The researchers, from the Dick Vet Equine Hospital, University of Edinburgh, Newcastle University, Istituto Zooprofilattico Sperimentale delle Venezie, and the University of Padova found 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 now underway to definitively identify the source of the toxin, in the hope that it will lead to novel treatments and improved diagnostics for this devastating disease.
The researchers think the toxin is 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.
Neurotoxic phospholipase A2 toxins are also present in the venom of many poisonous snakes, and 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 paralysed by snake venoms can aid recovery of horses from grass sickness.
The researchers say that the neurotoxin most likely also causes the apparently identical diseases (termed animal dysautonomias) which affect cats, dogs, hares, rabbits, llamas, alpacas and sheep, and that their finding contradicts the previous leading hypothesis that grass sickness is a form of botulism.
This breakthrough is summarised in an editorial in the January 2025 issue of the Equine Veterinary Journal (EVJ).
The editorial, by Bruce McGorum and colleagues, looks at the implications of their article 'Equine grass sickness is associated with major abnormalities in the ultrastructure of skeletal neuromuscular junctions2".
Bruce 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.”
Bruce's editorial is currently available on Early View, free for 12 weeks, and will also be published in the January 2025 print issue of the EVJ.
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