An epidemiological study of risk factors associated with the recurrence of equine grass sickness (dysautonomia) on previously affected premises.
Authors: Newton J R, Hedderson E J, Adams V J, McGorum B C, Proudman C J, Wood J L N
Journal: Equine veterinary journal
Summary
# Editorial Summary: Risk Factors for Equine Grass Sickness Recurrence Between 1997 and 2001, Newton and colleagues analysed questionnaire data from 305 UK premises that had experienced equine grass sickness (EGS), identifying which environmental and management factors predicted repeat cases; one-third of surveyed premises were classified as recurrent, with an overall incidence of 2.1 cases per 100 horses annually. Several practices significantly increased recurrence risk, notably larger horse populations, presence of younger animals, mechanical manure removal, loam and sand soils, and premises housing domestic poultry, whilst protective factors included chalk soil, hand-removal of droppings, pasture cutting, and co-grazing with ruminants. The findings support the toxico-infectious botulism hypothesis, as many risk factors relate to soil disturbance and grass contamination with *Clostridium botulinum*, suggesting that mechanical practices creating dust and soil contact present greater exposure risk than manual alternatives. For practitioners working on previously affected yards, these results indicate that switching from mechanical to hand-removal of droppings, implementing pasture management through cutting rather than grazing alone, and introducing ruminant co-grazing represent potentially practical modifications—though the authors note that controlled intervention trials are needed before definitive recommendations can be made to farm managers seeking to break the cycle of recurrence.
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Practical Takeaways
- •Farms with previous EGS cases can reduce recurrence risk by switching to hand droppings removal, introducing ruminant grazing, cutting grass regularly, and avoiding mechanical soil disturbance
- •High-risk premises (studs, liveries with young horses on loam/sand soils) should implement targeted biosecurity measures around pasture management to reduce Clostridium botulinum exposure
- •Consider soil type when assessing EGS risk on affected premises; chalk soils show protective effects while loam and sand increase recurrence likelihood
Key Findings
- •33% of premises with prior EGS cases experienced recurrence, with median incidence rate of 2.1 incidents per 100 horses per premises per year
- •Risk factors for recurrence included higher horse numbers, younger animals, stud farms, livery/riding establishments, loam and sand soils, domestic bird presence, and mechanical droppings removal
- •Protective factors included chalk soil, co-grazing with ruminants, grass cutting, and hand droppings removal
- •Findings support theory that EGS is toxico-infectious botulism related to soil contamination via Clostridium botulinum exposure