Equine West Nile virus cases almost triple in past month, exceed 500 in U.S.

In early September, 187 cases of equine West Nile virus were reported nationwide, according to the U.S. Geological Survey's disease maps. Now, just one month later, that number has soared to 518, according to a report posted on October 16. The U.S. Department of Agriculture Animal and Plant Health Inspection Service (USDA-APHIS) released a similar report in early October showing that the mosquito-borne virus is a much greater problem this year, having far surpassed 2011's report of 87 cases. The USDA states, however, that while this number is higher than the number of cases identified last year, it's very similar to the number of cases reported to officials years prior.

Amy Glaser, DVM, PhD, a West Nile virus expert and senior research associate at the Animal Health Diagnostic Center at Cornell University's College of Veterinary Medicine, isn't surprised by these numbers and actually expects a peak in caseload around this time each year. "Traditionally, the largest volume for equine West Nile virus occurs in September and October, right up until the first frost," she says. "That typically represents the peak of cases we see during any particular epizootic year."

At this time, only seven states remain free of the virus—Alaska, Delaware, Hawaii, Kansas, Maine, New Hampshire and West Virginia (Alaska and Hawaii have never reported West Nile virus). As of October 16, Texas holds the highest number of cases with 83, and Louisiana and Pennsylvania are close behind with 50 and 46, respectively.

The U.S. Centers for Disease Control (CDC) also reports that as of October 16, a total of 4,531 cases of West Nile virus disease have been identified in people, including 183 fatalities. Just one month ago, 1,993 cases were reported to the CDC, 87 of those resulting in death. Texas also has the most reported cases in people—a whopping 1,580, followed by California with 285.

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 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.

"The issue with West Nile is that it's here. It's not going away," says Glaser. "There's no way to tell what each year is going to look like, so it's always better to have good vaccine and antibody protection in the background, especially when you're dealing with an endemic disease of this nature."

Both USDA-APHIS and the American Association of Equine Practitioners (AAEP) 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.