Equis ISSN 2398-2977

Sarcocystis neurona

Synonym(s): S. neurona

Contributor(s): Prof Peter Timoney




  • Phylum: Apicomplexa.
  • Subphylum: Sporozoa.
  • Class: Coccidia.
  • Family: Sarcocystidae.
  • Genus:Sarcocystis.
  • Species:neurona.


  • Greek:sark- flesh. 'Sarcocyst' - cyst in (muscle) tissue, Greek:neuron- nerve.

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Clinical Effects



  • Obligatory predator-prey two-host life-cycle.
  • Sporocysts survive wide temperature extremes in the environment, but are killed by desiccation or high humidity.


Sexual stages in definitive host
  • Carnivore/omnivore (opossum?) ingests bradyzoite cysts in muscles of intermediate host (prey animal).
  • As much as 40-50% feral opossums found infected in endemic areas.
  • Bradyzoites liberated in intestine   →   pass into lamina propria   →   differentiate into macro- and microgametocytes   →   conjugation of gametes   →   oocysts sporulate within host   →   sporocysts passed in feces.

Asexual stages in intermediate host

  • Ingestion of sporocysts from environment   →   sporozoites released   →   invade intestinal wall   →   enter capillaries   →   two schizogonous (asexual) stages in endothelial cells.
  • Third asexual cycle in circulating lymphocytes   →   merozoites   →   penetrate muscle cells and CNS. This is the final development stage in horses.
  • In other intermediate hosts, merozoites encyst and divide via budding (endodyogeny)   →   bradyzoites contained in sarcocyst, usually in striated muscle   →   infectious for carnivorous definitive host.


  • Direct transmission does not occur between horses.
  • Horses infected by ingestion of sporocysts on pasture and in contaminated feed or water.
  • Definitive host (thought to be opossums) infected by ingestion of sarcocysts in muscle of intermediate host.

Pathological effects

  • The agent stimulates an inflammatory response in the central nervous sytem (CNS).
  • Initial perivascular cuffing, predominately with macrophages and lymphocytes, with occasional eosinophils and multinucleated giant cells.
  • Macrophage infiltration is seen in adjoining parenchyma.
  • Causes equine protozoal encephalomyelitis   CNS: Equine protozoal myeloencephalitis (EPM)  .
  • Progressive disease, lasting from days or months, to recumbency.
  • Organism can invade very specific focal areas of CNS, or produce a wide range of neurologic dysfunction resulting from multifocal, asymmetrical lesions in brain and/or spinal cord.
  • Causes necrotizing, non-suppurative encephalomyelitis with proliferative inflammation of grey and/or white matter.
  • Organisms (merozoites) seen most readily in cases that have been treated with glucocorticoids. Occur free in affected tissue or in rosettes within neurones and monocytes.
  • Presenting signs most commonly correspond to spinal cord damage.
  • Initial acute onset progressive ataxia or ill-defined lameness unresponsive to anti-inflammatories and not easily localized.
  • Upper and lower motor neurone signs possible.
  • Muscle atrophy, usually asymmaterical.
  • Cranial nerve dysfunction, eg unilateral lesions of trigeminal motor nerve   →   atrophy of masseter, digastiricus and temporalis muscles.
  • Areas of hypoalgesia or complete loss of cutaneous sensation.
  • Brain lesions may result in head tilt, depression, loss of menace response and facial paralysis.

Other Host Effects

  • Horse appears to be aberrant intermediate host.
  • Up to 50% of horses in the US have been exposed to the agent, based on serologic surveys.


Control via animal

Control via chemotherapies

Antimicrobial drugs
  • Sulfonamide (sulfadiazine or sulfamethoxazole, often in combination with trimethoprim) with pyrimethamine, given orally.
  • Potential for toxic side-effects.
  • At least 75% of severe cases improve on this treatment, but less than a third of these recover completely. At least 80% of mild cases recover completely.
  • Continue antimicrobial therapy for at least 90 days.
  • 25% of cases may relapse; these cases are more difficult to treat.
  • May be better to treat until immunoblot negative in cerebrospinal fluid (CSF).
  • Diclazuril given orally.

Anti-inflammatory and antioxidant drugs

  • If signs are severe or rapidly progressive.
  • Flunixin meglumine   Flunixin meglumine  and dimethyl sulfoxide   Dimethyl sulfoxide  twice daily for 3 days.
  • Oral vitamin E supplementation   Vitamin E  .
  • Brief course of dexamethasone   Dexamethasone  is recommended in severe cases by some authors, but may exacerbate infection.

Control via environment

  • Prevent ingestion of sporocysts in feed or water.
  • Deny access of opossums (thought to be definitive host) to horse feeding areas.

Other countermeasures

  • Cold-prepared commercial foods may contain sporocysts.
  • Avoid corticosteroid use in young horses if possible.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Witonsky S et al (2004) Sarcocystis neurona- specific Immunoglobulin G in the serum and cerebrospinal fluid of horses administered with S neurona vaccine. J Vet Intern Med 18 (1), 98-103 PubMed.
  • Fenger C K, Granstrom D E, Langemeier J L & Stamper S (1997) Epizootic of equine protozoal myeloencephalitis on a farm. JAVMA 210 (7), 923-927.
  • Granstrom D E, Dubey J P, Davis S W et al (1993) Equine protozoal myeloencephalitis - antigen analysis of cultured Sarcocystis neurona merozoites. J Vet Diagn Invest (1), 88-90 PubMed.
  • Hamir A N, Moser G, Galligan D T et al (1993) Immunohistochemical study to demonstrate Sarcocystis neurona in equine protozoal myeloencephalitis. J Vet Diagn Invest (3), 418-422 PubMed.
  • Bowman D D, Cummings J F, Davis S W et al (1992) Characterization of Sarcocystis neurona from a thoroughbred with equine protozoal myeloencephalitis. Cornell Vet 82 (2), 115 PubMed.
  • Dubey J P, Davis S W, Speer C A et al (1991) Sarcocystis neurona n. sp., the etiologic agent of equine protozoal myeloencephalitis. J Parasitol 77 (2), 212-218 PubMed.
  • Davis S W, Daft B N & Dubey J P (1991) Sarcocystis neurona cultured in vitro from a horse with equine protozoal myelitis. Equine Vet J 23 (4), 315-317.
  • Levine N D (1979) What is 'Sarcocystis' mucosa? Ann Trop Med Parasitol 73 (1), 91-92.