As reported by the British Equine veterinary Association (BEVA), 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 to be summarised in a special editorial in the January 2025 issue of the Equine Veterinary Journal (EVJ).
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 recognised 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 euthanised 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.
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 which 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.
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.
Intriguingly, such toxins are present in the venom of many poisonous 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 paralysed by snake venoms can aid recovery of horses from grass sickness.
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.”
