MRLS and Caterpillar Hairs

There is new evidence that the setae (hair-like projections) on Eastern tent caterpillars (ETC) can become embedded in the submucosal lining of the alimentary tract (from the mouth to the colon) of animals and cause microgranulomas (small, localized areas of inflammation only visible under a microscope). These sores could possibly allow bacteria normally found in the alimentary tract access to the circulatory system. Two recent research projects pointed to this discovery and a potential link to mare reproductive loss syndrome (MRLS).

Early in MRLS in 2001, researchers at the University of Kentucky's (UK) Livestock Disease Diagnostic Center (LDDC) noticed an unusual abundance of a couple of families of bacteria in aborted fetuses (Streptococcus and Actinobacillus). These bacteria are normally found throughout the alimentary tract of horses and naturally occur in the environment. However, there was never clear evidence how these bacteria were linked to MRLS.

In the first setae experiment headed by Karen McDowell, PhD, of UK's Gluck Equine Research Center, pigs were fed ETC in an effort to cause abortions similar to MRLS in broodmares. Abortions did occur as a result of feeding ETC to the pregnant pigs, but more importantly, Neil Williams, DVM, PhD, of the LDDC, who performed the histological (tissue) examinations of the pigs used in the experiment, noticed multiple small, localized areas of inflammation in some of the pigs. He discovered that all five pigs which had been fed caterpillars had these microgranulomas and none of the pigs with a diet free of ETC had the lesions.

Further study showed that the microgranulomas contained hair-like fragments identical to ETC setae.

He said the setae penetrated the protective mucosal layer and embedded in the submucosa, where the inflammatory reaction was intense. It is possible that there were hundreds of thousands, or even millions, of these microgranulomas in one animal. Williams said he only found these microgranulomas in the submucosal layer of the alimentary tract, and he didn't see them in any other organs or in placentas or fetuses of the pigs.

The reaction that causes the microgranuloma is similar to when a splinter gets in your finger. "It's a foreign body-type of reaction," he explained. "The body tries to wall it off."

The bulk of the blood supply to the alimentary tract lies in the submucosal layer, as does the lymphatic supply to the digestive tract. This breach of the protective barrier might offer a means of entry for bacteria into the bloodstream.

"We don't know if this is leading to abortions, or if the reaction is just there because the pigs were consuming caterpillars," said Williams.

In a follow-up experiment, hair remnants resembling ETC setae were found embedded in the submucosa of the digestive tract of one mare fed ETC. Examination of this mare represents the initial step of a controlled experiment with the purpose of investigating the role of the ETC exoskeleton (cuticle) in MRLS.

More Funding Needed

MRLS is estimated to have cost the Kentucky horse industry nearly $500 million in 2001 and 2002. Research spending has totaled nearly $2 million. The Kentucky Thoroughbred Association/Kentucky Thoroughbred Owners and Breeders and the Ag Development Board funded $694,615; the Grayson-Jockey Club Research Foundation funded $295,938; UK funded $80,000; and the USDA Agriculture Research Service funded $423,000 in 2002 and $410,000 in 2003.

"We desperately need money so that the trials can continue," said Richard Holder, DVM, president of the Kentucky Association of Equine Practitioners. "Anyone interested in contributing should contact Nancy Cox, MS, PhD, at the UK College of Agriculture (859/230-7759). The donations should be specified for MRLS research. We are extremely close to solving the MRLS problem."

$5 Million in MRLS Loans

The USDA's Farm Service Agency approved $5,407,739 in low-interest loans for breeders in Central Kentucky negatively impacted by MRLS. The program was developed by the Kentucky Thoroughbred Owners and Breeders and the American Horse Council. Fifteen out of 20 applications were approved.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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