Atypical myopathy is a severe and usually fatal disease that affects grazing horses. There is no predisposition based on sex, breed, or age. Horses are acutely affected by muscle necrosis, most often generalized, which can clinically resemble a stroke, even when they have not been exercising. The few early signs described are lethargy, decreased appetite, signs of colic, and stiffness. Horses are frequently found lying down or exhibiting marked stiffness in the hindquarters and are unwilling to move. Muscle tremors and localized or generalized sweating are commonly observed. The urine is often dark. Rectal temperature is often low.
Diagnosis is made by excluding other diseases and is based primarily on clinical signs and blood tests. Currently, there is no definitive diagnosis for atypical myopathy.
The cause of the disease was unknown for many years, but recent epidemiological studies have led to an advance in knowledge.
In Europe, a Danish study highlighted the presence of sycamore maples (Acer pseudoplatanus) in pastures where affected horses were kept. Some of these maples were infected with a fungus (Rhytisma acenirum). A Belgian study also supports this hypothesis.
In North America, a study identified a toxin, hypoglycin A, found in the seeds of the box elder (Acer negundo), which could play a role in the disease. In fact, a metabolite (residual product) of this toxin was found in the serum and urine of affected horses.
Ongoing studies will likely provide more information on the direct role of maple trees. As a precautionary measure, grazing should be avoided in meadows where maple trees are present, or the trees should be cut down. This recommendation is particularly important in pastures where cases of atypical myopathy have been reported.
Sycamore maple (Acer pseudoplatanus) (source: Wikipedia) Maple (Acer negundo) (source: Wikipedia)
For more information, please visit the website www.myopathieatypique.fr
References
Votion DM, Linden A, Saegerman C, Engels P, Erpicum M, Thiry E, Delguste C, Rouxhet S, Demoulin V, Navet R, Sluse F, Serteyn D, van Galen G, Amory H.: History and clinical characteristics of atypical myopathy in horses in Belgium (2000-2005). J Vet Intern Med. 2007 Nov-Dec;21(6):1380-91. Sas AM, van der Kolk JH, Dank M, Westermann CM. : Atypical myopathy: a review and des cription of the outbreak in the Netherlands during autumn 2009 and spring 2010. Tijdschr Diergeneeskd. 2012 Aug 1;137(8):514-21. Valberg SJ, Sponseller BT, Hegeman AD, Earing J, Bender JB, Martinson KL, Patterson SE, Sweetman L.: Seasonal pasture myopathy/atypical myopathy in North America associated with ingestion of hypoglycin A within seeds of the box elder tree. Equine Vet J. 2012 In press.
By Dr Valérie PICANDET, dipl. ACVIM/ECEIM
Specialist in internal medicine 