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MRLS mystery

By Rebecca Colnar, Article donated by the mane points horse resource center.

In the spring of 2001, horse enthusiasts across the country listened to news in horror. Central Kentucky horse farms were experiencing astronomical foal fetus deaths to the tune of millions of dollars. Approximately 1,020 late-term foal fetuses were presented to the state's Livestock Disease Center for examination.

The culprit was Mare Reproductive Loss Syndrome (MRLS), but experts were at a loss to explain what triggered the spontaneous abortions on such a mass scale. According to Dr. Neil Williams with the Livestock Disease Diagnostic Center, there were some consistent findings.

"Most of the fetuses were delivered at term or were aborted several weeks prior to the due date with no premonitory signs in the mares," Williams explains. He points out that there were lesions in the fetus and placenta, hemorrhages in the heart, as well as hemorrhages around

the umbilical cord. Interestingly, the tragic mystery affected many breeds of mares, not only Thoroughbreds.

In 2002, the losses occurred again, not with the same magnitude, although the timing and research findings were similar to 2001. The phenomenon has been recorded in Kentucky and southern Ohio, but not in any other states.

Researchers are looking at the association with a causative agent and the Eastern Tent Caterpillars.

In mid-spring 2002, researchers discovered a clue to solving the mystery in the guise of Eastern Tent Caterpillars. However, studies did not find what toxic component of the caterpillar caused fetal losses. There has been a correlation between the caterpillars, wild cherry trees and unique weather patterns, as well as a correlation between the caterpillar waste (frass) and MRLS. At press time, no mycotoxin or other possible pasture factors have been shown to be consistently related to MRLS. Currently, the study to find a link is still under way. Some factors being monitored include fungal mycotoxins, soil fungal counts, cyanide content of white clover and alkaloid content of tall fescue.

According to University of Kentucky Extension Entomologist Lee Townsend, researchers are looking at the association with a causative agent and the Eastern Tent Caterpillars (ETC). Until the causative agent is discovered, the main goal is to eradicate the tent caterpillar.

"Eliminating the Eastern Tent Caterpillars is our overall goal," Townsend says, explaining that broodmare owners should look over their pastures and assess the risks. The plan, called ERASE, stands for Exposure Reduction, Action Sites (where caterpillars are clumped together) and Elimination.

If you have cherry trees in your fields, along the edge or even close by, the wisest choice is to reduce the mare's exposure to them.

"Move your mare so she's not near any cherry trees," Townsend advises. "Keep in mind that caterpillars will move up to 100 feet away from the tree where they live."

He stresses that the wider a buffer you can have between pregnant mares and the infested trees, the better. If the tree is defoliated by caterpillars, it should be fenced off and the area around it not used for pregnant mares.

"You can use an insecticide to reduce the ETC population, especially focusing on the trees when the caterpillars are living in the tents," Townsend says. "Before they actually leave the tree, they tend to mass on the trunk. Once they leave, there's no way to control them. The greater the degree of control over the ETC, the less problem with mares with MRLS."

Limiting time on a pasture that has cherry trees and/or caterpillar exposure has not been enough to totally eliminate MRLS. If the environment can't be controlled it might be best to move the mares to a paddock and feed them quality hay.

Read the next horse diseases article on Monday Morning Disease.
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