Pennsylvania's First Case of WNV in 2012 Confirmed

A mild winter has been pleasant for residents of the Mid-Atlantic region. It has also created a favorable environment for mosquitoes. With mosquitoes comes the potential to contract West Nile virus (WNV), a debilitating and potentially deadly disease. The first case of the season of WNV was reported last week in a horse in Northampton County, Pa.

Entomologists are predicting that weather conditions could make this a particularly heavy mosquito season, and a two-pronged approach of vaccination and management is the most successful way to avoid the disease, according to Ray Sweeney, VMD, Dipl. ACVIM, chief of medicine at New Bolton Center, the large animal campus of the University of Pennsylvania's School of Veterinary Medicine.

First confirmed in the United States' horse population in 1999, WNV infection is responsible for equine clinical signs including flulike signs, 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%. The USDA Animal and Plant Health Inspection Service reported 83 cases of WNV in U.S. horses in 2011. Although the disease is now considered endemic to all of North America, risk of exposure varies with changes in populations of insect carriers.

Sweeney advises that the risk of contracting WNV can be lessened through both appropriate vaccination and careful management.

"Proper vaccination has been very effective in reducing the number of West Nile Virus cases in horses, from an outbreak situation a few years ago to practically no cases reported in horses in Pennsylvania in more recent years," says Sweeney. "There is a concern that if we let our guard down and stop vaccinating horses, we will begin to see more cases again."

Vaccination against WNV is very effective, he adds, but it doesn't last forever: "Horse owners are encouraged to check with their veterinarian to find out whether a booster vaccination is recommended."

Management practices also play a significant role in reducing exposure to the infected mosquitoes. Horse owners are advised to take the following steps:

  • Eliminate all standing water (i.e., mosquito breeding grounds) from the property, including puddles hiding in old tires, watering cans, gutters, and low-lying areas;
  • Arrange turnout schedules so that horses are in the barn at dusk and dawn, prime mosquito feeding times;
  • Keep mosquitoes from the barn through the use of fans, traps, and horse-friendly repellents; and
  • Use approved repellents on horses and any clothing such as fly sheets, as well.
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