Equine Reproductive Loss Syndrome (ERLS)

Other Names

Mare Reproductive Loss Syndrome (MRLS)
Eastern tent caterpillar-associated reproductive loss

What is Equine Reproductive Loss Syndrome?

Equine Reproductive Loss Syndrome (ERLS) — more commonly known as Mare Reproductive Loss Syndrome (MRLS) — is a reproductive disorder characterised by an abnormally high incidence of pregnancy losses in mares, including early fetal losses (EFLs) in the first trimester, late-term abortions (LFLs, typically occurring between 40 and 300 days of gestation), stillbirths, and the birth of weak, ill foals. The syndrome also encompasses non-reproductive complications in individual horses including unilateral uveitis (eye inflammation) and pericarditis (inflammation of the heart sac).

ERLS first gained widespread attention following major outbreaks in the Bluegrass region of Kentucky in 2001 and 2002, during which thousands of pregnancies were lost in a short period, representing an economic catastrophe for the Thoroughbred breeding industry. The link to Eastern Tent Caterpillars was subsequently established through extensive epidemiological and experimental research.

While the syndrome was first characterised in North America, similar associations between caterpillar exposure and reproductive losses have been investigated in other regions. Horse owners and stud managers in areas where tent caterpillars are endemic should be aware of the condition and implement monitoring protocols during high-risk periods.

Causes

The primary cause of ERLS is ingestion of or exposure to Eastern Tent Caterpillars (Malacosoma americanum). The mechanism is well established:

Eastern Tent Caterpillars build large silken tent nests in wild cherry (Prunus serotina), apple (Malus spp.), and crabapple trees in spring and early summer.
The caterpillars are covered in small, stiff hairs (setae) that have barbed tips. When horses graze contaminated pasture or eat from affected trees, these setal fragments are ingested.
The barbed setae penetrate the walls of the gastrointestinal tract — particularly the foregut — and carry bacteria (primarily Actinobacillus spp., Streptococcus spp., and Escherichia coli) through the gut wall and into the bloodstream.
Once in the bloodstream, these bacteria can seed the placenta and fetus, causing placentitis (placental infection), fetal bacteraemia, and ultimately fetal death or abortion.
In some horses, bacteria seed the pericardium or uvea, causing the non-reproductive complications described above.

The risk is greatest in spring when caterpillar populations are at their peak and mares in mid- to late first trimester are most vulnerable. The number of caterpillars present in an area varies enormously year to year depending on caterpillar population cycles and weather conditions — years with high caterpillar populations are associated with the most significant outbreaks.

Symptoms and Signs

Early fetal losses (EFLs): Mares that lose pregnancies before day 45 may show few clinical signs — the pregnancy simply ceases to develop and the fetus is resorbed or expelled unnoticed. Mares may return to oestrus unexpectedly.
Late fetal losses (LFLs): Abortion occurring after day 45, often in the second or third trimester. The aborted fetus may show signs of bacterial infection including pericarditis.
Premature foals: Some affected pregnancies result in the birth of weak, compromised foals (“dummy foals”) requiring intensive care.
Unilateral uveitis: Severe, painful inflammation of one eye — eye squinting, excessive tearing, cloudiness of the cornea, and sensitivity to light.
Pericarditis: Signs may include depression, elevated heart rate, abnormal heart sounds on auscultation, and ventral oedema (swelling under the chest and belly). Can be life-threatening if untreated.
Vulval discharge: Mares with placentitis may show a mucopurulent discharge from the vulva before abortion occurs.

Diagnosis

Reproductive ultrasonography: Regular monitoring of pregnancy via transrectal ultrasound during high-risk periods allows early detection of reduced fetal viability, fluid accumulation within the uterus, or placental thickening — all signs of developing placentitis.
Fetal and placental pathology: Post-mortem examination of aborted fetuses and placentas is essential to characterise the pathology. Bacterial culture of fetal tissues and placenta identifies causative organisms.
Blood tests: Haematology and biochemistry may show elevated white blood cell counts and inflammatory markers. Plasma fibrinogen is typically elevated in mares with active placentitis.
Progesterone measurement: Declining progesterone levels in a pregnant mare may indicate impending pregnancy loss.
Epidemiological investigation: Confirming the presence of Eastern Tent Caterpillars on the property, particularly in wild cherry and apple trees, is an important part of the diagnostic picture during an outbreak.

Treatment

There is no specific treatment that reverses the damage caused by caterpillar setal penetration. Management is supportive and aimed at reducing further risk:

Progesterone supplementation: Exogenous progesterone (e.g. Regu-Mate) may help maintain pregnancies at risk of loss in mares with declining progesterone levels. This should be administered under veterinary supervision.
Antibiotics: Broad-spectrum systemic antibiotics may reduce bacterial seeding of the placenta and fetus when placentitis is suspected — early treatment before clinical signs advance is most effective.
Anti-inflammatories: NSAIDs such as flunixin meglumine or pentoxifylline are used to reduce placental and uterine inflammation.
Treatment of pericarditis: Requires systemic antibiotics, NSAIDs, and potentially pericardial drainage under veterinary supervision. Cases should be referred to an equine cardiologist or internal medicine specialist if possible.
Treatment of uveitis: Topical and systemic anti-inflammatories, mydriatics (to dilate the pupil), and antibiotics as directed by your veterinarian. Uveitis can cause permanent vision loss if not treated promptly and aggressively.
Intensive neonatal care: Weak foals born to affected mares may require intensive supportive care including plasma transfusions, assisted feeding, and antibiotic therapy.

Prevention

Inspect all pastures and surrounding trees in spring for Eastern Tent Caterpillar nests, which appear as white silken tents in the branch forks of cherry, apple, and crabapple trees.
Remove and destroy caterpillar nests before the caterpillars disperse — ideally in early spring when the nests are small and the caterpillars have not yet spread across the pasture.
During high-risk years (when caterpillar populations are high), consider housing pregnant mares in paddocks away from trees that may harbour nests.
Inspect water troughs and buckets daily and remove any caterpillars that have fallen in.
Consider removing wild cherry trees from horse paddocks and their immediate surroundings, as these are the primary host tree.
Monitor all pregnant mares closely with regular reproductive ultrasounds during the peak caterpillar season (typically April–June in Kentucky and similar climates).
Report unexplained pregnancy losses to your veterinarian promptly — early investigation during an outbreak can protect other mares on the property.

Prognosis

The prognosis for individual mares that have experienced pregnancy loss from ERLS is generally good for future breeding — most mares can be successfully bred the following season once any underlying infection or uterine abnormality has resolved. Mares with pericarditis carry a more guarded prognosis depending on severity; some recover fully, while others develop chronic pericardial effusion requiring long-term management. Uveitis can cause permanent damage to vision, particularly if treatment is delayed. Prevention of caterpillar exposure in subsequent years is the most important factor in preventing recurrence.

When to Call the Vet

Contact your veterinarian immediately if a pregnant mare aborts, shows vulval discharge, develops eye inflammation, or appears depressed and has an elevated heart rate during spring or early summer — particularly if Eastern Tent Caterpillars have been observed on your property. Early intervention significantly improves outcomes. For related reading, see our entry on Equine Uterine Infections and equine reproductive health. Consult the British Equine Veterinary Association for guidance on reproductive health protocols.

Frequently Asked Questions

Is ERLS/MRLS a risk in the UK?

The primary cause — Eastern Tent Caterpillars (Malacosoma americanum) — is a North American species not present in the UK. However, the UK has its own tent caterpillar species and other caterpillar species that produce setae, and isolated cases of caterpillar-associated reproductive loss have been investigated in Europe. The risk is considerably lower than in affected areas of North America, but horse owners should remain vigilant and consult their veterinarian if unexplained pregnancy losses occur during spring caterpillar seasons.

Can affected mares breed successfully after experiencing ERLS losses?

In most cases, yes. ERLS does not cause permanent damage to the mare’s reproductive tract. Provided any underlying placentitis or uterine infection has fully resolved — confirmed through a veterinary examination and uterine culture — mares can usually be bred successfully the following season. Some mares may benefit from a uterine lavage before the next breeding cycle to ensure complete clearance of any infection.

Do all horses that eat caterpillars develop ERLS?

No. Not all horses that ingest caterpillars will develop significant illness or reproductive loss — individual susceptibility varies. The dose of setae ingested, the stage of pregnancy, the individual horse’s immune response, and the bacterial load on the caterpillars all influence whether disease develops. However, the risk is substantial enough during high-caterpillar years that exposure prevention is strongly recommended for all pregnant mares.

How do I recognise an Eastern Tent Caterpillar nest?

Eastern Tent Caterpillar nests appear as white to grey silken tent structures in the branch forks of trees — particularly wild cherry, apple, and crabapple. They typically appear in early spring and range from a few centimetres to 30cm or more in length as the season progresses. The caterpillars themselves are dark, hairy, and around 5cm long when mature, with a white stripe down their back. If you find these nests on trees adjacent to horse pastures, remove and destroy them immediately.