Equine protozoal myeloencephalitis (EPM) in horses is caused by single-celled protozoan parasites. This disease can infect any part of a horse’s central nervous system. Because of that, there are a myriad of clinical signs EPM can cause in a horse.
In this episode of “Farm Calls,” we talk to Nichola Pusterla, DVM, PhD, who is boarded in internal medicine and equine dentistry. He is a professor of equine internal medicine and dentistry at the University of California, Davis, School of Veterinary Medicine. His research focus is on equine infectious diseases. Pusterla also leads the UC Davis Equine Infectious Disease Research Laboratory.
About EPM in horses
EPM was first identified as caused by the protozoan parasite Sarcocystis neurona in the 1940s. Since then, researchers have proven that there are several protozoal parasites that can cause EPM.
However, the most common parasite that causes EPM is S. neurona.
The definitive host for S. neurona is the opossum. “Wherever the opossum lives, you can expect to see this protozoan in horses,” said Pusterla. “S. neurona especially loves the offspring of the opossum. You see more S. neurona sporocysts in the spring because of the opossum dynamics.”
The sporocysts are shed in the opossum feces, and the contaminate the environment. Horses inadvertently pick the parasites up when they are eating.
The other two protozoal organisms that can cause EPM in horses are N. hughesi and N caninum.
Positive or sick?
Depending on the region of the country, about 70% of horses will test positive for antibodies to the parasites that cause EPM. That doesn’t mean they have disease. It just means they have encountered one of the protozoan parasites.
“Scientists are still trying to understand why some horses get EPM and others don’t,” said Pusterla.
He added that with N. hughesi, broodmares did not have EPM, but foals had evidence of transplacental antibodies to the parasite.
Risk factors for EPM in horses
He noted that age plays a role in whether a horse will develop disease. Older horses are more susceptible. He also said there are regional and seasonal risk factors (where opossums are plentiful and spring when young opossums are around).
“Stress is also a risk factor,” said Pusterla.
More on EPM
Learn more about what researchers have learned about EPM, what horse owners need to watch for, and how to decrease the risk of your horse getting EPM by listening to this episode of EQUUS “Farm Calls.”
This episode of the EQUUS “Farm Calls” podcast is brought to you by Farnam–Your Partner in Horse Care. Visit farnam.com to learn more.