Box elder seeds implicated in Seasonal Pasture Myopathy

A toxin present in the seeds of the box elder tree has been found to cause Seasonal Pasture Myopathy.

Seasonal Pasture Myopathy (SPM), a highly fatal muscle disease, occurs particularly in the Midwestern United States and eastern Canada.

Horses with SPM suffer from severe, generalised muscle weakness. They are often unable to get to their feet, or only do so with difficulty. If they are still able to walk, they have a stiff gait - especially of the hindquarters. Affected animals have elevated heart rates, and show profuse sweating and muscle twitching.

The underlying disease process has been shown to be an acquired enzyme deficiency (MADD: multiple acyl-CoA dehydrogenases deficiency) in skeletal muscle.

A similar condition, Atypical Myopathy (AM), occurs in Europe.

Recent research, led by Dr Stephanie Valberg, of the University of Minnesota, showed that the seeds of the box elder tree (Acer negundo) contain hypoglycin A, which can cause SPM.

The study involved 12 horses with seasonal pasture myopathy The researchers examined farms on which SPM had occurred, looking for common factors, and compared them with unaffected farms. Seeds from box elder trees were present on all of 11 SPM-affected pastures and 61% of 23 control pastures.

Examination of box elder seeds from the SPM pastures showed they contained hypoglycin A, a branched-chain amino acid. Once ingested, hypoglycin A is metabolised into MCPA (methylenecyclopropylacetic acid) a toxic compound, which is known to cause multiple acyl-CoA dehydrogenase deficiency (MADD).

The researchers explain that the MCPA “would have a major impact on energy generation, particularly when horses are mobilising fat because of a negative energy balance.” They add ”a negative energy balance was probably promoted in SPM horses by offering little supplemental feed while horses were housed on overgrazed pastures, in some cases during inclement weather conditions.”

Writing in the Equine Veterinary Journal, they conclude: “For the first time, SPM has been linked to a toxin in seeds abundant on autumn pastures whose identified metabolite, MCPA, is known to cause acquired MADD, the pathological mechanism behind SPM and AM.”

“Further research is required to determine the lethal dose of hypoglycin A in horses, as well as factors that affect annual seed burden and hypoglycin A content in Acer species in North America and Europe.”

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