EQUINE PROTOZOAL myeloencephalitis (EPM) is a neurological disease in horses and is caused by the protozoan Sarcocystis neurona, which affects the brain and spinal cord. The protozoan requires two hosts to complete its life cycle, a definitive and an intermediate host. Opossums are the definitive hosts and cats, raccoons, armadillos, and skunks are intermediate hosts. Clinical signs of disease can include asymmetric incoordination (ataxia), weakness, depression, and muscular atrophy.
Diagnosis of EPM is difficult but can be based on clinical signs, a blood test, cerebrospinal fluid (CSF) analysis or response to treatment. The blood test is a non-invasive test that measures antibodies to EPM, but a positive test doesn’t necessarily signify current infection, only prior exposure to Sarcocystis. A negative blood test usually indicates that the horse is free of the disease.
The three FDA-approved treatments available for EPM are ReBalance, Marquis, and Protazil. ReBalance, a liquid combination of sulfadiazine and pyrimethamine, must be given for at least 90 days. This medication should be given at least one hour before feeding and can cause anemia with prolonged treatment. Marquis paste (ponazuril) and Protazil pellets (diclazuril) are given for 28 days. All of these drugs can minimize infection but do not kill the parasite so horses may relapse after stopping the medication. In addition to an approved treatment, the use of anti-inflammatory medications such as Banamine and phenylbutazone can reduce inflammation and limit further damage. Antioxidants such as Vitamin E can help with restoration of the nervous system.
While prevention of EPM is not possible, you can reduce the risk of infection by minimizing fecal contamination of food and water sources. Keep food and water troughs elevated and cleaned regularly. Make sure to secure all feed storage and trash containers and pick up/discard any fallen fruit to reduce the amount of scavengers. If you have any concerns about your horse or think your horse may have EPM, please contact your veterinarian.
column by DR. KATIE HENNESSY
BIO: Dr. Katie Hennessy graduated from the University of Illinois College of Veterinary Medicine in 2008 with a degree in large animal health and equine medicine. She completed an advanced internship at The Equine Medical Center of Ocala and is currently the owner and practicing veterinarian at Polk Equine. Her expertise ranges from small and exotic creatures to large animals, specializing in equine medicine.