With the cause of ‘Equine Grass Sickness'(EGS) eluding vets for more than a century, a research project has discovered it may be caused by a neurotoxic enzyme similar to toxins found in snake venoms.
The finding offers promise of new treatments for the disease, which leads to death in one per cent of animals grazing in high-risk areas and causes intestinal paralysis, colic, inability to swallow and muscle tremors and currently has no cure.
Unfortunately we have had first-hand experiences of this devastating disease at our farms. We have long been committed to supporting equine grass sickness research through funding, providing data and samples, and through sharing our skills and experience in nursing horses with a positive diagnosis. We truly welcome the news that a real step forward has been made in potentially identifying the cause of this disease and hope a prevention can be found soon.”
World Horse Welfare Belwade Farm Centre Manager Eileen Gillen.
The research, which was published recently in the Equine Veterinary Journal, involves experts at the Royal (Dick) School of Veterinary Studies’ Equine Hospital, who’s earlier studies which led to this breakthrough, were in part, funded by World Horse Welfare.
The entire research team and I are extremely grateful for the generous support given by World Horse Welfare for the first EGS studies that led up to this considerable breakthrough. Identifying a probable cause of EGS represents a significant milestone. 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 under way 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 grass during the cold and dry weather which typically occurs before cases of the disease are identified. Identifying a probable cause of EGS represents a significant milestone. We hope that this discovery will lead to novel treatments and improved diagnostics for this devastating condition.”
Professor Bruce McGorum BSc (Vet.Pathol.) BVM&S CertEIM DipECEIM FRCVS and Professor of Equine Medicine, RCVS and European Specialist in Equine Internal Medicine.
Toxin impact
Researchers examined evidence of damage in muscle tissue from horses affected.
Electron microscopic imaging revealed horses had major abnormalities in the structure of their neuromuscular junctions – sites of communication between nerves and muscles that are crucial for normal muscle function.
These abnormalities are believed to represent damage caused by a type of neurotoxic enzyme, termed phospholipase A2.
This toxin is believed to cause degeneration of the neuromuscular junction, leading to paralysis of the entire gastrointestinal tract – this results in inability to swallow, gastrointestinal distension, colic and constipation, coupled with muscle weakness and tremors.
Work is under way to identify the source of this toxin, which is expected to be a microbe such as a bacterium or fungus growing on pasture during cold and dry weather, which commonly precedes incidences of the disease.
Therapeutic insights
The toxin associated with EGS is also present in the venom of many poisonous snakes, and consequently there are many similarities between the signs of it and those of snakebites.
While there is no suggestion that venomous snakes cause EGS, it is possible some of the drugs under development to treat and promote nerve regeneration in people paralysed by snake venoms can also aid recovery of horses from EGS.
The neurotoxin newly linked to EGS is expected also to be the cause of apparently identical diseases, known as animal dysautonomias, affecting cats, dogs, hares, rabbits, llamas, alpacas and sheep.
This finding contradicts a previous theory that EGS may be a form of botulism.
“This work has clearly demonstrated the value of considering disease pathology, even subtle structural changes only visible at extremely high magnification, in the quest to identify the causes of disease, particularly those attributable to the action of toxins. The discovery of this distinct “signature” of toxin activity in EGS is a major “game-changer” in our understanding of this devastating disease”.
Professor Scott Pirie, R(D)SVS Equine Hospital
The study, published in Equine Veterinary Journal on open access, was carried out in collaboration with Newcastle University, Istituto Zooprofilattico Sperimentale delle Venezie, and the University of Padova.
It was funded by the Royal College of Veterinary Surgeon’s Charitable Trust and The Moredun Foundation Equine Grass Sickness Fund.
