Although equine protozoal myeloencephalitis (EPM) has been actively researched during the past decade, definitive scientific
knowledge of this disease remains as elusive as the opossums that transmit it. In almost all aspects of this disease—from
pathogenesis and diagnosis to treatment and prevention—gray areas remain.
That said, some risk factors and prevention strategies have been unveiled. Good news, right? Unfortunately, much of the information
that's been discovered about EPM has served only to generate more questions, and many practitioners feel less knowledgeable
about this disease now than they did five years ago.
Clinical signs of severe or advanced EPM are relatively straightforward and easy to see, including asymmetrical muscle loss,
varying degrees of weakness and ataxia and even recumbency. Evidence of early, low-level EPM, however, can be so insignificant
or subtle that it's often overlooked. Practitioners often depend on clients noticing small physical changes in their horses
[musculature, posture, gait and impulsion] or minor unexplained changes in personality or behavior (Photo 1, p 2). Additionally,
signs of EPM can mimic those of several other neurologic diseases or health conditions.
Moreover, methods of EPM diagnosis have been shown to be problematic and often inconclusive, so clinicians are often left
using a "better safe than sorry" strategy—that is, when in doubt, assume its EPM. This means some unaffected horses may also
be treated, and other conditions may be missed. But response to treatment seems to be the best diagnostic tool currently available.
Treatment options vary, and a number of drugs are available, but there's little consensus as to the absolute best product.
EPM cluster outbreaks
It now seems that cases of EPM are not as isolated as we originally thought. "EPM has traditionally been reported as a sporadic
disease," says William J.A. Saville, DVM, PhD, DACVIM, Department of Veterinary Preventive Medicine at The Ohio State University
College of Veterinary Medicine. It was thought that particular aspects of the immune system and environmental stressors led
certain individual animals to develop the disease, while most other horses did not.
While this remains true in most instances, Saville has identified cases of EPM cluster outbreaks where multiple horses in
the same area or on the same farm have developed the disease, which seems to imply that all horses, if exposed to the right
combination of stress and environmental factors, can develop EPM. "There are reports of EPM cases in Panama in which all affected
horses were stabled at the same location, and also a report of an outbreak on a farm in Kentucky," he reports.
At a horsemen's camp in Orinda, Calif., a number of horses developed EPM in 2010. According to an Orinda Horseman's Association
(OHA) news release, EPM is diagnosed in less than 0.5% of horses annually in the United States, and more than one case is
seldom seen at a particular farm.1,2 That said, the news release goes on to describe OHA pastures as hot spots for EPM. "In sharp contrast to the [national EPM]
incidence mentioned above, four horses out of a herd of 36 on the OHA pasture (11 percent of the total population) have been
diagnosed with EPM just in the last year," the report noted.1