Foal loss syndrome – 11/07/2004

  • October 1, 2013 at 7:57 pm #605

    I heard Paul Harvey talking about this a year or two ago, hope its
    not too old to post:

    On Kentucky Derby Saturday, May 5, 2001, 73 dead foals and fetuses
    were delivered to the Livestock Disease Diagnostic Center at UK’s
    Coldstream Research Campus on Newtown Pike. And while weekend
    deliveries of dead animals to the back door of the center is common,
    the tremendous number that day—about 10 times more than usual—was

    “I knew we had big trouble on our hands. In addition to these foals
    and fetuses, we were aware that too many foals in the Fayette County
    area were being born weak,” says Lenn Harrison, veterinarian and
    center director. Harrison worked alongside center pathologists
    practically non-stop that weekend—the first of many—to try to solve
    the puzzle of what was causing the death of so many foals and

    In the lab, pathologists performed a necropsy on each foal, looking
    at internal organs for signs of infection as well as taking blood
    and tissue samples for analysis. The tissue samples offered up the
    clue of rare bacterial infections in many aborted fetuses, but the
    pathologists concluded that these bacteria could not be solely
    responsible for the deaths. “As the necropsies continued, our belief
    got stronger that the bacteria were secondary to the specific cause
    of what was dubbed Mare Reproductive Loss Syndrome,” Harrison says.

    Within the next two days, the total number of dead foals and fetuses
    brought to the center grew to 276, and a team of over 100 experts
    from a variety of disciplines in the College of Agriculture joined
    the investigation. A group of highly experienced veterinarians made
    up a survey to be distributed right away to horse farms by the
    Kentucky Thoroughbred Farm Managers Club to try to determine the
    extent of the losses. Staff in the UK agricultural communications
    office also swung into action to make certain that the industry and
    the public were kept abreast of the situation. And a Web site,, was established to provide
    a continuing source of information for the public.

    By May 10, news of the syndrome had spread around the world: U.S.
    networks as well as European and Middle East news agencies were
    constantly calling UK for information about this syndrome that had
    the potential to disrupt the global horse industry. Four days later,
    May 14, farm managers who had returned questionnaires confirmed the
    worst: the syndrome was widespread, and it was continuing.

    “At this point, a couple of faculty members at Gluck [UK’s Gluck
    Equine Research Center] recalled that in 1980 and 1981 early fetal
    loss had occurred spontaneously and no cause had ever been
    determined,” Harrison says. “And in both years the losses abruptly
    stopped.” Reported losses during those years, however, were nowhere
    near the 2001 numbers.

    “A quick check of meteorological conditions then and now,” he
    explains, “showed a disturbing similarity of weather patterns that
    could affect pastures where the mares grazed.” March temperatures
    all three years were below normal, followed by above normal
    temperatures in April. Such a pattern would result in explosive
    biological activity in both plants and insects. The data also showed
    frost or freeze in the third week of April, followed by warm
    temperatures a few days later. What impact could those factors have
    on the pastures?

    “Although the convergence of weather factors in the three years with
    larger than usual abortion rates wasn’t a smoking gun, it was
    suggestive that something weather-related was responsible,” Harrison

    Perhaps, he thought, weather-dependent vegetation could be linked to
    a variety of possible disease-causing agents, including mycotoxins
    and ergot-type alkaloids (which are derived from fungi and can be
    poisonous to animals in high concentrations) and phytoestrogens
    (compounds produced by plants that mimic the hormone estrogen).
    Tests for poisoning from mycotoxins in feed, ergot-type alkaloids in
    pasture, and phytoestrogens appeared to be negative, but most of the
    tests were done after the syndrome had occurred.

    Because none of the obvious causes appeared plausible, the
    scientists turned their attention to still another possibility—that
    the villain might be the Eastern tent caterpillar, whose populations
    were high in each of the years in question. Harrison showed UK
    agronomist Jimmy Henning some information he’d run across on these
    prolific crawlers, in particular the fact that they feed on wild
    cherry leaves.

    Although the caterpillar-cyanide theory hasn’t held up in recent lab
    tests, the Eastern tent caterpillar’s role in the foal deaths is
    still being investigated. [photos by Steve Patton]

    And when Henning visited farms where mares had given birth to dead
    or dying foals or had aborted early-term fetuses, he discovered a
    striking pattern: most of the pastures had a large number of wild
    cherry trees. “I knew that cherry tree leaves carried the precursor
    to a poison—naturally occurring organic cyanide—that could cause
    death in horses and other animals, especially cattle,” says
    Henning. “I also realized that Eastern tent caterpillars, which were
    known to feed on the leaves of cherry trees and were seemingly
    immune to the poison, had been quite active during late April.”

    The caterpillars became even stronger suspects in a meeting of
    veterinarians, farm managers and the media on May 24. “At that
    meeting,” Harrison says, “several veterinarians confirmed that the
    incidence of early fetal losses as well as stillborn and dying foals
    had dramatically decreased during the previous week, paralleling the
    natural decline in caterpillar numbers.”

    The investigation into the possible role played by the caterpillars
    continued. In December 2001, entomology professor Bruce Webb made an
    interesting discovery: his research showed that cyanide does not
    accumulate in the caterpillars, so they are not likely to deliver
    appreciable amounts to horses. This finding does not mean, however,
    that the caterpillars were necessarily innocent bystanders. Research
    planned by Webb this spring will investigate potential indirect
    roles that the caterpillars may play. “They produce large amounts of
    waste as they feed,” Webb explains. “Much of this nutrient-rich
    material rains to the ground under infested trees and may serve as
    food for specific molds that produce toxins that might contribute to
    the syndrome.”

    As researchers close in on the causes of the foal-loss syndrome, the
    importance of their work is underscored by last spring’s sad
    numbers. Between April 28 and May 12, 418 aborted equine fetuses and
    stillborn foals were brought to the Livestock Disease Diagnostic
    Center for evaluation. During that same period in 2000, only 60
    fetuses and stillborn foals were received. On May 23, 2001, a
    University of Kentucky survey of 159 thoroughbred farm managers
    indicated that 678, or 21 percent, of 3,294 pregnant mares had
    experienced early fetal loss.

    So, what can be done now and in the future to make certain that such
    an aberration doesn’t occur again?

    Henning says that researchers have begun a complex environmental
    monitoring system they hope will give them advanced warning if
    conditions are ripe for this to happen again. About 13 farms are
    being tested on a rotating basis, with soil, grass, water and other
    samples collected, analyzed, and stored for later comparison should
    foals again begin to die in such numbers. Weather patterns are also
    being closely tracked, and a massive research initiative on the
    causal mechanisms of the syndrome is planned by the College of
    Agriculture in the coming months.

    Harrison says he hopes that the only thoroughbred problem he will
    have to deal with around Kentucky Derby time this year is which
    horse to bet on.

    Jeff Worley

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