Recently, Stephanie Valberg and colleagues identified ingestion of Acer negundo (box elder) seeds as a probable cause of seasonal pasture myopathy in the USA . Other recent work by Votion, Valberg and colleagues  and included in this issue of Equine Veterinary Journal has demonstrated that atypical myopathy in Europe is likely to have the same causal factor.
The chemical in maple seeds suspected to cause these myopathies, known as hypoglycin A (2S-2-amino-3-(2-methylidenecyclopropyl)-propanoic acid), is a nonproteinogenic amino acid, which, following ingestion and metabolic activation, becomes a potent inhibitor of acyl-CoA dehydrogenase, a key enzyme active in fatty acid metabolism.
At present it is not entirely clear what the risk factors for the occurrence of either form of myopathy are, as they pertain to the presence of different maple species in areas where horses reside. There are many more questions than there are answers. This editorial aims to summarise the current state of knowledge regarding plants that may play a role in this disease and suggest areas that need further study.
Maples are classified within the family Sapindaceae, known as the soapberry family, along with a number of other plants, such as horse chestnut (Aesculus species) and lychee (Litchi chinensis). Ackee (Blighia sapida), the national fruit of Jamaica, is a member of this family that contains hypoglycin A in its fruit. Hypoglycin A varies in concentration in ackee fruits based on maturity, with unripe fruits containing a larger quantity of this toxin and ripe fruits a much smaller concentration . When unripe fruit is ingested by humans, it may result in a disease known as Jamaican vomiting sickness, which in severe cases leads to death.
Jordan and Burrows  were the first to report a water-soluble toxin in the seeds and pods of ackee fruit. In 1954, this toxin was isolated and 2 amino acids identified by Hassall et al. , who named them hypoglycin A and hypoglycin B because of their hypoglycaemic activity. In 1958, Feng and Patrick  described the effect of hypoglycin A on animals, noting that it caused ‘drowsiness progressing to coma, and when large doses were given the animals died’. Besides ackee fruit, and now maple seed, we are not aware of any literature implicating any other plants contributing to hypoglycin A poisoning in mammals, though related compounds have been observed in other genera of Sapindaceae (including 2-amino-4-methylhex-4-enoic acid in Aesculus californica [California buckeye], and hypoglycin B, the γ-glutamyl dipeptide of hypoglycin A, in Billia hippocastanum).
Currently, there is very little information on the concentration of hypoglycin A in the seeds of various maples. One of the authors of this article (A.D.H.) has quantified hypoglycin A in 2 maple species, namely Acer negundo (box elder) and Acer pseudoplatanus (sycamore maple; A. D. Hegeman, unpublished results). This is consistent with work by Fowden and Pratt , who provided a qualitative description of a strong hypoglycin A response in these 2 species to their assay method. It is worth noting that, besides the 2 species mentioned above, both of which displayed a medium to medium-strong response, seeds from a number of other maples also showed a medium response or stronger to Fowden and Pratt's hypoglycin A assay, including Acer palmatum (Japanese maple), Acer saccharum (sugar maple) and Acer spicatum (mountain maple) and various cultivars and subspecies of these trees. Currently, we do not know whether seeds of these species are ingested by horses to any degree, nor have we had the opportunity to confirm Fowden and Pratt's observations of hypoglycin A levels with more quantitative testing.
We have isolated several hundred milligrams of purified hypoglycin A from A. negundo seed tissues and have confirmed by 1H nuclear magnetic resonance spectroscopy and mass spectrometry that the structure is identical to that of hypoglycin A obtained from ackee . Based on preliminary data from longitudinal and geographical quantitative studies of hypoglycin A levels in A. negundo seeds (currently under way in the laboratory of A.D.H.), we can say that even among seeds from the same tree there is wide variability in the amount of hypoglycin A. This variation within a tree and from tree to tree makes it difficult to assign environmental determinants that may influence seed toxicity. Owing to this variation, a high degree of sample replication is required to derive significant associations via statistical approaches.
While the most important factor that contributes to horses ingesting maple seeds that may contain hypoglycin A is undoubtedly the presence of trees that contain this toxin in their seeds in the area where the horse is feeding, another key factor is the interest of the horse in the seed, which is probably closely tied to the availability of other foods [1, 9]. Some of the other factors affecting the presence of dangerous maple seeds that deserve to be investigated more deeply include the prevalence of various maple species that might contain the toxin in their seeds in horse pastures. A thorough investigation of various maple tissues needs to be undertaken to discover whether seeds are the only place where hypoglycin A is found.
Other factors that are currently under investigation include the seasonality of this disease. It has been noted that this is a disease that is most likely to strike in the autumn [10-12]. We do not know if this is because at this time of year other food sources are less available, maple seeds are more prevalent or hypoglycin A is present in larger concentrations in seed. Some combination of these or other factors may be at work. Owing to the taxonomic relationship of maple to ackee, however, ripeness of fruit is obviously a possible contributor to hypoglycin A levels and is currently being investigated.
There are a number of factors contributing to fruiting in trees that also need to be examined for their contribution to hypoglycin A poisoning. First among these is the question of tree stress. It is unknown whether stress would increase seed load, but this has been suggested anecdotally. Trees can experience stress from a variety of different situations, ranging from compacted earth to deer rubbing against the bark, or even over- or underwatering. Besides seed load, we also do not know the impact of tree stress on hypoglycin A levels in seed; however, in other plant species abiotic stress can significantly increase biochemical defenses and raise the concentration of certain amino acids . Furthermore, if linked to the tar spot disease of Acer pseudoplatanus (sycamore maple), as has been suggested in other literature , cultural practices, such as the removal of leaves in autumn, may be a major influencing factor .
Populations of Acer that may contain hypoglycin A are present throughout the UK and USA and can be considered almost ubiquitous; however, in the UK, atypical myopathy has been reported to be most prevalent in the southern portion of England , where box elders are more numerous . If A. pseudoplatanus seeds were the predominant cause of the disease in the UK, then we would have expected reports of atypical myopathy to have been more common and widespread because of its widespread planting in the British Isles in the 18th century . Unfortunately, botanical surveys are largely restricted to plants growing in the countryside and not gardens, and this disease may be caused by cultivated, rather than wild, maples. As a result, further research is required on the specifics of affected pasture. In the near future, the authors hope to analyse the flora of horse pastures where atypical myopathy has been reported in order to find correlations between the presence of the disease and the plant species' abundance and growing conditions.
Clearly, additional study will be needed regarding the many factors that control the regulation of hypoglycin A production and the botanical and anatomical distribution within maple species to understand the periodic and seasonal variation in occurrence of this toxin and associated equine myopathies. It may be more important, however, to observe the interactions between horses and maple species in their pasture environments so that we can better understand the nature of the exceptional situations that lead to poisoning cases, because there are many examples of horses coexisting with these maple species without any problems.