Caterpillar control minimizes foal loss


LEXINGTON, Ky. – An unprecedented number of mares – more than 3,000 – aborted their foals in Kentucky in 2001.

Losses in 2002 were substantially lower and in 2003, “dramatically lower,” officials at the University of Kentucky report.

Researchers targeted eastern tent caterpillars as part of the cause and results recently reviewed in Kentucky indicated scientists are on the right track.

Little known syndrome. Last year several studies produced solid evidence that the presence of eastern tent caterpillars is strongly associated with the foal loss problem known as Mare Reproductive Loss Syndrome.

But precisely how the caterpillars were causing mares to lose their pregnancies was unknown.

Research this spring has brought the equine industry another step closer to solving the mystery.

“The main focus of this year’s research has been to zero in on the caterpillar and try to figure out what is the chemical or biological nature of the agent that causes MRLS,” said Nancy Cox, associate dean for research in the University of Kentucky College of Agriculture.

Exoskeleton toxic. Results from the university’s first-reported 2003 research trial, led by Bruce Webb and Karen McDowell, indicate the causative agent may be associated with the caterpillar’s exoskeleton, or outside cuticle.

This trial looked at what happens when different parts of the caterpillar are fed to pregnant mares, and the only treatments that caused abortions were the ones involving the outside cuticle.

Fetal losses occurred in five of five mares fed eastern tent caterpillars, and three of five mares fed exoskeleton (cuticle and related parts).

No losses occurred among mares fed internal parts of caterpillars.

“This is certainly evidence that we’re moving in a forward direction to narrow our search for the exact cause,” Cox said.

Another recent experiment demonstrated that irradiated eastern tent caterpillars can induce fetal loss in late-term pregnant mares.

This suggests mare reproductive loss syndrome is caused by a non-infectious agent in caterpillars (irradiation at sufficient levels has been shown to kill infectious agents such as viruses and bacteria).

Pasture management. Researchers made significant progress this year in terms of pasture management recommendations and suggestions for caterpillar control.

“After an intensive program of monitoring farms in 2002 and 2003, we were able to reduce the importance of certain factors, such as cyanide and mycotoxins, and at the same time gain a better understanding of other risk factors such as tall fescue endophyte, which is still under study,” said Jimmy Henning, assistant extension director for agricultural and natural resources.

“We learned a lot about what was going on in horse pastures the last two springs, and were able to pass that information along to farm managers.”

Insecticide study. Studies determined that certain reduced-risk pesticides effectively eliminated all sizes of ETC larvae in wild cherry trees.

Cherry trees are a preferred host tree for the insect. Micro-injection of certain insecticides into the base of trees also gave good control when applied at the right time.

Another study showed that a pyrethroid insecticide containing permethrin provided an effective “barrier” treatment along fence lines to control large crawling tent caterpillars once they had left the trees and were wandering prior to cocoon formation.

2003 numbers. Foal loss numbers from all causes are down dramatically in 2003.

Recent reports from the University of Kentucky’s Livestock Disease Diagnostic Center show equine fetal submissions (accessions) for January through May, 2003 are only half the number for the same period in 2002.

In May alone accessions dropped by 86 percent.

“Other than a very few isolated foal losses that we consider at, or below, normal levels we’ve basically seen nothing that even approaches the problems of 2001 and 2002, and that’s very good news,” said Lenn Harrison, diagnostic center director.

More research. Meanwhile, researchers remain vigilant. Cox said science is a steady process that gradually eliminates various possibilities and continually narrows the focus.

The work doesn’t stop until the main question is answered.

“Given the discovery that the Eastern Tent Caterpillar is the causative agent for mare reproductive loss syndrome, we would be irresponsible not to figure out everything we can about the mechanisms of the disease,” she said.

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