FieldDxHypoglycin A intoxicationClinical signs, differential diagnosis, treatment and prevention in horses.
Overview
A frequently fatal muscle disease of grazing horses ("atypical myopathy"), most common in autumn. Hypoglycin A, a toxin in sycamore maple and box elder (Acer spp.) seeds/seedlings, which irreversibly blocks fatty-acid metabolism enzymes in muscle.
Clinical signs
- Frequently fatal myopathy.
- Clinical signs include acute-onset muscle weakness, recumbency, and myoglobinuria/hemoglobinuria.
- Biochemical evaluation reveals a characteristic, profound increase in serum creatine kinase (CK) and aspartate aminotransferase (AST) activity.
- Necropsy reveals widespread myodegeneration in postural and respiratory skeletal muscles and the myocardium.
- Special stains for lipid reveal excessive lipid storage in the heart, diaphragm, and other oxidative postural muscles.
- Accumulated serum short- and medium-chain acylcarnitines, urine organic acids, and glycine conjugates typical of multiple acyl-CoA dehydrogenase deficiency are present.
- Survival rate is extremely low (fewer than 25% survive).
Predisposing factors
- Ingestion of Hypoglycin A from seeds/leaves of certain plants while grazing.
Look-alikes to rule out
Treatment
Supportive therapy is the mainstay, including administration of antioxidants (e.g., vitamin C, vitamin E, riboflavin) and intravenous fluids containing dextrose to maintain hydration and renal perfusion.
Drugs mentioned: Riboflavin Vitamin C Vitamin E
Prevention
1. Deny access to the toxin source: fence off or remove sycamore maple (Acer pseudoplatanus) and box elder (Acer negundo) trees and their drip zone from all grazing paddocks. 2. Remove or collect fallen seeds ("helicopters") and pull emerging seedlings; do not turn horses out onto pasture carrying visible seed or seedling burdens. 3.
Source
A Textbook of the Diseases of Cattle, Horses, Sheep, Pigs and Goat by ELSEVIER (Page 1505, index 553, 548) 71, 111; The Merck Veterinary Manual 11th Edition (Pages 1185–1190) 112. Col H added from: Bochnia et al., "Hypoglycin A Content in Blood and Urine Discriminates Horses with Atypical Myopathy...", PLOS ONE 2015, https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0136785; and Sander et al., J Vet Diagn Invest 2016, https://journals.sagepub.com/doi/10.1177/1040638715624736 (a dedicated Merck Veterinary Manual atypical myopathy page was not retrieved — pending verification)
MSD Veterinary Manual — Plants Causing Myopathies in Horses (accessed 2026-08-11)
Condensed and attributed, never reproduced whole. Open the full entry in the app.