The Mystery of the Fallen Horses

An interesting article from my vet-

The Mystery of the Fallen Horses


Researchers may have caught the culprit in a decade-old mystery

By Becky Beyers


Horses were dying, and nobody could figure out why.



Seemingly healthy animals put out to graze in pastures suddenly got very sick and died within a few days. Their muscles stiffened, they had difficulty walking or standing, breathed rapidly and finally laid down and died. It didn’t happen often, but over the course of a few decades, veterinarians saw enough similar cases to think that a common cause was to blame.


Stephanie Valberg, director of the university’s Leatherdale Equine Center and Adrian Hegeman, associate professor in the Department of Horticultural Science, teamed up to find the cause of mysterious horse deaths. The culprit: A chemical found in boxelder tree seeds. When horses eat the seeds, they get sick and often die.

Photos by David Hansen



Scientists from CFANS and the university’s College of Veterinary Medicine now believe they’ve caught the killer and have clues as to how to prevent it.



“This phenomenon—Seasonal Pasture Myopathy—has been described for decades,” says Stephanie Valberg, director of the university’s Leatherdale Equine Center and a professor of veterinary medicine. “It’s the worst-case scenario when you have horse owners asking ‘what can I do?’ and there are no good answers.” The illness is fatal in about 90 percent of cases.



Until last fall, the cause was a mystery. Some experts blamed a perennial herb plant that’s a known toxin to horses that eat it—whitesnake root—but tests of the mysterious dead horses’ livers showed no sign of that toxin.



Scientists in Europe, where the phenomenon is even more prevalent, noticed that the victims were often younger horses and that they tended to die in October or November. But still, no one could explain the phenomenon.



Valberg wanted to learn more about the environments where Minnesota horses had died. “To me, a horse farm has wood fences around it and no trees,” she says, but in the fall of 2011, when she began visiting farms in southeastern Minnesota, she discovered that’s not always the case. She found sloping pastures filled or ringed with trees. Boxelder trees. Lots and lots of boxelder trees. Could they be the culprit?



After 11 pasture inspections, boxelders were clearly a common theme throughout all the deaths; in fact, one pony that died had been in a small enclosure where boxelders were the only plants it could reach. Valberg and her team traced weather patterns around the time of the deaths and found that in many cases, high winds a few days before the horses got sick had blown seeds from the boxelders into the pastures, where the affected horses may have gobbled them up. The researchers thought they had found their killer, but why boxelder seeds are so toxic was still unclear. European colleagues at first blamed a fungus on the trees’ leaves. Valberg’s hunch was that something in the seeds was toxic, and she found an obscure 40-year-old study that suggested an amino acid might be the cause, but she needed help from colleagues with expertise in plants.



So she contacted horticultural scientist Jeff Gillman and plant pathologist Bob Blanchette in CFANS. They, in turn, connected her with Adrian Hegeman, an associate professor in horticultural science whose expertise is in plant metabolomics. He started investigating, ran across the same 1972 study, and arrived at a similar conclusion: the boxelder seeds could be the culprit. “It made so much sense,” he says now. Further testing of tissue samples from horses that died proved the theory was correct: a metabolite of hypoglycin A, the toxin found in the seeds, could be found in all the horses’ tissues. The toxin gets into horses’ livers and muscles and blocks the metabolism of fats; the telltale sign is through testing horses’ blood or urine, and if a specific pattern of fats are there, it’s a confirmed case. The researchers’ work has been published in veterinary journals and is getting attention from around the world.



But even with a likely suspect, investigators still have questions about the deaths: Why do only some horses get sick? Do other trees make similar toxins? What environmental and weather factors might play a role?



Last fall, Valberg went to Switzerland, where colleagues are studying the phenomenon and hilly pastures are lined with Acer trees—which are in the same genetic family as the Western Boxelder so common to Minnesota. She collected leaves and seeds from as many trees as she could, which now are being analyzed in Hegeman’s lab to see if they contain similar toxins. It’s possible that tar spot, a fungus often found on similar trees, also might play a role. Hegeman, Gillman and Blanchette will use their expertise to help explore that possibility and to share best practices with horse owners.



Meanwhile, Valberg and Krishona Martinson, an associate professor in the Department of Animal Science, are working with horse owners statewide via University of Minnesota Extension to collect more information about horses that get sick and the conditions around their illnesses and deaths. Early analysis shows that younger horses and those who are new to a pasture are more susceptible, for example, but it’s not clear whether older horses developed immunity or simply learned not to eat the seeds.



It’s also unclear just how widespread the phenomenon is, Valberg says. In many cases, by the time a horse owner calls the local veterinarian, it’s too late for the horse to be brought to a university clinic. The horse is buried with no post-mortem testing, so nobody knows for sure what killed it. “Most of the cases we do diagnose are near vet schools,” she says, because owners and local veterinarians may not want to spend the time or money on post-mortem testing, especially if travel is required. “We simply don’t have a handle on how common this is in Minnesota.”



Ultimately, the research team expects to develop tests that can diagnose seasonal pasture myopathy earlier and to share ways that horse owners can prevent future cases. “We want to help the people who have lost horses,” Valberg says. “They really want to know what happened and how to prevent it from affecting their other horses.”



Minimizing your horses’ risk

•Minimize the number of boxelder trees and seeds accessible to the horses, through trimming trees or moving fences. This is especially important in fall, when trees are shedding seeds. “We don’t need everybody to cut down all their boxelders,” Valberg says, but be careful about how reachable they are for horses and how far seeds spread



•Prevent overgrazing of pastures by moving the horses often, or by supplementing their diet with hay or grain so they don’t eat weeds, seeds and trees



•Reduce the number of hours a horse spends on pasture each day in the fall or after a heavy wind or rain if boxelder seeds are present



•Don’t pasture young horses or those that are new to a farm in an area with lots of boxelders in the fall or early spring.