Equine Protozoal Myeloencephalitis
Learn what the condition is, how it may be detected early, how it is treated or managed, and which breeds or species are linked to it.
Condition snapshot
A concise guide to the condition’s pattern, detection, management and urgency.
This snapshot is a general guide, not a diagnosis or treatment plan. New, severe or worsening signs require veterinary assessment.
About the condition
Equine protozoal myeloencephalitis, or EPM, is infection of the central nervous system by Sarcocystis neurona or, less commonly, Neospora hughesi. It occurs mainly in the Americas. Opossums are the definitive host for S. neurona and contaminate feed or water with sporocysts; horses are aberrant hosts and do not transmit EPM directly to one another.
Signs vary with the location of inflammatory lesions in brain or spinal cord. Asymmetric weakness and ataxia, toe dragging and focal neurogenic muscle wasting are especially suggestive. Other horses develop facial paralysis, head tilt, difficulty swallowing, seizures, altered behaviour or urinary dysfunction. Disease can progress gradually, fluctuate or worsen after stress, making it easy to confuse with lameness.
Exposure is common in endemic regions, while clinical nervous-system invasion is much less common. A positive blood antibody test therefore proves exposure, not active EPM. Cervical compression, trauma, EHM, degenerative myelopathy and other infections can produce similar signs and must be excluded.
Approximately many treated horses improve, but complete recovery is less common and relapse can occur. Prognosis worsens with severe deficits, recumbency and long duration before treatment. EPM is rare outside the Americas unless a horse previously lived there, so travel history is critical. Prevention focuses on protecting feed and water from opossum faeces; deworming other horses or isolating the patient does not interrupt direct transmission because there is none.
Sarcocystis neurona uses opossums as definitive hosts; horses acquire sporocysts from contaminated feed or water but do not normally pass infection to one another. The patchy lesions explain marked asymmetry and focal muscle atrophy. Because exposure antibodies are common in endemic areas, attributing every uneven gait to EPM leads to missed orthopaedic and compressive disease.
Travel history can be decisive for horses now living outside the Americas.
Screening & Early Detection
A complete neurological examination grades ataxia, weakness, cranial-nerve function and muscle atrophy, then localises lesions. History includes residence or travel in the Americas, feed storage, wildlife access, onset and progression. Safe handling is essential because an ataxic horse can fall.
Serum antibody is useful when negative or very low, because this makes EPM less likely, but a high titre alone has poor specificity. Paired serum and cerebrospinal-fluid antibody testing with a validated ratio assesses intrathecal antibody production and provides much stronger support. CSF must be collected cleanly because blood contamination can distort interpretation.
Radiography, myelography, CT or other imaging investigates cervical compression; blood and CSF tests assess EHV and regional diseases. Definitive diagnosis requires demonstration of organisms in nervous tissue after death, so ante-mortem diagnosis remains probabilistic. Repeat antibody titres are not recommended for deciding treatment duration. Objective neurological grades and gait video provide better follow-up evidence than serum titre change.
Serum and CSF are collected close together so the antibody ratio reflects the same immune state.
Treatment & Management
Approved antiprotozoal options include ponazuril, diclazuril or sulfadiazine combined with pyrimethamine, given for the product- and case-specific duration. Administration details affect absorption, and prolonged sulfonamide-pyrimethamine treatment can suppress folate metabolism and blood-cell production, requiring monitoring. Failure to improve prompts reconsideration of diagnosis and adherence rather than automatic indefinite combination therapy.
Anti-inflammatory medication is used early when treatment-related inflammation could worsen deficits. Vitamin E supports antioxidant status, while evidence for commonly promoted immune stimulants is limited. Ataxic horses need safe footing, reduced handling, protection from falls and a controlled rehabilitation programme. Severely dysphagic or recumbent horses require intensive nursing and realistic welfare review.
Clinical improvement, neurological grading and function guide duration and return to work. Residual deficits can persist even after organisms are controlled, and relapse is possible for years. Feed rooms are secured against opossums, spilled grain and carcasses removed, and water protected from faecal contamination. Opossums should not be handled or fed to cats on the property, but indiscriminate wildlife poisoning is unsafe and unnecessary.
Rehabilitation advances only when repeat neurological examinations show that balance can support the next exercise stage.
Breeds or species affected
Akhal-Teke
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Anglo-Arabian
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Arabian
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Barb
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Caspian
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Kathiawari
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Marwari
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Shagya Arabian
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Thoroughbred
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American Quarter Horse
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American Paint Horse
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Appaloosa
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Australian Stock Horse
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Brumby
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Waler
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Morgan
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Standardbred
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American Saddlebred
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Tennessee Walking Horse
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Missouri Fox Trotter
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Rocky Mountain Horse
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Peruvian Paso
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Criollo
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Mustang
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Canadian Horse
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Andalusian
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Lusitano
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Lipizzaner
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Friesian
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Dutch Warmblood
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Hanoverian
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Holsteiner
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Oldenburg
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Westphalian
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Trakehner
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Selle Français
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Belgian Warmblood
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Danish Warmblood
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Swedish Warmblood
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Swiss Warmblood
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Australian Warmblood
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Irish Sport Horse
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Cleveland Bay
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Württemberger
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Zangersheide
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Knabstrupper
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Orlov Trotter
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Camargue
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Icelandic Horse
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Pony of the Americas
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Quarter Pony
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Miniature Horse
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Clydesdale
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Shire
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Percheron
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Belgian Draft
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Suffolk Punch
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American Cream Draft
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Ardennes
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Breton
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Boulonnais
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Comtois
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Dutch Draft
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North Swedish Horse
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Norwegian Fjord
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Haflinger
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Gypsy Cob
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Irish Draught
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Welsh Pony and Cob
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Connemara Pony
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New Forest Pony
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Dartmoor Pony
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Exmoor Pony
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Shetland Pony
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Highland Pony
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Fell Pony
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Dales Pony
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Australian Pony
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Finnhorse
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Black Forest Horse
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