Science As Detective Work: The Case of the Dying Racehorse Foals

It is sometimes difficult to tell the difference between scientific investigation and good detective work. This spring, scientists were called in to solve a mystery that Sherlock Holmes would have enjoyed.

Our mystery starts in April in Kentucky, the Bluegrass State. At this time of year, the lush pastures of Kentucky’s many horse farms become filled with thoroughbred mares and their leggy foals. Kentucky is the acknowledged center of thoroughbred horse breeding in the United States, a 1.2 billion dollar industry. Normally about 10,000 thoroughbred foals are born in Kentucky each year.

This April, in horse farms all over Kentucky, newly pregnant mares began to abort unborn fetuses in the first trimester of pregnancy. Foals also began to die unexpectedly soon after birth, some surviving only a few days. A certain number of stillborn or weak foals is to be expected, but not this wave of death.

There were few clues as to the cause of the problem. It seemed to affect all horse breeds and bloodlines, and so probably isn’t due to bad genes. A similar problem arose in 1980-81, when many unexplained horse deaths occurred. That mystery has never been solved.

Veterinarians dubbed the illness causing hundreds of Kentucky mares to deliver stillborn or weak foals “mare reproductive loss syndrome.”

How widespread is mare reproductive syndrome? An alarming 497 of this year’s foals had died by May 18. The number of lost fetuses is unknown, but much larger — roughly one out of 20 foals due to be born this spring has aborted or been stillborn. During May, the University of Kentucky Diagnostic lab was receiving 20+ dead foals/fetuses a day. Financial losses are estimated to be at least $225 million. Only now is the wave of foal death subsiding.

Breeders, after eliminating heredity and veterinary procedures as possible causes, focused on pastures. The weather this spring in Kentucky has been abnormal, with warm weather followed by a hard freeze and unusually dry conditions. This suggests a working hypothesis: Perhaps the unusual weather has encouraged mold to grow on the grass the horses have been eating. Mold can produce powerful mycotoxins, poisons that could easily harm the reproduction of animals eating the toxins.

Testing this hypothesis, University of Kentucky Agriculture Department investigators looked for mold and mycotoxins on the grass of pastures where foals were dying. They didn’t find any. The mycotoxin hypothesis is wrong.

The investigators did find something else, however, something unexpected. Cyanide.

Cyanide is a deadly poison to all vertebrates. In high concentrations, cyanide binds to a protein called cytochrome oxidase that carries out a key step in respiration. Like throwing a monkey wrench into the gears of an engine, this causes respiration to screech to a halt, no longer able to manufacture the energy-rich ATP molecules that fuel the body’s activities. Within minutes, the brain dies.

The low cyanide concentrations found in Kentucky grass were not enough to kill an adult horse outright, but were plenty enough to kill fetuses and weaken foals.

How did cyanide get onto Kentucky’s pastures? Sherlock Holmes would have loved the answer: caterpillar feces! Here is the train of events that transpired, as the University of Kentucky scientists have pieced it together.

Black cherry trees are common all over Kentucky, and their leaves naturally contain traces of cyanide. Analyzing the leaves, the scientists discovered that the warm dry weather followed by a frost had caused the cyanide levels in the tree leaves to be much higher than usual.

Eastern tent caterpillars eat the leaves of black cherry trees. They are ferocious eaters, and can defoliate a tree in days, eating every leaf. Agronomists have reported a heavy infestation of Eastern tent caterpillars in Kentucky this spring. Laden with cyanide from the leaves they had eaten, the caterpillars left cyanide-laced feces on the surrounding grass. Eating the grass poisoned the mares, killing their foals.

The caterpillar infestation is over, and so is the problem, for this year. In future, breeders will know what to look for. This case is solved.

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