As most parts of the nation are in the midst of the mosquito-borne disease season, the USDA Animal and Plant Health Inspection Service (APHIS) is keeping a running tally of the number of confirmed cases of West Nile virus (WNV), Eastern equine encephalitis (EEE), and Western equine encephalitis (WEE) reported within the United States.

In the tally, which was last updated Sept. 16, APHIS' National Animal Health Surveillance System (NAHSS) reports that 28 cases of WNV have been confirmed in the U.S. in 2011. Leading the way in case counts are California and Pennsylvania, both of which have five confirmed cases in 2011. Arizona, Indiana, and Texas have all reported two cases of WNV thus far, and one case has been reported in Florida, Georgia, Iowa, Kentucky, Massachusetts, Minnesota, Mississippi, Nevada, New York, Oregon, Virginia, and Wisconsin.

In 2010 NAHSS reported 125 confirmed WNV cases in 28 states. Clinical signs of WNV include flulike conditions where the horse seems mildly anorexic and depressed; fine and coarse muscle and skin fasciculations (twitching); hyperesthesia, or 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 (incoordination on one or both sides, respectively). Equine mortality rate can be as high as 30-40%.

As of Sept. 16, NAHSS reported 23 cases of EEE since the beginning of the year. Nine cases have been confirmed in New York so far, followed by three in Florida and Louisiana. One EEE case has been reported in each of the following states: Minnesota, Mississippi, and North Carolina. At the time of the last NAHSS update, only five cases of EEE had been reported in Wisconsin. However the number of confirmed cases has since risen to 25, bringing the national total thus far to 43 cases.

In 2010 NAHSS reported 247 equine cases of EEE from 18 states. 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.

There have been no cases of WEE confirmed in the U.S. thus far in 2011.

The American Association of Equine Practitioners recommends all horses be vaccinated for WNV, EEE, and WEE as a part of their core vaccination regimen. In addition to vaccination owners can take other steps to minimize the mosquito populations near horses by eliminating their breeding and resting areas. For example, reduce or eliminate sources of stagnant or standing water, remove muck from areas near the horses, stable your horses during peak mosquito periods (i.e., dawn and dusk), use equine-approved mosquito repellants, place fans inside the barns or stalls to maintain air movement (as mosquitoes don't fly well in wind), and avoid using incandescent bulbs inside stables at night. Instead, place incandescent bulbs away from the stables. This will attract the mosquitoes to areas outside the stables.

About the Author

Erica Larson, News Editor

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More