West Nile Virus Reported in Horses Nationwide

West Nile Virus Reported in Horses Nationwide

Studies have shown that the WNV vaccine has a substantial effect on preventing disease.

Photo: Erica Larson, News Editor

As autumn approaches, some owners are ready to start thinking about phoning their veterinarians to set up their horses' fall vaccinations. And because the number of West Nile virus (WNV) cases confirmed in U.S. horses continues to rise slowly, consider requesting a WNV booster.

According to preliminary data released Sept. 5 by the U.S. Department of Agriculture Animal Health Monitoring and Surveillance, 64 cases of WNV have been confirmed in 23 states thus far in 2013. Some recently confirmed cases include:

California—As of Aug. 16, the California Department of Food and Agriculture (CDFA) reported that two additional horses in that state had tested positive for WNV. The affected horses are "a 1-year-old unvaccinated Andalusian colt in Sacramento County and a 4-year-old Morgan mare of unknown vaccination status in Placer County. Both horses are recovering."

The CDFA's statement also indicated that a total of eight horses have been confirmed positive for WNV in 2013. They were located in Kern, Placer, Riverside, Sacramento (3), and Stanislaus (2) Counties. Four of those were euthanized.

In 2012, California confirmed 22 equine WNV cases.

Delaware—The Delaware Department of Agriculture announced Aug. 30 that a Kent County horse had tested positive for WNV, the first case in that state since 2003. Delaware State Veterinarian Heather Hirst, DVM, MS, said in a statement that the horse was recovering well.

Michigan—On Sept. 5 the Michigan Department of Agriculture and Rural Development reported WNV in two horses, one residing in Kent County and the other in Ingham County. In 2012, four Michigan horses tested positive for the disease.

New Mexico—A Sept. 5 report from the Albuquerque Journal indicates that two horses residing in Lea County tested positive for the virus. Both horses are recovering, the report said. In 2012, 10 horses in New Mexico tested positive for WNV.

Wyoming—As of Aug. 29, the Wyoming Department of Health's West Nile virus activity report indicated seven equine WNV cases confirmed in that state this year. According to the report, equine cases were confirmed in Campbell, Laramie, Natrona, Platte, and Sheridan counties. In 2012, Wyoming reported five equine WNV cases.

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%.

Studies have shown that the WNV vaccine has a substantial effect on preventing disease. The American Association of Equine Practitioners (AAEP) recommends vaccinating all foals and horses against WNV. For horses residing in the northern United States veterinarians recommend vaccinating in the spring prior to peak mosquito levels. In the south, where mosquito populations are present year-round, horses might be vaccinated more frequently. In addition to geography, age and exposure play an important role in deciding how often to vaccinate horses. The AAEP's complete vaccination guidelines are available online

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 TheHorse.com. Learn More