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Equine Protozoal Myeloencephalitis (EPM) Vaccine StudiedBy Kentucky Equine Research Staff · November 13, 2017

Vaccination remains one of the most important ways to help protect horses against disease. While some vaccines, like rabies, prevent disease, others are intended to aid in disease control. A vaccine against Sarcocystis neurona, a causative agent for equine protozoal myeloencephalitis (EPM), has been available since 2000, but its efficacy has been unclear. A recent study reveals researchers might be headed back to the lab.

According to the study*, approximately 50% of all horses in the United States come into contact with S. neurona, and an estimated 14 of every 10,000 clinical cases are attributed to EPM each year. Even with treatment, only 70-75% of horses show an improvement in clinical signs. Further, relapses occur commonly, and residual neurological deficits are frequent.

“Because there is no cure for EPM, disease prevention plays a prominent role in protecting horses,” said Laura Petroski, B.V.M.S., a veterinarian for Kentucky Equine Research (KER).

“To help protect horses against EPM caused by S. neurona, a vaccine was developed and conditionally licensed. Although the product was considered safe, post-vaccination challenges to determine the efficacy of the product were not performed,” Petroski explained.

One research team successfully developed an equine model of EPM to test the vaccine. Researchers recruited 70 healthy, neurologically intact horses void of antibodies against S. neurona—an indicator that none of the included horses had been exposed to the pathogen previously. A subset of horses was vaccinated, and all horses were subsequently administered S. neurona sporocysts, the infective form of the protozoa.

The research team found that almost all horses exposed to S. neurona sporocysts developed neurological signs consistent with EPM, including the vaccinated group, prompting researchers to conclude that the vaccine did not protect horses from developing neurological deficits.

Experts recommend limiting opossum access to barns, sealing concentrates and other feeds or supplements in bins with tight-fitting lids, and using haynets or feeders, even in fields, to limit exposure to S. neurona.

Vitamin E is a valuable component of the EPM treatment regime, as the EPM disease cycle includes an oxidative phase. “Experts recommend administering between 5,000 and 10,000 IU of natural vitamin E per day for horses with EPM because of its proven antioxidant action. KER offers Nano•E, a natural form of vitamin E that is water-soluble and rapidly absorbed,” shared Petroski.

*Saville, W.J.A., J.P. Dubey, A.E. Marsh, et al. 2017. Testing the Sarcocystis neurona vaccine using an equine protozoal myeloencephalitis challenge model. Veterinary Parasitology. 247:37-41.