Changes in gut fungi seen in horses with equine grass sickness – study

Equine grass sickness is a generally fatal multiple systems neuropathy of unknown cause in grazing horses.
Equine grass sickness is a generally fatal multiple systems neuropathy of unknown cause in grazing horses. Photo: The Royal (Dick) School of Veterinary Studies, University of Edinburgh

Equine grass sickness in horses is associated with alterations in the fungal community in the gastrointestinal tract, researchers have found.

Equine grass sickness is a generally fatal multiple systems neuropathy of unknown cause in grazing horses. An apparently identical disease occurs in cats, dogs, rabbits, hares, sheep, alpacas and llamas.

The condition kills about 1 to 2% of horses that graze affected premises in Britain annually.

Many risk factors for the disease are consistent with it being a pasture mycotoxicosis — caused by the ingestion of food containing toxic byproducts produced by certain fungi.

Researchers in a University of Edinburgh study set out to identify fungi in the gastrointestinal tract of affected horses that might be responsible for the disease.

Samples were collected post mortem from up to five sites within the gut of affected horses for molecular-based analysis. In all, 150 samples from 54 affected horses were collected.

The results were compared with those from 67 samples taken from 31 unaffected horses euthanised for reasons unrelated to the study.

Faecal samples were also collected from 48 unaffected control horses that had been co-grazing with horses that came down with equine grass sickness.

Bruce McGorum and his fellow researchers, writing in the journal Animal Microbiome, found evidence of a very rich and diverse range of fungi at all five sites in all the horses tested — richer than that of humans and mice.

Analysis indicated that the vast majority had been ingested, with most phylotypes ingested environmental microfungi, agaricoids and yeasts.

The richness of the fungal community — the mycobiota — varied throughout the gastrointestinal tract and was greater in the diseased horses.

They identified 56 key phylotypes that had increased abundance and high prevalence in samples taken from horses with the disease.

The increased abundance in affected horses could reflect increased gut colonisation by opportunistic pathogenic fungi, but more likely reflected ingestion of increased numbers of these fungi in plants, litter and soil while grazing, they said.

“Indeed there is evidence to suggest that equine grass sickness horses are exposed to increased numbers of a wide range of diverse environmental microbes.”

Further work is required to determine whether these fungi produced neurotoxic substances that could play a role in the development of equine grass sickness, or whether their association with the condition is coincidental.

Many key phylotypes were extremophiles — adapted to live in extreme environments — or were predicted to produce toxic substances, the authors noted.

They acknowledged it was possible that the alterations in the fungal gut community associated with equine grass sickness could be a consequence, rather than a cause, of the disease.

The study team said their work is the first reported molecular characterisation of the gastrointestinal mycobiota of grazing horses, with key phylotypes associated with the condition identified.

The study team comprised McGorum, Zihao Chen, Laura Glendinning, Luanne Hunt, Alasdair Ivens, John Keen, Scott Pirie, Toby Wilkinson and Gerry McLachlan, all with the University of Edinburgh; Hyun Gweon, with the University of Reading; and Joanne Taylor, with the Royal Botanic Garden Edinburgh.

McGorum, B.C., Chen, Z., Glendinning, L. et al. Equine grass sickness (a multiple systems neuropathy) is associated with alterations in the gastrointestinal mycobiome. anim microbiome 3, 70 (2021).

The study, published under a Creative Commons License, can be read here

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