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Equine Protozoal Myeloencephalopathy and Vitamin EBy Kentucky Equine Research Staff · August 23, 2016

Equine protozoal myeloencephalopathy (EPM) is a progressive and degenerative protozoal disease most commonly caused by Sarcocystis neurona, though Neospora hughesi has also been implicated. Opossums contract the protozoa from other species, including raccoons and cats, and as a definitive host, opossums carry the organism through a final developmental stage before transmitting it to the horse via contaminated water or feedstuffs.

After a horse ingests the protozoa, it travels throughout the body via the bloodstream. In most cases, the horse’s immune system will combat the organism, and the horse will remain healthy. However, if the protozoa crosses the blood-brain barrier, it attacks the central nervous system, which consists of the brain and spinal cord and controls the body and mind. Infection may result in a number of symptoms depending on which areas of the nervous system it attacks.

Symptoms and Diagnosis

EPM has similar symptoms to many neurologic diseases and can, therefore, be difficult to diagnose. Early signs may look like a limb lameness but, as the disease progresses, it can result in gait abnormalities, weight loss, sensory loss, head tilting, seizures, and difficulty swallowing.

If the horse is left untreated, the protozoa will continue to attack the central nervous system, the disease will progress, and the horse will die. Postmortem cross-sections of spinal cord show extensive damage, including necrosis.

There are multiple tests available to diagnose EPM, and some give more information than others. The traditional Western blot analysis will detect the presence of S. neurona in cerebrospinal fluid and serum; however, the test cannot distinguish an old infection from a new one. Additionally, it cannot detect N. hughesi or reveal the concentration of protozoa.

Veterinarians may also use a polymerase chain reaction (PCR) to search for the protozoa’s DNA. Similar to the Western blot, it will confirm the presence of a protozoa, though it does not tell the concentration or current status of the infection. Researchers at the University of California, Davis, developed a comprehensive analysis called the indirect immunofluorescence antibody test (IFAT). IFAT identifies the presence of S. neurona or N. hughesi, detects the concentration of the protozoa, and reveals if the infection is currently active.  


If a horse tests positive, a veterinarian can prescribe different antiprotozoal drugs with treatment protocols ranging from 28 days to 120 days. Many horses return to normal function over time, but some still may exhibit neurologic symptoms.

Some veterinarians recommend the use of vitamin E for horses diagnosed with neurological problems such as EPM. Vitamin E is a collective term for a family of eight stereoisomers found in plants. Each of the eight forms has a different methylation group that affects its bioavailability. The stereoisomers contain two tocopherols and tocotrienols, each of which consist of four similar compounds distinguished as alpha, beta, gamma, and delta. The most bioavailable source is natural d-alpha-tocophorol. This is not to be confused with dl-alpha-tocophorol—a synthetic and less-available source. A high-quality supplement contains natural vitamin E instead of synthetic.

Supplementing a natural vitamin E source may help horses recover. Vitamin E is important for healthy nerve function. It also serves as an antioxidant to quench free-radical damage from tissue degradation. In addition, it strengthens the immune system, which may make it better at fighting off the protozoa. Nano•E, developed by Kentucky Equine Research (KER), is a highly absorbable source of d-alpha-tocopherol. It comes in a palatable liquid, making it easy to mix into grain.

Kathleen Crandell, Ph.D., senior nutritionist at KER, states, “The vitamin E recommendations for EPM range from 5,000-10,000 IU per day through the course of treatment. The American Association of Equine Practitioners (AAEP) recommends 10,000 IU/day. Unfortunately, there have been no studies on whether horse owners could offer a lower amount to diagnosed horses if the product contains natural, micellized, or nanodispersed versus synthetic forms,” she commented, “but physiologically it would make sense that you should be able to.”

The important point, Crandell emphasized, is that natural vitamin E be part of the nutritional treatment plan for horses with neurological disease.