Environmental influences on mare reproductive loss syndrome: Do they fit with a toxin as the causative agent?
Authors: Burns Stuart J, Westerman Albert G, Harrison Lenn R
Journal: Journal of equine veterinary science
Summary
Mare Reproductive Loss Syndrome (MRLS), characterised by abortion outbreaks linked to Eastern Tent caterpillars (ETC; *Malacosoma americanum*), has long presented a diagnostic and management challenge for equine practitioners. Burns and colleagues applied Westerman's Correlates of Causal Strength of Evidence to test whether a toxin-mediated mechanism could explain the syndrome's epidemiology, correlating known environmental triggers with biological plausibility. The analysis supported a toxin hypothesis: higher ambient temperatures compressed caterpillar hatching into tight windows, creating population density spikes that drove increased consumption of black cherry leaves—which contain prunasin, a precursor compound that the caterpillars metabolise into a protective alkaloid toxin. This toxin contaminated pasture and water during off-tree feeding periods, accumulating to toxic concentrations in dry conditions but being diluted by rainfall, which correspondingly reduced abortion incidence and severity. Although the pathogenesis remains unconfirmed, the study provides a coherent environmental model linking caterpillar biology, plant chemistry, weather patterns and clinical outcomes—suggesting that targeted surveillance during high-risk periods (hot, dry springs) and pasture management to reduce caterpillar populations warrants consideration alongside traditional supportive care in MRLS-affected premises.
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Practical Takeaways
- •During warm, dry weather conditions when Eastern Tent caterpillar populations are high, increase monitoring of pregnant mares for signs of reproductive loss and consider management changes to reduce pasture exposure
- •Proximity to Black Cherry trees and environmental conditions (heat, drought) that promote both caterpillar activity and toxin accumulation on forage should be considered risk factors for MRLS
- •Rainfall provides natural dilution of potential toxin contamination; supplemental feeding or water sources from alternative locations may reduce exposure during high-risk periods
Key Findings
- •MRLS is associated with Eastern Tent caterpillars (Malacosoma americanum) and a potential toxin mechanism fits causal criteria using Westerman's Correlates of Causal Strength of Evidence
- •Higher ambient temperatures and lower precipitation correlate with MRLS events by increasing ETC population density and toxin accumulation on pasture
- •Black Cherry tree leaves contain prunasin, a precursor to a caterpillar-produced toxin that contaminates pasture and water during off-tree feeding
- •Rainfall dilutes toxin contamination and reduces clinical effects, while dry conditions allow toxin accumulation and increase exposure risk