25 Cases of EEE Confirmed in Wisconsin Horses

Twenty-five cases of Eastern equine encephalitis (EEE) have been confirmed in Wisconsin horses since mid-August, mostly in north central Wisconsin, State Veterinarian Robert Ehlenfeldt, DVM, said Friday (Sept. 23). It has been detected in Price, Lincoln, Taylor, Clark, Marathon, and Dunn counties to date.

A viral disease, EEE affects the central nervous system and is transmitted to horses by infected mosquitoes. The fatality rate for EEE in horses is 75-95%. The course of EEE can be swift, with death occurring two to three days after onset of clinical signs despite intensive care. Horses that survive might have long-lasting impairments and neurologic problems.

Clinical signs of EEE include moderate to high fever, depression, lack of appetite, cranial nerve deficits (facial paralysis, tongue weakness, difficulty swallowing), behavioral changes (aggression, self-mutilation, or drowsiness), gait abnormalities, or severe central nervous system signs, such as head-pressing, circling, blindness, and seizures.

Ehlenfeldt advised horse owners to call a veterinarian if their horses show any signs of central nervous system disease: depression, loss of appetite, drooping eyelids and lower lip, aimless wandering and circling, blindness, and sometimes paralysis.

"EEE looks a lot like other diseases of the central nervous system--other strains of encephalitis, West Nile virus, equine herpes virus-1, even rabies," Ehlenfeldt said. "That's why it's important that you call a vet and get a definite diagnosis if possible. You need to know if other animals are at risk, or if there's a human health risk, because the same mosquitoes that infect horses may infect humans in some cases."

More cases are likely to be confirmed, Ehlenfeldt said. He noted that fairly warm weather is predicted for next week (in Wisconsin) that might allow mosquitoes to be active. The risk remains until overnight temperatures routinely fall below freezing. In addition, some preliminary results are still awaiting confirmation at the National Veterinary Services Laboratories.

Ehlenfeldt noted It might be too late for initial vaccines to be effective-- it takes several weeks for immunity to build up--but boosters are effective in a matter of days for horses that have already been vaccinated, so owners should consider that option. Owners should also eliminate any standing water that they can, and keep horses in the barn between dusk and dawn if possible, to limit their exposure to mosquitoes.

Wisconsin last experienced a large-scale outbreak of EEE in 2001, when 69 cases were confirmed. Since then, cases have occurred sporadically.

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