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Atypical Myopathy – Horse Health

Atypical MyopathyAtypical Myopathy

Atypical Myopathy

Written by Hannah Briggs

Atypical myopathy is a devastating disease that can occur in horses and donkeys of any age and breed. It occurs as a result of ingesting sycamore seeds and saplings, these contain the toxin hypoglycin A which causes severe damage of the postural, cardiac and respiratory muscles. Prevalence of the disease is greatest in the Autumn with 93% of cases being seen during this season due to ingestion of the seeds, however, increasing numbers have been observed related to ingestion of the saplings.

Horses that have ingested sycamore seeds will show a variety of signs relating to the damage of the skeletal, cardiac and respiratory muscles. These can include mild to moderate colic symptoms, depression, reluctance to move, weakness, inability to lift head and lying down. Some horses are found to have died suddenly but it is more likely that you will see clinical signs first. 70-90% of horses showing signs will not survive and most are euthanised once a diagnosis is confirmed.


Symptoms include, depression, reluctance to move, weakness, inability to lift head and lying down


Very dark brown urine is a significant clinical feature, in combination with muscle weakness, that a horse has atypical myopathy. It is produced due to acute renal failure as a result of toxic substances released during muscle breakdown being absorbed into the blood stream and damaging the kidneys. The injury to the kidneys results in very dark urine but in extreme or end stage cases the horse may produce no urine at all.

Grazing should be restricted or removed in fields that inhabit sycamore trees

Grazing should be restricted or removed in fields where sycamore seeds and saplings may be present

Horses that are displaying signs of atypical myopathy will need treatment quickly and usually require hospitalisation as the nursing care they require is very intensive. They will require large volumes of intravenous fluids in order to help maintain and support kidney function and assist in the removal of toxins from the blood stream. Easily palatable food or nutrition via their intravenous fluids will be required as these horses are often unable to eat or recumbent. A recumbent horse is a challenge in itself as due to their size and weight they quickly get sores over any bony prominence – knees, hocks, fetlocks, hips and stifles, which can become necrotic and infected.

Atypical myopathy can sometimes be confused with other ailments including colic or azoturia (tying up). It is vital that atypical myopathy is quickly identified, remember that just because there aren’t any sycamore trees in your field, it doesn’t mean that you can exclude this as a potential problems. Sycamore seeds can be carried in windy dry conditions over a fair distance.


On average one tree produces 10 000 seeds and they travel on average 30-80 metres


Prevention should be our main aim as once a horse has been affected the prognosis is very poor. Ideally horses should not be grazed on paddocks with sycamore trees or where seeds and saplings are present. In reality this can be very difficult to achieve without moving yards. Horses are more at risk during autumn and are more likely to eat harmful plants if there is a lack of food. Meaning when the grass stops growing and the grazing is poor, horses are far more likely to forage other plants. Provide your horse with hay out in the field and ensure there is always more piles of hay then there are the number of horses. Areas with sycamore trees should be fenced off if they can’t be avoided. Paddocks should be thoroughly checked for seeds as on average one tree produces 10 000 seeds and they travel on average 30-80 metres. Saplings should be pulled up and seeds collected and burnt.

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