04/05/2007 11:00PM

Cause of foal loss still not certain

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LEXINGTON, Ky. - It's been six years since mare reproductive loss syndrome cost Kentucky's commercial Thoroughbred breeding industry an estimated third of its annual foal crop. But researchers still are working to identify exactly how the disease occurred and how it unfolds.

The general consensus - and one that central Kentucky breeders are heeding - is that exposure to an unusually large infestation of Eastern tent caterpillars in the spring of 2001 caused thousands of mares to abort their early-term fetuses or produce sickly late-term foals. That's what researchers at the University of Kentucky in Lexington have concluded, and breeders have responded by taking aggressive measures against the insects, which hatch and form white, silky, tentlike nests in cherry and crabapple trees in the early spring.

"Mare reproductive loss syndrome is caused by setae" - stiff hairs - "from Eastern tent caterpillars that lodge themselves in the gastrointestinal tracts of horses and cause normal bacteria in the upper GI tract of the mare to infect the fetus through a mechanism we don't understand to this point," said Dr. Kyle Newman of Venture Laboratories in Lexington, who has conducted research with UK scientists.

The caterpillar theory has been controversial, with some researchers in other locations asserting that mares will not willingly ingest caterpillars. But UK studies, according to a release written by Dr. Karen McDowell, show otherwise and have established a link between the caterpillars and bacteria that can kill fetuses.

"Epidemiological and field studies conducted by UK researchers demonstrated that MRLS was associated with unprecedented populations of Eastern tent caterpillars, wild black cherry trees, and waterfowl on horse farms in Kentucky," McDowell wrote. "A series of studies over the next five years has subsequently revealed that horses will inadvertently eat the caterpillars and that the caterpillar hairs embed into the lining of the alimentary tract. Once that protective barrier is breached, normal alimentary tract bacteria may gain access to and reproduce in sites with reduced immunity, such as the fetus and placenta. Fetal death from these alimentary tract bacteria is the hallmark of MRLS."

In their cooperative research, Newman and UK scientists found that pregnant mares exposed to the caterpillar hairs developed cloudy fetal fluid, a sign of MRLS, whereas mares not exposed did not.

In necropsies on mares both exposed and unexposed to caterpillars, "we found that mares with MRLS symptoms had higher concentrations of these bacteria in certain parts of the gastrointestinal tract than the control mares," Newman said.

Thanks to UK's work, the caterpillar is widely considered to be the culprit in MRLS, or at least a significant contributing factor leading to the disease. In Oneida County, N.Y., on April 11, Cornell University is hosting a seminar for horse breeders on the insect's relationship to MRLS.

And breeders in New Jersey and Florida last summer reported a higher rate of abortion among mares exposed to the Eastern tent caterpillar; UK conducted pathology reports on those aborted foals and confirmed that the results were consistent with MRLS. The numbers were not at the levels seen in Kentucky in 2001, but those 2006 abortions reminded Kentucky horsemen that the disease should stay on their radar screen now that the Bluegrass State's Thoroughbred crop has rebounded.

But there is still more to learn.

"For any arguments that anyone can come up with for why caterpillars can't cause abortions, I can come up with 10 reasons for why all of this research says it does make a definitive link," Newman said. "The story makes very good sense."

Oregon State study points to viruses

One can still find debate over whether the Eastern tent caterpillar is the definitive agent, or the only agent, for MRLS. Dr. Alvin Smith of Oregon State University has a different theory, which he and another researcher, Dr. Andreas Kurth, aired in the American Journal of Veterinary Research last April. They described "a strong association" between infection with a virus called vesivirus and MRLS. If correct, that would mark the first time vesivirus has been described as an abortion-causing agent in horses, but Smith says that the evidence points to the virus's presence among mares who aborted in 2006.

In a study of 112 horses, Smith and Kurth found that 64 percent of pregnant mares who had aborted their fetuses or been in areas with high abortion rates in the 2001 MRLS outbreak tested positive in blood tests for antibodies to vesivirus. Among horses with no reported abortion troubles, the positive rate was 40opercent.

Vesivirus, which is found most commonly in ocean species and is part of a family of viruses called calicivirus, is known to cause abortions in other species. Smith doesn't deny the caterpillar theory but believes that if vesivirus isn't the cause of MRLS, it is likely a contributing factor to abortions in horses. He recommends that breeders be aware of vesivirus in any case.

"I'm not totally convinced that tent caterpillars and caliciviruses are entirely separate entities here," he said. "I'm reasonably convinced that the horses in Kentucky are getting pretty substantial exposure to calicivirus a lot of the time and, in all probability, year in and year out, calicivirus will be contributing to some part of the abortions that are seen and go without specific diagnoses."

* The first under-tack preview for Keeneland's April 17 juvenile sale will take place Monday at Keeneland at 10:30 a.m.

* The Barretts May 2-year-old sale catalog is available online at www.barretts.com. The one-day sale of more than 400 horses takes place May 15 at Fairplex Park in Pomona, Calif.

* TheGreatestGame.com, an initiative to recruit new owners for Thoroughbred racing, has been rebranded as The Racing Game. The website, , provides information on such topics as buying a horse, selecting a trainer, and navigating licenses and taxes.