New research on equine grass sickness (EGS) has been published this month, greatly improving our understanding of this devastating disease.
The free, special collection in the Equine Veterinary Journal (EVJ) contains four separate studies and can be viewed at http://bit.ly/2dC7Drr
Here’s what the research reveals.
The new research reports novel risk factors for the disease, identifying key differences between EGS and botulism – a point that questions the hypothesis that EGS is caused by neurotoxins from Clostridium botulinum. It also reports a novel diagnostic technique and suggests that monitoring weight loss could help to predict whether individual horses with chronic EGS are likely to survive.
Which horses does grass sickness affect?
Grass sickness almost exclusively affects grazing horses and the UK has the highest incidence of the disease in the world. It’s estimated that the disease kills between 1 and 2% of horses in the United Kingdom annually.
What does grass sickness do?
As a result of damage to parts of the nervous system that control involuntary functions the disease causes gut paralysis.
Is grass sickness fatal?
Acute and sub-acute cases of equine grass sickness are invariably fatal while approximately 55% of chronic cases can survive and return to a useful working life.
When does grass sickness strike?
Cases are more common in spring.
What causes the condition?
Despite more than 100 years of research, supported predominantly by the The Moredun Foundation Equine Grass Sickness Fund (www.grasssickness.org.uk), the cause of EGS remains unknown.
1. ‘Equine grass sickness in Scotland: A case-control study of environmental geochemical risk factors’
In terms of risk factors, this study suggests that the high incidence of the disease in Eastern Scotland may be partly associated with the particular composition of macro and trace elements in the soil in the fields on which horses graze. Further work is required to determine whether interventions to alter concentrations of particular elements in soil could potentially reduce the risk of EGS.1
2. ‘Equine grass sickness, but not botulism, causes autonomic and enteric neurodegeneration and increases soluble N-ethylmaleimide-sensitive factor attachment receptor protein expression within neuronal perikarya’
This study identified key differences between EGS and botulism, which questions the currently favoured hypothesis that EGS is caused by neurotoxins from Clostridium botulinum, a bacterium which commonly inhabits soil. The study suggests that EGS is unlikely to be caused by neurotoxins from this bacterium and concludes that further investigation of an alternative cause for EGS is needed.2
3.‘Neuronal chromatolysis in the subgemmal plexus of gustatory papillae in horses with grass sickness’
In the this study, characteristic degeneration of nerves was identified in small biopsies collected from tongues of EGS horses during post mortem examination. Examination of these biopsies accurately differentiated control horses from EGS cases. While further validation of this technique is required, it could potentially provide a relatively non-invasive method of confirming the diagnosis in a live horse.3
4. ‘Bodyweight change aids prediction of survival in chronic equine grass sickness’
While around 55% of horses with chronic EGS survive, objective criteria for predicting survival of these cases are currently lacking. This study reported that non-survivors had greater rate and magnitude of bodyweight loss than survivors. Survival prediction curves were published to allow veterinary surgeons and horse owners to use body weight data to help determine whether an individual horse with chronic EGS was likely to survive or die.4
What the experts say
Professor Bruce McGorum, Head of the Equine Section at the Royal (Dick) School of Veterinary Studies and Roslin Institute in Edinburgh, has been leading much of the research. He said: “Given that our recent research suggests that EGS is unlikely to be caused by neurotoxins from Clostridium botulinum we are now moving on to determine whether EGS is caused by ingestion of mycotoxins produced by pasture fungi. We are very pleased that The Horse Trust (www.horsetrust.org.uk) has provided funding for this three-year investigation."
Professor Celia Marr, Editor of the Equine Veterinary Journal said: “While the cause of equine grass sickness continues to evade us, these papers present an optimistic step in the right direction for the eventual prevention of this dreadful disease. It is only with the support of the Moredun Foundation Equine Grass Sickness Fund and more recently that of The Horse Trust that our world-leading veterinary researchers are able to continue to unravel the mystery.”
1 Equine grass sickness in Scotland: A case-control study of environmental geochemical risk factors C. E. Wylie, D. J. Shaw, F. M. Fordyce, A. Lilly, R. S. Pirie, B. C. McGorum
2 Equine grass sickness, but not botulism, causes autonomic and enteric neurodegeneration and increases soluble N-ethylmaleimide-sensitive factor attachment receptor protein expression within neuronal perikarya B. C. McGorum, S. Scholes, E. M. Milne, S. L. Eaton, T. M. Wishart, R. Poxton, S. Moss, U. Wernery, T. Davey, J. B. Harris, R. S. Pirie
3 Neuronal chromatolysis in the subgemmal plexus of gustatory papillae in horses with grass sickness B. C. McGorum, R. S. Pirie, D. Shaw, N. Macintyre, A. Cox
4 Bodyweight change aids prediction of survival in chronic equine grass sickness
R. C. Jago, I. Handel, C. N. Hahn, R. S. Pirie, J. A. Keen, B. E. Waggett, B. C. McGorum