Equine protozoal myelitis is a progressive degenerative neurologic disease of horses. The causative agent is Sarcocystis neurona, a protozoal agent that infiltrates the central nervous system of the horse creating variable neurologic symptoms. Other causative agents have also been implicated in causing the disease as well including Neosporum caninum and Neosporum hughesi.
Sarcocystis neurona is a single celled protozoal agent. The definitive host for the organism is the opossum with raccoons, cats, skunks and armadillos being intermediate hosts. The horse is a dead-end host and cannot transmit the disease. Horses contract the disease from exposure to feed and water contaminated with opossum feces. While the exact mechanism of contamination and central nervous system invasion is not known, it is believed that between 25%-65% of the horses in our area have been exposed and as such have antibodies to the disease. It is felt that the horse can mount an attack against the disease until it gains access to the central nervous system at which time the horse will develop neurologic symptoms. The incubation period from the time of exposure to clinical signs is also unknown at this time.
Clinical signs of EPM can be quite variable depending on the location in the central nervous system that the organism gains access and sets up house as well as the magnitude of the infection. Symptoms vary from simple muscle atrophy to fulminate neurologic disorder. Typically horses will have muscle atrophy of the sacral/glutteal muscles or muscles of the face (masseter muscles), asymmetric or unilateral neurologic deficits. Ataxia (incoordination), spasticity (stiffness and stilted gaits), lameness, difficulty swallowing, abnormal sweating, loss of sensation of various muscle groups, head tilt, poor balance and generalized weakness.
As you can see from the symptoms of the disease they are vast and can be consistent with many neurologic conditions, which makes definitive diagnosis very difficult. Blood panels are of little or no help in diagnosing the condition. Circulating blood titers for the organism are of value when either negative to rule the condition out or very high to strongly support the condition. Titers in the middle may simply indicate a horse that has been exposed but does not have the infectious condition. Central spinal fluid (CSF) taps can be performed but must be done without blood contamination, which will give false positive results. For this reason, we typically recommend CSF taps be performed in a hospital environment to give the best results possible.
Treatment for the condition has had some recent advances. For many years the condition was treated with Daraprim and sulfa drugs along with supportive agents such as folic acid and vitamin E. In recent years two FDA approved drugs were available for the treatment of the condition. Currently only one remains, “Marquis” made by Bayer Corporation is the recommended treatment for EPM. Marquis is an antiprotozoal agent that is administered for 28 days. It is an oral gel preparation that is formulated in a tube with each tube treating for 7 days. It must be noted that the treatment is usually very successful at killing the organism but neurologic signs may remain due to irreparable damage to the nerve tissue.
Studies indicate that while 70% of treated horses show clinical improvement, only 25% return to original function. Relapses have been documented and are a result of either re-infection, latent organism emerging or persistence of the disease (lack of 100% kill).
Prevention of the disease is difficult. Prevention practices for the horse owner include keeping a clean environment for the horses and minimize the draw for opossums to visit. Keeping tight lids on the feed bins and keeping the feed area clean will help reduce the draw for opossums to visit. Clean waterers regularly. Pick up any “road kill” to discourage opossums as well.
In conclusion, many questions remain unanswered with regards to the cause, pathogenesis, occurrence, treatment and diagnosis of EPM. It must always be considered a potential cause any time a horse shows neurologic symptoms. Ruling EPM in as a diagnosis usually requires a blood test or CSF sample. Equally important are other tests such as radiographs and myelograms to rule out other causes of neurologic disease.