Atypical myopathy (sycamore poisoning or seasonal pasture myopathy) is a disease caused by the ingestion of sycamore seeds (‘helicopters’), seedlings or leaves and is often fatal. During autumn the seeds fall onto pasture and the subsequent green seedlings appear in Spring.
Sycamore poisoning can be fatal
Hypoglycin A (HGA) toxin found within the seeds and seedlings causes the disease, by slowing the energy production in muscles, including the heart. Levels of HGA vary from seed to seed and tree to tree and with thresholds of toxicity varying amongst horses, predicting whether an exposed horse will become ill is virtually impossible.
Onset of symptoms can be rapid, with horses occasionally being found dead in their field. Often the first sign is colic, quickly followed by muscle tremors and weakness, leading to a dropped head and the horse may lie down but be unable to get back up as the muscles have become so weakened. Once the breathing and heart muscles are affected, death can follow, and euthanasia should be considered.
Dark red-brown urine is a tell-tale sign of atypical myopathy, as there are only a small handful of diseases which cause this symptom. If your vet suspects atypical myopathy, they will usually take a blood sample in order to test for HGA toxin as well as to check the muscle enzyme levels. Increased muscle enzymes reflect damage to muscles, which may be a result of sycamore poisoning if HGA levels are also high in the blood.
Can Atypical myopathy be treated?
The first 24-48 hours are critical and emergency hospitalisation and treatment will increase chances of survival. During this time, horses may deteriorate before showing signs of improvement. After this period, prognosis begins to creep up in most cases. However, overall prognosis is guarded with many vets predicting a 50:50 chance of survival.
Hospitalisation results in the most success because vast quantities of intravenous fluids may be given rapidly, to protect the kidneys from the damaging toxins and prevent dehydration. 24-hour nursing care also ensures better pain management by analgesic infusions; this is not possible to the same extent on a yard.
Will my horse ever be the same again?
Horses who survive the first week tend to make a full recovery. Almost all horses who survive return to their former selves with very little or no trace of the disease, but how long recovery takes depends on the individual horse. Patience, commitment and intensive treatment are required.
How can I help prevent the disease?
Pasture management is key to prevention. Minimise the risk of ingestion of sycamore – ‘SPRING’ into action:
‘Horses know what not to eat’ is commonly heard on yards but is not true. There are all too many cases of atypical myopathy and other plant toxin-related diseases seen every year. When the first-choice forage (grass) is scarce, horses are more likely to scavenge sycamore leaves, ragwort and other toxic plants.
It is also possible to test your grazing for levels of the toxin in the leaves, seeds and seedlings. Finding out if plant material contains HGA and the levels of any toxin present, can help you understand the risk to your horse to and how manage it accordingly.
Do other trees contain this toxin?
The Sycamore is a member of the Acer family of trees, of which there are over 25 other species. Research has been conducted as to whether any of the other Acer species contain HGA toxin. The most common of the family in Europe are the Norwegian and field maples, neither of which have been found to contain HGA. However, other Acer trees found to contain HGA include box elder and maple. As does the ackee. Research has also confirmed that common non-Acer family trees such as oak and ash do not contain HGA. Many of these trees have similar looking leaves, so do consult a specialist to identify which trees may pose a threat to your horse.