Some say the incidence of equine protozoal myeloencephalitis (EPM) is increasing, while others say it's decreasing. Who's right? At the 2010 American Association of Equine Practitioners convention, held Dec. 4-8 in Baltimore, Md., one presenter sought to answer that question. Frank Andrews, DVM, MS, Dipl. ACVIM, director of the Louisiana State University Equine Health Studies Program, discussed a retrospective evaluation of 17 years' worth of horses' records from 15 university equine hospitals, coupled with an online survey of veterinarians.

The research team found that the incidence of EPM diagnosis (confirmed by positive test results for the protozoa) in those university hospitals' populations has not increased from 1990 to 2007 (despite a spike from 1995 to 1997, when the Western blot test first came into use). The proportional EPM morbidity rate, or rate of EPM cases within the hospital population, was found to be 0.88% over the study period.

The rate of EPM diagnosis "peaked in 1997 with a 1.95% incidence," commented Andrews. "This is very common in epidemiology, when the number of cases goes way up when a new test is found. Also, at least two pharmaceutical companies were working on products to treat EPM at that time, so more horses were potentially being enrolled in university studies of the disease.

"The actual incidence could be higher if you consider nonclinical or subclinical infection, or horses that have the disease but are not diagnosed," he added.

Andrews noted that Standardbreds, Thoroughbreds, Tennessee Walking Horses, and males were more likely to be diagnosed with the disease. Most of the affected horses in the study were 2-15 years old.

The second part of the study involved a recent survey of 221 veterinarians, 76% of whom felt that the incidence of EPM in their practices had decreased or stayed the same over the last two to four years. More than 43% of those veterinarians said they diagnose EPM without using laboratory testing, relying instead on neurologic exam results alone or in conjunction with the horse's response to EPM medication.

Andrews said the group's next goals are to document the incidence of EPM since 2007 and conduct a more thorough survey study of veterinarians on EPM incidence, diagnosis, and treatment.

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.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More