Revised:  05/22/2008

Memo for May 10, 2007













New Jersey Department of Agriculture



To: Veterinarians

From: Nancy E. Halpern, D. V. M. State Veterinarian

Date: May 10, 2007

Re: Mare Reproductive Loss Syndrome


Mare Reproductive Loss Syndrome (MRLS) was first recognized in 2001 as an outbreak of fetal deaths, foals born weak, and late-term abortions in Kentucky. A strong association exists between the presence of unusually large numbers of eastern tent caterpillars and MRLS. Pregnant mares experimentally fed the caterpillars typically abort within days.


MRLS was confirmed in Florida during March and April of 2006 and northern New Jersey in June of 2006.


While research has shown that eastern tent caterpillar is responsible for MRLS, debate remains over the mechanism of action that leads to abortion. Factors implicated in the pathophysiology of MRLS include plants eaten by the caterpillar, biochemical interactions produced by the caterpillar, or its setae or hairs that embed in the lining of the gastrointestinal tract, compromising the lining of the gastrointestinal tract, allowing the normal bacteria within the tract to cross into the bloodstream and affect the placenta and fetus.


In a recent report from the University of Kentucky, Lee Townsend, College of Agriculture entomologist, states that eastern tent caterpillars and forest tent caterpillars are developing normally this spring. Townsend also indicates that although high populations of caterpillars aren’t widespread this year, there are abundant populations in some areas. The forest tent caterpillar is similar to the eastern tent caterpillar and scientists have not ruled out the possibility that it could cause problems similar to the eastern tent caterpillar.


Freezing temperatures at night do not adversely affect the caterpillars because they are an early spring insect and are used to erratic weather patterns such as abnormal warm or cold spells; and because the caterpillars are well-protected by hair on their bodies and by forming aggregates with other caterpillars in tents.


Over the next two to three weeks the caterpillars will begin to aggregate in tents on the main trunk branches of trees. The caterpillars are more easily destroyed at this time because they are grouped together. When the caterpillars leave the trees and disperse, control becomes more difficult.


Control of the tent caterpillars is best accomplished in early spring because the caterpillars are still small and susceptible to control measures. Measures for controlling tent caterpillars include:

  • Identify and remove egg masses from trees.
    • Prevent exposure of pregnant mares to the caterpillars, including dead caterpillars and their excreta/feces.


  • Use foliar sprays to reduce caterpillar populations. Examples of foliar sprays include:
    • Bacillus thuringiensis (Bt) based products - these products must be eaten by the small caterpillars to be effective (ineffective in larger caterpillars); caterpillars are not killed by direct contact with the spray. Spray on foliage around active nests. Bt residues on foliage can be broken down by sunlight in three to four days so it is important to assess control and re-treat if necessary.
    • Bifenthrin (Talstar) or Carbaryl (Sevin) based products - these products are effective when eaten or with direct contact. Spray on foliage or tents. The residual life of carbaryl is about a week; that of bifenthrin is at least two to three weeks.


  • Kill caterpillars by injecting trees with Bidrin (Inject-A-Cide “B” or 2 % Abacide).
  • Reassess sites every 5 days to assess caterpillar populations.

The entire report released by University of Kentucky can be found at the following link:


For information about tent caterpillar control, contact the Rutgers Cooperative Extension agricultural agent in the county in which you are located. Contact information is available at


Contact your veterinarian if your horses exhibit any unusual signs of infertility, abortion or weak foal births. Although MRLS is not a required reportable disease, the Division of Animal Health requests your cooperation in reporting any unusual incidents of the signs described above to (609)-292-3965.






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