REVIEW paper: mare reproductive loss syndrome.
Authors: Sebastian M M, Bernard W V, Riddle T W, Latimer C R, Fitzgerald T D, Harrison L R
Journal: Veterinary pathology
Summary
# Mare Reproductive Loss Syndrome: An Environmental and Infectious Disease Challenge During spring 2001, Kentucky experienced a devastating epidemic of reproductive failure characterised by early and late fetal loss, fibrinous pericarditis, and uveitis—collectively termed mare reproductive loss syndrome (MRLS)—with concurrent outbreaks in Ohio, West Virginia, and Tennessee causing an estimated $500 million in losses across 2001–2002. The syndrome presented with particular diagnostic difficulty, as affected mares showed no clinical signs preceding abortion, though necropsy of late-term fetuses revealed bronchopneumonia and funisitis, whilst early losses displayed no pathological findings; non-haemolytic *Streptococcus* and *Actinobacillus* species accounted for 65% of bacterial isolates from aborted fetuses. Epidemiological investigation and controlled experimental exposure identified eastern tent caterpillars (ETC) as the causative trigger, with direct exposure to living caterpillars or administration of ETC material reproducibly inducing both early and late fetal losses in pregnant mares, though similar exposure failed to reproduce disease in mice, rats, or goats. Two mechanisms are proposed: either ETC-derived toxins compromise fetal immunity allowing opportunistic bacterial superinfection, or caterpillar setae breach gastrointestinal integrity leading to bacteraemia and transplacental infection. Practitioners encountering abortion storms during caterpillar season should assess pasture infestation history, as subsequent reports from Australia, Florida, and New Jersey confirm MRLS remains a significant international reproductive hazard requiring environmental management alongside supportive care and judicious antimicrobial therapy for secondary bacterial infections.
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Practical Takeaways
- •Monitor pastures for eastern tent caterpillars during spring breeding season and remove infested vegetation to reduce reproductive loss risk
- •Pregnant mares showing no clinical signs but aborting should be investigated for secondary bacterial infections and caterpillar exposure history
- •MRLS recurrence has been documented internationally (Australia 2004, Florida/New Jersey 2006), so remain alert for similar syndromes in your region
Key Findings
- •MRLS epidemic in 2001-2002 caused approximately $500 million in economic losses across multiple US states
- •Nonhemolytic Streptococcus spp. and Actinobacillus spp. accounted for 65% of organisms isolated from aborted fetuses during MRLS outbreaks
- •Epidemiologic and experimental studies demonstrated a strong association between eastern tent caterpillar (ETC) exposure in pastures and MRLS in pregnant mares
- •Two proposed mechanisms: ETC toxin with secondary bacterial invasion, or ETC hair-induced gastrointestinal breach leading to bacteremia