Atypical Myopathy

Written by Briony Witherow MSc, BSc, RNutr.

This month’s blog focuses on a potential pasture threat that faces our horses in spring. Atypical Myopathy (also known as sycamore myopathy) has seemingly increased in prevalence over recent years and many horse owners will be aware of the associated risk. This blog focuses on appraising risk, awareness of symptoms and how to identify offending trees.

What is Atypical Myopathy?

Atypical Myopathy is a serious and sometimes fatal muscle disorder resulting from the ingestion of Sycamore seeds (helicopters), leaves or seedlings. In the UK, the tree species that toxicity is attributed to is the Sycamore Acer tree (Acer pseudoplatanus).Its seeds, leaves and seedlings contain cyclopropylamino acids, hypoglycin A (HGA) and methylenecyclopropylglycine (MCPG) which have an adverse effect on the function of skeletal, respiratory, and cardiac muscle.

Key Symptoms

• Reluctance to move, muscle weakness, tremors, and stiffness.
• Lethargic/Quiet demeanour
• Sweating
• Dark red/brown urine

Seasonal Risk of Atypical Myopathy

The highest risk period for Atypical Myopathy is suggested to be Autumn (76% of cases) through seed ingestion, followed by Spring (24% of cases), through seedling ingestion. The risk period in the UK for Spring typically spans a month either side of March, and for Autumn, a month either side of October – creating two three-month higher risk periods.

Risk Assessment

The potential risk to your horse will be contingent on whether sycamore trees are located within or in the surrounding area of your horse’s pasture or turnout area. It is important to note that not all Acer tree varieties contain dangerous toxins – therefore, the presence of the familiar helicopter seeds or acer leaf shape requires closer inspection to differentiate from non-harmful species.

See below, three common UK species of Acer – note that only the Sycamore Tree is thought to contain harmful toxin, while the Field Maple and Norway Maple do not.

To remember how to differentiate between the three, remember the THREE S’s: Seed Shape, Leaf Size and Silhouette.

Reducing Risk

• Identify any Sycamore trees in proximity to pasture and consider the following:
  • Avoid grazing pasture in proximity of Sycamore trees at times of high risk (Spring and Autumn).
  • Fence trees and surrounding areas, with consideration that seeds can travel ~200m from the original tree.
  • If you cannot avoid grazing fields which share proximity to Sycamore trees, reduce pasture time to less than six hours per day during high-risk periods; avoid over-stocking pasture to ensure plentiful grass; do not feed hay/bucket feed from the floor; avoid feeding near the tree line; consider trough placement and check regularly (the toxin is soluble in water).
  • It is thought that Sycamore trees vary in their level of toxicity and lab testing for the toxin (HGA) can form part of a management solution.

Take Home Points

• If you suspect your horse has ingested Sycamore seeds, leaves, or seedlings, contact your vet immediately.
• In the event that Atypical Myopathy is suspected, remove horses from shared pasture and consider testing non-symptomatic horses with your vet.
• Brush up on your tree knowledge and get familiar with the trees within and surrounding your horse’s pasture. See if you can identify any Acer varieties and differentiate between them.
• Walk pasture and turnout areas frequently during high-risk periods and employ preventative strategies