LEXINGTON, Ky. -- After nearly a month of pursuing theories that resulted in dead ends, researchers said Thursday that the wild black cherry tree is responsible for the mare reproductive loss syndrome that has swept central Kentucky farms this spring and caused at least 1,200 abortions in broodmares.

According to preliminary findings presented by Gluck Equine Research Center scientists, Eastern tent caterpillars, which eat black cherry tree leaves, probably helped deliver cyanide or cyanogenic compounds - sugars that can produce cyanide under certain conditions - from the cherry trees to mares at pasture.

Among the possible "perfect conditions" that may have caused leaf wilt and the release of cyanide is frost, part of the hot-to-frozen weather pattern that hit central Kentucky in mid-April.

"The toxin is cyanide, the source is the black cherry tree," Gluck toxicologist Dr. Tom Tobin told about 900 breeders who gathered at an informational meeting Thursday in Keeneland's sale pavilion. "We know the leaves came from the black cherry trees very shortly before the first episodes of early fetal losses [on April 26]. Precisely how delivery of the cyanide to the mares occurred is unclear, but there is preliminary evidence of cyanide in [aborted] fetuses." Pathologists found cyanide in the heart muscles of aborted fetuses.

Tent caterpillars were long suspected as sources of the problem by local horsemen, who found infestations in pastures, barns, and water troughs. The caterpillars initially tested negative for cyanide. But Dr. Lenn Harrison, director of Lexington's Livestock Disease Diagnostic Center, said that "a retrospective inquiry" into that result suggested that even a slight delay in caterpillar testing may have allowed the caterpillars to pass the cyanide from their systems - a process he said takes only six hours. Testing of a second round of caterpillars revealed that they were "strongly positive" for cyanide, Harrison said.

Dr. Scott Smith, dean of the University of Kentucky's college of agriculture, emphasized that "these observations are preliminary and must be confirmed, and further validation is essential. A great deal of work still needs to be done."

The syndrome first became apparent to veterinarians on April 26 as an unusually large number of mares began producing dead or critically ill foals. Since then, researchers have struggled to find an answer to the cause, investigating cherry trees, caterpillars, and grass toxins.

Mycotoxins produced by mold or fungus on pasture grass were initially thought to be the problem, but they now appear unlikely as the primary cause of the abortions. Dr. Jimmy Henning stressed that pasture grasses have tested negative for mycotoxins and said that "no Kentucky hay baled before Kentucky Derby Day has been found to contain any fusarium mycotoxins," the type that includes zearalenone, an initial suspect in the abortions.

From April 28 through Thursday afternoon, the diagnostic center has received a total of 532 aborted or stillborn foals or fetuses thought to have been affected by the syndrome. The Gluck center reported on May 10 that 678 mares had aborted early-term fetuses at around 60 days of pregnancy, but the center has not publicly updated those figures since then, and the number is thought to have grown significantly.

As part of the effort to obtain an accurate count of foal losses, The Jockey Club has asked breeders to submit their live foal/no foal reports as quickly as possible after their mares foal.

Dr. Tom Riddle of the Rood and Riddle Equine Hospital noted that, at his practice, the percentage of early fetal loss was higher, running at 40 percent of mares bred in February. Another 30 percent of February-bred mares Riddle saw showed cloudy fluid in the placentas around their fetuses, indicating that 70 percent of the mares may have been exposed to the cyanide.

But he noted that the losses appear to be slowing: 39 percent of mares he checked from May 4-10 had lost their early pregnancies, but that percentage dropped to 17 percent from May 11-17 and to just 8 percent from May 18 through Thursday.