Rain Brings Out Mosquitoes, Heightened Equine Disease Risk

Heavy rains over the past few weeks have led to an explosive mosquito population, 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 cooler spring delayed a mosquito outbreak in parts of the state, but now that things are heating up it creates the perfect breeding environment for mosquitos,” says Assistant State Veterinarian Darlene Konkle, DVM, of the Wisconsin Department of Agriculture, Trade, and Consumer Protection. “Cases of EEE have now emerged in southern states and WNV has already been detected in a bird in Wisconsin."

Horses require two doses of the vaccination initially, and then boosters at least annually. "We recommend working with your veterinarian, so you get the best plan for your horse and advice about additional boosters later in the season," Konkle says.

Both WNV and EEE are caused by viruses transmitted by mosquitoes, and both can cause encephalitis, or inflammation of the brain. EEE kills about 90% of horses that it strikes, and WNV kills in more than a third of all equine cases. Clinical signs are similar for both diseases and include depression, appetite loss, drooping eyelids and lower lip, fever, weakness, twitching, paralysis or lack of coordination, aimless wandering, circling, and blindness.

Neither virus is contagious between horses. While humans can become infected by both WNV and EEE, it does not pass between people and horses. Mosquitos biting warm-blooded animals is the only route of transmission.

Besides vaccination, Konkle 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, and 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.
  • Consider keeping horses in the barn when mosquitoes are most active.
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