Setae and MRLS

Studies to determine the cause of mare reproductive loss syndrome (MRLS) have led investigators to associate the syndrome with exposure to the Eastern tent caterpillar. Although the exact cause of MRLS remains unknown, some component of the outer covering of the caterpillar (its “exoskeleton” or “integument”) appears to be involved.

In the first study to implicate the integument as a causal agent in MRLS, investigators fed one group of pregnant mares integuments that had been separated from caterpillars and another group the insides of the caterpillars. Half of the mares fed integuments aborted their foals while none of those fed only the insides aborted. A second study conducted by another group of researchers produced similar results--68% of horses fed integuments aborted while none of those fed the insides of the caterpillars aborted.

It presently isn’t known if the property of the integument that causes the problem is a microbe, a toxin, some mechanical property of the integument, or possibly even a combination of these. One candidate is the hair-like covering of the caterpillar’s body, the seta (pronounced see-tay).

Caterpillar Protection

The integument of caterpillars is an anatomically complex structure consisting of protuberances of many sorts. The largest and most conspicuous of these are the setal hairs. Some species of caterpillars--such as the pine processionary of southern Europe--have brittle, sharply pointed setae that are hollow and filled with a poison. When a processionary is handled, the setae puncture the victim’s skin and break open, spilling their contents and causing an intense itching dermatitis. The setae can cause conjunctivitis if they get into the eye and pharyngitis or bronchitis if setal fragments are inhaled.

In addition to its poisonous hairs, the processionary caterpillar has softer, non-poisonous hairs that are similar in appearance to those found on tent caterpillars. Recent studies have shown that a proteinaceous component of these hairs can cause an allergic response capable of escalating into a life-threatening bout of anaphylactic shock in sensitized individuals that come in contact with even the smallest fragment of a seta.

In contrast to the processionary caterpillar, the setae of the tent caterpillar are not poisonous and there is no apparent toxic-irritative effect from handling the caterpillars. There is, however, the potential for the setae to cause an allergic response similar to, but less intense than, that reported for the processionary. Some individuals who have had repeated close contact with tent caterpillars have developed skin rashes and experienced mild respiratory irritations. While it might seem that some sort of allergic response to the setae of the tent caterpillar could underlie MRLS, there have been no reports of any allergy-like symptoms associated with MRLS.

See setae and other caterpillar images here

How then might setae be involved in MRLS?

Setae and Horses

The first attempt to establish a small animal model for MRLS used mice. When researchers injected pregnant mice with a saline solution containing setal fragments, 60% of the experimental animals reabsorbed all their fetuses and a subsequent necropsy showed that they had acute suppurative inflammation of their placentas. In contrast, only a single fetus was reabsorbed in control mice and no infections were found. However, the lead researcher, Manu Sebastian, PhD, said the results were inconclusive because of the small number of mice used in the study.

A second attempt to induce MRLS-like symptoms in an animal, other than the horse, involved the pig. In this study, investigators found setae in the digestive tracts of pigs fed tent caterpillars. Based on this observation alone, the investigators were unable to draw any definitive conclusions regarding the possible involvement of setae in MRLS.

However, one possible, but as yet undocumented scenerio, is that inflammation of the wall of the digestive tract at the point where a seta lodged might allow bacteria that are normally confined to the digestive tract to move into the blood stream and to find there way to the placenta, infecting the tissue and promoting an abortion.

One of the first researchers to draw attention to the possible involvement of setae in MRLS was Thomas Tobin, MVB, MSc, PhD, MRCVS, DABT, of the University of Kentucky’s Gluck Equine Research Center. While he acknowledges that that setae might compromise the integrity of the intestinal wall and provide a path of entry for an infectious agent as outline above, Tobin also champions a more elaborate and comprehensive hypothesis which he terms the Septic Penetrating Setal Emboli hypothesis (SPSE). He hypothesizes that setae not only penetrate the intestinal wall, but that they and the bacterial that ride on them work their way into the bloodstream and travel in the vessels to target sites where they penetrate the target organ and dump their load of infectious bacteria. According to this hypothesis, the prime target, due to its size, is the maternal/fetal placentation interface; other targets include the heart and the eye, accounting for the small incidences of pericarditis and uveitis associated with MRLS.

Tobin acknowledges that at this stage in our understanding of MRLS, his SPSE hypothesis must be considered one of several viable working hypothesis, which include the possibilities that an as yet unidentified toxin or a microbial agent associated with the integument or the setae are the primary culprits.

References
1. TheHorse.com, January, 2003. Study conducted by W. Bernard and M. LeBlanc of the Rood and Riddle Equine Hospital and T. D. Fitzgerald of the State University of New York at Cortland. (See article #4042.)
2. TheHorse.com, June, 2003. Study conducted by UK scientists: K. McDowell (Department of Veterinary Science), Bruce Webb, and Walter Barney (Department of Entomology), and N. Williams and M. Donahue (Livestock Disease Diagnostic Center). (See article #4442.)
3. Sebastian, et al. 2003. A laboratory animal model of mare reproductive loss syndrome: preliminary evaluation of a mouse model. Proceedings of the First Workshop on Mare Reproductive Loss Syndrome. UK Agriculture Experiment Station Publication SR-2003-1:51-53.
4. TheHorse.com, September, 2003. Study conducted by UK scientists, K. McDowell (Department of Veterinary Science), N. Williams, M. Donahue, and D. Bolin, (Livestock Disease Diagnostic Center), M. Lindemann and J Monegue (Animal Sciences), B. Webb (Department of Entomology), B. Lynn (Department on Chemistry) and K. Newman (Venture Laboratories). (See article #2690.)

About the Author

Terry Fitzgerald

Terry Fitzgerald, PhD, is a distinguished university professor of biological sciences at the State University of New York College at Cortland who wrote the book The Tent Caterpillars.

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