Equine ATYPICAL MYOPATHY, also known as ATYPICAL MYOGLOBINURIA, is a frequently fatal condition affecting grazing horses in autumn and/or spring. Outbreaks of the disease seem to appear after occurrence of specific climatic conditions.
The condition has been recognised in 1984. Since then several clinical cases have been described in different European countries. Belgium was first confronted with an outbreak in 2000. In France, the first cases have been described in fall 2002
The lack of a comprehensive scientific literature about clinical signs, diagnostic tools and epidemiology of atypical myopathy induced Veterinary Faculty of Liège to dedicate the first part of its research on the collection and analysis of all available information related to Belgians cases in order to inform, under an appropriate form, horses owners as well as equine practitioners (veterinaries) (see “Publications”).
Epidemiological studies have, since 2006, been enlarged to all European cases, thanks to many Veterinary Faculties and AMAG members.
The collection and analyse of information enabled to deepen clinical signs and diagnostic tools knowledge, improve preventive measures and suggest symptomatic treatment.
Unfortunately, no curative treatment is available at this time, but supportive therapy, partially based on the recently identified pathological process in muscular mitochondria from affected horses, may be recommended.
The cause (aetiology)
The cause of atypical myopathy has been recently attributed to the action of a toxin named hypoglycin A. This toxin may be contained in the seeds of several trees of the genus Acer (i.e. the maple trees) and in the past, botanical surveys carried out over the meadows of confirmed Belgian cases of atypical myopathy have consistently reported the presence of sycamore maples (Acer; Aceraceae; Acer pseudoplatanus) in the vicinity of horses.
Figure 1 – Samares of sycamore maple: this is a double samara, each comprising a seed and a membranous wing for dispersal via wind, spin like a helicopter propeller
Figure 2 – Samaras of sycamore maple tree (Acer ; Aceraceae ; Acer pseudoplatanus). Belgium, October 2014
In the United States, the box elder tree (Acer; Aceraceae; Sapindaceae, Acer negundo), another species of maple trees, has been consistently found in cases of seasonal pasture myopathy (a pathological condition similar to atypical myopathy). The box elder is not a European native species but it tends to naturalize.
Figure 3 – Samaras of box elder tree (Acer; Aceraceae ; Sapindaceae ; Acer negundo)
Figure 4 – Samaras of box elder tree (Acer; Aceraceae ; Sapindaceae ; Acer negundo)
NEW: plant testing
Following research that was supported by The Horse Trust, the Comparative Neuromuscular Diseases Laboratory of the Royal veterinary College of London (UK) now offer testing of seeds, leaves and seedlings for the Hypoglycin A toxin known to cause atypical myopathy. Please go the “Plant sample testing” for further information.
Should a curative treatment be developed in time, it will still remain crucial to prevent the disease outbreaks by setting specific measures.
When facing a horse with a tentative diagnosis of atypical myopathy, a symptomatic treatment can be put in force. It will not attack directly the cause of the disease, but will fight to decrease its clinical effects. Currently, the recommended treatment focuses on (1) limiting further muscle damage, (2) restoring the circulating volume, (3) favouring elimination of the toxin, (4) correcting acid-base and electrolyte disturbances, (5) provinding energetic substrates that can be used by the “diseased” muscle and, (6) alleviating pain if present.
Facing a case of atypical myopathy, some measures have to be taken by the veterinarian. The owner has also a great role to play to help its horse affected by atypical myopathy (see the heading: “Alert: facing a case of atypical myopathy, measures to be taken by the horse’s owner).
According to literature, atypical myopathy has a mortality rate around 74%. However, this rate differs significantly from one country and from time to time. For example, during the fall of 2006 the mortality rate dropped to 40% (vs. 70% in other countries).
Factors in favour of a recovery
Preliminary analysis of European cases shows that medical treatment, more specifically vitamins and antioxidants injections (see “Alert: facing a case, the veterinarian”) increases survival chances.
Nevertheless, in regard to the high fatality rate, any treatment should be attempted only when the horse’s suffering appears limited.
Prognostic factors for survival are: normal mucosae, no sign of severe respiratory distress, and standing position most of the time, normal rectal temperature ( 37°C < Rectal temperature ≤ 38.5°C), and normal abdominal transit.
Obese horses appear to be less at risk for the condition. It is worth noting survivors have an uneventful recovery and surprisingly only showed minimal muscle damages. Anyhow cardiac system has to be monitored for several months after clinical recovery.
Factors unfavourable to a recovery
A lying down horse, abundant sudation, anorexia (the horse doesn’t want to eat), tachycardia (high cardiac frequency; ≥ 45 beats/ Min), tachypnée (high respiratory frequency; ≥ 15 resp./ Min) respiratory difficulties. High acido-basic blood disbalance decreases horse’s survival probability.