MRLS outbreak in Washington raises new scientific questions

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Jul 01, 2004

It was Saturday May 5, 2001. A usually festive time in the Bluegrass, as "My Old Kentucky Home" played at Churchill Downs, and 17 horses battled in the 127th running of the Kentucky Derby.

Down the road in Lexington, and on farms in surrounding counties, it was not a happy time.

A tragedy was unfolding.

Young foals were battling for their lives - a mysterious toxin had already claimed more than 300 foals , and before the carnage was over an estimated $ 225 million was lost in the Central Kentucky horse industry from a deadly disease , Mare Reproductive Loss Syndrome ( MRLS ).

MRLS, eventually linked to an overwhelming infestation of Eastern Tent Caterpillars (ETC), which occurred between April and June of 2001 and 2002 in Central Kentucky, produced both early and late fetal losses. Early fetal loss (EFL) occurred within 35 and 100 days after mares were bred. In mares that underwent ultrasound examination during this period, fetal death followed by expulsion of the fetus associated with the presence of abnormal echogenic fluid (cloudy and flocculent) around the fetus.

Late fetal loss (LFL) occurred as abortion during the last trimester of pregnancy. LFL was associated in many, but not all, cases with a swollen and engorged placenta (premature placental separation or "red-bag" syndrome). Foals born alive were sometimes weak and required intensive veterinary care. In addition to reproductive losses, a number of cases of pericarditis (inflammation within the sac surrounding the heart) and severe unilateral uveitis (inflammation of one eye) were associated with the occurrence of MRLS.

May 2003, in Kitsap County, Dr. Kenneth Feigner, DVM noticed a curious similarity to that previous period and raised the possibility of MRLS in Western Washington, thousands of miles from Kentucky.

Exploring the possible linksWhat links western Washington with Central Kentucky?

According to Feigner, in years past he would see an abortion every year or two. Half the time he can determine a cause. But spring of 2003 was very unique. "Last year we had four abortions and one foal that was born very compromised, and in each case in the pastures where these mares were kept were very heavily infested with Western Tent Caterpillars (WTC)," Feigner stated.

According to Gary Wolbert, Wolbert's Olympic Horticultural Spray Services, sometimes the Eastern Tent Caterpillar (ETC) can be found in western Washington, though the overwhelming predominance is of the Western species. Regardless, the WTC and ETC are probably not uniquely different from each other in their physiology and eating habits. Therefore, the WTC well might affect horses similarly, and the potential for their involvement in MRLS in Washington as seen with the ETC in Kentucky is probable. In all of the aborting mares there was no other abnormality that Feigner could find. He did a necropsy on a couple of them and sent samples to Washington State University for diagnostic workup.

"It was only after you see one or two, and you say it's just a matter of chance, but by the third one and more it seems too weird," Feigner says. Feigner also sent in some tissue samples and discussed the situation with veterinarians in Kentucky, those working on the follow-up to the MRLS problem there.

"They asked if we could send them large volumes of frozen and alcohol-preserved caterpillars for analysis. The frozen ones to be used in subsequent animal testing to see if they could induce abortion in mares, the alcohol-preserved ones for toxicity testing." Feigner has yet to hear back from Kentucky as to the fate of those bugs. "Kitsap County in 2003 was absolutely overrun with caterpillars, Feigner says. "I've never seen anything like it. The sprayers agreed they'd never seen anything like it either."

"One thing I did notice was that during the year before this outbreak, in the Fall 2002, there was a huge number of WTC moths. One could see millions of them broiling in the lights." The insects ranged from Silverdale to Hansville. Vast areas of alders looked like they were in the middle of winter, not summer, because they were totally stripped bare.

Last Fall 2003, he saw almost no WTC moths, and was hoping the sprayers would be wrong, and this year would be not nearly as bad.