Rain Increases Risk of WNV, EEE among Wisconsin Horses

After nearly two months of drought conditions, mosquitoes have wasted little time hatching after recent rains, which elevates the need for horse owners to vaccinate their animals against Eastern equine encephalitis (EEE) and West Nile virus (WNV), two mosquito-borne diseases that together have stricken hundreds of horses in Wisconsin since 2001.

"The drought we have experienced in the last couple of months may have given us a false sense of security when it comes to mosquitoes," says State Veterinarian Robert Ehlenfeldt, DVM, of the Wisconsin Department of Agriculture, Trade, and Consumer Protection. "Cases have now emerged in Illinois and Michigan, demonstrating that the viruses may be ready to re-emerge in Wisconsin."

Horses require two doses of the vaccination initially, and then boosters at least annually. "We don't recommend administering the vaccinations yourself," Ehlenfeldt says. "Work with your veterinarian, so you get the best formulation for your horse and advice about additional boosters later in the season."

Clinical signs for WNV include flulike signs, where the horse seems mildly anorexic and depressed; fine and coarse muscle and skin fasciculation (twitching); hyperesthesia (hypersensitivity to touch and sound); changes in mentation (mentality), when horses look like they are daydreaming or "just not with it"; occasional somnolence (drowsiness); propulsive walking (driving or pushing forward, often without control); and "spinal" signs, including asymmetrical weakness. Some horses show asymmetrical or symmetrical ataxia. Equine mortality rate can be as high as 30-40%.

A viral disease, EEE affects the central nervous system and is transmitted to horses by infected mosquitoes. Its fatality rate 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.

Neither of the viruses is contagious between horses. While humans can also be infected by both WNV and EEE, the disease is not transmissible between people and horses. Mosquitos biting warm-blooded animals is the only route of transmission.

Besides vaccination, Ehlenfeldt recommends taking other steps to limit horses' exposure to mosquitoes:

  • Remove items from surrounding property that could collect stagnant water such as old tires, tin cans, plastic containers;
  • Keep rain gutters clean and draining properly;
  • Clean and chlorinate swimming pools, outdoor saunas, and hot tubs, and drain water from pool covers;
  • Turn wading pools and wheelbarrows upside down when not in use;
  • Empty and replace water in birdbaths at least once a week; and
  • Consider keeping horses in the barn from dusk to dawn, when mosquitoes are most active.
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