June 25, 2003 | By: Haven Miller

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. 

Results from UK’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, led by Manu Sebastian and Bill Bernard, demonstrated that irradiated Eastern Tent Caterpillars can induce fetal loss in late-term pregnant mares.  This suggests MRLS 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).  The study, conducted jointly by UK and Rood & Riddle Equine Hospital, was the first experiment in late-term pregnancies which reproduced a pathological finding similar to naturally-occurring MRLS.

“There’s been tremendous cooperation by our equine industry in achieving these results,” Cox said.  “We appreciate the support we’ve received from the Kentucky Association of Equine Practitioners, the Kentucky Thoroughbred Association, and the Kentucky Thoroughbred Farm Managers Club in helping design and conduct these experiments, as well as provide funds.”   

UK researchers also 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 in the UK College of Agriculture.  “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.”

Studies conducted by Dan Potter of UK’s entomology department in late winter and spring 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.  This work was supported by the Grayson-Jockey Club Research Foundation.

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.

“Armed with the results of UK experiments linking MRLS with exposure to Eastern Tent Caterpillars and new information on caterpillar behavior and control, farm managers developed aggressive management plans that minimized exposure of pregnant mares in 2003,” said Lee Townsend, UK Extension entomologist.  “On top of that, mother nature provided invaluable help with a caterpillar-specific disease that dramatically reduced ETC numbers.  It certainly proved to be a winning combination.”

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, LDDC director.

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 MRLS, we would be irresponsible not to figure out everything we can about the mechanisms of the disease,” she said.  “Knowing how the caterpillar affects pregnancy will help our ability to manage pregnancy in mares in many other scenarios.”


Nancy Cox, 859-257-3333