Clinical expression of lolitrem B (perennial ryegrass) intoxication in horses.
Authors: Johnstone L K, Mayhew I G, Fletcher L R
Journal: Equine veterinary journal
Summary
# Perennial Ryegrass Staggers in Horses: Clinical Signs and Diagnostic Challenges Whilst perennial ryegrass staggers is commonly cited as a neurological concern in equine practice, detailed clinical descriptions and diagnostic approaches have been lacking. Johnstone and colleagues exposed seven horses to perennial ryegrass seed and hay containing 2 ppm lolitrem B over two weeks, documenting neurological and clinical changes through standardised examination and video analysis. All horses developed variable tremor (notably including ocular tremor) and symmetrical vestibular ataxia, with signs worsening during activity and exacerbated by blindfolding; intriguingly, plasma lolitrem B levels did not correlate with clinical severity, whilst urine levels remained undetectable. Secondary signs including limb oedema, heel lesions and serous nasal discharge appeared in more severely affected animals, potentially attributable to ergovaline co-toxins rather than lolitrem B alone. For practitioners managing grazing horses, these findings clarify the distinctive movement disorder and variability in clinical presentation, though the poor correlation between measurable toxin levels and clinical signs highlights why diagnosis currently relies on history, clinical observation and pasture analysis rather than individual horse testing—a diagnostic gap the authors suggest plasma ELISA testing may eventually address.
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Practical Takeaways
- •Recognize perennial ryegrass-associated tremor and ataxia as a variable clinical syndrome; affected horses may show subtle signs during rest but marked incoordination during movement or when vision is compromised
- •Clinical severity does not reliably correlate with plasma lolitrem B levels, so diagnosis cannot be ruled out based on toxin concentration alone
- •Manage affected horses by removing access to contaminated ryegrass pasture; monitor for associated dermatological and respiratory signs that may indicate multi-toxin exposure
Key Findings
- •All 7 horses exposed to 2 ppm lolitrem B developed variable tremor and ataxia within 2 weeks
- •Tremor was action-related and included subtle rapid nystagmus; ataxia involved truncal sway and irregular limb placement that worsened with blindfolding
- •Plasma lolitrem B levels increased during exposure but did not correlate with clinical sign severity; urine levels showed no detectable change
- •Secondary signs including limb swelling, heel lesions, and serous nasal discharge were observed in severely affected horses, possibly due to ergovaline co-exposure