Another new word has been added to the equine world--setae (prounced see-tay). Why? Horses have hair, caterpillars have setae. There are some researchers who hold the belief that these hair-like projections on the skin of Eastern tent caterpillars (ETC) might have something to do with all the health problems seen in horses in Kentucky and surrounding states in 2001 and 2002. What has become known as mare reproductive loss syndrome (MRLS) actually includes six distinct problems--early fetal loss, late fetal abortion, the birth of ill foals (with up to 50% mortality), pericarditis (fluid around the heart), unilateral endophthalmitis (infection in one eye), and encephalitis (inflammation in the brain) caused by the bacteria <I>Actinobacillus<I>.

Could all of this be caused by the tiny hairs of a caterpillar?

It’s well-known in the world of entomologists that those setae on the caterpillars can be problematic, even lethal. (See article #4609.) But the ETC are considered a benign type, although there are some humans who get a rash if they touch the caterpillars or are exposed to them over a period of time.

But how do the caterpillars cause all of these different problems in some horses, and other horses seem to be immune?

At the first workshop on MRLS in August 2002, held at the Gluck Equine Research Center in Lexington, Ky., researchers and veterinarians presented facts, figures, and ideas on what they had discovered, lived through, and observed the past two breeding seasons.

At that point, researchers and veterinarians had cooperated to discover it was something on the outside (integument) of the caterpillar that was causing the problems. Whether it was a toxin, bacteria, or mechanical cause is still unknown.

Tom Tobin, MVB, MSc, PhD, MRCVS, DABT, at the Gluck Center, gave a short presentation on how he thought the setae might be involved in these syndromes. He used this analogy: “Dentists working in my mouth put me on prophylactic antibiotics immediately because I have a heart murmur, and they don’t want to risk a bacterial vegetative endocarditis (bacteria traveling from the mouth to the heart and causing a problem). Likewise, something happens when horses are exposed to caterpillars in that we suddenly have oral commensal bacteria (those bacteria normally found in the mouth) appearing shortly thereafter at multiple locations in the body.”

He also pointed out that bacteria carried into--or allowed into--the horse’s body via eating or breathing the caterpillar setae was different from having those caterpillars ground up and tubed directly into the horse’s stomach, as was done in several experiments that resulted in abortions, but none of the other syndromes.

These setae have small barbs that could possibly attach to the walls of the GI tract and cause irritation, possibly enough to allow those tag-along bacteria to take up residence and spread. If that barbed setae found its way into a blood vessel, it could be carried to any number of places in the horse’s body, especially places where there is a lot of blood flow, such as the uterine artery. Via that route it could be transmitted into the blood supply to the fetus.

Numbers Don’t Lie
Information from a more recent paper submitted to a scientific journal by researchers and veterinarians including Manu Sebastian, DVM, MS, a pathologist with the University of Kentucky's Livestock Disease Diagnostic Center, Marie Gantz, a statistical consultant in the College of Agriculture, and Tobin, created a statistical analysis of the association between ETC and abortions.

This model showed that high levels of ETC exposure produce intense, focused outbreaks of abortions closely linked in time and place to dispersing ETC (as occurred in Central Kentucky in 2001). With less-intense exposure, the lag time is longer and abortions tend to spread out over time and can occur out of phase with ETC exposure, obscuring identification of this syndrome and the role of the ETC in years with fewer caterpillars. This model theorizes that ETC doses anywhere between 5 and 500 grams/day of ETC will, if maintained, sooner or later abort 100% of exposed pregnant mares. This is not likely in a field situation where the exposure to ETC is transient and short-lived.

But, based on these hypotheses, the presence of any caterpillars is a threat to pregnant mares and presumably other horses.

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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