The mosquito-borne neurologic disease is present throughout the United States.
In 2008, 179 equine cases were detected in the United States, with 41 occurring in Washington. In 2009, the number of cases in the country increased to 241, and 30 percent of those (72 horses) resided in Washington.
About one-third of horses die
The majority of cases in Washington were detected in Benton, Yakima and Kittitas counties with Grant, Franklin and Adams counties reporting a few as well. The first positive horses were reported in July and the last in September, which coincides with the mosquito season in these areas.
West Nile virus is maintained within the wild bird population. Birds do not typically become ill as a result of the infection, but mosquitoes that feed on infected birds can then transmit the virus to humans and horses.
Signs of the disease in horses can be mild, manifested as lethargy, change in behavior, muscle twitching, hind-end weakness or loss of coordination. Signs can be much more severe, however, and progress to paralysis, seizures or even coma. West Nile virus kills about one-third of infected horses that display clinical signs.
There is no specific treatment for West Nile virus. With supportive care, many horses survive the infection, but some horses can have residual issues with weakness or lack of coordination.
“These vaccines can only be administered by your veterinarian,” she said. “So even if you perform routine vaccinations yourself, it is important to have your veterinarian out once a year, just prior to start of the mosquito season, to have your horse vaccinated.
It is unclear why case numbers continue to rise in central Washington. Data collected from the majority of the diagnosed horses in 2008 and 2009 indicates that they were not vaccinated appropriately against the virus.
Beyond yearly vaccination, horse owners can further protect themselves and their horses by decreasing mosquito exposure. It helps to eliminate stagnant water sources from the property and stable horses at dawn and dusk when mosquito activity is at its peak.