Horse owners worried about equine protozoal myeloencephalitis (EPM) take note: In the next few months, there could be several new products reaching the market designed to treat or prevent this neurological disease caused by a single-celled protozoal parasite. Some will be medications approved by the FDA for treatment; one potentially will be a daily feed additive that like a daily dewormer will protect the horse constantly at a low dose. A vaccine undergoing USDA scrutiny would be the first product of its kind for EPM. While the vaccine is controversial among some researchers, it could offer a means of protecting some horses.

At a meeting of the Kentucky Thoroughbred Farm Managers Club on May 2, Fort Dodge Animal Health presented the results of field testing of its EPM vaccine. EPM is detected by blood serum and cerebrospinal fluid samples showing the presence of antibodies. (A positive blood serum only means the horse has been exposed to the parasite, not that it has the disease.) The new vaccine was field tested in 897 horses in seven states. According to Rocky Bigbie, DVM, a representative of Fort Dodge, 99.6% of the horses were reaction-free.

The vaccine research data is being reviewed by the USDA for a conditional license. Bigbie says the conditional license "does assure safety, as the safety standards (of the USDA) are the same as with a fully-licensed product." The USDA reviews the vaccine for purity, potency, safety, and in the case of a conditional license, reasonable expectation of efficacy. It is estimated that an initial injection will be followed by a booster given every six months or yearly, depending on the prevalence of EPM in the area.

Concerns expressed from the industry include the effect of the vaccine on a horse with clinical signs of EPM, the absence of a challenge model, and the problem of seroconverting a vaccinated horse and making diagnosis more difficult.

Bigbie explains that if the protozoa has invaded the spinal cord, chances are that the vaccine will not help.

In terms of seroconversion (testing positive for antibodies to EPM) of a horse without disease, "There is a protein band on the Western blot test that should help differentiate be-tween vaccination positives and field-exposed positives," he says.

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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