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University of Illinois Extension

West Nile Virus in Horses and Humans: A Veterinary Connection

January 17, 2013

Though the practice of medicine for animals may seem as remote from human health care as a colicky horse is from a colicky infant, human and veterinary interests converge on issues related to public, animal, and environmental health. "Zoonotic" diseases—infectious diseases that can be passed between animals and people—represent one area in which experts at the University of Illinois College of Veterinary Medicine in Urbana are working from various perspectives to improve health.

Dr. Marilyn O'Hara Ruiz, a geographer by training and not a veterinarian, directs the veterinary college's GIS and Spatial Analysis Laboratory, which uses spatial science to examine the problems of disease transmission in an ecological setting. The zoonotic disease West Nile virus (WNV), which is transmitted by mosquitoes and affects primarily birds, horses, and people, has been a focus for Dr. Ruiz for nearly a decade.

WNV was first detected in the United States in 1999 in New York. The virus spread westward across the country, carried by avian hosts. Illinois was particularly hard hit by WNV, with 877 human cases reported, when the virus appeared in the state in 2002. In most years since then, Illinois had fewer than 100 human cases, but in 2012 there had been 249 cases and 10 deaths reported as of November 14, according to the Illinois Department of Public Health, which has supported some of Dr. Ruiz's work on WNV.

"The mosquitoes that transmit the virus actually prefer to feed on—and thus infect—birds, especially perching birds, rather than humans and horses," Dr. Ruiz explains. "Cats and dogs are not typically going to be sickened by WNV, but horses and humans are more susceptible."

Indeed, this year in Illinois there have been seven reported equine cases of WNV.

Dr. Scott Austin, an equine veterinarian at the University of Illinois Veterinary Teaching Hospital who is board certified in large animal internal medicine, looks at WNV from a clinical perspective. He says it is important to safeguard horses from this disease, although only a small percentage of the horses exposed to WNV actually develop signs of disease.

"Four vaccines that are greater than 95 percent effective against West Nile virus are on the market," says Dr. Austin.

Other ways to reduce the risk of WNV for horses and people are to avoid being outdoors during periods of high mosquito activity, such as the early morning and evening, and to eliminate stagnant water where the vector mosquitoes breed.
"Catch basins are important breeding sites for the Culex mosquitoes that carry the virus," says Dr. Ruiz.

In a study published in 2012, she found that low rainfall was associated with greater numbers of mosquitoes, because excess rain flushes the premature larvae from the catch basin. She also found that mosquito larvae develop more quickly when both air and water temperatures are high.

A horse that is exposed to WNV may develop signs of disease within 7 to 10 days, according to Dr. Austin. Common signs include fever, head tilt, drooping ears, muscle tremors, weakness, and altered behavior, such as anxiousness, hyperexcitability, or lethargy.

"Supportive treatment is available," says Dr. Austin, "so if you suspect your horse has WNV, you should see your equine veterinarian."

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