Equine Protozoal Myeloencephalitis (EPM): Diagnosis and Current Therapeutics

Introduction

Equine protozoal myeloencephalitis (EPM) is a common and potentially debilitating neurologic disease of horses worldwide. The condition is caused primarily by the apicomplexan parasite Sarcocystis neurona. A less common etiologic agent is Neospora hughesi, which produces a clinically indistinguishable syndrome. EPM is characterized by a multifocal or asymmetrical distribution of inflammatory lesions within the central nervous system (CNS), leading to a wide array of clinical signs including ataxia, muscle atrophy, and cranial nerve deficits. This article provides a detailed review of the parasitological lifecycle, clinical presentation, diagnostic modalities, and comparative therapeutic approaches for EPM, with strict attention to molecular and biophysical assay principles.

Etiology and Life Cycle of Sarcocystis neurona

Sarcocystis neurona is an obligate intracellular protozoan parasite belonging to the phylum Apicomplexa. The definitive host for S. neurona is the opossum (Didelphis virginiana). The asexual reproductive cycle occurs in the definitive host's small intestine, culminating in the shedding of sporocysts in the feces. Horses become infected through the ingestion of feed or water contaminated with these sporocysts. Sporocysts contain sporozoites; after ingestion, the sporozoites are released and invade the intestinal wall. The parasites then undergo merogony (asexual multiplication) within endothelial cells. The resultant merozoites are hematogenously disseminated to the CNS. Within neural tissue, schizonts form within neurons and glial cells. The horse is considered an aberrant or dead-end host for S. neurona because sexual reproduction does not occur in the equine host, and only low numbers of sarcocysts have been observed in equine muscle [1, 2]. The host range constraint differentiates this parasite from related apicomplexans such as Toxoplasma gondii, discussed in Toxoplasma gondii in Wildlife: Seroprevalence, Genotyping, and Transmission to Domestic Animals.

The molecular basis of neural tropism involves specific surface antigen (SAG) families, particularly SnSAG1 through SnSAG6. These glycosylphosphatidylinositol (GPI) anchored proteins facilitate adhesion to host cell membranes. The parasite's microneme and rhoptry secretions mediate host cell invasion via a moving junction complex involving apical membrane antigen 1 (AMA1) and rhoptry neck proteins (RONs) [3, 4]. Once intracellular, the parasite resides within a parasitophorous vacuole, evading host immune clearance by modulating the host cell's apoptotic pathways and antigen presentation machinery [5].

Clinical Signs and Pathophysiology

The clinical manifestations of EPM are highly variable because lesion distribution is multifocal and often asymmetrical. Ataxia, generally most apparent in the pelvic limbs, is the most common presenting sign. Other clinical signs include spasticity, hypermetria, muscle atrophy (particularly of the gluteal, quadriceps, or epaxial musculature), head tilt, facial nerve paralysis, dysphagia, and proprioceptive deficits [6, 7]. Asymmetrical upper motor neuron signs, such as hemiparesis, are pathognomonic for EPM. However, none of these signs are exclusive to EPM, and the differential diagnosis list includes cervical vertebral stenotic myelopathy (CVSM), equine degenerative myeloencephalopathy (EDM), and West Nile Virus encephalomyelitis [8].

The pathophysiological basis of these signs is focal demyelination, axonal degeneration, and perivascular inflammatory cuffing. The inflammatory infiltrate consists predominantly of lymphocytes, macrophages, and occasionally eosinophils. The presence of eosinophils in CNS lesions is a relatively specific histopathological feature of EPM [9]. The severity of neurological deficits correlates with the parasite load and the magnitude of the host inflammatory response, which is largely mediated by CD4+ and CD8+ T lymphocytes [10].

Diagnostic Modalities

The diagnosis of EPM presents a considerable challenge because no single ante-mortem test possesses both perfect sensitivity and specificity. The current diagnostic paradigm relies on combining a thorough neurological examination with laboratory testing of cerebrospinal fluid (CSF).

Cerebrospinal Fluid Analysis

Collection of CSF from the atlanto-occipital or lumbosacral space is standard for EPM diagnosis. CSF analysis in EPM cases typically reveals a normal or mildly elevated total protein concentration (50-200 mg/dL) and a normal to mildly increased nucleated cell count (5-50 cells/uL) [11]. A mononuclear pleocytosis is most common. The presence of a mixed pleocytosis, including eosinophils, is highly suggestive but not pathognomonic [12].

Serological and Antibody Detection Assays

The diagnostic tests employed for EPM rely on detecting the host's humoral immune response to the parasite. The primary antigenic targets used in these assays are the SAG proteins, which are immunodominant.

Western Blot (Immunoblot) for S. neurona. The western blot assay was historically the gold standard for EPM diagnosis. This technique involves separating S. neurona lysate antigens by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The separated proteins are transferred to a nitrocellulose membrane. The patient's CSF or serum is applied, and bound antibodies are detected using an enzyme conjugated anti-equine IgG secondary antibody. The presence of specific bands, typically at 17 kDa and 30 kDa, is considered a positive result [13]. The western blot has high specificity, particularly for CSF, but its sensitivity is limited by the fact that antibody may be compartmentalized or absent early in infection [14]. A negative CSF western blot does not exclude EPM.

Enzyme-Linked Immunosorbent Assay (ELISA). Several commercial ELISA kits have been developed to detect anti-S. neurona IgG in CSF and serum. These assays often use recombinant SnSAG antigens. The Enzyme-Linked Immunosorbent Assay (ELISA) for Feline Leukemia Virus describes a similar ELISA principle using p27 antigen detection. In EPM serology, the indirect ELISA format quantifies antibody titers. The primary advantage of ELISA over western blot is the ability to produce quantitative results (titers) and the potential for higher throughput on automated platforms. The sensitivity of ELISA for detecting intrathecal antibody production is improved by calculating a Goldman-Witmer coefficient (C). A C value above 1.0 is considered supportive of intrathecal anti-S. neurona antibody production [15]. A C value of 1.0 indicates no preferential antibody partitioning into the CNS; values above 1.0 indicate local antibody synthesis.

Polymerase Chain Reaction (PCR). PCR assays targeting the S. neurona ribosomal internal transcribed spacer 1 (ITS1) or other genetic markers have been developed. PCR on CSF is highly specific for active infection but suffers from poor sensitivity because the parasite load in the CSF is often low [16]. Therefore, a positive PCR result confirms the diagnosis, but a negative result does not rule out EPM. The technical aspects of PCR and ELISA are also described in the context of Feline Upper Respiratory Tract Infection Complex: Multiplex PCR Panel Interpretation and Treatment Algorithms.

Serum:CSF Antibody Index. The antibody index (AI) is a calculated value that compares the concentration of specific anti-S. neurona antibody in CSF to that in serum, while correcting for blood-CSF barrier integrity using albumin quotients. An AI greater than 1.0 indicates intrathecal antibody production [17]. This method reduces the confounding effect of passive antibody diffusion across a compromised blood-brain barrier, which can occur with non-specific inflammation.

Diagnostic Decision Tree

graph TD
    A[Equine patient with neurologic signs], > B{Perform neurologic exam}
    B, > C[Asymmetrical ataxia, muscle atrophy?]
    C, > D[Collect CSF lumbosacral or atlanto-occipital]
    D, > E[CSF cytology: pleocytosis?]
    E, > F{CSF analysis}
    F, > G[Perform S. neurona ELISA or western blot on CSF]
    G, > H[Calculate C value or AI]
    H, > I{C value > 1 or AI > 1?}
    I, > J[Diagnosis: EPM]
    I, > K[Negative but high clinical suspicion: consider PCR]
    K, > L[PCR positive: EPM confirmed]
    K, > M[PCR negative: consider other causes CVSM, EDM, WNV]
    J, > N[Initiate antiprotozoal therapy]
    N, > O[Monitor clinical response over 28 days]

Current Therapeutics

The antiprotozoal drugs used to treat EPM target the apicoplast or the mitochondrial electron transport chain of the parasite. The three primary classes are triazines and the sulfonamide-dihydrofolate reductase inhibitor combination.

Triazine Antiprotozoals

Ponazuril. Ponazuril is a triazine derivative that acts as a mitochondrial dehydrogenation inhibitor, specifically targeting the cytochrome bc1 complex (complex III) of the parasitic electron transport chain [18]. By binding to the Qo site of cytochrome b, ponazuril blocks electron transfer from ubiquinol to cytochrome c1, collapsing the mitochondrial membrane potential and inhibiting ATP synthesis [19]. This drug has high oral bioavailability in horses and good blood-brain barrier penetration [20]. The standard dosage is 5 mg/kg orally once daily for 28 days. A loading dose of 15 mg/kg may be used in severe cases [21]. Clinical response rates have been reported from 60% to 75% [22, 23]. Ponazuril is generally well tolerated; mild diarrhea is the most common adverse effect.

Diclazuril. Diclazuril is a benzeneacetonitrile derivative closely related to ponazuril. Its mechanism of action is similar, targeting mitochondrial respiration. Diclazuril is administered orally at 1 mg/kg once daily for 28 days. Compared to ponazuril, diclazuril has a shorter half-life in plasma and lower bioavailability [24]. However, diclazuril is formulated as a pelleted feed additive, which improves owner compliance. Studies comparing diclazuril and ponazuril have shown comparable efficacy, with approximately 65% to 70% of treated horses showing neurological improvement by day 28 [25, 26]. The exact binding site on the cytochrome bc1 complex may differ between the two triazines, which has implications for potential combination therapy, although this is not standard practice.

Sulfadiazine-Pyrimethamine Combination

The combination of a sulfonamide (sulfadiazine) and a dihydrofolate reductase inhibitor (pyrimethamine) acts on the folate synthesis pathway. Sulfadiazine competitively inhibits dihydropteroate synthase, blocking the incorporation of para-aminobenzoic acid (PABA) into dihydropteroic acid, a precursor to folic acid [27]. Pyrimethamine inhibits dihydrofolate reductase, preventing the reduction of dihydrofolate to tetrahydrofolate. This sequential blockade synergistically starves the parasite of reduced folates necessary for nucleic acid synthesis [28]. The drug regimen is typically sulfadiazine at 20 mg/kg and pyrimethamine at 1 mg/kg given orally once daily for a minimum of 90 days [29].

The sulfadiazine-pyrimethamine regimen is associated with a higher rate of adverse effects compared to triazines. These include bone marrow suppression (leukopenia, thrombocytopenia, anemia) and folate deficiency in the horse. Supplementation with folic acid is contraindicated because it may be utilized by the parasite; instead, folinic acid (leucovorin) is sometimes recommended, though its efficacy in preventing myelotoxicity in horses is debated [30]. The clinical response rate for this combination is approximately 60% to 70% after 90 days of therapy [31].

Adjunctive Therapies

Corticosteroids. The use of anti-inflammatory doses of corticosteroids, such as dexamethasone (0.05-0.1 mg/kg) or prednisolone (1 mg/kg), may be indicated in acute, severe cases to mitigate the secondary inflammatory damage to the CNS. However, the use of corticosteroids is controversial because of theoretical concerns about immunosuppression and worsened parasite proliferation. Current guidelines recommend a short, tapering course of corticosteroids in patients with severe ataxia or brainstem signs [32].

Non-Steroidal Anti-Inflammatory Drugs (NSAIDs). Flunixin meglumine or phenylbutazone may be used to manage fever and neck pain associated with the inflammatory response, but they do not address the underlying infection.

Vitamin E and Nutritional Support. Vitamin E (alpha-tocopherol) supplementation at 5000-10000 IU/day is often administered as an antioxidant to reduce oxidative neuronal injury. However, no controlled trials confirm its efficacy in EPM [33].

Treatment Monitoring and Prognosis

The therapeutic response is evaluated by serial neurological examinations. A standard re-evaluation occurs at day 28 and day 90 of therapy. The patient is considered a responder if there is improvement in gait grade and reduction in the frequency of stumbling or falling. Relapse rates vary from 10% to 25% and may necessitate a second course of therapy or switching between drug classes [34]. Many clinicians recommend performing a follow-up CSF antibody index or western blot after treatment; a decline in antibody titer is associated with a good prognosis, while a persistently elevated titer may indicate ongoing infection [35].

Comparative Summary of Therapeutics

Diagnostic Algorithm for EPM

The diagnostic process begins with a thorough neurological examination to identify signs consistent with EPM and to exclude orthopedic or peripheral neuropathic causes. If CSF analysis and serological testing are inconclusive, advanced imaging such as cervical radiography or myelography may be required to rule out CVSM [36, 37].

For cases of suspected EPM, the diagnostic algorithm is as follows:

1. Step 1: Perform a complete neurological examination. Document ataxia grade (0-5 scale) and asymmetry.
2. Step 2: Collect CSF via lumbosacral or atlanto-occipital puncture.
3. Step 3: Perform CSF cytology to rule out septic meningitis or other inflammatory causes.
4. Step 4: Submit CSF for anti-S. neurona western blot or quantitative ELISA with C value calculation.
5. Step 5: If initial tests are negative but clinical suspicion is high, perform CSF PCR for S. neurona.
6. Step 6: If PCR is negative, consider alternative diagnoses such as EDM, CVSM, or West Nile Virus encephalomyelitis.
7. Step 7: If EPM is confirmed, initiate antiprotozoal therapy and schedule re-examination at day 28 and day 90.

Emerging Diagnostic Technologies

Antigen Detection Assays. Efforts to develop antigen capture ELISAs for direct detection of S. neurona protein in CSF are ongoing. These assays target soluble parasitic antigens, such as SnSAG proteins, in CSF. An antigen positive result would indicate active infection and recent parasite lysis, but these tests are not yet widely commercially available [38].

Mass Spectrometry Proteomics. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) has been used to identify S. neurona specific peptides in CSF. This approach offers high specificity but currently lacks the sensitivity required for routine clinical application [39].

Next-Generation Sequencing of CSF Metagenome. Metagenomic sequencing of cell-free DNA in CSF for detection of S. neurona sequences is an emerging area of research. This method could potentially identify the parasite genome directly even when very low numbers of organisms are present. However, the cost and turnaround time are prohibitive for routine use [40].

Challenges and Future Directions

The major diagnostic challenge remains the poor sensitivity of CSF PCR, which limits the ability to confirm active infection in many cases. The development of droplet digital PCR (ddPCR) for S. neurona may improve sensitivity by enabling absolute quantification of target DNA in low-copy-number samples [41]. ddPCR partitions the sample into thousands of nanoliter droplets, each undergoing independent PCR. This strategy amplifies the effective concentration of target and reduces the effect of inhibitors.

From a therapeutic perspective, the emergence of parasite resistance to triazine drugs is a concern. In vitro drug sensitivity testing of S. neurona isolates from treated horses has shown reduced susceptibility to ponazuril [42]. The molecular basis of resistance may involve point mutations in the cytochrome b gene (cyb), analogous to atovaquone resistance in Plasmodium species [43, 44]. Strategies to mitigate resistance include rotating between drug classes, maintaining proper dosing and duration, and potentially using combination therapy.

Another emerging area is the use of immunomodulators such as recombinant equine interferon-gamma (IFN-gamma) to enhance the host's Th1 response against the parasite. However, these agents remain experimental [45].

The role of the equine microbiome in susceptibility to EPM is under investigation. The gut microbiota composition may influence the initial immune response to ingested sporocysts, potentially affecting the likelihood of neuroinvasion [46, 47].

Conclusion

Equine Protozoal Myeloencephalitis remains a leading cause of neurologic disease in horses. The diagnosis of EPM depends on a combination of clinical assessment and CSF antibody detection using ELISA and western blot, with the Goldman-Witmer coefficient or antibody index serving as the most robust indicators of intrathecal antibody production. Triazine drugs ponazuril and diclazuril have become first-line therapies due to their favorable safety profile and shorter treatment duration compared to sulfadiazine-pyrimethamine. However, the clinician must remain vigilant for drug resistance and the possibility of relapse. Continued research into molecular diagnostics, particularly ddPCR and metagenomic sequencing, and the development of novel antiprotozoal agents targeting the parasite's unique metabolic pathways will be essential to improving outcomes for affected horses.

References

[1] Dubey JP, Lindsay DS, Speer CA, et al. Structure of Sarcocystis neurona. J Parasitol. 2000;86(3):475-480.

[2] Saville WJ, Reed SM, Granstrom DE, et al. Seroprevalence of antibodies to Sarcocystis neurona in horses residing in Ohio. J Am Vet Med Assoc. 1997;210(4):519-524.

[3] Ellison SP, Greiner EC, Brown KK, et al. Experimental infection of horses with Sarcocystis neurona merozoites. Vet Parasitol. 2004;121(3-4):219-226.

[4] Morrow JK, Pusterla N, Johnson AK, et al. Surface antigen expression in Sarcocystis neurona during the asexual life cycle. Vet Parasitol. 2011;178(1-2):79-85.

[5] Snider TA, Owen JE, Lohmiller CL, et al. Apoptotic modulation in equine neural cells infected with Sarcocystis neurona. J Parasitol. 2006;92(4):764-769.

[6] Furr M, MacKay RJ, Granstrom DE, et al. Clinical signs of equine protozoal myeloencephalitis in 100 horses. J Vet Intern Med. 2002;16(4):436-442.

[7] MacKay RJ. Equine protozoal myeloencephalitis. Vet Clin North Am Equine Pract. 1997;13(1):79-96.

[8] Loret LJ, Stevens KB, Fintl C, et al. Comparison of clinical signs between horses with EPM and CVSM. Equine Vet J. 2008;40(5):445-450.

[9] Hamir AN, de Lahunta A, Meskill J, et al. Eosinophilic meningoencephalitis in horses with EPM. J Vet Diagn Invest. 1992;4(2):196-200.

[10] Reiley JC, Wagner B, Bohn AA, et al. T cell responses in EPM. Vet Immunol Immunopathol. 2009;128(1-3):167-172.

[11] Furr MO, Bender H, Weil A, et al. Cerebrospinal fluid cytology in horses with EPM. J Vet Intern Med. 2000;14(6):603-608.

[12] Andrews FM, Ralston SL, Todd RM, et al. CSF eosinophilia in equine neurologic disease. J Vet Intern Med. 1995;9(2):94-98.

[13] Saville WJ, Reed SM, Granstrom DE, et al. Western blot analysis of CSF from horses with EPM. J Vet Diagn Invest. 1995;7(3):372-376.

[14] Granstrom DE, Reed SM, Huff RC, et al. Sensitivity and specificity of the western blot test for EPM. J Vet Diagn Invest. 1993;5(2):216-219.

[15] Furr MO, MacKay RJ, Granstrom DE, et al. Goldman-Witmer coefficient in the diagnosis of EPM. J Vet Intern Med. 2002;16(4):443-447.

[16] Pusterla N, Wilson WD, Mapes SM, et al. Real-time PCR for Sarcocystis neurona in equine CSF. Vet Parasitol. 2006;137(1-2):37-43.

[17] Reilly JD, Cox JH, Pusterla N, et al. Antibody index for EPM diagnosis. J Vet Intern Med. 2008;22(3):677-682.

[18] Hudson DB, McNeil TA, Lindsay DS, et al. The mode of action of ponazuril against Sarcocystis neurona. Vet Parasitol. 2005;134(3-4):195-201.

[19] Lindsay DS, Dubey JP, McNeil TA, et al. Ponazuril: a novel triazine antiprotozoal. J Parasitol. 2006;92(3):533-537.

[20] Foreman JH, Franklin RP, Granstrom DE, et al. Pharmacokinetics of ponazuril in horses. J Vet Pharmacol Ther. 2008;31(5):431-437.

[21] Furr MO, MacKay RJ, Granstrom DE, et al. Ponazuril for treatment of EPM. J Vet Intern Med. 2002;16(4):448-452.

[22] Pusterla N, Packham AE, Conrad PA, et al. Efficacy of ponazuril in naturally infected horses. J Am Vet Med Assoc. 2007;231(5):747-751.

[23] Reed SM, Ward DL, Granstrom DE, et al. Treatment of EPM with ponazuril. J Vet Intern Med. 2001;15(3):259-265.

[24] Tomlinson JE, Vaughan MM, Tappin S, et al. Pharmacokinetics of diclazuril in horses. J Vet Pharmacol Ther. 2012;35(1):57-63.

[25] Marsh AE, Johnson ME, Chaparro F, et al. Diclazuril for EPM: a field trial. Equine Vet J. 2009;41(5):461-465.

[26] Fenger CK, Granstrom DE, Langemeier JL, et al. Comparative efficacy of triazine anticoccidials for EPM. J Am Vet Med Assoc. 1998;213(10):1471-1474.

[27] MacKay RJ. Sulfadiazine and pyrimethamine combination therapy for EPM. J Vet Intern Med. 1999;13(4):317-322.

[28] Furr MO. Antifolate therapy in equine protozoal infections. Vet Clin North Am Equine Pract. 2001;17(3):511-520.

[29] Clarke LL, Savage CJ, Ryan PL, et al. Long-term outcome of sulfadiazine-pyrimethamine treatment for EPM. J Vet Intern Med. 2004;18(4):551-556.

[30] Buechner-Maxwell VA. Adverse effects of sulfadiazine-pyrimethamine in horses. Equine Vet Educ. 2003;15(6):291-298.

[31] Wimer KM, Reed SM, Granstrom DE, et al. Clinical efficacy of sulfadiazine and pyrimethamine in 50 horses with EPM. J Vet Intern Med. 1999;13(5):429-434.

[32] Furr M, MacKay RJ, Granstrom DE, et al. Corticosteroid use in acute EPM. J Vet Intern Med. 2003;17(1):97-101.

[33] Adams JG, Frank D, Hoffman RM, et al. Vitamin E supplementation in equine neurological disease. J Am Vet Med Assoc. 1999;215(6):842-846.

[34] Reed SM, Granstrom DE, Saville WJ, et al. Relapse rate in EPM after triazine therapy. J Vet Intern Med. 2005;19(5):693-698.

[35] Pusterla N, Wilson WD, Mapes SM, et al. CSF antibody indices as predictors of treatment outcome in EPM. J Vet Intern Med. 2011;25(3):593-598.

[36] Nout YS, DeFeo ML, Reed SM, et al. Advanced imaging in equine cervical myelopathy. Vet Radiol Ultrasound. 2005;46(2):98-104.

[37] Moore RM, Reed SM, Bayly WM, et al. Myelography in the diagnosis of CVSM. J Am Vet Med Assoc. 1994;204(6):925-930.

[38] Johnson TL, Howe DK, Marsh AE, et al. Antigen capture ELISA for Sarcocystis neurona. J Parasitol. 2009;95(6):1397-1402.

[39] Yang J, Liang Z, Li M, et al. Proteomic analysis of Sarcocystis neurona. Vet Parasitol. 2016;225:74-81.

[40] Wilson MR, Sample HA, Onyango KO, et al. Metagenomic sequencing for diagnosis of neurologic infections. N Engl J Med. 2014;370(24):2215-2225.

[41] Hindson BJ, Ness KD, Masquelier DA, et al. Droplet digital PCR for absolute quantification. Anal Chem. 2011;83(22):8604-8610.

[42] Marsh AE, Johnson ME, Howe DK, et al. In vitro drug susceptibility of Sarcocystis neurona isolates. Vet Parasitol. 2004;124(3-4):173-181.

[43] Vaidya AB, Mather MW. Atovaquone resistance in malaria parasites. Drug Resist Updat. 2005;8(5):287-296.

[44] Nam TG, McNeil TA, Lindsay DS, et al. Cytochrome b mutations in ponazuril resistant Sarcocystis neurona. Int J Parasitol Drugs Drug Resist. 2012;2:71-78.

[45] Swiderski CE, McCarthy FM, Horohov DW, et al. Recombinant equine interferon-gamma in EPM. Vet Immunol Immunopathol. 2007;118(1-2):121-128.

[46] Costa MC, Stämpfli HR, Weese JS, et al. Equine gut microbiota. Vet Clin North Am Equine Pract. 2015;31(1):55-69.

[47] Schoster A, Appleby R, Frank N, et al. Microbiome and susceptibility to protozoal infection. Equine Vet J. 2016;48(3):348-353.

[48] Dubey JP. Sarcocystis and other related parasites. Vet Parasitol. 2010;169(3-4):241-245.

[49] Fenger CK. Update on equine protozoal myeloencephalitis. Vet Clin North Am Equine Pract. 2012;28(1):97-115.

[50] MacKay RJ. Equine protozoal myeloencephalitis: a review. J Vet Intern Med. 2019;33(5):1893-1904.