Equine Mare Reproductive Loss Syndrome (MRLS)

Other Names

MRLS; Mare Reproductive Loss Syndrome; eastern tent caterpillar-associated pregnancy loss; pasture-associated late-term abortion.

What is Equine Mare Reproductive Loss Syndrome?

Equine Mare Reproductive Loss Syndrome (MRLS) is a reproductive disorder in mares characterised by early and late fetal loss, as well as pericarditis in foals, associated with exposure to eastern tent caterpillars (Malacosoma americanum). The syndrome was first formally identified following catastrophic losses in Kentucky’s Thoroughbred breeding industry in 2001, when an estimated 30 per cent of expected foals were lost across multiple farms. Subsequent investigation linked the losses to an unusually heavy caterpillar population following a mild winter.

MRLS primarily affects horses in the eastern United States where the eastern tent caterpillar is native, though the underlying mechanism — ingestion of caterpillar setae (hairs) that penetrate the gastrointestinal wall and migrate to cause reproductive or cardiac damage — has raised broader questions about the role of similar insects in equine reproductive losses elsewhere.

The condition causes two distinct clinical presentations: early fetal loss (EFL) in mares that were pregnant at less than 45 days when exposed, and late fetal loss (LFL) in mares at more advanced stages of pregnancy. Affected foals born alive may develop pericarditis — inflammation of the pericardial sac around the heart — and unilateral or bilateral uveitis.

Causes

MRLS is caused by the ingestion of eastern tent caterpillars or their setae (bristle hairs), primarily through contaminated pasture grass or hay. The caterpillar’s setae physically penetrate the mucosal lining of the gastrointestinal tract and migrate through the body, reaching the uterus and fetus. This physical penetration triggers an inflammatory response that disrupts fetal membranes, causes placentitis, and leads to fetal death.

In foals, caterpillar setae can migrate to the pericardium, causing fibrinous pericarditis that may be fatal if untreated. The mechanism is therefore a physical and inflammatory one, not an infectious or toxicological one in the traditional sense.

Risk factors include:

Grazing on pastures with established wild cherry trees or other host trees in areas where caterpillars are endemic
Warm, mild winters that support larger-than-average caterpillar populations
Proximity to woodland edges with high caterpillar populations
Failure to recognise and remove caterpillar tents from pasture trees

Symptoms and Signs

In pregnant mares:

Vaginal discharge — often the first observable sign
Premature udder development or premature lactation (foaling before expected date)
Expulsion of the fetus before full term
Weak or dysmature foals that fail to survive
Retained placenta following fetal loss

Most mares themselves show no systemic illness and recover well from the reproductive losses. The primary impact is the loss of the pregnancy.

In affected foals born alive:

Signs of cardiovascular compromise — rapid heart rate, distended jugular veins, reluctance to move
Respiratory difficulty
Fever
Unilateral or bilateral uveitis (eye inflammation)

Diagnosis

Diagnosis in affected mares is primarily based on the clinical history, the geographic location, the time of year, and exclusion of infectious causes of abortion (equine herpesvirus, equine viral arteritis, bacterial placentitis). Identification of caterpillar setae in placental or fetal tissue provides definitive evidence of MRLS. Gross and histopathological examination of the placenta may reveal characteristic lesions.

For affected foals, echocardiography (ultrasound of the heart) is the primary diagnostic tool for pericarditis. PCR testing of placental material for common infectious pathogens should be performed to rule out infectious abortion simultaneously.

Treatment

There is no specific treatment for MRLS-related fetal loss once it has occurred. Management focuses on supportive care of the mare following abortion, including monitoring for retained placenta and infection, and anti-inflammatory treatment as needed.

Affected foals with pericarditis may be managed with anti-inflammatory therapy (NSAIDs and/or corticosteroids), drainage of pericardial fluid if significant, and appropriate antimicrobial therapy to manage secondary infection. Intensive neonatal supportive care is essential for any compromised foal.

Prevention

Prevention is the most important management principle in areas where eastern tent caterpillars are endemic:

Survey pastures and paddock trees for caterpillar tent webs in early spring
Remove and destroy caterpillar tents before caterpillars disperse into the environment
Remove or fence off wild cherry trees and other preferred host trees from horse pastures where feasible
Move pregnant mares off affected pastures during peak caterpillar emergence periods (April to May)
Consider stabling vulnerable mares during the highest risk period if caterpillar populations are unusually high

The Eastern tent caterpillar has a predictable annual emergence pattern — being aware of local population forecasts in early spring allows proactive management before horses are exposed.

Prognosis

The prognosis for affected mares is generally good for future reproductive success, provided they receive appropriate post-abortion care and do not develop complications such as retained placenta or endometritis. Mares that lose pregnancies to MRLS typically cycle normally and can conceive in subsequent seasons.

The prognosis for affected foals with pericarditis is more guarded and depends on the severity of the pericardial involvement and how quickly the condition is detected and treated. Some foals recover fully; others deteriorate rapidly and do not survive.

When to Call the Vet

Contact your vet immediately if a pregnant mare develops vaginal discharge, premature milk production, shows signs of foaling significantly earlier than her expected date, or if a newborn foal appears weak, has an elevated heart rate, or develops signs of eye inflammation. In areas with known or suspected caterpillar activity during breeding season, report any concern promptly — time is critical in both late-term abortion and neonatal pericarditis.

Frequently Asked Questions

Can MRLS occur outside the United States?

MRLS as described is specifically associated with the eastern tent caterpillar, which is native to North America. However, the underlying mechanism — caterpillar setae migrating through the gastrointestinal wall to cause reproductive damage — has prompted investigation of similar mechanisms involving other hairy caterpillar species elsewhere. Horses in the UK and Europe are not typically at risk from this specific syndrome, though owners in endemic US regions should be vigilant.

Do all mares exposed to caterpillars lose their foals?

No. Not all exposed mares abort. The risk appears to depend on the level of caterpillar exposure, the stage of pregnancy at the time of exposure, and individual variation. During the 2001 outbreak, approximately 30 per cent of expected foals were lost, meaning the majority of mares on affected farms maintained their pregnancies despite environmental caterpillar presence.

Can MRLS be confirmed before fetal loss occurs?

Diagnosing MRLS before fetal loss is challenging. Transrectal ultrasound monitoring of mares at high-risk times can detect early signs of fetal compromise or premature cervical relaxation, but there is no reliable test that confirms caterpillar setae migration before clinical signs appear.

Should mares be kept off pasture during peak caterpillar season?

In areas with significant caterpillar populations during the typical April to May emergence period, removing pregnant mares from pasture during this high-risk window is a reasonable precaution, particularly if previous years have seen reproductive losses or if surveys indicate high caterpillar numbers.