Wisconsin Horse Dies from Equine Encephalomyelitis

MADISON -- A horse in Dodge County has died from Eastern equine encephalomyelitis, a mosquito-borne disease that is easily prevented by vaccination, Wisconsin State Veterinarian Dr. Robert Ehlenfeldt said today.

Tests at the Wisconsin Veterinary Diagnostic Laboratory showed that a 1-year-old quarter horse died in Juneau July 28, one day after it went down, paralyzed by the neurological infection. The horse had not been vaccinated for Eastern equine encephalomyelitis, or EEE.

Mosquitoes may also transmit EEE to humans, but it does not pass between humans and horses. Infections in humans range from a mild illness with flu-like symptoms to severe, and sometimes, fatal illness. There is no vaccination available for humans, and no human cases have been reported in Wisconsin this year.

There have also been two cases of another mosquito-borne disease, West Nile Virus, in Wisconsin horses this summer -- one 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. The vaccines are effective, and you should be vaccinating your animals every year -- preferably in the spring before the first mosquito hatch," Ehlenfeldt said. "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."

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. If animals have already been infected, the vaccines are not effective.

Most of the vaccines can be purchased from a veterinarian and injected by the horse owner, but 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."

Along with vaccination, Ehlenfeldt recommended removing standing water where mosquitoes breed and keeping animals inside from dusk to dawn as the best means of preventing West Nile virus and equine encephalomyelitis.

Mosquito-borne illnesses typically appear in horses in mid-summer to early fall. Signs in horses include depression, appetite loss, drooping eyelids and lower lip, aimless wandering and circling, blindness and sometimes paralysis, particularly in EEE. There is no cure; the infections must run their course. EEE is less common, but more dangerous to horses, with a mortality rate of 90-100 percent. The mortality rate for WNV in horses is 30-35 percent.

Wisconsin experienced a major outbreak of EEE in 2001, when 67 horses suffered from confirmed or presumed positive cases and even more were suspected. Most of the cases were in northwestern Wisconsin.

West Nile virus first struck horse in Wisconsin in 2002, when 167 cases were confirmed. Since then, the state has seen two dozen or fewer cases annually.

To protect humans from EEE and WNV, the Wisconsin Department of Health Services recommends:

• Using effective repellants on clothing and exposed skin. Mosquitoes may bite through thin clothing, so spraying clothes with a repellent containing permethrin or DEET will give extra protection. These repellants are the most effective and most studied.
• Wearing light-colored clothing, and wear long-sleeved shirts and long pants
• Removing mosquito breeding areas such as containers, tires, or other objects where water can collect
• Changing the water in bird baths every couple of days, and draining standing water from swimming pool and hot tub covers or other surfaces where it collects
• Keeping window screens in good repair
• Keeping gutters clean and drains, ditches and culverts clear of debris and weeds
• Avoiding the outdoors at dawn and dusk when mosquitoes are most active

Learn more about mosquito control. 

Source: Wisconsin Department of Agriculture, Trade and Consumer Protection press release