Atypical Myopathy in Horses: Welfare and Prevention
Disease Overview
Atypical myopathy (AM) in horses is caused by hypoglycin A (HGA), a toxin found in seeds, seedlings, and helicopter seeds (samaras) of sycamore (Acer pseudoplatanus) and related maples. HGA disrupts mitochondrial fatty acid oxidation in muscle cells, causing rhabdomyolysis. It primarily affects the postural and respiratory muscles. Outbreaks occur in autumn (seed fall) and spring (seedling emergence). Mortality rates are high (up to 75% in untreated cases).
Welfare Consequences
AM causes acute, severe suffering: affected horses develop muscle weakness, trembling, recumbency, difficulty raising the head, myoglobinuria (dark urine), rapid heart rate, and respiratory distress. Most affected horses cannot stand and experience extreme distress. Even with intensive treatment, many die within 24-72 hours of symptom onset. The speed of onset and severity of suffering make prevention the only meaningful welfare strategy.
Risk Factors and Grazing Management
Risk is highest when horses graze on pasture with sycamore trees during seed fall (September-November) or seedling emergence (April-May). Hungry horses on sparse pasture eat seeds and seedlings more readily. Key preventive measures: fencing horses away from sycamore trees and their seed fall area; removing seeds from pasture where possible; providing additional hay when pasture is sparse in autumn and spring; and monitoring weather conditions (wind increases seed dispersal).
Diagnosis and Treatment
Diagnosis is based on clinical signs, elevated muscle enzymes (CK, AST, LDH), history of sycamore exposure, and if available plasma HGA measurement. Treatment is intensive supportive care: IV fluids, glucose supplementation (supporting alternative metabolic pathways), B-vitamin supplementation (riboflavin, carnitine support mitochondrial function), anti-oxidants, and nursing for recumbent horses. Prognosis is guarded; horses that survive the first 5 days have improving chances.
Owner Education and Awareness
Owner education is critical for prevention. Identifying sycamore trees on or adjacent to grazing land, understanding seasonal risk periods, and knowing early signs of AM allows rapid intervention. Autumn is the highest risk period, but year-round awareness is needed. Contacting a vet immediately if AM is suspected maximises chances of survival. Reporting cases to the Atypical Myopathy Alert network contributes to understanding of outbreak patterns.