Equine protozoal myeloencephalitis (EPM)



March 2017



Signalment (JPC# 2019041):  A 9-year-old, standardbred mare


HISTORY:  This mare had a history of multiple skin nodules for the past two and a half years.  In recent months she had been treated with systemic corticosteroids.  Posterior paresis developed to the extent that the horse required assistance to stand.  Grossly, the lumbar spinal cord was swollen, friable and gray.


HISTOPATHOLOGIC DESCRIPTION:  Spinal cord, lumbar:  Within both the ventral white and gray matter, there is perivascular cuffing with low to moderate numbers of lymphocytes, macrophages, fewer plasma cells and neutrophils, that multifocally extends into the surrounding neuropil.  Multifocally, similar inflammatory cells infiltrate the leptomeninges and ventral nerve roots, mixed with hemorrhage, fibrin and cellular debris.  Predominantly within the ventral funiculus of the white matter, myelin sheaths are dilated and often contain cellular debris and/or gitter cells (digestion chamber).  There are moderate numbers of swollen and hypereosinophilic axons (spheroids) as well as swollen astrocytes with amphophilic cytoplasm (gemistocytes).  Within the gray matter, there are multifocal glial nodules, hemorrhage, and occasional swollen neurons with central chromatolysis (degeneration).  Occasionally, within the white matter, there are few 10x40 μm diameter schizonts with an indiscernible outer wall that contain many 2x4 μm, oval, basophilic merozoites.  A golden-brown, finely granular pigment is often present within neuronal sarcoplasm (lipofuscin).


MORPHOLOGIC DIAGNOSIS:  Spinal cord, lumbar:  Meningomyelitis and radiculitis, lymphoplasmacytic and histiocytic, multifocal, moderate, with hemorrhage, necrosis, axonal degeneration, gliosis and protozoal schizonts, standardbred, equine.


ETIOLOGIC DIAGNOSIS:  Sarcocystal meningomyelitis


CAUSE:  Sarcocystis neurona


CONDITION:  Equine protozoal myeloencephalitis (EPM)


CONDITION SYNONYMS:  Equine protozoal meningomyelitis; protozoal encephalomyelitis (PE)



·         Sarcocystis sp. is an apicomplexan protozoal parasite

·         Neospora caninum and Neospora hughesi can cause identical lesions

·         Most common in young adult horses (1–6 years of age); highest incidence in thoroughbreds, standardbreds and quarter horses

·         Exposure is widespread (32-89% seropositive), but prevalence of disease is much lower (<1%)

·         Most cases appear in the summer and fall


·         Horses are both aberrant dead-end and intermediate hosts, but are usually dead-end hosts (i.e., do not commonly develop sporocysts in tissue) and lesions are limited to the brain and spinal cord

·         S. neurona can parasitize all regions of the equine CNS and may be found in neurons, mononuclear cells and glial cells

·         Cell mediated response important for immunity; steroid administration invariably worsens the clinical signs

·         The factors affecting neuroinvasion and neurovirulence are unknown

·         Ingestion of oocysts in feed contaminated by the definitive host (opossum) > sporocysts excyst > extra-intestinal merogony (two generations; exact locations unknown) > infection of leukocytes > entry into CNS (actual route unknown, presumed leukocyte trafficking) > infection of neurons, macrophages, neutrophils and glial cells > necrosis > denervation atrophy of affected muscles

·         Although reproduction in arterioles and capillaries occurs in other Sarcocystis sp., this has not been demonstrated in horses

·         Sarcocystis neuronasurface antigen (SnSAG1)expressed on merozoites is involved with infection and immunity; only merozoites expressing SnSAG1 were able to experimentally reproduce disease; vaccination against this protein reduced severity of disease



·         Obligatory two-host life cycle with cysts usually in muscles of wild and domestic herbivores

·         Natural intermediate hosts:  armadillos, sea otters, raccoons, skunks and cats

·         Definitive hosts:  North American opossum(Didelphis virginiana) and South American opossum (D. albiventris) shed in feces

·         Carnivore eats sarcocysts in flesh of intermediate host > zoites released > invade intestinal epithelium > sexual reproduction > oocysts (each contains two sporocysts) > shed as infected oocysts/sporocysts in feces

·         Intermediate host ingests contaminated feed or water > sporocytes excyst > sporozoites released > penetrate intestinal epithelium > asexual reproduction (schizogony) > schizonts and merozoites > final stage of differentiation (formation of sarcocysts; usually in skeletal muscle)



·         Variable; depends on area of CNS affected

·         Acute to slowly developing, obscure lameness, asymmetric ataxia, or focal muscle atrophy

·         Clinical pathology:  no consistent changes in CBC or serum chemistry

·         CSF analysis:  inconsistent changes include increased protein concentration, mononuclear pleocytosis, protozoal agent, eosinophils (rare), or normal if lesions are small and buried deeply in the brain parenchyma



·         Gross lesions confined to CNS

·         Multifocal and asymmetric/random areas of gray-brown to dark yellow discoloration/necrosis with hemorrhage, swelling and liquefaction 

·         Frequency of lesions:  spinal cord > brainstem > rest of brain

·         Destruction of both grey and white matter



·         Multifocal areas of necrosis,hemorrhage, malacia with aggregation of gitter cells and gliosis in both grey and white matter

·         Large numbers of lymphocytes, histiocytes, plasma cells and fewer eosinophils and neutrophils that form thick perivascular cuffs, especially in meninges

·         Chronic cases:  Predominantly histiocytes with occasional eosinophils and multinucleated giant cells

·         Axonal swelling or loss with spheroids and digestion chambers

·         Organism:  resembles Toxoplasma gondii; few and often difficult to find; may be found in neurons, astrocytes, eosinophils and macrophages; easier to visualize in neurons than inflammatory cells

o   Schizonts:  oval or irregularly round, thin walls (<0.5 μm), 20 μm diameter, contain a few basophilic ovoid merozoites and may resemble a macrophage or degenerated host cell

o   Merozoites:  5 μm x 1.5 μm, single, centrally located vesicular nucleus formed centrally or peripherally in schizont, often around a residual body; crescent shaped to round and often within aggregates in host cell

o   Sarcocysts:  500 μm long and 40 μm wide, 1-2 um thick cyst wall; contain 5 μm long bradyzoites



·         Schizonts and merozoites:  In host cell cytoplasm; no parasitophorous vacuole

·         Merozoites:  Either slender or stumpy, lack rhoptries, micronemes, a conoid, apical rings, and polar rings

o   The lack of rhoptries contrasts Sarcocystis with other apicomplexan genera including Toxoplasma, Isospora, Eimeria, Bestoitia, Hammondia, and Neospora

o   The bradyzoites of Sarcocystis apparently do have rhoptries but merozoites do not



·         IHC:  Aid in identification of schizonts and merozoites since often difficult to find

·         Definitive antemortem diagnosis is challenging and there are frequent false positive results

·         Serum and CSF antibody analysis:  ELISA detection of antibodies against S. neurona surface antigens (SnSAG2, 4/3 serum:CSF titer ratio)

·         Calculating antibody coefficients between CSF and serum (antibody index (AI) and C-value) helps rule out seropositivity without CNS infection

·         PCR:  Low sensitivity; may be useful in conjunction with other information




·         Cervical vertebral myelopathy

·         Equine herpes virus myelitis (EHV-1)

·         Equine motor neuron disease (EMND)

·         Rabies (Rhabdoviridae; lyssavirus)

·         Western and eastern equine encephalomyelitis (Flaviviridae; alphavirus)

·         West Nile virus (Flaviviridae; flavivirus)

·         Idiopathic polyradiculoneuritis (cauda equina syndrome)

·         Rhodococcus equimyeloencephalitis (secondary to vertebral osteomyelitis)

·         Leukoencephalomalacia [Fusarium moniliforme (fumonisin B1 toxicosis)]



·         Toxoplasma gondii

·         Neospora sp.

·         Other Sarcocystis sp.

·         Microsporidian parasites



·         Protozoal encephalomyelitis:  Sporadic in cattle and sheep

·         S. neurona-like disease (encephalomyelitis) occurs in a variety of mammals including cats, mink, raccoons, skunks, Pacific harbor seals, ponies, dogs, skunks and Southern sea otters; may involve microbes similar to S. neurona

o   Has emerged as important cause of mortality in marine mammals (esp. sea otters)



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