Assessment of tree-associated atypical myopathy risk factors in Acer pseudoplatanus (sycamore) seeds and leaves.

Background: Sycamore tree-derived hypoglycin A (HGA) toxin causes atypical myopathy (AM), an acute, equine pasture-associated rhabdomyolysis but incidence fluctuates.

Objectives: Investigate whether tree or environmental factors influence HGA concentration in sycamore material and are associated with AM relative risk.

Study design: Retrospective and experimental prospective study.

Methods: UK sycamore population, seed production and AM incidence data were obtained. HGA concentration was measured in seeds from trees from 10 different central UK locations. The effect of tar spot infection, seed maturity, tree trunk girth, location (urban/countryside), AM cases within 130 m, soil type, facing direction of seeds on the tree and year on seed HGA concentration was examined. HGA concentration was compared in whole and homogenated seeds stored in different ways.

Results: HGA concentration in sycamore seeds was not associated with tree tar spot infection, location, trunk girth, seed weight or branch-facing direction but HGA concentration in sycamore seeds varied significantly and in parallel year on year in the same trees. Trees in the same vicinity tended to have similar HGA concentrations in their seeds when compared with those from farther afield. Seed production estimates were positively correlated with regional AM case incidence (τb = 0.3; p = 0.007). HGA sycamore seed concentration remained stable as seeds matured, but HGA declined in leaves as they wilted in autumn. Warmer and wet storage resulted in higher HGA concentrations in seed homogenates but not in whole seeds. HGA was detected in water containing sycamore seeds for 48 h.

Main limitations: Lack of accurate weather data; sampling restricted to central England.

Conclusions: Tree factors that were investigated did not affect HGA concentration in sycamore seeds but HGA concentrations varied year on year. AM incidence is related to seed production; conditions that mimic browsing and ingestion increased seed HGA concentration. HGA toxicity could occur from contaminated water sources.