As a grad student who is writing my dissertation on the Tanganyika Laughter Epidemic of 1962, I am partial to one particular Radiolab segment … on the Tanganyika Laughter Epidemic of 1962.

For those of you who don't remember all the way back to season four, producer Ellen Horne traveled to Bukoba, Tanzania, to find out about a fifty-year-old outbreak of hysterical laughter that spanned about ten months; thousands came down with it, mostly young girls, each laughing and crying for hours at a time. Ellen heard of many possible causes. Stress, puberty, … even caterpillars. And not just caterpillars, but dead caterpillars. The ghosts of caterpillars.

You see, the same area that fell prey to the laughing sickness had, about a year or so before, also been the site of an infestation of caterpillars, said Gertrude Rweyemamu, herself a victim of the 1962 epidemic and a participant in our 2008 show. The members of the farming community exterminated the little pests, the hungry hungry hippos of the soil. Rweyemamu explained that an elder of her community concluded the epidemic was the dead caterpillars' revenge. But Rweyemamu wasn’t sold. She shrugged off the revenge theory, offering that if caterpillars were the culprits, perhaps they had some sort of bacterium in them that made people crazy.

To be fair, it was an offhand remark. And, as Ellen reported, blood tests at the time ruled out all possible germy explanations. But it got me thinking: could a caterpillar start an epidemic? Is that even possible? Sure, fleas gave us the bubonic plague. And ticks cause Lyme disease. But, what about nature’s fuzzy little plush toy, the caterpillar? They seem so friendly and benign. What was on their rap sheet?

Turns out, a caterpillar epidemic recently broke out in the United States. So recently, actually, I was surprised I hadn’t heard about it.

It all started in 2001, in the bluegrass pastures of central Kentucky. That year, starting a week and a half before the Kentucky Derby, local newspapers began reporting a rash of horse miscarriages. Not just one or two. Not even one or two dozen. The miscarriages numbered in the hundreds, and eventually, in the thousands.

In three short weeks (a period described, in the title of a paper written by University of Kentucky veterinary science professor Thomas Tobin, as the: “2001 Kentucky Equine Abortion Storm”), 20-30 percent of the region's mares lost their foals, either as early- or late- term fetuses, stillborns, or, in some cases, just a few days after they were born. Most accounts of what became known as “Mare Reproductive Loss Syndrome” were coolly clinical, but somehow as I was reporting out the topic I couldn’t help feel how epic and tragic and extreme it all was, as if pulled from the pages of the Old Testament.   

In a state with a billion-dollar horse industry, the loss of thousands of thoroughbred foals was financially crippling; not just in the short-term to breeders, but also down the line to stable owners, to feed and hay sellers, to blacksmiths, to veterinarians, to racetracks. According to the University of Louisville's department of equine business, the costs all tallied up to $336 million (or $85,142 per thoroughbred foal lost). Add to that the incalculable emotional toll on the mares and their owners. Kentuckian Amy Graves, whom I spoke with in 2011, lost foals from two of her own mares. She summarized the loss to the horses, the owners, and the area in one word: "devastating."

As government and private veterinary scientists began to research mare reproductive loss syndrome, their gaze soon turned to a plague of caterpillars, which were at the time blanketing Kentucky’s Ohio Valley. Two inches long, fully grown. Spiky brown hair. Black, with a white racing stripe. The eastern tent caterpillar was one scary mammajamma, the Terminator of caterpillars. And they were everywhere.

"It was like a carpet,” Graves said of the caterpillars covering her hometown of Versailles, Kentucky. “I couldn't walk through the grass without crunching them." 

She remembers one spring day she went to use her barbecue and found caterpillars covering the lid.

"The grill looked like it grew hair." 

In data collected both from the field in 2001 and in a series of experiments over the next two years, researchers found that when they piped the caterpillars directly into the stomachs of six pregnant mares, all six aborted within five days; if they killed the bacteria on the caterpillars first, only three of six aborted, though it took longer, between eleven and twenty-four days.

But how exactly could a caterpillar out in nature harm a fetus wrapped in a mother's womb?

Well, the caterpillars were all over the pastures, and they even got into the feed, where the mares unknowingly gulped them down. After that, the caterpillars' body, covered in tiny, barbed hair (or “setae”), made it into the equine gastrointestinal tract. There, these tiny hairs, even just fragments of them, pierced through the intestines, carving microscopic holes; in some cases, the hairs then slipped through, entering into the bloodstream, where they would ride around the body and "randomly lodge in distant tissues," wrote Tobin.

Along for the ride on these tiny hairs were, to quote one journal article, “bacterial ‘hitchhikers,’” any of a number of species of Streptococcus or Actinobacillus. All of the mare’s most robust organs fought off these intruders -- all, that is, but the most immunologically vulnerable: the placenta. The barbs spiked their way into this critical organ -- the connection between mother and baby – and researchers conclude that the combination of the breached placenta and the influx of bacteria triggered the miscarriage. The penetrating setae hypothesis, like mare reproductive loss syndrome itself, is, “biologically unique and without precedent in the biological and medical literature,” according to Tobin’s article. 

More than a decade after the fact, every time Graves thinks about the eastern tent caterpillar, she gets tingles down her spine. And to this day, she squishes any and every tent caterpillar she sees, lest it make its way down the gullet of a pregnant mare.

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Thanks to scientist Thomas Tobin for his heebie-jeebie inducing photo of eastern tent caterpillars swarming a bucket in Kentucky, during the 2001 outbreak. Kentucky horse owner Graves confessed she didn't have any photos of her own yard or pastures because it was "too gross to even want to document."