EEE in Wisconsin: Officials Say to Take Precautions

Eastern equine encephalitis has struck again in northwest Wisconsin, two years after a major outbreak that killed at least 42 horses (see article #2852), prompting the acting state veterinarian to urge horse owners again to vaccinate and re-vaccinate their animals.

Tests at the Wisconsin Veterinary Diagnostic Laboratory and the National Veterinary Services Laboratory showed that an 8-year-old Belgian mare died in Turtle Lake on July 26 from the mosquito-borne Eastern equine encephalitis (EEE). The horse had been vaccinated for EEE two days earlier, after another horse on the same farm had died. That horse was not tested. Two remaining horses on the farm are well so far.

"Just like West Nile virus, EEE is preventable in horses with vaccination," said Robert Ehlenfeldt, DVM, acting state veterinarian. "The vaccination is effective, and you ought to be vaccinating your animals every year--preferably in the spring before the first mosquito hatch. Most of the EEE vaccine manufacturers also direct on their labels that horses should be re-vaccinated in the event of an outbreak. This was a case of a horse being vaccinated too late, but we could have a long mosquito season ahead of us. 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."

The EEE vaccine requires 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. If animals have already been exposed, the vaccine is not effective.

Like West Nile virus, EEE is caused by an "arbovirus"--short for "arthropod-borne." Mosquitoes carry these viruses, which in horses often develop into encephalitis, which is an inflammation of the brain. However, EEE is more dangerous to horses, with a mortality rate of 90-100 percent. The mortality rate for WNV in horses is about 30 percent. Arboviruses are not contagious between people, or between horses--only from mosquito to warm-blooded animal. Mules, donkeys and birds can also be infected.

The southeastern United States has been experiencing a major outbreak of EEE this summer, with hundreds of horses dying and some human cases reported. Two years ago, Wisconsin experienced its first major outbreak of the disease in memory. Confirmed cases of EEE were found in Ashland, Barron, Bayfield, Chippewa, Forest, Marathon, Polk, Rusk, Sawyer, Shawano, St. Croix, Taylor, Vernon, Washburn, and Winnebago counties in 2001; most of the cases were in northwestern counties, however.

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