Cause of the disease

Atypical myopathy is an acute poisoning resulting from the ingestion of toxins produced by certain trees of the genus Acer, i.e., "maples." The two implicated toxins are hypoglycin A (HGA) and methylenecyclopropylglycine (MCPrG), but other toxins may contribute to the pathological process (1, 2, 3). These toxins are present in several species of maples but not in every one of them. In Europe, the main toxic source is the sycamore maple (Acer pseudoplatanus). In the United States, the toxic source is the boxelder maple (Acer negundo).

The main sources of poisoning: Equines primarily become intoxicated by ingesting the fruits known as samaras (Figure 1) and seedlings (Figure 2 a and b) of the sycamore maple. These two toxic sources account for the seasonal nature of atypical myopathy and its emergence as clinical series in the autumn and spring. In the autumn, samaras fall to the ground due to stormy conditions and/or naturally when they mature, hence becoming accessible for ingestion (Figures 3). In the spring, samaras that have fallen on the ground germinate, and the seedlings of the sycamore maple are the primary source of intoxication during this period (Figures 4).

^{Figure 1. The fruits of the sycamore maple are double samaras, with a samara being a dry fruit whose wings allow for wind dispersal. The two samaras are connected by their fleshy part containing the seed. The size, shape, and angle between the two samaras of the same fruit allow for species determination. Photo credit: D-M. Votion}

^{Figure 2a, b. Emergence of young sycamore maple seedlings. Their development begins with the germination of the seed and the emergence of a pair of cotyledons (Figure 2a) and then two juvenile leaves (Figure 2b). The shape of the cotyledons and juvenile leaves allows for species differentiation. Photo credit: D-M. Votion}

^{Figure 3. Samaras of the sycamore maple fallen on the ground. Photo credit: D-M. Votion}

^{Figure 4. Seedlings of the sycamore maple. Photo credit: D-M. Votion}

Additional sources of poisoning: Various studies have shown that hypoglycin A (and probably other toxins) is also present in other parts of the tree. These additional sources can therefore contribute to poisoning without being the cause alone. These secondary sources are: