If laboratory tests are inconclusive, a positive response to treatment may be an indicator that a horse is indeed infected. If, on the other hand, the horse keeps getting worse even as he receives a full course of treatment, he probably doesn't have EPM. A reasonable argument can and often is made that response to treatment is one of the best diagnostic tools available," says Mackay, who starts a horse on a course of anti-protozoal medication when he strongly suspects EPM but can't make an immediate, definitive diagnosis, or when he is awaiting consultation with a specialist. If the horse improves with medication, EPM could very well have been the cause, Mackay says. "You can always stop treatment if something else is uncovered."
4. Continue to look for alternative causes.
Even when you are certain your horse has EPM and he is undergoing treatment for the disease, Fenger advises that you watch for clues that might reveal another source for the problem. "I've seen horses with serious musculoskeletal prob- lems kept in work simply because the owners were convinced that they had EPM. People treat horses for a nonexistent disease and continue to strain another obscure injury that they never really looked for," she says. "Racehorses have been known to break down because early warning signs of skeletal problems are attributed to EPM."
Until more is known about EPM, questions about when to test horses for the disease will linger and debates over how to interpret the results will continue. Beyond the widespread agreement that a clinically normally horse need not be tested, there are no hard and fast rules. A concerned horse owner is best off learning the facts about EPM, discussing the situation with a trusted veterinarian and making a decision based on the horse's individual signs and circumstances.
