Atypical Myopathy

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Atypical Myopathy

Sycamore Poisoning, Seasonal Pasture Myopathy

Atypical Myopathy Overview


Atypical myopathy (AM) also known as seasonal pasture myopathy (SPM), is a frequently fatal disease in horses caused by eating sycamore seeds ("helicopers") or seedlings from the sycamore
(Acer pseudoplatanus) and/or box elder (Acer negundo) trees. The seeds contain hypoglycin A, which is metabolized into a toxic substance known as MCPA (methylenecyclopropylacetic acid). MCPA prevents energy from being produced within the horses' muscle cells which result in damage to the muscles in the body. The severity of the disease varies from horse to horse, as it depends on the amount of toxin within the seeds and seedlings, amount eaten, and individual susceptibility of the horse.
Affected horses may develop a wide range of clinical signs and may initially include lethargy, loss of appetite and reluctance to work.

AM affects full-time pastured horses and is more frequently reported in the autumn, immediately following inclement weather such as cold, humidity and rain. Horses that develop AM are usually kept in sparse pastures with an accumulation of dead leaves, dead wood and trees in or around the pasture and are often not fed any supplementary hay or feed.

Symptoms

Red or brown urine
94%
Weakness
93%
Recumbency or reluctance to move
87%
Stiffness
82%
Depression
76%
Tachycardia
74%
Trembling
70%
Sweating
61%
Congested mucous membranes
61%
Difficulty swallowing
56%

Diagnosis

  • History
  • Clinical signs
  • Physical exam
  • Laboratory tests

While waiting for your veterinarian

  • Horses should be removed from the pasture as soon as possible and stabled and kept as quite and comfortable as possible and should avoid physical effort and stress. The health of other horses in the same pasture should be evaluated.

Treatment

Supportive care: individually adapted fluid therapy and monitoring of digestive, respiratory, cardiac, and urinary systems.

Prognosis

Guarded to poor. The estimated mortality rate is 89%.

Scientific Research

General Overviews

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Age Range

Younger horses less than 3 years of age are more susceptible.

Risk Factors

  • Sycamore trees on property
  • Turning horses out in sparse pastures with contain lots of dead leaves and trees
  • Not knowing what tree species are in horse pastures
  • Not providing horses with enough grass to eat while turned out in pasture

Seasonality

WinterSpringSummerAutumn