On-Going MRLS Studies Focus on Eastern Tent Caterpillar

A research project begins well before the experiment starts. That was the case of the recently completed collaborative effort at the University of Kentucky that associated the Eastern tent caterpillar and its frass (excrement) and Mare Reproductive Loss Syndrome (MRLS). Work on that project began last fall, even before funding was finalized. The study had cost about $250,000 at the mid-way point.

Dr. Nancy Cox, associate dean of research at the University of Kentucky's College of Agriculture, is coordinating the research efforts aimed at solving the mystery of MRLS. "This (MRLS) is one of the highest priorities in the college," she said.

Cox coordinates the various individuals involved in the projects, and is responsible for acquiring funding. The caterpillar/MRLS study was funded by several sources, including the College of Agriculture, the USDA Agricultural Research Services, and individuals and organizations within the Thoroughbred industry.

Projects coordinated by Cox dealing with MRLS at the university include:

-- Preliminary observations on insecticide safety with broodmares, by Karen McDowell, MS, PhD;

-- MRLS: Are estrogens or anti-estrogens involved, by McDowell;

-- Evaluating novel, reduced-risk insecticides for managing Eastern tent caterpillar outbreaks on horse farms, by Daniel A. Potter, PhD, and Lee H. Townsend, PhD;

-- Eastern tent caterpillar feeding in horse pastures: Behavoiral and ecological aspects, by Lynne K. Rieske-Kinney, PhD;

-- Assessment of direct and indirect interactions between Eastern tent caterpillars and mares as contributing factors to MRLS, by Bruce Webb, BA, MS, PhD;

-- Investigations of mycotoxins as a possible cause of MRLS, by Kyle Newman, PhD, Venture Labs;

-- Evaluation of the rate of cyanide and its metabolites in MRLS, by Thomas Tobin, MVB, MS, PhD, MRCVS, DABT;

-- An experiment to define the relationship of prunasin/mandelonitrille, hydrogen cyanide, and certain bacteria to MRLS and related conditions, by Lenn R. Harrison, DVM;

-- Study of bacteria associated with MRLS, James M. Donahue, PhD; and

-- The influence of MRLS on fetal growth, Lucas Pantaleon, DVM, Hagyard-Davidson-McGee veterinary group (Lexington, Ky.).

Collaborative efforts also include weather monitoring, field and forage monitoring, and sampling done the University of Kentucky Livestock Disease Diagnostic Center (LDDC).

The caterpillar project was planned in late 2001 because there was a need to have the project ready to go as soon as the caterpillars in Kentucky started hatching this spring. The research was conducted on the university's Spindletop Farm with pregnant mixed-breed mares leased from Eastern Kentucky.

More on the Caterpillar/MRLS Study
Pasture plots of 20-by-200 feet were used for this study. The pens were separated by 20 feet. For 10 days, horses were put on the plots for six hours and allowed to graze. The horses were divided into three groups -- one group had caterpillars and their frass placed in the field each day; one group only had caterpillar frass put in the field each day; the third group was kept away from caterpillars and frass as much as possible, but handled and sampled the same way as the other two groups.

Each day, the plots were reduced by 20 feet, and more caterpillars and/or frass added. This means the concentration of exposure increased each day until by Day 8, the horses were in a 20- by 40-foot pen. Researchers put at least 10,000 caterpillars on the plot per day. (Little know fact: It takes about 900 recently hatched caterpillars to make one pound, but it only takes 250 mature caterpillars to make a pound.)

Webb, a professor of entomology in the University of Kentucky's College of Agriculture, oversaw the research project. He said the College of Ag Engineering designed "traps" from PVC plastic pipe cut in half (nearly a mile of pipe). Most caterpillars were not able to crawl around the C-shaped curve. An added precaution to this barrier to keep caterpillars in the pen was the addition of TangleFoot (a sticky substance) on the top of the pipe to prevent escape.

Webb said he wanted to do this study because the survey done by the university last year implicated Eastern tent caterpillars. He thought this basic research would either point to an association, or rule out an association between the caterpillars and MRLS. Web said his initial thoughts were that caterpillars were not involved in the syndrome, but the results of the experiment changed his mind.

"We needed to try and reproduce the syndrome," said Webb. "There are only five to six weeks of caterpillars each year (in Kentucky). We had people working seven days a week on the farm with the mares and in the laboratory."

Assisting Webb in caterpillar collection and storage were Claire Collins, who will be starting her PhD work in the fall, and Walter Barney, who earned an entomology degree in 1988 and came back to UK to work on this study.

Webb felt it was important to use "Kentucky-bred" caterpillars in this research to most closely mimic the natural environment. Therefore, he had students collecting caterpillars from along roadsides, from nurseries, and from farms. He had about 160 pounds of caterpillars left in the laboratory prior to finishing his initial research project. (See Article Quick Find #3465 at www.TheHorse.com for details on this research.)

The experiment showed a statistically significant correlation between early fetal loss and caterpillar frass. Webb and his assistants have collected almost 50 pounds of frass and stored it for future study.

What They Learned
"I think it is the caterpillars that are actively feeding and producing frass that are the problem," said Webb. "So we need to control caterpillars early while they are still eating."

What's Next?
Webb wants to immediately repeat this study of correlating caterpillars and their frass with MRLS while there are still caterpillars in the right stages of development. However, since it is the feeding stages of the caterpillar that Webb thinks are the most problematic to horses, he might use frass rather than caterpillar exposure.

He said he also wants to irradiate the frass and kill any biologic agents it contains to see if there is a toxin or biologic agent (bacteria, fungus, virus, etc.) that is causing the problem.

Webb also said the excess rain this spring was probably contributing to the lower loss rates. He thinks the rain "drives" the frass into the soil.

The timeline looking at Eastern tent caterpillar growth and movement and MRLS occurrences last year was as follows:
-- March 31, caterpillar eggs start to hatch.
-- April 21 through mid-May, caterpillars start wandering
-- April 25, by this time black cherry trees are defoliated
-- April 28-May 23, foal loss numbers rose.

"We probably have had MRLS around at low levels for a long time," said Webb, "but maybe now we don't have to live with it." By Kimberly S. Graetz