Seasonal pasture myopathy/atypical myopathy in North America associated with ingestion of hypoglycin A within seeds of the box elder tree.
Authors: Valberg S J, Sponseller B T, Hegeman A D, Earing J, Bender J B, Martinson K L, Patterson S E, Sweetman L
Journal: Equine veterinary journal
Summary
# Editorial Summary: Box Elder Seeds and Atypical Myopathy Seasonal pasture myopathy (SPM) in North America has long puzzled equine practitioners, but this landmark 2013 investigation finally identified its culprit: hypoglycin A, a toxic amino acid present in box elder tree seeds (*Acer negundo*). Valberg and colleagues compared pastures and management practices across 12 affected horses, 11 SPM farms, and 23 control properties, then analysed seeds using gas chromatography–mass spectrometry and identified the hypoglycin A metabolite MCPA (methylenecyclopropylacetic acid) in affected horses' serum and urine via tandem MS. Serum acylcarnitine and urine organic acid profiles in SPM cases showed the characteristic pattern of multiple acyl-CoA dehydrogenase deficiency (MADD)—the same metabolic dysfunction documented in atypical myopathy cases elsewhere—establishing the mechanistic link between toxin ingestion and clinical disease. Critically, box elder seeds appeared on all SPM pastures and 61% of control properties, suggesting that pasture management factors are equally important: SPM horses had significantly longer turnout, more overgrazed swards, and less supplemental feed than controls. Whilst the lethal dose threshold for hypoglycin A in horses remains undefined, these findings suggest that risk mitigation strategies should encompass both reducing seed access during autumn (particularly on depleted pastures) and maintaining adequate forage quality and quantity to minimise consumption of fallen seeds.
Read the full abstract on PubMed
Practical Takeaways
- •Identify and monitor box elder trees on autumn pastures; consider removing or restricting horse access to trees bearing seeds during fall months
- •Implement supplemental feeding and avoid overgrazing during autumn to reduce reliance on seed-contaminated forage, particularly for at-risk horses
- •Consider MADD (evidenced by acylcarnitine abnormalities) as the pathological mechanism when evaluating horses with atypical myopathy in autumn months
Key Findings
- •Box elder tree (Acer negundo) seeds were present on all 12 SPM pastures and 61% of 23 control pastures in autumn
- •Hypoglycin A, a known toxin causing acquired MADD, was identified in box elder seeds by GC-MS analysis
- •The hypoglycin A metabolite MCPA was detected in conjugated form in serum and urine of all 7 tested SPM horses
- •SPM horses had longer pasture turn-out time, more overgrazed pastures, and less supplemental feeding compared to control horses