WNV: A Threat to Unvaccinated Horses

As spring approaches and temperatures rise, growing mosquito populations will increase the risk for deadly mosquito-borne diseases, including West Nile virus (WNV) and equine encephalomyelitis (sleeping sickness).

Since its discovery in the United States in 1999, WNV has spread rapidly across the country and poses a significant health threat to humans, horses, and other animals.

"The North American West Nile virus epidemic persists," said Lyle Petersen, MD, MPH, Director of Vector-Borne Infectious Diseases for the Federal Centers for Disease Control and Prevention, when addressing the seventh National Conference on West Nile Virus in the United States held in San Francisco, Calif., in February.

Unseasonably warm weather and mild winter conditions, which have been experienced in many parts of the country this year, may lead to an increased threat of WNV. Torrential rains in other parts of the United States further complicate the risk.

West Nile virus was found in mosquitoes in January in Baton Rouge, La. In California, four counties have already reported birds testing positive for the disease, a finding that is "early in the season," according to Gary Erbeck, director of the San Diego County Department of Environmental Health.

Randy Phillips, who is helping to coordinate the WNV response at the Clark County Health Department in Vancouver, Wash., says there's an increased chance of West Nile virus in northern Oregon and southwest Washington. "Mosquitoes may be more prevalent due to the high rains and mild temperatures we've had this year," says Phillips.

Guidelines from the American Association of Equine Practitioners (AAEP) state proper vaccination of previously non-vaccinated horses involves administration of two doses of vaccine three to six weeks apart. After completion of the first series of vaccinations, horses should be vaccinated semi- annually or more frequently, depending on the geographical risk. Annual revaccination is best completed in the spring, prior to the onset of mosquito season. Horse owners should contact their veterinarians as soon as possible to evaluate threat levels in their areas and determine if their horses' vaccines are current to ensure they will have maximum protection against the disease.

More Prevention Measures

Limiting exposure to mosquitoes is fundamental in helping prevent the spread of WNV and sleeping sickness. According to the USDA, the following precautions may help reduce the risk of West Nile around homes and stables:

  • Keep horses stabled during dawn and dusk when mosquitoes are most active.
  • Turn off lights that attract mosquitoes.
  • Use fluorescent lights, which do not attract mosquitoes.
  • Keep screens in stable windows.
  • Make sure roof gutters drain properly.
  • Turn over plastic wading pools and wheelbarrows when not in use.
  • Empty water collecting in buckets, tarps, pool covers and tires.
  • Clean water troughs and birdbaths at least once a week.
  • Use mosquito repellent.

Early season mosquito control, in particular, can interfere with the life cycle of mosquitoes that feed on infected birds. By controlling mosquitoes early in the season, spring and summer populations will be reduced.

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