Since it first appeared in the United States in 1999, in New York City, West Nile virus has spread to all but four states in the continental U.S. Idaho got its first confirmed case last summer, a horse that had traveled through West Nile-infected states. Fully expecting that this summer will bring the state its first human cases of West Nile infection, UI Extension in March brought the state’s extension educators up to date on the disease and how to avoid contracting it.

“It’s established. It’s here. It’s something that people will have to deal with,” said UI entomologist Marc Klowden. Local mosquitoes acquire the virus from infected birds when they bite them. The mosquitoes then infect people and other mammals during subsequent bites.

Horses are even more vulnerable than humans. A total 14,717 cases of West Nile infection has been confirmed nationwide in horses, including 4,300 horses dead or euthanized. As of December 2002, confirmed human cases totaled 3,949, with 254 fatalities. Chilling as those numbers may seem, fewer than 20 percent of infected people show any symptoms at all, and most people’s symptoms are mild and flulike. Fewer than one percent of infections—usually in the very old, very young, or immunocompromised—develop into full-blown West Nile encephalitis.

Tips for avoiding infections

To avoid infection, steer clear of mosquitoes, said Ed Bechinski, UI extension integrated pest management coordinator. “The best protectant for the individual person is standard DEET repellant.” Choose formulations containing no more than 40 percent DEET (no more than 10 percent for children ages 2 to 12). Klowden advises staying away from mosquito areas at dusk and dawn, the prime biting hours.

To reduce mosquito numbers around the home, drain pools of standing water. Mosquitoes lay their eggs and develop into adults in pools as small as those left by rain in flowerpots or wheelbarrows. Whole ponds can be treated with reduced-risk insecticides such as Bt formulations that specifically target immature mosquitoes. A vaccine against the virus is available for horses, but not for people. “Horse owners should visit with their vets right now,” advises Bechinski.

The good news, says Klowden, is that as the virus spreads, people are developing protective antibodies against it. So, while an epidemic today, “it will be a nonevent in about 10 years.” For more information, people can contact the UI extension educators in their county.