05/25/2001 12:00AM

Dial 'C' for cyanide, cherry tree, caterpillar


LEXINGTON, Ky. - Dr. Jimmy Henning, professor of agronomy at the University of Kentucky, has what seems to be a peaceful profession, researching and teaching about pasture grasses and similar topics.

But last week was different. Heading up a team to investigate the relationship between pastures and an outbreak of mare abortions, Henning found a critical link that pointed to the culprit in mare reproductive loss syndrome: wild black cherry trees. Since April 26, when central Kentucky veterinarians first began reporting a mysterious wave of broodmare abortions, Henning has been at the forefront of the investigation, something like an agronomist detective thriller. His work with grass and soil put him in a perfect position to research mycotoxins, the fungus-produced grass poisons that researchers initially put forward as a leading suspect.

"But I did not want to believe the mycotoxin theory, because it presents unsolvable problems," Henning said Friday at his office in UK's agriculture department. "There's no good, practical solution for them. So I was trying to find some other cause."

Central Kentucky's heavy infestation of Eastern tent caterpillars, though a popular theory with many farm owners, seemed unrelated to fetal loss at first, when laboratory testing of caterpillars showed no cyanide, a poison that cherry leaves can produce under certain conditions. "It was always one of the theories, but it kind of fell out of vogue, partly because no mammal had ever been known to eat a caterpillar," Henning said.

But on May 15, Dr. Lenn Harrison of the Livestock Disease Diagnostic Center showed Henning some previous caterpillar research done by Dr. Terrence Fitzgerald.

"It showed that caterpillars can eat cherry leaves and create a compound more toxic than the leaves' original compound," Henning said. "It made the caterpillar theory more feasible. But it still was not a great theory, because we knew that there had been caterpillar infestations last year with no outbreaks. We knew we had to go farm to farm and mare to mare and find a correlation."

From May 16 to May 18, Henning and a team from UK's agronomy, animal science, and plant pathology departments began some old-fashioned gumshoe legwork. On foot, they scoured four major Thoroughbred nurseries for cherry trees, both in pastures and along heavily wooded borders where farmers could overlook caterpillar-eaten branches. They interviewed breeders and obtained fetal-loss records for potentially affected mares.

Among the things breeders told them was that caterpillars had been on the move since mid-April, just before the abortions hit, invading foaling barns, water troughs, and paddock fences. On one farm, a horse - contrary to Fitzgerald's expectations - did eat a caterpillar and came down with wobbling symptoms consistent with cyanide poisoning.

The "eureka moment" came, Henning said, on the last farm. In a field that surrounded a substantial grove of wild cherries, already denuded by caterpillars, mares had lost 13 of 13 pregnancies. But in a field which had no cherry tress in it or near its borders, all 30 mares had had no fetal loss as of May 18.

"It was a huge relief," Henning said. "For the first time in three weeks, I relaxed."

On May 19, Henning's relief grew when half of the submitted pasture samples returned from a laboratory with negative test results for mycotoxins -- effectively banishing Henning's worst fear.

The next step was to look again for cyanide traces in both the caterpillars and the aborted fetuses submitted to the diagnostic center. This time, thanks to quicker testing that didn't allow caterpillars to excrete cyanide from their systems, the Eastern tent caterpillars showed strong positives for cyanide. And, finally, on the night of May 23, tests on fetal heart tissue also were positive for cyanide - a crucial piece of evidence.

Another factor that implicated the caterpillars and the cherry trees were old UK records that showed Eastern tent caterpillar infestations and hot-to-frozen weather patterns, like those seen this year, before the last major round of early fetal loss back in 1980 and 1981.

Scientists still need to confirm that cherry trees and caterpillars have caused the problem by trying to duplicate results in horses using caterpillars and their excrement. For now, Henning said, farmers can protect their mares in future seasons by spraying trees against caterpillars, removing cherry trees from pastures and borders, and, if necessary, taking mares off infested pastures at the height of the infestation.

"Several farms we examined also found it helpful to mow their fields when the caterpillars were moving," he added. Henning's field report noted that mowing might have been protective by "disturbing either the larvae or driving the excrement to the bottom of the pasture canopy or both."

"The pasture has been exonerated," Henning said. "I was in denial about mycotoxins, and I guess this is a time when denial worked."