EPM Round Table

Editor’s note: This synopsis of a special EPM Round Table video available on EquiManagement.com is brought to you by Merck Animal Health.

Credit: Amy Dragoo

Equine protozoal myeloencephalitis (EPM) was first identified as a disease entity—although not by that name—in 1964, by Dr. Jim Rooney of the University of Kentucky. It then became the disease du jour for more than a decade before falling out of the limelight. However, the disease did not go away.

In 2015, industry experts thought it was time to take a new look at an “old” disease and discuss what progress has been made, and what still needs to be done.

EPM Round Table Participants

The moderator of the Round Table was Steve Reed, DVM, DACVIM, of Rood and Riddle Equine Hospital in Kentucky. The panelists were: Monica Aleman, MZV, PhD, DACVIM, of the UC Davis College of Veterinary Medicine; Amy L. Johnson, DVM, DACVIM (LAIM & Neurology), of the University of Pennsylvania’s New Bolton Center; Rob McKay, BVSc(Dist), DACVIM, PhD, of the University of Florida; Nicola Pusterla, DrMedVeT, Med- Vet, DACVIM, of the UC Davis College of Veterinary Medicine; and Wendy Vaala, VMD, DACVIM, of Merck Animal Health.


The panel participants agreed that EPM is still an important disease in U.S. horses. While clinical disease only occurs in about 1% of exposed horses, exposure can range from 50% to more than 80% in some areas, and there is an expected 10- 20% relapse rate. The panelists also noted that there is more than one cause for EPM.

The panelists discussed that not all neurologic horses have EPM, that not all horses have the same risk factors for EPM, and that not all horses with EPM have the same clinical signs. What these researchers and practitioners have discovered over the years is that “the more we know, the more complex this disease seems.” They reminded vets that EPM is still the second-most prevalent cause of neurologic problems after CVM, and in some areas it is the most prevalent.

They noted that at first, only the singlecelled parasite Sarcocystis neurona was identified as a cause, but researchers at UC Davis discovered that Neospora hughesi also was a cause. Reed said that it has been proven that N. hughesi is not just a West Coast problem.

A study of 5,200 serum samples across 18 states found that 79% of healthy horses tested positive for S. neurona. Pusterla said that N. hughesi was found in 34% of healthy horses tested across the United States.

All of the participants agreed that compromise of a horse’s immune system and stress were key in predisposing a horse to disease, and that a variety of locations in the CNS could be compromised.

Pusterla said that not every horse has the same risk, and that young performance horses have a higher risk. He said there is a pattern of horses that live in an area perceived to have a low infection rate being moved to high-risk areas and being put into exercise, then showing neurologic signs within a few weeks. Panelists discussed whether at-risk horses should be put on a prophylactic treatment to mitigate the response of exposure.

The panel agreed that vets need to be “retrained” on diagnosing EPM. “ ‘Rest, treat and horse improves’ is not a gold standard for diagnosis,” said Pusterla.

Pusterla also noted that N. hughesi has been shown to cross the placenta and infect the fetus.

They discussed the “ratio test” to determine disease, “but even that is not foolproof,” said McKay. The ratio test compares antibody titers from blood and CSF. Another point raised by the panelists was the need for validation of sampling labs. They all agreed that you need to “treat the horse, not the titers,” and that use of FDA-approved products was the smartest course of action.

Watch the whole video Round Table here.

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