About Atypical Myopathy
Atypical Myopathy is a disease that destroys the skeletal muscle, respiratory muscles and the muscle of the heart caused by the ingestion of hypoglycin A, a toxin that is found in the "helicopter" seeds of the European sycamore tree (Acer pseudoplatanus). The toxin hypoglycin A is also found in the box elder tree (Acer negundo) causing Seasonal Pasture Myopathy (SPM) in America. The breakdown of muscle tissues causes waste products to be released into the blood stream which can lead to fatal kidney failure.
The concentration of hypoglycin A in sycamore seeds varies between seeds from the same tree, and in the seeds from different trees so there is no correlation between the quantity of seeds eaten and the severity of the disease. There has been a surge in Atypical Myopathy in recent years so all sycamore trees should be viewed as a danger to horses.
Atypical Myopathy predominantly occurs during the autumn (October-December) when seeds are cast from the trees but can also occur in spring when saplings are eaten, and the risk is higher during periods of wet and windy weather when sycamore seeds are carried further. Young horses (under 3 years of age) and older horses (over 20 years of age) appear to be more susceptible, as are horses kept on over-grazed and sparse pasture. Factors contributing to horses being poisoned by hypoglycin A are the availability of the seed in the pasture combined with a lack of adequate grazing or supplementary forage.
Symptoms of Atypical Myopathy
The symptoms of Atypical Myopathy can include lethargy, weakness and stiffness, muscle tremors and twitching, reluctance to move, depression, sweating, high heart rate, hypothermia and low temperature, breathing difficulties, colic, red or purple gums and dark urine. Horses may frequently lay down and be reluctant to stand or have trouble getting up. The disease may easily be mistaken for colic.
The symptoms of Atypical Myopathy occur suddenly and progress rapidly often developing into difficulty in breathing and heart problems and can rapidly turn fatal with death occurring within 72 hours. Once the clinical symptoms are present, prognosis is poor with only a 10%-25% survival rate.
Treatment of Atypical Myopathy
Once Atypical Myopathy is suspected, urgent veterinary care is needed. Whilst waiting for the vet do not move the horse as excessive movement may cause further muscle damage, keep the horse warm, offer sugar and sugar water and if the horse urinates try to collect a urine sample for diagnosis.
There is no antidote available but some horses diagnosed early can recover with intensive 24 hour care given at an equine hospital where the horse can be treated with intravenous fluids, pain-killers, vitamins and antioxidants to correct dehydration, support the respiratory system, prevent further muscle damage, reduce the toxins and support renal function. However the survival rate is low and even with intensive veterinary treatment, severely affected horses may die or may require euthanasia.
Prevention of Atypical Myopathy
Atypical Myopathy can be avoided by ensuring that there are no sycamore trees in fields where horses are grazed or in the surrounding area and that any natural streams passing through pastures are fenced off, as these can carry sycamore seeds that could be eaten by horses.
Where sycamore trees are present in pasture and it is not possible to remove them, it is best to fence off an area around the tree where the seeds and leaves fall, reduce the amount of time horses are turned out during October-December, regularly remove fallen leaves and seeds and feed of plenty of hay above the ground in haynets or hay racks when turned out especially where pasture is poor. This will all help to reduce the consumption of sycamore seeds.
