10 Horses Dead From West Nile

West Nile virus (WNV) has topped headlines in the Northeast for the better part of 2000, and now has sickened at least 15 horses in the states of Connecticut, New York, New Jersey, Massachusetts, and Rhode Island. As the magazine went to press, only five of those horses were recovering; the others died or were euthanized. Officials are familiarizing themselves with WNV in many ways as they learn more about the epidemiology of the deadly disease. Information gathering includes a written and serological survey of the stables where positive horses have been found.

In the meantime, horse owners in the Northeast are waiting for the first frost to kill off the mosquitoes that carry the virus. But officials feel the virus is not going to spread much farther before the winter.

“I don't expect to see equine cases in Maryland and Pennsylvania, but I can't rule it out,” said Randall Crom, DVM, staff veterinarian for the United States Department of Agriculture's Animal and Plant Health Inspection Service (APHIS), and Coordinator for West Nile Virus Issues. (WNV has been found in dead birds in Maryland and Pennsylvania.) “I think the peak period for equine exposure has passed. That's not to say there couldn't be some cases of West Nile in horses that will have onset later than today's date. But in looking at what happened last year, we only had two horses, if you calculate back from their date of onset, that would have had exposure to the mosquitoes after the first of October.”

Last year was the first time WNV emerged in the Western Hemisphere; it appeared in birds, horses, and humans. Twenty-five horses in New York tested positive for WNV in 1999, and nine of those horses died or were euthanized. This year, the first positive equine case again was in New York. Since then, positives have been found in the five states listed above, but all clinical cases have been at separate farms, except three cases in Orange County, NY.

“The USDA has asked states with affected horses to do some follow-up surveillance, which includes blood testing horses in the same general area,” said Susan Littlefield, VMD, Rhode Island Public Health Veterinarian. The surveys also collect information about the premise and mosquito activity around the affected horses. Officials hope to link positive mosquitoes to equine cases so they can target the species that spreads WNV. They also chart seropostive horses in affected barns. The Rhode Island positive horse was a Miniature which exhibited a four-leg lameness. “All the horses in this barn were negative, but this one horse,” explained Littlefield. “This was a very random, hit-or-miss type of thing, which is scary,” she added.

Will West Nile be back next year again?

Peter Timoney, FRCVS, PhD, of the University of Kentucky Gluck Equine Research Center, said, “We know from work with this virus that it can infect a very wide species of birds, mammals, and other species of vertebrates in a totally new ecological environment. We don't know what kind of relationships it will strike up with the native fauna that exist in the country. It's a remarkably adaptable virus.” Timoney explained that prime vessels for overwintering the virus are live mosquitoes, transovarial activity in mosquitoes, migrating birds, and bats (two species of which have been found with WNV).

“The fact that I believe we'll see new cases next year does not necessarily mean that WNV will be in the U.S. forever,” said Crom. “So far, to our knowledge, there is no tropical or sub-tropical reservoir cycle that has developed in the Western Hemisphere, as is the case in Africa, which serves as a source for outbreaks in temperate climates of the Eastern Hemisphere.”

The quest for answers about WNV won't die down after the first mosquito-killing frost. Crom explained that the USDA and other agencies will study disease epidemiology, surveillance will continue, and a pharmaceutical company is hard at work on a West Nile Virus vaccine.

“It is possible that we could have a vaccine, or at least a conditional license vaccine, by next mosquito season,” said Crom. “It depends on what product comes forward and what timeframe that happens in. Once USDA gets a WNV vaccine application for horses, we will do everything we can to expedite the processing of that application and its potential approval.”

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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