Status: The vaccine, from Fort Dodge Animal Health, has conditional licensure. This type of licensing is given if a new vaccine is considered to be of special need (the disease has a significant impact on the horse population) and shows a reasonable expectation of efficacy (there's reason to believe it might work). The vaccine must still be approved by each state; it was available in 32 states at press time. (Check with your veterinarian to find out whether it's available in your state.)
Effectiveness: The vaccine has been shown to stimulate an antibody (disease-fighting) response against the EPM-causing organism. However, it's not yet clear whether these antibodies will actually protect your horse against the disease. Horses that contract EPM naturally produce antibodies to fight the disease, but they don't always win the fight.
Safety: To be approved, the vaccine had to meet standard safety requirements, and it appears to be very safe. Fort Dodge administered it to almost 900 horses and followed them for 1 year after vaccination. Very few had pain or swelling at the injection site, and there were no other significant complications.
Diagnostic dilemma: To diagnose EPM, vets test for disease-specific antibodies in the blood and fluid that surround a horse's spinal cord. Since the vaccination stimulates antibody production, it can make the already-difficult diagnosis even more complex. If you vaccinate your horse against EPM, and he later contracts the disease, a clear diagnosis might be impossible.
Cost: Approximately $30 to $40. (Check with your vet.)
Barb Crabbe is an Oregon-based equine practitioner specializing in performance horses.