Mare Reproductive Loss Syndrome (MRLS)

A case of Mare Reproductive Loss Syndrome (MRLS) was diagnosed at the Animal Health Diagnostic Center, Cornell University. Samples were submitted in late June 2006 from a farm in northwestern Massachusetts, very close to the New York border. The diagnosis has been confirmed by pathologists at the Kentucky state laboratory, recognized experts in this disease.

Please review the information below.

Cornell University Animal Health Diagnostic Center

Mare Reproductive Loss Syndrome (MRLS) and Eastern Tent Caterpillars (ETC)

MRLS has been linked to the consumption of Eastern Tent caterpillars (ETC) by pregnant mares. Eastern Tent caterpillars prefer wild cherry, apple and crabapple. They are found regularly in NYS and surrounding states. In some areas or years ETC may be more plentiful than in others. The clinical syndromes associated with ETC consumption include early pregnancy loss, late-term abortions, foals born weak and septic, pericarditis, uveitis, laminitis and oral ulceration. Some of these health consequences not related to pregnancy have been seen in non-pregnant mares and in male horses.

Horse Owners: Take Action

Inspection of pastures and surrounding trees for the presence of Eastern Tent caterpillars is critical. The only way to protect the pregnant mare or other horse is to remove her from contact with the caterpillars. If the caterpillars are eliminated before they crawl out of the tree nests, contact with horses can be eliminated. Later, when the caterpillars may be crawling on vegetation, buildings, fences and other objects, it may be necessary to confine horses to the barn, and carefully inspect water buckets and troughs for the presence of caterpillars.

Excellent information about Eastern Tent Caterpillars and pictures are available at:

At this time, it is strongly recommended that all abortions and foal deaths receive a post-mortem evaluation. There are many different causes of abortion or newborn foal death.

Early pregnancy loss can be detected by ultrasonographic evaluation.

For more information, consult the University of Kentucky web page on MRLS at

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