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Working to Solve the Foal Deaths Mystery

by Jeff Worley

Photo of Eastern tent caterpillarClick here for the most recent research on the caterpillar/foal death connection. [photo by Steve Patton]

On Kentucky Derby Saturday, May 5, 2001, 73 dead foals and fetuses were delivered to the Livestock Disease Diagnostic Center at UK's Coldstream Research Campus on Newtown Pike. And while weekend deliveries of dead animals to the back door of the center is common, the tremendous number that day—about 10 times more than usual—was alarming.

"I knew we had big trouble on our hands. In addition to these foals and fetuses, we were aware that too many foals in the Fayette County area were being born weak," says Lenn Harrison, veterinarian and center director. Harrison worked alongside center pathologists practically non-stop that weekend—the first of many—to try to solve the puzzle of what was causing the death of so many foals and fetuses.

In the lab, pathologists performed a necropsy on each foal, looking at internal organs for signs of infection as well as taking blood and tissue samples for analysis. The tissue samples offered up the clue of rare bacterial infections in many aborted fetuses, but the pathologists concluded that these bacteria could not be solely responsible for the deaths. "As the necropsies continued, our belief got stronger that the bacteria were secondary to the specific cause of what was dubbed Mare Reproductive Loss Syndrome," Harrison says.

Within the next two days, the total number of dead foals and fetuses brought to the center grew to 276, and a team of over 100 experts from a variety of disciplines in the College of Agriculture joined the investigation. A group of highly experienced veterinarians made up a survey to be distributed right away to horse farms by the Kentucky Thoroughbred Farm Managers Club to try to determine the extent of the losses. Staff in the UK agricultural communications office also swung into action to make certain that the industry and the public were kept abreast of the situation. And a Web site,, was established to provide a continuing source of information for the public.

By May 10, news of the syndrome had spread around the world: U.S. networks as well as European and Middle East news agencies were constantly calling UK for information about this syndrome that had the potential to disrupt the global horse industry. Four days later, May 14, farm managers who had returned questionnaires confirmed the worst: the syndrome was widespread, and it was continuing.

"At this point, a couple of faculty members at Gluck [UK's Gluck Equine Research Center] recalled that in 1980 and 1981 early fetal loss had occurred spontaneously and no cause had ever been determined," Harrison says. "And in both years the losses abruptly stopped." Reported losses during those years, however, were nowhere near the 2001 numbers.

"A quick check of meteorological conditions then and now," he explains, "showed a disturbing similarity of weather patterns that could affect pastures where the mares grazed." March temperatures all three years were below normal, followed by above normal temperatures in April. Such a pattern would result in explosive biological activity in both plants and insects. The data also showed frost or freeze in the third week of April, followed by warm temperatures a few days later. What impact could those factors have on the pastures?

"Although the convergence of weather factors in the three years with larger than usual abortion rates wasn't a smoking gun, it was suggestive that something weather-related was responsible," Harrison says.

Perhaps, he thought, weather-dependent vegetation could be linked to a variety of possible disease-causing agents, including mycotoxins and ergot-type alkaloids (which are derived from fungi and can be poisonous to animals in high concentrations) and phytoestrogens (compounds produced by plants that mimic the hormone estrogen). Tests for poisoning from mycotoxins in feed, ergot-type alkaloids in pasture, and phytoestrogens appeared to be negative, but most of the tests were done after the syndrome had occurred.

Because none of the obvious causes appeared plausible, the scientists turned their attention to still another possibility—that the villain might be the Eastern tent caterpillar, whose populations were high in each of the years in question. Harrison showed UK agronomist Jimmy Henning some information he'd run across on these prolific crawlers, in particular the fact that they feed on wild cherry leaves.

And when Henning visited farms where mares had given birth to dead or dying foals or had aborted early-term fetuses, he discovered a striking pattern: most of the pastures had a large number of wild cherry trees. "I knew that cherry tree leaves carried the precursor to a poison—naturally occurring organic cyanide—that could cause death in horses and other animals, especially cattle," says Henning. "I also realized that Eastern tent caterpillars, which were known to feed on the leaves of cherry trees and were seemingly immune to the poison, had been quite active during late April."

The caterpillars became even stronger suspects in a meeting of veterinarians, farm managers and the media on May 24. "At that meeting," Harrison says, "several veterinarians confirmed that the incidence of early fetal losses as well as stillborn and dying foals had dramatically decreased during the previous week, paralleling the natural decline in caterpillar numbers."

The investigation into the possible role played by the caterpillars continued. In December 2001, entomology professor Bruce Webb made an interesting discovery: his research showed that cyanide does not accumulate in the caterpillars, so they are not likely to deliver appreciable amounts to horses. This finding does not mean, however, that the caterpillars were necessarily innocent bystanders. Research planned by Webb this spring will investigate potential indirect roles that the caterpillars may play. "They produce large amounts of waste as they feed," Webb explains. "Much of this nutrient-rich material rains to the ground under infested trees and may serve as food for specific molds that produce toxins that might contribute to the syndrome."

As researchers close in on the causes of the foal-loss syndrome, the importance of their work is underscored by last spring's sad numbers. Between April 28 and May 12, 418 aborted equine fetuses and stillborn foals were brought to the Livestock Disease Diagnostic Center for evaluation. During that same period in 2000, only 60 fetuses and stillborn foals were received. On May 23, 2001, a University of Kentucky survey of 159 thoroughbred farm managers indicated that 678, or 21 percent, of 3,294 pregnant mares had experienced early fetal loss.

So, what can be done now and in the future to make certain that such an aberration doesn't occur again?

Henning says that researchers have begun a complex environmental monitoring system they hope will give them advanced warning if conditions are ripe for this to happen again. About 13 farms are being tested on a rotating basis, with soil, grass, water and other samples collected, analyzed, and stored for later comparison should foals again begin to die in such numbers. Weather patterns are also being closely tracked, and a massive research initiative on the causal mechanisms of the syndrome is planned by the College of Agriculture in the coming months.

Harrison says he hopes that the only thoroughbred problem he will have to deal with around Kentucky Derby time this year is which horse to bet on.