What is atypical myopathy, or sycamore poisoning, and what you need to know

Written by Hannah Briggs

Atypical myopathy, or sycamore poisoning, 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, which contain the toxin hypoglycin A that causes severe damage to the postural, cardiac and respiratory muscles.

Over recent years, more instances of Atypical Myopathy (AM) have been recorded. Prevalence of the disease is greatest in the Autumn with around 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.

Despite many number of horses in the same herd being diagnosed with AM, it is not a contagious disease and the association between the two is due to the same grazing areas. Warmer autumnal weather is considered to increase the growth of the toxin in the seeds and saplings.

This article looks at:

• Symptoms of Atypical Myopathy
• Treatment of Atypical Myopathy
• Prevention of Atypical Myopathy

Symptoms of Atypical Myopathy

Horses that have ingested sycamore seeds will show a variety of signs relating to the damage to 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.

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

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Very dark brown urine is a significant clinical feature of Atypical Myopathy, 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 bloodstream 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.

Treatment of Atypical Myopathy

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 bloodstream. 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 problem. Sycamore seeds can be carried in windy dry conditions over a fair distance.

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

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Prevention

Prevention should be our main aim as once a horse has been affected by Atypical Myopathy 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 are always more piles of hay than 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.