U.S.

A Working Theory

May 30, 2001 / 4:19 PM EDT / AP

Scientists now believe they know why more than 500 foals died and mares spontaneously aborted several thousand more early pregnancies on central Kentucky's horse farms this spring.

Their work, however, is far from finished as they work to explain their theory conclusively and make sure they can help prevent a future outbreak of Mare Reproductive Loss Syndrome.

"We've come a long way in a fairly short period of time, but there's still a lot of things we don't understand," Dr. Thomas Tobin, a toxicology and pharmacology researcher at the University of Kentucky's Gluck Equine Research Center, said Wednesday.

"We're confident we've zeroed in on cyanide and cyanogenic compounds as the problem. Now we have to determine how the mares ingested the poison."

Researchers and veterinarians began surveying more than 150 farms on Wednesday in an attempt to determine the exact conditions present on the farms during the last few weeks of April. The statistical data collected then will be analyzed by local scientists as well as those at the United States Department of Agriculture.

The on-site visits also will focus on the proximity of cherry trees to pastures in which pregnant mares may have grazed.

The prevailing theory suggests mares somehow ingested cyanide produced from the leaves of the trees, which are found on many of the region's horse farms.

"The extreme environmental conditions we had this spring, including several days of hard frost, may have damaged the leaves and made them even more toxic," Tobin said.

Low levels of cyanide, which inhibits the body's ability to receive and use oxygen, were found in the tissue of several fetuses. The poison may have caused foals to struggle for oxygen inside the womb.

"It wouldn't take a large amount of poison to affect an early-term embryo or a near-term foal," Tobin said. "We know that wilted leaves or broken branches from a black cherry tree are lethal to cattle and sheep."

Scientists also believe that Eastern tent caterpillars, which nest in the cherry trees and are immune to the poisonous chemicals in their leaves, may have played a role.

Tent caterpillars, which in Kentucky have now cocooned and are growing into moths, will be shipped from New York for study. Researchers will feed cherry tree leaves to caterpillars and monitor how the insects digest the toxins and how the toxins might be secreted and ingested by horses, Tobin said.

"We expect to have significant data from those tests back in about six to eight weeks," he said. "We hope they will be able to tell us exactly what, if any, part the caterpillars played in this scenario."

Scientists originally thought toxins generated by fungal- or mold-based agents in pasture grasses might have been behind the outbreak. But tests on hundreds of grass samples have been mostly negative for such toxins, Tobin said.

More than 100 scientists and staff from the University of Kentucky's College of Agriculture along with veterinarias, farm managers, leaders in the equine industry and other governmental and private agencies have worked around the clock for nearly a month to find the cause of the outbreak.

"The cooperative scientific and technical progress, made under the most urgent conditions in essentially a three-week period, is unprecedented anywhere," said Scott Smith, dean of the University of Kentucky College of Agriculture. "This effort has been a model of communication and cooperation, and will continue to be."

Current estimates show that up to 5 percent of this year's foal crop and as much as 29 percent of the 2002 crop may have been lost to the syndrome. Kentucky produces about 10,000 thoroughbred foals each year.

David Switzer, executive director of the Kentucky Thoroughbred Association, estimated the economic impact on the state's billion-dollar thoroughbred industry at between $200 and $225 million.

Those numbers are based on estimates of approximately 500 dead foals this year and 2,000 spontaneous abortions of horses that would have been born next spring.

The foals lost this year would have generated about $37 million in stud fees and sales revenue, and those lost that would have been born next year about $150 million, Switzer said.

Another $35 million to $38 million of the total number is lost income for companies that do business with horse farms.

"I think those numbers are on the conservative side," Switzer said. "If you don't have as many foals, you don't buy as much feed or hire as many stable workers.

"It's going to trickle down and hit everyone connected with the horse industry. Nobody is going to be untouched."