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Persistence of Sarcocystis neurona and histopathology in horses with equine protozoal myeloencephalitis.
Abstract: Currently, little is known about the exact role of immunopathology in equine protozoal myeloencephalitis (EPM), including the ability to persist after anti-protozoal treatment. The main objectives of this study were to determine whether is present in the CNS in horses with EPM, including previously treated cases, and to evaluate the associated histopathology and immune response. For this study, control ( = 10) and horses with EPM ( = 9) were confirmed based on our inclusion criteria. Based on our preliminary data, we classified horses with EPM and clinical signs for >6 months as chronically affected ( = 5) while horses with clinical signs for <6 months were classified as acutely affected ( = 3). Histopathology changes between acute and chronic horses were identified. Identification of was performed postmortem using PCR and IHC. Histopathology analysis with IHC identified presence significantly more than PCR ( < 0.03) when directly compared. Histopathology analysis revealed that horses chronically affected with EPM had increased degenerative changes in their CNS compared to acutely affected horses when necropsy reports were compared. Our study indicates potential important histopathology differences between horses of varying clinical sign duration and the potential persistence of after anti-protozoal treatment. Additional studies are warranted to further elucidate the role of in disease, including the use of "acute" and "chronic" as clinical definitions for EPM.
Copyright © 2026 Helber, Wagner, Leeth, LeRoith, Cecere, Lahmers, Andrews, Hay, Werre, Johnson, Clark, Pusterla, Reed, Lindsay, Taylor, Estell, Furr, Mackay, Del Piero, Carossino, Pandaleon, Weatherford, Ramirez-Barrios, Zimmerman and Witonsky.
Publication Date: 2026-04-15 PubMed ID: 42063422PubMed Central: PMC13124509DOI: 10.3389/fvets.2026.1787994Google Scholar: Lookup The Equine Research Bank provides access to a large database of publicly available scientific literature. Inclusion in the Research Bank does not imply endorsement of study methods or findings by Mad Barn.
- Journal Article
Summary
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Overview
- This study investigates whether the parasite Sarcocystis neurona persists in the central nervous system (CNS) of horses affected by equine protozoal myeloencephalitis (EPM), including those previously treated.
- It also evaluates the histopathological changes and immune responses associated with the infection, comparing acute and chronic cases of EPM.
Background and Objectives
- Equine Protozoal Myeloencephalitis (EPM): A neurological disease in horses caused by the protozoan parasite Sarcocystis neurona.
- Immunopathology Knowledge Gap: There is limited understanding about how the immune system contributes to EPM pathology and whether the parasite persists in the CNS after treatment.
- Main Objectives:
- To detect the presence of Sarcocystis neurona in the CNS of horses with EPM, including those previously treated with anti-protozoal drugs.
- To compare histopathological differences and immune responses between acute and chronic cases of EPM.
Methodology
- Subjects:
- 10 control horses without EPM.
- 9 horses diagnosed with EPM, confirmed by inclusion criteria.
- Classification:
- Acute cases: clinical signs present for less than 6 months (n=3).
- Chronic cases: clinical signs present for more than 6 months (n=5).
- Detection Techniques: Postmortem samples analyzed using:
- Polymerase Chain Reaction (PCR) to detect parasite DNA.
- Immunohistochemistry (IHC) to detect parasite proteins and localize parasite presence.
- Histopathological Examination: Tissue samples studied to evaluate degenerative changes and inflammation in the CNS.
Key Findings
- Parasite Detection:
- IHC identified the presence of Sarcocystis neurona at significantly higher rates than PCR (p < 0.03), suggesting IHC may be more sensitive or effective in detecting persistent parasites in tissue.
- Evidence of the parasite was found in CNS tissue of horses even after anti-protozoal treatment, indicating potential persistence.
- Histopathological Differences:
- Chronic EPM cases exhibited more pronounced degenerative changes in CNS tissue compared to acute cases.
- This suggests ongoing damage or cellular deterioration in longer-term disease.
- Clinical Implications:
- Findings indicate that EPM may involve persistent infection and prolonged immune-mediated tissue damage.
- Using the duration of clinical signs (<6 months vs >6 months) as “acute” and “chronic” clinical definitions may be useful in future research and treatment strategies.
Conclusions and Future Directions
- The study provides evidence that Sarcocystis neurona can persist in horses’ CNS despite treatment, and chronic cases show increased tissue degeneration.
- Histopathological differences between acute and chronic EPM suggest different disease dynamics or phases, which could influence diagnosis and management.
- Further research is needed to:
- Better understand the role of persistent infection in EPM pathogenesis.
- Clarify immune mechanisms involved in chronic tissue damage.
- Refine clinical definitions and approaches for diagnosing and treating acute versus chronic EPM.
Cite This Article
APA
Helber L, Wagner B, Leeth CM, LeRoith T, Cecere TE, Lahmers KK, Andrews FM, Hay AN, Werre SR, Johnson AL, Clark CK, Pusterla N, Reed SM, Lindsay DS, Taylor SD, Estell KE, Furr M, Mackay RJ, Del Piero F, Carossino M, Pandaleon K, Weatherford S, Ramirez-Barrios R, Zimmerman K, Witonsky SG.
(2026).
Persistence of Sarcocystis neurona and histopathology in horses with equine protozoal myeloencephalitis.
Front Vet Sci, 13, 1787994.
https://doi.org/10.3389/fvets.2026.1787994 Publication
Researcher Affiliations
- Department of Large Animal Clinical Sciences, Virginia- Maryland College of Veterinary Medicine, Virginia Tech Blacksburg, Blacksburg, VA, United States.
- Department of Population Medicine and Diagnostic Sciences, Cornell University, Ithaca, NY, United States.
- Department of Animal and Poultry Sciences, Virginia Tech, Blacksburg, VA, United States.
- Department of Biomedical Sciences and Pathobiology, Virginia- Maryland College of Veterinary Medicine, Virginia Tech, Blacksburg, VA, United States.
- Department of Biomedical Sciences and Pathobiology, Virginia- Maryland College of Veterinary Medicine, Virginia Tech, Blacksburg, VA, United States.
- Department of Biomedical Sciences and Pathobiology, Virginia- Maryland College of Veterinary Medicine, Virginia Tech, Blacksburg, VA, United States.
- Equine Health Studies Program, Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, United States.
- Department of Animal and Poultry Sciences, Virginia Tech, Blacksburg, VA, United States.
- Department of Population Health Sciences, Virginia- Maryland College of Veterinary Medicine, Virginia Tech, Blacksburg, VA, United States.
- Department of Clinical Studies - New Bolton Center, University of Pennsylvania School of Veterinary Medicine, Kennett Square, PA, United States.
- Peterson and Smith Equine Hospital, Ocala, FL, United States.
- Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, Davis, CA, United States.
- Rood and Riddle Equine Hospital, Lexington, KY, United States.
- Department of Biomedical Sciences and Pathobiology, Virginia- Maryland College of Veterinary Medicine, Virginia Tech, Blacksburg, VA, United States.
- Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Purdue University, West Lafayette, IN, United States.
- Department of Large Animal Clinical Sciences, Marion duPont Scott Equine Medical Center, Virginia- Maryland College of Veterinary Medicine, Virginia Tech, Leesburg, VA, United States.
- Department of Physiological Sciences, Center for Veterinary Health Sciences, Oklahoma State University, Stillwater, OK, United States.
- Department of Large Animal Clinical Sciences, University of Florida, Gainesville, FL, United States.
- Department of Pathobiological Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, United States.
- Department of Pathobiological Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, United States.
- Department of Large Animal Clinical Sciences, Virginia- Maryland College of Veterinary Medicine, Virginia Tech Blacksburg, Blacksburg, VA, United States.
- Department of Large Animal Clinical Sciences, Virginia- Maryland College of Veterinary Medicine, Virginia Tech Blacksburg, Blacksburg, VA, United States.
- Department of Biomedical Sciences and Pathobiology, Virginia- Maryland College of Veterinary Medicine, Virginia Tech, Blacksburg, VA, United States.
- Department of Biomedical Sciences and Pathobiology, Virginia- Maryland College of Veterinary Medicine, Virginia Tech, Blacksburg, VA, United States.
- Department of Large Animal Clinical Sciences, Virginia- Maryland College of Veterinary Medicine, Virginia Tech Blacksburg, Blacksburg, VA, United States.
Conflict of Interest Statement
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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