Processionary caterpillar setae and equine fetal loss: 2. Histopathology of the fetal-placental unit from experimentally exposed mares.
Authors: Todhunter K H, Cawdell-Smith A J, Bryden W L, Perkins N R, Begg A P
Journal: Veterinary pathology
Summary
# Processionary Caterpillar Setae and Equine Fetal Loss: Understanding the Placental Pathology Pregnant mares experimentally exposed to processionary caterpillar (*Ochrogaster lunifer*) material demonstrated rapid setal fragment migration into fetal membranes, with fragments detected in the allantochorion within 2–22 days of exposure and present in 92% of aborted fetuses. Histopathological examination of 13 aborted fetuses, 3 fetuses from treated euthanised mares, and membranes from 8 foals revealed setal fragments embedded throughout the placental unit—in chorionic villi and stroma, allantois vasculature, and umbilical structures—with individual fetuses harbouring between 1 and 7 fragments distributed across multiple anatomical locations. Acute to chronic active inflammation was universal across all aborted and euthanised fetuses, whilst 95% of examined membranes showed amnionitis, funisitis, and allantoitis, with concurrent pneumonia and bacterial presence in 95% and 90.5% of specimens respectively. These findings establish direct setal migration as the primary initiating mechanism in equine amnionitis and fetal loss (EAFL), suggesting that clinical cases presenting with unexplained third-trimester abortion or placental inflammation during processionary caterpillar season warrant investigation for setal contamination and supportive antimicrobial therapy targeting secondary bacterial colonisation.
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Practical Takeaways
- •Processionary caterpillar exposure during pregnancy is a significant risk factor for abortion — mares grazing in areas with these caterpillars should be monitored closely and kept away from infested vegetation during breeding season
- •The rapid migration of caterpillar setae (within 2 days) suggests that early intervention may be possible if exposure is suspected, though prevention through pasture management is the most practical approach
- •Secondary bacterial infection is common in EAFL cases, so any pregnant mare showing signs of illness after potential caterpillar exposure warrants urgent veterinary evaluation and appropriate supportive care
Key Findings
- •Caterpillar setal fragments were found embedded in placental tissue in 92% of aborted fetuses (11/12), with fragments appearing in membranes within 2-22 days of mare exposure
- •Acute to chronic active inflammation was present in 100% of aborted and euthanized fetuses, with amnionitis, funisitis, and allantoitis in 95% of examined membranes
- •Pneumonia occurred in 95% of fetal specimens and bacteria were present histologically in 90.5% of cases, suggesting secondary bacterial infection following setal migration
- •Direct setal migration into fetal membranes within 2 days of exposure appears to be the primary initiating mechanism for EAFL rather than systemic toxemia