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Equine Protozoal Myeloencephalitis Practice Tips

equine featured free large animal
Equine protozoal myeloencephalitis (EPM) has 2 causative organisms- Sarcocystis neurona (most common) and Neospora hughesi
  • Sarcocystis neurona is transmitted by opossums.
  • We don’t know how Neospora hughesi is transmitted yet!

When to suspect it:

  • For a horse to be considered as an EPM suspect, they MUST have neurologic deficits!
  • Neurologic deficits from EPM are classically multifocal, and associated with asymmetric muscle atrophy, but they can be ANYWHERE and do ANYTHING within the CNS.
  • Mini donkey featured in the picture below presented with seizures and was diagnosed with EPM! She made a complete recovery. 


Clinical signs (neurologic disease) + paired serum/CSF titers for the causative agents of EPM.
  • Validated tests (labs): IFAT (UC Davis), or SAG 2,4/3 ELISA (EDS).
  • SAG 2,4/3 ELISA is slightly preferred, but either test is good.
  • Disclaimer- There is a lab (Pathogenes) that runs a SAG 1,5/6 ELISA. This test is NOT recommended for diagnosis of EPM. 

Why paired serum and CSF? Why not just serum?
  • High titers in blood does NOT equal active neurologic disease!
    • In some parts of the US, 80+% of horse will test positive for EPM in blood(!!), but very few will have neurologic disease from EPM.
  • CSF titers will tell you if there is active inflammation associated with the causative agents of EPM in the central nervous system.
  • A recent study out of U Penn showed that, in the majority of cases, it’s actually cheaper in the long run to pay for the CSF tap and paired titers, than it is to empirically treat for EPM based on serum titers alone! (This way you’re avoiding $$$ treatments for cases that don’t need it.)
  • Caveats to this:
    • The only time to consider blood EPM titers helpful without CSF- if blood titers are NEGATIVE, then that is effectively a rule out to eliminate EPM from your differential list (99.9% of the time)
    • Rarely, we sample CSF so acutely that they won’t have high titers… yet


FDA approved treatments are outlined below. In general, ALWAYS recommend a recheck neuro exam in the last week of treatment, to decide if a longer course is necessary (it often is). Repeat blood and/or CSF testing is NOT recommended to determine treatment length.

  • Ponazuril (Marquis)- paste
    • 28 day course length.
      • First dose only: 3x standard dose (15mg/kg loading dose, then 5mg/kg every day after that)- this helps a steady state be reached faster.
    • Administer with ½ cup vegetable oil to increase bioavailability.
    • No adverse effects reported at therapeutic doses.
  • Diclazuril (Protazil)- pellets
    • For treatment- standard dose is 1 mg/kg PO q24h.
    • For prevention (small studies, relatively recent, so take it with a grain of salt):
      • In a 2022 study, 1 mg/kg PO q7d achieved target blood concentrations.
      • In a 2018 study, 0.5 mg/kg PO q3-4d (2x/week) achieved target blood concentrations.
    • No adverse effects reported at therapeutic doses.
  • Sulfadiazine/Pyrimethamine (ReBalance)- liquid
    • 90 day course minimum (up to 270 days.)
    • Bone marrow suppression is possible, particularly with prolonged courses of treatment.
  • NSAIDs- +/- during the first week of EPM treatment
    • There can be worsening in neuro signs in the first week of treatment, from the inflammation associated with protozoa die-off. Consider NSAIDs during the first week of treatment (ideally check creatinine first.) 



  • Majority of horses treated with an FDA-approved treatment are expected to improve by at least 1-2 grades of ataxia.
    • A complete recovery is often but not always possible (the worse the case, the more likely there will be residual defects, but ultimately impossible to predict.)
  • Relapses are possible – months to years later
    • Currently impossible to predict which cases will relapse, and when.



Limit access of wildlife (opossums) to equine properties (pastures, barns, feed storage areas) (aka, easier said than done.)


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 About the Author: Sarah Humphreys, DVM, DACVIM (LAIM)


Dr. Sarah Humphreys is a large animal internal medicine specialist, and as part of her training she completed a rotating internship at University of Pennsylvania’s New Bolton Center, a rotating fellowship at Oregon State University, and her residency at University of California Davis. Sarah started Humphreys Veterinary Consulting in 2022 to offer teleconsulting services for large animal veterinarians with internal medicine cases, and  she loves teaching veterinarians and students alike.

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