The good news is if
the Eastern tent caterpillars were responsible in some way for this spring's outbreak of foal loss, that they are gone now, cocooned and growing into the next generation of moths. The bad news is that next year, they could be back in the same numbers, according to Dr. Terry Fitzgerald, a distinguished university professor of biological sciences at the State University of New York College at Cortland. Fitzgerald was contacted by researchers at the Gluck Equine Research Center for his knowledge and help in this area.
"This was an excellent year for caterpillars," said Fitzgerald, who has studied the Eastern tent caterpillar since 1976. "Next year should be a very big year, too, where I am located (New York). Then another year there should be a lot, then the numbers will crash because of a bacteria or virus or weather."
He said the Eastern tent caterpillar develops over time into a very large population, then the population "crashes." It takes four or five years for them to appear again, then about every 10 years the population cycles back to an "outbreak" in the number of caterpillars.
"The weather plays a big role," he said about the survival of the caterpillars. "I remember in 1996 they emerged at a time when it was warm, then it was very cold for three weeks and the entire population was wiped out. Last year we began to see them back again. The females fly in from other areas and lay eggs."
When asked if he believed the caterpillar could cause the extreme problems seen in Kentucky and other states this spring, Fitzgerald said, " Cyanide has that potential, and the caterpillars have cyanide in them (from eating black cherry tree leaves). And if it got into the horse it could harm the horse. There are a lot of unknowns in the Kentucky situation. The pathway is not known, and there is no precedent in literature."
The caterpillars were looked at early in the outbreak this spring, then the theory was put on the back burner while other, pasture-related testing was being done. When those studies showed that mycotoxins probably were not the major cause, interest was renewed in the caterpillars.
"We think there is an association with cherry trees and tent caterpillars and the problems we've seen, based on examinations of pastures of farms badly affected," said Dr. David Powell of the Gluck Equine Research Center and one of the leaders in the research team. "We have noticed an association with the presence of cherry trees (the favored meal of tent caterpillars) in pastures where mares lost fetuses, and the presence of massive numbers of caterpillars."
Eastern tent caterpillars prefer to eat the young leaves of the black cherry tree, which is native to most of the Eastern United States. Healthy leaves of cherry trees contain prunasin, a cyanide precursor that is non-toxic. But when the leaves are damaged, the prunasin molecule is split and free cyanide (also called prussic acid or hydrocyanic acid) is liberated. In Fitzgerald's book The Tent Caterpillars
, he states, "The cyanide produced by these plants in response to injury is a potent toxin that affects both vertebrate and invertebrate herbivores. When freshly picked, young black cherry leaves contain up to 2,000 mg of cyanide per kilogram of leaves, some 30-40 times the average fatal dose for humans."
Fitzgerald explained that the concentration of cyanide varies from leaf to leaf on a single tree, and also seasonally. Leaves that contain as much as 2,500 ppm of cyanide in April contain less than 50 ppm by late October.
It isn't known how the caterpillar detoxifies the cyanide it ingests from the cherry tree leaves.
"The Eastern tent caterpillar is not only unaffected by cyanide, but one study suggest that the caterpillar may actually use the compound to its advantage. Eastern tent caterpillar larvae common regurgitate defensively when attacked by predators or parasitoids. Regurgitated juices collected from caterpillars that have just eaten contain both benzaldehyde and cyanide at the same concentration in which they occur in the leaves. It has been suggested that the Eastern tent caterpillar may arm itself intentionally by feeding preferentially on the youngest host leaves, which have the highest benzaldehyde potential."
He also said that birds and small mammals tend to not eat the Eastern tent caterpillars, probably more because of the hairs than anything else.
There are suggested pathways for the cyanide to get from the cherry trees through the caterpillars and to the horses that are being studied.
Fitzgerald's lab is looking at whether caterpillar excreta contains cyanide, or whether it has been broken down and is harmless. There also is the question of whether the cyanide can somehow escape the caterpillar's body if, for example, they drown in a water tank.
While Eastern tent caterpillars aren't known to have any other toxins in their systems, and don't excrete hormones, they do use chemicals for social pathways. Like ants, they leave a chemical trail when they find food to lead the colony to the source. One chemical has been identified as a steroid, but it is used in "vanishingly small quantities," said Fitzgerald. He said the level used in making these trails is 100 billionth of a gram.
Fitzgerald said if they find traces of cyanide in samples from live horses or from fetuses, then the horse probably ate a plant containing cyanide or somehow picked it up from the plant. "My original thinking was that horses were browsing small sprouting cherry trees. Birds eat the cherries, then the seeds are passed in the bird feces. So under electric wires or along fence lines you'll see lots of young cherry trees growing." He said it could be possible if fields and fencerows were not being mowed this spring because of the drought that the young cherry trees might have sprouted and been grazed by the horses. He reminded that the younger the cherry tree leaves, the more cyanide they contain.
"There have been cases of horses killed by eating cherry trees," said Fitzgerald. He cited the case of when hurricane Fran in 1996 toppled thousands of trees in North Carolina. Horses grazed on the leaves of the fallen trees and died.
Fitzgerald was skeptical that the caterpillars were irritating the horses grazing around them. Oral ulcers and small, blister-like sores around the mouth have been seen on some horses. Fitzgerald said the only time he can remember tent caterpillars causing a reaction to human skin is when he had students dissecting 40,000 of them, and the students did get skin rashes.
"The Eastern tent caterpillar is relatively innocuous," said Fitzgerald.
For now, testing will continue to see if cyanide or some product broken down from cyanide can be found in equine samples.