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Incidence of equine West Nile virus rising steadily across the nation


A number of states reporting confirmed infections in late summer; veterinarians advising horse owners to vaccinate.

Although most of the nation has been struggling with drought conditions this summer, the population of mosquitoes—the vector responsible for transmitting West Nile virus—has still found a way to make its presence known. As of Sept. 4, 187 cases of equine West Nile virus had been reported this year, according to the United States Geological Survey’s disease maps, and the numbers are steadily climbing. Alabama, Arizona, Arkansas, California, Colorado, Florida, Georgia, Idaho, Illinois, Indiana, Iowa, Kentucky, Louisiana, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nevada, New Jersey, New Mexico, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, South Carolina, South Dakota, Tennessee, Texas, Washington and Wyoming have all reported confirmed cases of the viral disease, with Louisiana topping the list at 26 cases.

Additionally, human cases of West Nile virus have been rising at an equally concerning rate, with the Centers for Disease Control and Prevention (CDC) reporting 1,993 cases as of early September this year, including 87 deaths, the highest number reported since 1999 when the disease was first detected. More than 70 percent of those cases have been reported from six states (Texas, Mississippi, Louisiana, Oklahoma, South Dakota and Michigan) and almost half of the cases are from Texas, the CDC reports.

West Nile virus is currently endemic across the United States and readily transmissible from birds—the reservoir hosts for the virus—to humans, horses and other mammals by mosquitoes. Direct horse-to-horse transmission is unlikely. According to the U.S. Department of Agriculture Animal and Plant Health Inspection Service (USDA-APHIS), 10 percent to 39 percent of unvaccinated horses will develop clinical signs when infected with West Nile virus and 30 percent to 40 percent will die. Most infected horses exhibit neurological signs, which may include ataxia, circling, hind limb weakness, muscle fasciculation, change in mentation, somnolence, listlessness and anorexia.

Amy Glaser, DVM, a senior research associate and West Nile virus expert at the Animal Health Diagnostic Center at Cornell University’s College of Veterinary Medicine, urges vaccination of horses in affected areas. “This year has seen an elevated level of West Nile virus activity in many geographic areas, including parts of the Northeast, Midwest and Texas,” she says. “We all need to be aware of the presence of the virus and to take measures to avoid or prevent mosquito exposure. Horses should be vaccinated for West Nile virus and for eastern equine encephalitis virus.”

Both USDA-APHIS and the American Association of Equine Practitioners (AAEP) also recommend annual vaccination as part of a standard wellness protocol for horses. Additionally, USDA-APHIS recommends that horse owners take appropriate measures to reduce mosquito exposure with the use of insect repellents and to eliminate mosquito breeding grounds—such as stagnant water sources, manure and weeds—when possible. Detailed information about vaccination products and schedules is available at www.aaep.org.

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