Researchers have identified the probable cause of equine grass sickness as a neurotoxin, with similar actions to snake venom toxins but produced by pasture microbes. The breakthrough has the potential to lead to novel treatments and improved diagnostics for this devastating disease. Key aspects of the work are summarized in a special editorial in the January 2025 issue of the Equine Veterinary Journal (EVJ).
The editorial, by Bruce McGorum and colleagues, highlights the findings and implications of their article Equine grass sickness is associated with major abnormalities in the ultrastructure of skeletal neuromuscular junctions*. It is currently available on Early View and will also be published in the January 2025 print issue of the EVJ.
What Is Equine Grass Sickness?
Equine grass sickness, a previously enigmatic disease which kills approximately 1% of horses grazing in high-risk premises, was first reported in Tayside in 1907. Since then, it has been recognized throughout the UK, northern Europe, South America, and very infrequently in North America. However, the disease remains most prevalent in Scotland. It causes rapid degeneration of nerves leading to paralysis of the entire gastrointestinal tract, inability to swallow, abdominal pain, muscle weakness, drooping eyelids, sweating, salivation, and muscle tremors. As there is currently no effective treatment for the most severe (acute and sub-acute) forms of the disease, horses must be euthanized on humane grounds, typically within 24-48 hours of showing abnormal signs. In contrast, approximately 50% of horses with the mild (chronic) form of grass sickness survive with nursing and supportive care.
Causative Neurotoxin in Equine Grass Sickness
Researchers from the Dick Vet Equine Hospital, University of Edinburgh, Newcastle University, Istituto Zooprofilattico Sperimentale delle Venezie, and the University of Padova believe they have identified the neurotoxin that causes equine grass sickness. Electron microscopy revealed that affected horses have major abnormalities in the structure of their neuromuscular junctions, microscopic sites of communication between nerves and muscles that are crucial for normal muscle function. These abnormalities are believed to represent a toxin specific signature, effectively a “smoking gun,” for a neurotoxic enzyme termed a phospholipase A2.
Work is underway to definitively identify the source of this toxin. It is most likely to be produced by a microbe such as a bacterium or fungus growing on the horse’s pasture during the cold and dry weather which commonly precedes the disease.
Potential Connection to Snake Venom
Intriguingly, neurotoxic phospholipase A2 toxins are present in the venom of many venomous snakes. Consequently, there are many similarities between the signs of grass sickness and those of snake envenomation. While there is no suggestion that venomous snakes cause grass sickness, it is hoped that some of the drugs that are currently being developed to treat and promote nerve regeneration in people paralyzed by snake venoms can aid recovery of horses from grass sickness.
This neurotoxin most likely also causes the apparently identical diseases (termed animal dysautonomias) which affect cats, dogs, hares, rabbits, llamas, alpacas, and sheep. This finding contradicts the previous leading hypothesis that grass sickness is a form of botulism.
A Significant Breakthrough
Professor Bruce McGorum, R(D)SVS, said: “Identifying a probable cause of equine grass sickness represents a significant breakthrough. We hope that this discovery will lead to novel treatments and improved diagnostics for this devastating condition. We are very grateful for the generous support we have received from horse owners, veterinary surgeons, scientists, charities, and funding bodies.”
“Further work is underway to determine the source of this neurotoxin; it is likely to be produced by a microbe such as a bacterium or fungus which grows on the horse’s pasture during the cold and dry weather which precedes the disease.”
“EVJ is incredibly proud to play a small part in getting this information to vets and horse owners,” said Professor Celia Marr, Editor of the EVJ. “Speaking as an equine clinician, I know that this is a really important step towards addressing the high losses associated with equine grass sickness.”
Read the Full Article
The editorial can be found here. It is free to read for 12 weeks.
The article Equine grass sickness is associated with major abnormalities in the ultrastructure of skeletal neuromuscular junctions can be found here.
Related Reading
- Equine Grass Sickness: A Global Concern?
- Mitigation Strategies for Fescue Toxicosis in Broodmares
- Blue-Green Algae Toxicity in Horses
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