Horses at Risk of Equine Protozoal Myeloencephalitis
This neurologic disease was first recognized in the 1970s. S. neurona was confirmed as the causative organism in 1991. Equine Protozoal Myeloencephalitis is a protozoal disease that causes inflammation of the brain and/or spinal cord in horses, leading to various degrees of neurological deficits. The primary organism responsible for this disease is Sarcocystis neurona.1 Neospora hughesi (pronounced Hughes-see-I) is sometimes identified as the causative organism, and very rarely, Neospora caninum. All of these organisms are in the coccidia family. Both S. neurona and N. hughesi use the opossum for the definitive host.
Interestingly, EPM is a disease of the “Americas” or western hemisphere. As far as we know now, it does not occur in horses in Europe or Australia, unless those animals have spent time in North or South America.
It is generally thought that up to 53% of horses can be sero-positive where the opossum is present in the US, meaning they have been exposed to the disease, even if they don’t show clinical signs.2 The protozoa is a thin shelled organism – it doesn’t survive well in some environments, especially arid or cold climates.
Horses develop clinical disease more when they are stressed – shipping around in trailers, competing at shows or participating in events, etc. However less than 1% develop the disease.1
Like many other animal diseases that are affected by environmental influences, some regions of the country have a higher incidence of the disease than others. While 53% of the general horse population can be sero-positive, the number of those horses showing neurologic disease will vary greatly. As you might expect, areas like Oklahoma that are warm and humid will have a higher incidence of sero-positive horses with clinical symptoms of neurologic disease. California is arid and therefore not conducive to the survival of the protozoa in the environment. There have also not been reports of many opossums in this region. However, there are many horses that travel to California for racing, breeding and horse shows, that have been exposed to EPM in other regions of the country. That is why you will see a significant number of sero-positive horses with neurologic disease in that area of the country.
- Regional prevalence of sero-positive horses that are showing signs of Neurologic Disease3
- California 35%
- Oklahoma 80%
- Texas 65%
- Kentucky 66%
- Florida 56%
- New York 57%
- Reed S. Neurology is Not a Euphemism for Necropsy: A Review of Selected Neurologic Diseases Affecting Horses, Proceedings of the 54th Annual Convention of the AAEP, San Diego, CA December 2008 pg 78-109
- Saville WJ, Reed SM, Granstrom DE, et al, Seroprevalence of antibodies to Sarcocystis neurona in horses residing in Ohio. J Am Vet Med Assoc 1997; 210(4):519-524
- West, C. Equine Protozoal Myeloencephalitis: Less Common Than We Thought?, The Horse.com, March 22, 2005, Article #5608
