EPM: Not So Common?

"Equine protozoal myeloencephalitis (EPM) is a common cause of neurological disease of horses in North and South America, and results from a protozoal infection with Sarcocystis neurona or Neospora hughesi (less commonly)," said Steve Reed, DVM, Dipl. ACVIM, of The Ohio State University (OSU), at the Western Veterinary Conference held Feb. 20-24 in Las Vegas, Nev. But it might not be as common as was previously thought.

"Why bother EPM testing?" Reed asked. Studies show equine S. neurona exposure rates as only about 52%, he said, but many vets assume that nearly all horses have been exposed and begin treating suspect cases instead of spending time and money on a test they think will be positive.

"But what if that horse is seronegative (indicating no exposure to the parasite)?" he asked. "Do we spend hundreds if not thousands of dollars of the client's money to treat a negative horse on a therapeutic trial? An S. neurona Western blot test (WBT) on (blood) serum is cheap and easy, with very few false negatives. With a very acute onset you might get a false negative, but there are very few of those."

Although complete results are not yet available, Reed discussed a study of EPM exposure being conducted by OSU and Equine Biodiagnostics/IDEXX. The researchers evaluated serum WBT results in neurological patients presented for diagnosis from 2000-2003.

They found that the incidence of seropositive (exposed) horses was as follows: California 35%, Oklahoma 80%, Texas 65%, Kentucky 66%, Florida 56%, and New York 57%.

In addition, "The actual seropositives in the general population may be much lower," Reed said. Most of the study horses were tested because they exhibited neurologic signs, so they don't represent a random sample and might be expected to have a much higher incidence of any pathogens causing neurologic signs than the general horse population. "Seropositive rates of horses with CNS (central nervous system) disease are about twice as high as the general horse population," he added.

Jennifer Morrow, PhD, a scientist and equine consultant with Equine Biodiagnostics/IDEXX, who is working on this study, notes that in working with IDEXX sales representatives, who talk to veterinarians, in many regions, "It has become clear to us that there is a misconception that all samples test positive, so many practitioners don't test suspected cases anymore. There is a lot of value to a negative result, probably more so than a positive result. It's a small thing to help make a better diagnosis."

What's Next?

It is certain that new tests need to be developed that will provide clinically relevant information, such as possible predispositions of horses to develop clinical EPM. "We are working on an assay at Ohio State that we believe might give an indication of how recently a horse was exposed and/or infected," Reed said.

He also discussed some of the basics of what is known about this sometimes elusive disease. For more information, see www.TheHorse.com/ViewArticle.aspx?id=5608.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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