Evidence for marsh mallow (Malva parviflora) toxicosis causing myocardial disease and myopathy in four horses.

REASON FOR PERFORMING THE STUDY: Investigation of toxicosis caused by Malva parviflora was required after 4 horses from the same farm developed severe muscle fasciculations, tachycardia, sweating and periods of recumbency leading to death or euthanasia after ingesting the plant.

OBJECTIVES

To describe historical, clinical, clinicopathological and pathological findings of 4 horses with suspected M. parviflora toxicosis. The role of cyclopropene fatty acids (found in M. parviflora) and mechanism for toxicosis are proposed.

STUDY

DESIGN

Case series.

METHODS

Historical, physical examination, clinicopathological and pathological findings are reported. Due to similarities with atypical myopathy or seasonal pasture myopathy acyl carnitine profiles were performed on sera from 2 cases and equine controls. Presence of cyclopropene fatty acids was also examined in sera of 2 cases.

RESULTS

M. parviflora had been heavily grazed by the horses with little other feed available. Horse 1 deteriorated rapidly and was subjected to euthanasia. Horse 2 was referred to hospital where severe myocardial disease and generalised myopathy was determined; this horse was subjected to euthanasia 36 h after admission. Horse 3 died rapidly and Horse 4 was subjected to euthanasia at onset of clinical signs. Post-mortem examinations performed on 3 horses revealed acute, multifocal cardiac and skeletal myonecrosis. Myocyte glycogen accumulation was absent when examined in Horse 2. Acyl carnitine profiles revealed increased C14-C18 acyl carnitine concentrations in cases relative to controls. Cyclopropene fatty acids were detected in sera of cases but not controls.

CONCLUSION

These findings suggest aetiology different to that of atypical myopathy or seasonal pasture myopathy. We hypothesise that cyclopropene fatty acids in M. parviflora interfere with fatty acid β-oxidation in horses in negative energy balance, causing the clinical signs and abnormal acyl carnitine profiles. These equine cases suggest a pathophysiological course that closely mimics the human genetic condition very long chain acyl CoA dehydrogenase deficiency.