Evidence for marsh mallow (Malva parviflora) toxicosis causing myocardial disease and myopathy in four horses.
Authors: Bauquier J, Stent A, Gibney J, Jerrett I, White J, Tennent-Brown B, Pearce A, Pitt J
Journal: Equine veterinary journal
Summary
# Editorial Summary: Marsh Mallow Toxicosis in Horses A case series from Australia documented four horses from a single farm that developed acute myocardial and skeletal myopathy after grazing heavily on marsh mallow (*Malva parviflora*) when alternative forage was scarce, presenting with severe muscle fasciculations, tachycardia, profuse sweating and recumbency that rapidly progressed to death or euthanasia within hours to 36 hours of clinical onset. Post-mortem examination of three horses revealed multifocal acute necrosis affecting both cardiac and skeletal muscle, distinguishing this toxicosis from atypical myopathy (which typically shows glycogen accumulation in muscle fibres); biochemical analysis identified elevated medium and long-chain acyl carnitine concentrations in affected horses alongside the presence of cyclopropene fatty acids in their serum—compounds unique to *Malva parviflora* that were absent in control animals. The researchers propose that cyclopropene fatty acids disrupt the β-oxidation pathway for fatty acid metabolism in horses experiencing negative energy balance, essentially mimicking the pathophysiology of human very long-chain acyl-CoA dehydrogenase deficiency and explaining the acute cardiac and muscle damage observed. This finding has direct relevance to equine practitioners managing pasture composition and nutrition, particularly in regions where marsh mallow establishes; recognising the risk factors (limited forage availability forcing reliance on toxic weeds) and the acute clinical presentation (rapid onset myocardial and myopathic signs without glycogen storage disease markers) enables earlier intervention and prevention through pasture management.
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Practical Takeaways
- •Malva parviflora (marsh mallow) should be considered a toxic plant for horses, particularly when heavily grazed with limited alternative forage available in negative energy balance situations.
- •Clinical presentation of cyclopropene fatty acid toxicosis includes muscle fasciculations, tachycardia, profuse sweating and recumbency, progressing rapidly to death; differentiate from atypical myopathy by absence of glycogen accumulation.
- •Remove affected horses from pastures containing Malva parviflora immediately and ensure adequate nutrition to prevent negative energy balance states that may predispose to toxicosis.
Key Findings
- •Four horses exposed to heavily grazed Malva parviflora developed severe muscle fasciculations, tachycardia, sweating and recumbency, resulting in death or euthanasia.
- •Post-mortem examination revealed acute, multifocal cardiac and skeletal myonecrosis in 3 cases without glycogen accumulation.
- •Acyl carnitine profiles showed increased C14-C18 concentrations in affected horses compared to controls.
- •Cyclopropene fatty acids were detected in sera of affected horses but not in controls, suggesting interference with fatty acid β-oxidation.