Mare reproductive loss syndrome

Overview

Mare Reproductive Loss Syndrome (MRLS) is a reproductive disease that primarily affects pregnant mares, leading to spontaneous abortions, early fetal loss, and other pregnancy-related complications. The syndrome gained widespread attention during a major outbreak in Kentucky, USA, in 2001, when thousands of mares experienced unexplained pregnancy losses over a short period. Since then, MRLS has become a subject of intense veterinary research, especially in regions with large equine populations.

MRLS can result in a variety of reproductive issues including early embryonic death (EED), late-term abortions, fibrinous pericarditis in fetuses, and weak foals. The condition poses significant economic implications for the equine breeding industry, especially in thoroughbred horse farms. It is now recognized as a multifactorial syndrome, with environmental and biological factors contributing to its occurrence.

Causes

The exact cause of MRLS was unknown for many years, but subsequent research has linked the syndrome primarily to exposure to certain environmental factors, particularly:

• Eastern tent caterpillars (Malacosoma americanum): Ingestion or exposure to these caterpillars is the most strongly implicated factor. Caterpillar hairs (setae) can damage the gastrointestinal tract, allowing bacteria to enter the bloodstream and reach the uterus and fetus.

• Bacterial infection: Following GI damage, bacterial translocation may lead to infection and inflammation of the placenta or fetal membranes.

• Pasture management: Mares grazing in areas heavily infested with caterpillars, especially near cherry or black cherry trees (which are preferred hosts for caterpillars), are at greater risk.

• Seasonal patterns: MRLS outbreaks typically occur in spring, correlating with the life cycle of the eastern tent caterpillar.

Other potential contributing factors such as mycotoxins, nutritional deficiencies, or viral agents have been considered but have not shown consistent causal links.

Symptoms

The primary symptoms of MRLS are reproductive in nature and can vary depending on the stage of pregnancy and the extent of exposure. Common clinical signs include:

• Early fetal loss: Occurs between 40 and 80 days of gestation and may be identified through ultrasonography or signs of return to estrus.

• Late-term abortion: Spontaneous loss of pregnancy occurring in the second or third trimester, often without prior warning signs.

• Stillbirths or weak foals: Foals born alive may be weak, septicemic, or non-viable shortly after birth.

• Placental abnormalities: Inflammation, thickening, or discoloration of the placenta may be observed.

• Fetal pericarditis: In aborted fetuses, inflammation of the pericardial sac (fibrinous pericarditis) is a common finding on necropsy.

Mares affected by MRLS generally appear systemically healthy, with no signs of illness aside from reproductive failure.

Diagnosis

Diagnosis of Mare Reproductive Loss Syndrome is typically based on clinical signs, epidemiological factors, and ruling out other causes of abortion. The following diagnostic steps are commonly used:

• Reproductive history: A sudden increase in abortions or early fetal losses in a group of pregnant mares raises suspicion of MRLS.

• Ultrasound examination: Used to detect early fetal death, abnormalities in fetal development, or placental detachment.

• Postmortem examination of aborted fetuses:

  • Fibrinous pericarditis

  • Placentitis or funisitis (inflammation of the umbilical cord)

  • Histological findings consistent with bacterial infection

• Environmental assessment: Presence of tent caterpillars, cherry trees, or other known risk factors in the pasture.

• Exclusion of other infectious causes: Testing for viral agents (e.g., equine herpesvirus, equine viral arteritis), bacterial infections, or fungal pathogens to rule out alternative diagnoses.

A definitive diagnosis of MRLS is often reached through pattern recognition in outbreak settings combined with supportive histopathological findings.

Treatment

There is no specific treatment for MRLS once fetal loss has occurred. Management focuses on prevention and mitigation of further losses. Strategies include:

• Environmental control:

  • Removal or fencing off of cherry trees to limit caterpillar habitat

  • Clearing eastern tent caterpillar nests from trees and pastures

  • Rotating pastures during caterpillar season (typically spring)

• Reducing exposure risk: Keeping pregnant mares in caterpillar-free areas during peak months (April–May in the U.S.).

• Monitoring pregnancies: Frequent ultrasounds and veterinary check-ups to detect early signs of fetal loss or complications.

• Supportive care: For weak foals or mares that have recently aborted, fluid therapy, antibiotics, and nutritional support may be required.

Research into potential prophylactic interventions continues, but no effective vaccine or medical preventive has been established.

Prognosis

The prognosis for individual mares affected by MRLS depends on the timing and severity of exposure. Key prognostic factors include:

• Mares with early fetal loss: May return to estrus and potentially be bred again within the same season.

• Late-term abortions: Often result in greater physical and emotional toll on the mare and breeder but typically do not lead to long-term reproductive damage.

• Foals born alive: May have poor viability and require intensive neonatal care; survival is uncertain.

Most mares that recover from MRLS-related abortion can successfully carry future pregnancies, especially if preventive measures are taken. Farm-wide prognosis improves significantly with early recognition of outbreaks and implementation of pasture and caterpillar control strategies.

MRLS remains a significant concern for the equine industry in endemic areas, but with proper environmental management and awareness, the risks can be effectively minimized.