Atypical Myopathy by Dr Kate Maxwell BSc (Hons) BVSc MRCVS GPCert(EqP)

Atypical myopathy (also known as “sycamore poisoning”) is an acute, severe muscle disease which is fatal in up to 75% of cases. This disease is usually seen in the autumn, in horses and ponies kept at pasture. It is associated with the ingestion of sycamore seeds and saplings that contain the toxin “Hypoglycin A” which disrupts energy metabolism in predominantly the leg, breathing and heart muscles.

History

This condition typically occurs in the autumn following wet, stormy weather and also in the spring after an autumn with a lot of outbreaks. Younger adults are most commonly affected. Cases are usually identified at pasture, they can be standing or recumbent on initial presentation and there is usually no history of recent exercise. Commonly there will be evidence of sycamore trees, their saplings and/or seeds on the current and/or surrounding pastures.

Clinical signs

Clinical signs result from an inability of the leg, breathing and heart muscles to use energy sources effectively. Patients often then develop multiple organ failure. Affected individuals are exceedingly dull and anxious, being reluctant to move, raise their head and may be unable to stand up. They may sweat and can show muscle tremors. They usually present with an increased heart and breathing rate. Horses who are unable to stand may become cold to the touch. Urine from affected individuals may be orange to dark brown/red in colour.

Diagnosis

Diagnosis of the condition is typically made based on history, clinical signs, blood tests, urine samples, with or without muscle biopsy.

Blood tests and urine samples are used to determine the level of muscle damage by analysing the levels of products released from damaged muscle fibres. Muscle samples can be looked at under the microscope to definitively confirm the presence of muscle damage.

Treatment

Atypical myopathy is often fatal and, to date, no specific cure has been established.  Instituted therapy aims to limit further muscle damage, provide rehydration, restore normal energy usage in affected muscle, provide vitamins and anti-oxidants to the body and alleviate pain if present.

Affected animals need emergency intensive care to maximize the chance of survival. Individuals should be moved to the nearest available stable or veterinary clinic. Intravenous fluids are the mainstay of therapy to which additional substances and drugs are added. These aim to rehydrate the patient, normalize energy metabolism and flush out the toxins from the blood. Painkillers are administered frequently. Vitamins and antioxidants have been shown to improve survival rates and so these are administered in high doses. Additional nursing care such as tempting with food and water, grooming and frequent human interaction is important, in particularly in those horses who are unable to stand.  Some cases may require slings to support them to stand and lift their heads.

Individuals who survive longer than 5 days have a markedly improved prognosis.

Prevention

• Prevent access to sycamore trees, their saplings and seeds.
• If possible, remove sycamore leaves, saplings and seeds from pasture.
• Provide additional forage and reduce stocking density in fields during high-risk periods.
• Limit grazing time on pastures with previous cases.
• Ensure access to fresh drinking water from water troughs.
• Provide access to a mineral/vitamin supplement and salt block.