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Equine veterinary journal2024; doi: 10.1111/evj.14063

Equine grass sickness is associated with major abnormalities in the ultrastructure of skeletal neuromuscular junctions.

Abstract: Equine grass sickness (EGS) is a frequently fatal multisystem neuropathy of equids. The aetiology is unknown; proposed causes include toxicoinfection with Clostridium botulinum and a mycotoxicosis. The effect of EGS on the organisation and structural integrity of the skeletal neuromuscular junction (NMJ), the target of botulinum neurotoxins (BoNTs), is unknown. Objective: To compare the organisation and structural integrity of skeletal NMJs from EGS horses, control horses and one horse with a presumptive diagnosis of botulism. Methods: Blinded, retrospective case control. Methods: NMJs in samples of diaphragm or intercostal muscle from six EGS horses, three control horses and one equine botulism case were compared using electron microscopy, morphometry and confocal light microscopy. Results: A significantly higher percentage of EGS NMJs had abnormal morphology (EGS 72.2%, 95% CI 55.6-84.4; Controls 6.9%, 1.7-23.8; OR 35.1, 8.47-244.8; p < 0.001). EGS NMJs had a significantly lower mean volume fraction occupied by synaptic vesicles (SVs) (EGS 18.7%, 12.6-28.0; Controls 36.3%, 20.8-63.4; p = 0.024). EGS NMJs had evidence of accelerated SV exocytosis and SV depletion, accumulation of neurofilament-like material in terminal boutons and/or bouton degeneration. NMJs from the botulism horse had dense packing of SVs towards the presynaptic membrane active zone, consistent with BoNT intoxication, but had absence of the abnormalities identified in EGS NMJs. Conclusions: Group sizes were limited by difficulties obtaining suitably processed samples. Ages of control and EGS horses differed. Botulism was diagnosed based on clinical and post mortem findings. Conclusions: EGS is associated with major changes in skeletal NMJ ultrastructure that are inconsistent with the effects of BoNTs. SV depletion may reflect increased exocytosis coupled with reduced repopulation of SVs via anterograde axonal transport and endocytosis, consistent with the action of an excitatory presynaptic toxin and/or neurotransmitter reuptake inhibitor. Skeletal NMJs represent a previously unrecognised target for the toxin that causes EGS.
© 2024 The Authors. Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.
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Publication Date: 2024-02-01 PubMed ID: 38301732DOI: 10.1111/evj.14063Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

The research study investigates the abnormalities in the neuromuscular junctions of horses affected by Equine grass sickness (EGS), a pathogen whose cause is not completely understood, but is often fatal for horses. The study demonstrates that this disease is linked with significant alterations in the microscopic structure of these critical components of the nervous system, distinguishing it from other conditions, such as botulism.

Objective and Methods of the Research

  • The primary objective of the study was to compare the organisation and structural integrity of skeletal neuromuscular junctions in healthy horses and those affected by EGS. Additionally, it studied the condition of a horse presumed to have botulism.
  • The researchers adopted a blinded, retrospective case control method. They analyzed neuromuscular junctions in samples of diaphragm or intercostal muscle from six EGS horses, three control horses, and one botulism-affected horse. They utilized techniques such as electron microscopy, morphometry, and confocal light microscopy for their analysis.

Results of the Study

  • The research found that a significantly higher percentage of neuromuscular junctions from EGS affected horses displayed abnormal morphology. The study further reports that synaptic vesicles – small sacs that store and release neurotransmitters in the nervous system – occupy a smaller mean volume in EGS neuromuscular junctions.
  • Neuromuscular junctions of EGS horses showed an accelerated rate of synaptic vesicle exocytosis (process where a cell transports molecules out) and their depletion, an accumulation of neurofilament-like material in terminal boutons, and/or bouton degeneration.
  • In contrast, neuromuscular junctions from the botulism horse featured dense packing of synaptic vesicles near the presynaptic membrane’s active zone, which is in line with botulinum neurotoxin effects. However, the abnormalities found in EGS junctions were absent.

Conclusion and Implications of the Findings

  • The study acknowledges limitations in the results, suggesting that the limited number of samples and the difference in age between control and EGS horses may have impacted the results.
  • The major finding is that EGS results in significant changes to skeletal neuromuscular junction microstructures, these changes are inconsistent with the effects of botulinum neurotoxins.
  • The synaptic vesicle depletion in the EGS neuromuscular junctions may reflect an increased rate of exocytosis teamed with a decreased replenishment of synaptic vesicles. This could indicate the action of a presynaptic toxin which creates perverse excitement or inhibition of neurotransmitter reuptake, leading to overstimulation of the nervous system.
  • The study, therefore, suggests that skeletal neuromuscular junctions represent an unrecognised target of the unknown toxin that triggers EGS, inviting further investigation in this direction.

Cite This Article

APA
McGorum BC, Davey T, Dosi MCM, Keen JA, Morrison LR, Pirie RS, Shaw DJ, Harris JB. (2024). Equine grass sickness is associated with major abnormalities in the ultrastructure of skeletal neuromuscular junctions. Equine Vet J. https://doi.org/10.1111/evj.14063

Publication

Equine veterinary journal
ISSN: 2042-3306
NlmUniqueID: 0173320
Country: United States
Language: English

Researcher Affiliations

McGorum, Bruce C
  • Royal (Dick) School of Veterinary Studies and The Roslin Institute, University of Edinburgh, Roslin, UK.
Davey, Tracey
  • Electron Microscopy Research Services, Newcastle University, Faculty of Medical Sciences, Newcastle upon Tyne, UK.
Dosi, Miranda C M
  • Royal (Dick) School of Veterinary Studies and The Roslin Institute, University of Edinburgh, Roslin, UK.
Keen, John A
  • Royal (Dick) School of Veterinary Studies and The Roslin Institute, University of Edinburgh, Roslin, UK.
Morrison, Linda R
  • Royal (Dick) School of Veterinary Studies and The Roslin Institute, University of Edinburgh, Roslin, UK.
Pirie, R Scott
  • Royal (Dick) School of Veterinary Studies and The Roslin Institute, University of Edinburgh, Roslin, UK.
Shaw, Darren J
  • Royal (Dick) School of Veterinary Studies and The Roslin Institute, University of Edinburgh, Roslin, UK.
Harris, John B
  • Medical Toxicology Centre and Institute of Neuroscience, Newcastle University, Faculty of Medical Sciences, Newcastle upon Tyne, UK.

Grant Funding

  • Royal College of Veterinary Surgeons Trust Blue Sky Research Fund
  • The Equine Grass Sickness Fund

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Citations

This article has been cited 3 times.
  1. Atkins CN, Hahn CN, McGorum BC. Comparison of Dysautonomia Across Species: Current Knowledge and Future Research Opportunities. J Vet Intern Med 2025 Jul-Aug;39(4):e70140.
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  2. Slavik K, Whitlock R, Johnson A. Equine botulism. Equine Vet J 2026 Mar;58(2):333-347.
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  3. Harte T, Smith D, Moore J, Wells B. Review of published research on primary dysautonomia of domestic animals. Vet Rec 2026 Jan 3;198(1):e30-e40.
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