ATYPICAL Myopathy or Sycamore poisoning is a seasonal condition of the grazing horse. The disease occurs in horses that consume sufficient quantities of the natural toxin hypoglycin A (HGA). In Ireland, seeds and seedlings from Sycamore trees (Acer pseudoplatanus) are the primary source of HGA toxin. It is therefore no surprise that disease incidence peaks in autumn and spring, related to ingestion of seeds and seedlings. Relatively more cases occur in autumn and are attributed to ingestion of samaras. Samaras are winged fruits or ‘helicopters’, containing seeds. During years when there are a high number of cases in autumn, cases tend to be seen the following spring.

Toxin concentrations in seeds and seedlings in spring

HGA concentrations in Sycamore seeds and seedlings can vary greatly, even between trees from the same location. Toxin levels in seeds increase throughout early spring, peaking in April when seeds begin to germinate. Levels are particularly high in newly emerged seedlings and gradually fall throughout the remainder of the spring.

Year-to year variation

As well as a seasonal component, the number of cases reported varies each year and relevant factors include the following:

Weather conditions

A warm summer the year prior to flowering is associated with “bumper cropping” in Sycamores. Stormy weather in autumn is an obvious contributor to samara dispersal. Rainfall is another risk factor. This makes sense as the HGA toxin is water soluble and has been detected at significant levels in water collected from wet seedlings.

Grass availability

The availability of fresh grass in the spring is a potential explanation for the finding that relatively more clinical cases occur in autumn, when seeds are on the pasture, compared with spring when seedlings are plentiful. It is believed greater intake of seedlings is more likely to occur on over-grazed pasture where seedlings protrude above the level of the grass. The same may be true when weather conditions favour slow grass growth in early spring despite the rapid appearance of seedlings.

Individual horse response

Some horses appear to be more sensitive to the toxin than others and not all horses in a grazing group will succumb to the disease. While animals of any age can be affected, younger horses are thought to be pre-disposed.

Other sources of HGA toxin

Some horses may be exposed to lower levels of toxin over a relatively long period e.g., via hay or haylage made from pasture containing Sycamore seeds or seedlings. Water sources are another potential contamination source.

Effect on the horse

Ultimately HGA toxicity prevents the horse from generating energy efficiently. Muscles most affected include the heart, and those used for breathing and standing/posture. Signs include the following:

  • Reluctance to move and progressive stiffness.
  • Lying down excessively.
  • Dark red-brown urine.
  • Changes to breathing rate and effort.
  • Affected animals can suffer a great deal of pain. Death rates are high, up to 75%. There is no specific anti-dote and treatment consists of intensive supportive care. As such, prevention is key.


  • Identify trees within and close to grazed fields, bearing in mind that samaras can disperse a surprising distance from the mother tree.
  • Test for HGA in seeds or seedlings from suspect trees. Samples can be sent to the Royal Veterinary College, Comparative Neuromuscular Diseases Laboratory. RVC Comparative Neuromuscular Diseases Laboratory Diagnostic Services.
  • If feasible collect seeds, remove seedlings or prevent horses from accessing affected areas, using fencing or stabling.
  • If attempting to destroy young seedlings be aware that seedlings still contain the toxin at significant levels after herbicidal spraying or mowing. If possible, a collection system should be used when mowing and the gathered material burnt.
  • Be aware that stormy autumn weather or felling of trees may result in heavy contamination of pasture with seeds.
  • Avoid over-grazing and provide supplementary feeding in the form of hay, haylage or concentrate feeds, particularly during autumn and spring.
  • Where possible, provide access to mains water rather than natural sources and ensure that any stationary water source is not situated under a Sycamore canopy.
  • If you have had previous cases on your premises, it may be worthwhile to restrict grazing during periods of peak risk to less than six hours a day, particularly if weather conditions have been particularly wet and windy.
  • Some useful resources:

  • Sycamore - Tree Guide UK - Sycamore tree identification
  • Royal Veterinary College: information-and-advice/fact-files/atypical-myopathy