Atypical myopathy: Millie's story

A devastating case of atypical myopathy in Norfolk prompted us to raise awareness of the dangers to grazed horses. Atypical myopathy (AM) is caused by the ingestion of the toxin hypoglycin-A commonly found in the seeds of sycamore trees, leading to the destruction of muscle tissue including the heart. It is thought that bacterial or fungal toxins prevalent in mild climates (autumn and spring) are additional attributing factors for AM.

Although rare, recent reports have indicated that most cases of AM result in a fatality within 72 hours of symptoms being observed, which include the sudden onset of weakness or stiffness, coupled with a dark coloured urine or difficulty urinating, difficulty standing or walking, difficulty breathing, depressed demeanour and muscle tremors. However, a recent case in Norfolk – from clinical signs to euthanasia – progressed within less than four hours.

Millie, a 10-year-old 15hh cob, started showing colic-like symptoms on Saturday 1st November last year, prompting an emergency call to Knotts Yard Veterinary Practice by a friend of Millie’s owner who was temporarily caring for her. Millie was observed as experiencing stiffness, sweating, discomfort when standing and repeatedly lying down, as well as muscle tremors, low temperature and high heart rate. Blood samples taken by the vet quickly confirmed atypical myopathy, and despite treatment Millie deteriorated rapidly and was sadly put to sleep in the early evening.  

Pippa Childs BVetMed MRCVS of Knotts Yard said: “Prompt diagnosis and treatment give the best chance of survival but Millie’s case illustrates how quickly these horses can deteriorate. A blood test was taken and run immediately at the lab in our surgery, confirming AM. I went straight back to Millie planning to get her onto intravenous fluids and refer her into a hospital facility; however, she had deteriorated so quickly that euthanasia was the kindest option for her. It was only four hours between the initial call out and Millie’s euthanasia”.

Subsequent immediate action was taken to protect Millie’s companions following AM diagnosis, which involved cordoning off grazing where fallen sycamore seeds were present and close observation of the remaining horses for the next 72 hours (the alert period).


Millie (centre) with field mates Mason (left) and Passandra

Owner Julie Field’s other horses, two Shetland ponies Mason and Passandra, and thoroughbred cross Harvey, have all been rehomed to her from Redwings, making this case particularly compelling. Commenting on the case, our Chief Executive Lynn Cutress said: “We are devastated for Julie. We feel a real responsibility to raise awareness of this devastating disease for all owners to ensure horses are protected”.

Following the rise in reported cases of AM, as identified by the Royal Veterinary College, we are eager to provide practical advice to reduce exposure to hypoglycin-A toxicity. “Millie had been grazed in the same paddock for two and a half years and other horses had been kept in this paddock for over 12 years without experiencing any problems. Millie’s case sadly shows that even if horses have been historically grazed close to sycamore trees, they are always still at risk of contracting AM,” commented Redwings Head of Veterinary and Care, Nicky Jarvis.

“We urge owners to be particularly vigilant in autumn and spring months, and emphasise the importance of effective pasture management in protecting their horses; such as providing water from the mains supply, restricting access to pasture that is tree lined or covered with dead leaves that could hide sycamore seeds, and to ensure hay is free of mould and not given on the ground in a humid environment”.

Since her heart-breaking experience, Julie says she is greatly concerned that despite the recent rise in reported incidents the public may still rest on their laurels and she also calls for the veterinary sector to provide more evidence on what environmental factors increase risk. “I’m worried that horse owners may not take the risk of atypical myopathy seriously and so I want Millie’s story to raise awareness in order that something good may come out of this tragic situation. I am also pleased that Redwings has made a concerted effort in raising awareness of AM, in particular with their issuing of guidance to their Guardians”.

For horse owners seeking further advice on atypical myopathy, please call our welfare helpline on 01508 481008. However, if you suspect your horse may have the condition, call your vet immediately.