Infectious Diseases Forum at AAEP Convention

The world is keeping its eye on infectious diseases because of the terroristic use of anthrax—and the potential use of other biologics--as agents of war against humans. However, veterinarians deal on a daily basis with diseases that can have deadly effects on humans and horses. Included in that list is West Nile virus (WNV), which was a major topic of discussion in the forum.

WNV currently has been found in mosquitoes, birds, horses, or humans in 27 states, according to Bill Saville, DVM, PhD, Dipl. ACVIM, of The Ohio State University. Five of 44 human cases have resulted in death, and the fatality rate in horses has averaged about 24% in those horses with clinical signs of WNV. One veterinarian said that in Florida, many more horses are dying from WNV than the official reported numbers indicate.

In the three years since its introduction into the United States, WNV has spread from one state in the Northeast (New York) all the way to the border of Texas. There had been 416 cases of equine WNV as of Nov. 24, most of which occurred this year in Florida.

“Ohio has been very aggressive in its vector control education of the public,” said Saville, who attributes the fact that Ohio has had no equine or human cases to that education process. “We have been on the road nine or 10 months this year (2001), and have used the 1,500 extension people in Ohio as well as video conferencing in the state to spread the word.”

Saville said that because of the worry the public has with pesticide use, the Ohio education has centered around environmental clean-up of potential mosquito breeding grounds and use of larvacides in standing water. He said mosquito dunks were safe when used in tanks used to water livestock, and many horse farms in Ohio used them.

The Ohio State will share its educational process of mosquito control with the public through three educational CDs. One is for horse owners, one is for households without horses, and one is for veterinarians. A fourth CD will be produced for physicians to help them educate clients. (The Horse will announce the availability of these CDs--which will be sold by The Ohio State at a low cost to cover production and handling--on its web site at

There are only six products licensed by the FDA to control adult mosquitoes, and the general public is wary of the spraying. That is why Saville and his colleagues have focused on environmental control of breeding sites and killing of larvae.

Peter Timoney, FRCVS, PhD, head of the Gluck Equine Research Center at the University of Kentucky, led a discussion that centered around testing of horses for WNV. He said the IgM Elisa test that is used as a screening for antibodies against WNV has proven to be a reliable diagnostic test that reflects recent infection to WNV, but should be followed up with a serum plaque reduction neutralization test to confirm the diagnosis. However, he indicated, there appears to be a small population of horses which will not develop a detectible IgM response, but will be positive on the serum neutralization test. It was previously thought that only vaccinated horses did not develop a detectable IgM response, but vaccinated horses do significant neutralizing antibody responses after the second dose of the inactivated vaccine.

When the question was raised about whether humans could acquire WNV without mosquito transmission, it was stated that it was a possibility, although a remote one. There has been proven transmission from bird to bird, possibly through grooming, and transference of the virus from one generation of mosquitoes to the next (from mother to offspring). It is thought that virus transmission could be possible through breaks in the skin from birds to humans. Therefore, it was stated that veterinarians should warn clients who are removing dead birds to wear disposable gloves, or at least turn a garbage bag inside out to pick up the bird and avoid direct contact.

The conditionally licensed WNV vaccine produced by Fort Dodge Animal Health was thought to be providing protection to horses. However, that cannot be proven scientifically until a challenge test can be done in horses. And a challenge test cannot be done until the federal government allows Fort Dodge to use live virus to challenge vaccinated horses under controlled conditions. The delay is due to the fact that WNV is a virus that also can affect humans, and there is no human vaccine against the virus. Therefore having a living virus around susceptible humans during research is a risk.

A Fort Dodge representative said of the more than one million doses of WNV vaccine that have been distributed, there have been no cases of WNV in horses which had their initial course of two doses of the vaccine administered three to six weeks apart and were at least three weeks beyond the second vaccination. This was true even in horses vaccinated in the face of the WNV outbreak in the panhandle of Florida.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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