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  Discussion on Seasonal pasture / atypical myopathy assoc.with the Box Elder tree
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DrO

Posted on Friday, Nov 23, 2012 - 9:13 am:

This may be very big for areas that suffer from the mysterious Seasonal Pasture / Atypical Myopathy. Though still only associated and the cause not proven, Box Elders were found in 100% of the pastures where the disease occurred and there is a known mechanism for the myopathy with ingestion of the seeds. It looks promising and until proven otherwise we would recommend you keep up (mow?) the Box Elder seeds from the pasture or at the least provide plenty of alternative forage.
DrO

Equine Vet J. 2012 Sep 28.
Seasonal pasture myopathy/atypical myopathy in North America associated with ingestion of hypoglycin A within seeds of the box elder tree.

Valberg SJ, Sponseller BT, Hegeman AD, Earing J, Bender JB, Martinson KL, Patterson SE, Sweetman L.

Department of Veterinary Population Medicine, College of Veterinary Medicine, University of Minnesota, St Paul, Minnesota, USA.
Abstract

REASONS FOR PERFORMING STUDY:
We hypothesised that seasonal pasture myopathy (SPM), which closely resembles atypical myopathy (AM), was caused by ingestion of a seed-bearing plant abundant in autumn pastures.
OBJECTIVES:

To identify a common seed-bearing plant among autumn pastures of horses with SPM, and to determine whether the toxic amino acid hypoglycin A was present in the seeds and whether hypoglycin metabolites were present in SPM horse serum or urine.

METHODS:
Twelve SPM cases, 11 SPM pastures and 23 control farms were visited to identify a plant common to all SPM farms in autumn. A common seed was analysed for amino acid composition (n = 7/7) by GC-MS and its toxic metabolite (n = 4/4) identified in conjugated form in serum [tandem mass spectrometry (MS/MS)] and urine [gas chromatography (GC) MS]. Serum acylcarnitines and urine organic acid profiles (n = 7) were determined for SPM horses.

RESULTS:
Seeds from box elder trees (Acer negundo) were present on all SPM and 61% of control pastures. Hypoglycin A, known to cause acquired multiple acyl-CoA dehyDrOgenase deficiency (MADD), was found in box elder seeds. Serum acylcarnitines and urine organic acid profiles in SPM horses were typical for MADD. The hypoglycin A metabolite methylenecyclopropylacetic acid (MCPA), known to be toxic in other species, was found in conjugated form in SPM horse serum and urine. Horses with SPM had longer turn-out, more overgrazed pastures, and less supplemental feeding than control horses.

POTENTIAL RELEVANCE:
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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