The disease in a nutshell

Atypical myopathy is a frequently fatal environmental intoxication that can affect all types of equids (horses, ponies, donkeys, mules, zebras…) with acess to pasture. In Europe, this intoxication is linked to the ingestion of seeds and seedlings of the sycamore maple tree (Acer pseudoplatanus).

Indeed, Equine atypical myopathy is a myopathy (i.e., muscle disease) affecting equids staying in pasture most of the day (the majority of cases were in pasture for more than 6 hours a day). This extremely severe intoxication is linked to the presence of certain trees from the Acer genus, in particular the maple tree (Acer pseudoplatanus) in Europe (1) and the negundo tree (Acer negundo) in the United States of America (2).

The Acer genus includes more than 500 species of maples around the world. Several species can be toxic to equids. In Belgium, the three mostly encountered Acer species are the sycamore maple tree (Acer pseudoplatanus), the Norway maple (Acer platanoides) and the field maple (Acer campestre). The Norway and the field maple are completely harmless for equids.

The causes of the emergence of atypical myopathy via a tree considered as native remain unknown.

Atypical myopathy is a seasonal intoxication: three-quarters of cases are identified in autumn and a quarter in spring. This notion of "season" is not strictly correlated to the seasonal calendar as defined in the northern hemisphere: indeed, some "autumn" cases are recorded from mid-September while "spring" cases can declare themselves beyond June 21. However, it should be noted that 94% of cases occur between October 1 and December 31 for so-called “autumn” cases (Figure 1) and between March 1 and May 31 for “spring” cases (Figure 2).

^{Figure 1. Most cases resulting from the ingestion of samaras occur between October 1 and December 31 (3).}

^{Figure 2. Most cases resulting from the ingestion of seedlings occur between March 1 and May 31 (3).}

The toxicity of maple trees responsible for atypical myopathy is explained by the presence of toxins including hypoglycine A and methylenecyclopropylglycine. These toxins are more concentrated in the seeds, called samaras, which fall to the ground in autumn and are therefore likely to be ingested by an equine which thus risks being poisoned (Figure 3). In the spring, these seeds germinate and turn into seedlings, which are cause of spring cases if ingested (Figure 4).

^{Figure 3. Sycamore maple samaras (Acer pseudoplatanus). Samaras that fall to the ground in autumn are the source of the toxins responsible for atypical myopathy. Photo credit: DM. Votion}

^{Figure 4. Seedlings of the sycamore maple tree (Acer pseudoplatanus). Sycamore maple seedlings emerging in the spring are the toxic source responsible for atypical myopathy. Photo credit: DM. Votion}

To date, there is no curative treatment. Nevertheless, the results of the clinical research make it possible to guide veterinarians as to the supportive treatments (link to "treat a case") to be administered. The absence of an antidote and the high mortality rate make prevention the best strategy to avoid atypical myopathy.