes, there's been a bumper crop this year. We know that West Nile virus is going to show up every summer, and now we know we have mosquitoes carrying the EEE virus. Mosquitoes stick around until the first killing frost, so we could have months ahead of us when there's a risk of mosquito-borne illness. If you haven't vaccinated your horses for both EEE and WNV, do it now. If you've already vaccinated them, consult your veterinarian about boosters," Ehlenfeldt said.

Initially, the vaccines require two doses two to four weeks apart. Previously vaccinated horses develop protective antibodies within four days of being re-vaccinated. Horses that have never been vaccinated need two weeks to build up enough antibodies to protect them, Ehlenfeldt said.

"We always recommend that veterinarians administer vaccinations and get a look at the animal to check for clinical signs--especially this late in the season, when most mosquito-borne infections could already be present," Ehlenfeldt said. If a horse is already infected, a vaccine will not be effective.

Along with vaccination, Ehlenfeldt recommended removing standing water where mosquitoes breed, as well as keeping animals inside from dusk to dawn as the best means of preventing WNV and EEE.

EEE is less common, but more dangerous to horses, with a mortality rate of more than 90%. The mortality rate for WNV in horses is 30-35%.

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