FieldDxEquine amnionitis and fetal lossClinical signs, differential diagnosis, treatment and prevention in horses.
Overview
Mare reproductive loss syndrome: abortions, stillbirths and weak foals with inflammatory disease linked to eastern tent caterpillar exposure. The cause is unconfirmed; proposed mechanisms are caterpillar setae carrying gut bacteria, or a toxin from caterpillars that eat black cherry leaves.
Clinical signs
- Early and late-term abortions, mostly at 40-80 days of gestation and some up to 140.
- Stillbirths and weak foals that die within days.
- An abortion or a dead fetus on ultrasound is usually the first sign; mares rarely show signs of impending labour.
- Few mares have colic, fever or purulent vulvar discharge.
- Fibrinous pericarditis and unilateral uveitis in horses of any age.
- Amnionitis and neutrophilic funisitis, with a thickened, discoloured placenta and cord.
Predisposing factors
- Pasture exposure to eastern tent caterpillars during outbreaks.
- Pregnant mares on affected farms.
- Regions with MRLS in earlier years.
Treatment
- No treatment exists for the affected fetus once loss has occurred; care for the mare is supportive, watching for retained placenta or metritis after an abortion.
- Management is preventive: insecticide control and pasture management to reduce mare exposure to Eastern tent caterpillars, the exposure most strongly associated with the syndrome.
Prevention
1. Manage pastures to control eastern tent caterpillar numbers. 2. Keep pregnant mares away from caterpillar exposure.
Source
MSD Veterinary Manual — Equine Amnionitis and Fetal Loss (accessed 2026-08-12)
MSD Veterinary Manual — Abortion in Horses (accessed 2026-10-29)
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