Atypical myopathy is a seasonal illness affecting horses and ponies, most commonly in Spring and Autumn. Animals can become very ill very quickly and it is often fatal if not caught early. Survival rates from large outbreaks have been reported to be 26-70%.
Clinical signs can include depression, weakness, stiffness, recumbency (laying down/unable to rise), trembling, sweating, dark urine, congested mucous membranes, high heart rate and difficulty in breathing. Body temperature is usually not increased.
The most common cause is the ingestion of sycamore seeds (Acer pseudoplatanus). With sycamore seeds, the chemical believed to cause the illness is known as Hypoglycin A. The level of Hypoglycin A can vary from plant to plant and from region to region and year to year. The Royal Veterinary College offers a test for Hypoglycin A which may be worth considering if you have large numbers of sycamore trees around your paddocks and or previous history of cases of atypical myopathy. Click here for this information.
The higher numbers of cases in Autumn are thought to be linked to the weather. Prior to a number of large outbreaks that were studied in detail, the weather conditions have been reported variously to be “windy”, “very wet and cold”, ‘‘stormy, cold, and humid”, ‘‘stormy” or ‘‘stormy and humid”. Outbreaks in Belgium have been characterised by a lack of solar radiation, an excess of precipitation or high relative humidity, and no frost in the preceding days. In the USA, outbreaks have been associated with an absence of severe frost.
RVC Fact sheet – Download here.
Acorns from the oak Quercus robur contain tannins (tannic acid) and gallotannins which, if eaten in sufficient amounts can cause horses and ponies to become seriously ill. The amount of acorns varies from year to year due to preceding Summer weather conditions. Acorns are bitter and horses generally avoid eating them, but if they are present in very large numbers and or the pasture is poor, they may be eaten in sufficient amounts to make horses ill. There is also evidence that some horses and ponies actually like the taste and will seek out acorns. In addition, some horses and ponies appear to be more susceptible and may become ill after eating smaller numbers of acorns.
Signs that horses and ponies have been eating acorns include seeing the acorn husks in faeces. The signs of acorn poisoning in horses and ponies may include depression, dehydration, lethargy, reduced appetite, increased recumbency (lying down), colic, bloody loose droppings, constipation, mouth ulcers and kidney disease.
A large crop of acorns and strong winds increase the risk of acorn poisoning as there will clearly be more on the ground.