Atypical Myopathy

Author: Katherine Murray BVMedSci BVM BVS CertAVP MRCVS

What is atypical myopathy?

‘Atypical’ means irregular or abnormal and ‘myopathy’ is a general term used to describe a disease or disorder affecting skeletal muscles. So ‘atypical myopathy’ is an abnormal disorder affecting the muscles, it is thought to be fatal in approximately 75% of cases.

What causes it?

It is caused by ingestion of: - Sycamore seeds or leaves that fall onto

pasture in Autumn and Winter - Sycamore seedlings that germinate in Spring

- Specifically, it is caused by ingestion of the

Hypoglycin A toxin within the plant.

Hypoglycin A (HGA) toxin

- This toxin is found in some but not all species of sycamore trees, the European Sycamore (Acer Pseudoplanatus) is the one currently linked to UK cases

- The concentration of the toxin varies between plants

- Research shows some horses are more susceptible to the toxin than others

- HGA slows or stops energy production in skeletal and cardiac (heart) muscles.

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What are the clinical signs?

- Weakness of the skeletal muscles including:

• Difficulty walking

• Muscle tremors

• Difficulty breathing

• Recumbency (unable to stand)

- Heart issues (due to cardiac muscle weakness)

- Depressed with a low head carriage

- Colic signs but often appetite remains

-Dark red or brown urine

Normal Abnormal

The dark urine is due to muscle damage. A product called myoglobin is released from damaged muscle cells and is removed from the body in the urine, giving it a dark red/brown colour (container on right).

If you are suspicious that your horse may be showing any of the clinical signs listed above call your vet immediately.

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How is it diagnosed?

Diagnosis is based on - History eg. Time of year - Environment eg. Are sycamore trees

within or close to grazing areas?

- Clinical signs (listed above)

- Clinical examination (heart and respiratory rate, temperature etc.)

- Blood samples submitted for laboratory tests

eg. Testing for high muscle enzymes which would indicate muscle damage

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Can it be treated?

Up to 75% of cases can be fatal HOWEVER early diagnosis and treatment will increase chances of survival. Clinical signs often get worse before they improve so even if signs are mild, transportation to a hospital to start intensive treatment is recommended to give your horse the best chance of survival.

Treatment includes:

- Intravenous fluid therapy (IVFT) – very large volumes are needed to:

• Protect the kidneys from long-term damage

• Prevent ongoing dehydration

- Pain relief – ‘multimodal analgesia’

• A combination of various types of intravenous pain relief may be required

• Often strong pain relief such as opioids are required which cannot be administered outside of a hospital setting

- Supplementation of vitamins

• Carnitine, vitamin B1 and vitamin B2 may support muscle cell function

• Vitamin C and Vitamin E are anti-oxidants and may be a useful adjunctive treatment

The recovery period is long, often several months. However, studies show that horses which survive to day 5 after the onset of clinical signs are likely to recover fully with no long-term side effects.

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Prevention – How to reduce the risk

Given that the disease is caused by ingestion of sycamore seeds and leaves avoid this by:

- Reducing the chance of your horse scavenging for alternative food sources if grazing is tight in Autumn and Spring:

• Provide supplementary forage

• Reduce stocking density

- Remove fallen sycamore leaves and seeds from pasture or fence off affected areas

- Test your pasture for the presence of Hypoglycin A toxin

- The ‘helicopter’ seed shape means that the seeds can travel around 200 yards so check neighbouring areas also for high-risk plants

If you have any concerns about your horse, pony or donkey please do not hesitate to contact the practice on 01635 39039.

References

Olga Witkowska-Pilaszewicz, et al.(2019) ‘Equine atypical myopathy – a review’, Animal Science Papers and Reports, 37(3), pp: 233-242.