Can A Horse That Has Had EPM Have A Relapse?

Equine protozoal myeloencephalitis (EPM) is a potentially devastating disease caused by a protozoa that horses ingest. Once the horse has been infected, the organisms can migrate to the central nervous system, causing inflammation and tissue damage to the spinal cord and brain.

A horse that has EPM can be incoordinated, lame or weak. Treatment involves the use of antiprotozoal or antiparasitic medication like diclazuril, ponazuril or sulfadiazine and pyrimethamine. These can reduce or eliminate the signs of EPM. Most cases of EPM respond to treatment, but horses may need more than one round of treatment weeks or months after their initial treatment.

EPM-causing protozoa are good at evading the immune response and surviving immune attacks, which makes them hard to eradicate. Additionally, a horse’s immune response in the central nervous system is not as strong, additionally complicating treatment. Because of this, horses that have had EPM can relapse, though it is unclear how often this happens. One study showed a relapse in about 8 percent of horses within 90 days after the initial EPM treatment concluded. Evidence of relapses after longer periods of time also occur.

Multiple factors are involved in an EPM relapse, including the dose and type of drug used, as well as the variability of the individual horse to clear the organisms. A veterinarian caring for a horse that has another episode of EPM should investigate lengthening the duration of treatment.