Mare reproductive loss syndrome (MRLS) is a syndrome consisting of equine abortions and three related nonreproductive syndromes which occur in horses of all breeds, sexes, and ages. MRLS was first observed in the U.S. state of Kentucky in a three-week period around May 5, 2001, when about 20–30% of Kentucky's pregnant mares suffered abortions. A primary infectious cause was rapidly ruled out, and the search began for a candidate toxin. No abortifacient toxins were identified.
In the spring of 2001, Kentucky had experienced an extraordinarily heavy infestation of eastern tent caterpillars (ETCs). An epidemiological study showed ETCs to be associated with MRLS. When ETCs returned to Kentucky in the spring of 2002, equine exposure to caterpillars was immediately shown to produce abortions. Research then focused on how the ETCs produced the abortions. Reviewing the speed with which ETCs produced late-term abortions in 2002 experiments, the nonspecific bacterial infections in the placenta/fetus were assigned a primary driving role. The question then became how exposure to the caterpillars produced these non-specific bacterial infections of the affected placenta/fetus and also the uveitis and pericarditis cases.
Reviewing the barbed nature of ETC hairs (setae), intestinal blood vessel penetration by barbed setal fragments was shown to introduce barbed setal fragments and associated bacterial contaminants into intestinal collecting blood vessels (septic penetrating setae). Distribution of these materials following cardiac output would deliver these materials to all tissues in the body (septic penetrating setal emboli). About 15% of cardiac output goes to the late-term fetus, at which point the septic barbed setal fragments are positioned to penetrate placental tissues which lack an immune response. Bacterial proliferation, therefore, proceeds unchecked and the late-term fetus is rapidly aborted.
Similar events occur with the early-term fetus, but as a much smaller target receiving an equivalently smaller fraction of cardiac output, the early-term fetus is less likely to be "hit" by a randomly distributing setal fragment. Since this MRLS pathogenesis model was first proposed in 2002, other caterpillar-related abortion syndromes have been recognized, most notably equine amnionitis and fetal loss in Australia, and more recently, a long-recognized relationship between pregnant camels eating caterpillars and abortions among the camel pastoralists in the western Sahara.
History
What became MRLS was first noted on April 26, 2001, by Dr. Thomas Riddle of Lexington, Kentucky, who observed an unusual number of equine in utero early fetal deaths in 60-day-old fetuses he was examining by ultrasound for sex determination. These early fetal losses were soon followed by a sequence of numerous early and late fetal losses and, recognized somewhat later, coincident and relatively small numbers of pericarditis, unilateral uveitis, and encephalitis syndromes occurring in horses of all ages and sexes.
The fetal loss numbers were large. During the three weeks around the first of May 2001, about 20–30% of Kentucky's pregnant mares suffered abortions. Of foals conceived in the spring of 2001, about 2000 were lost, the so-called early fetal losses (EFLs). Of foals conceived during the spring of 2000, and then close to term, at least 600 were lost, the so-called late fetal losses (LFLs). Based on these overwhelming reproductive losses, the syndrome was named the mare reproductive loss syndrome (MRLS).
MRLS was defined as including four syndromes: (1) EFLs, (2) LFLs, (3) unique unilateral uveitis, and (4) pericarditis syndrome. An associated encephalitis syndrome was not included in the original case definition. The unusual pericarditis cases and the unique single-eye uveitis cases were observed in horses of all ages, breeds, and sexes, were independent of any state of pregnancy, and that they occurred at the same time as the early and late fetal loss syndromes. From the first, therefore, MRLS was clearly not simply a pregnancy-related syndrome.
The total economic loss to Kentucky and the racing industry for the 2001 MRLS season has been estimated at $336–500 million.
Coincident with the MRLS syndrome in May 2001, Kentucky was experiencing an extraordinarily heavy infestation of eastern tent caterpillars (ETCs). As part of an extensive and multifaceted investigation spearheaded by the University of Kentucky, a rigorous epidemiological survey by Dr. Roberta Dwyer and her associates soon confirmed an association of MRLS with the presence of the caterpillar. Yet to come, however, was scientific proof that the caterpillars were the cause of MRLS, and the proposed mechanism by which they affected horses.
Clinical signs MRLS was initially characterized by four syndromes: (1) EFLs, (2) LFLs and the nonreproductive syndromes, (3) unilateral uveitis, (4) pericarditis, and later (5) Actinobacillus encephalitis. MRLS was observed in mares of all breeds and ages. Early and late fetal losses were observed within the first and last trimesters of pregnancy, respectively. For EFLs, clinical signs from the sick mares included pus-like discharge from the vulva and fetal membranes protruding from the vulva, as the fetuses were located in either the vagina or vulva. One to three days prior to the EFLs, several mares showed mild colic symptoms, abdominal straining, or low-grade fever. Within a week of abortion, inspections indicated moderate to severe inflammation within the uterine region. Performing ultrasounds revealed either dead fetuses or live fetuses with slow heart rates and lethargic movements. All the fetuses, both alive and dead, were surrounded by cloudy amniotic fluid.
Clinical signs for LFLs included explosive parturition, dystocia, foaling while standing, premature placenta separation, and foals either stillborn or born weak. Placentas were observed to carry a pale brown hue as opposed to their usual dark reddish-brown color. The umbilical cords were thick, dull, yellowish, and inflamed. The weak foals were often incapable of breathing on their own and required resuscitation. These foals were also observed to be dehydrated and hypothermic, with irregular heartbeat and respiration. The majority of these foals did not survive past four days.
One finding observed exclusively in MRLS was unilateral uveitis; initially, affected horses expressed inflammation around a single eye, along with fluid accumulation on the corneas, anterior and posterior chambers due to the inflammation. The fluid in the anterior chambers exhibited tan to yellow hues, and often was accompanied by hemorrhaging around the surface of the iris.
Cause
The eastern tent caterpillar infestation in the spring of 2001 was extraordinary. It was not uncommon to observe more than a dozen caterpillars per square foot of lawn or pasture. One photograph shows a water bucket completely engulfed with ETCs. The pasture presence of ETCs was thought to be related to the MRLS abortions, leading to ETC epidemiological studies. ETC eggs are preferentially laid in webs on black cherry trees, and the primary food for ETCs is black cherry tree leaves.
By June 2001, Kentucky ETCs were no longer available for research, and would not reappear until the following April/May. MRLS research in the interim focused on the possible role of black cherry tree-related cyanide as the proximal cause of MRLS. The leaves of black cherry trees are known to be toxic due to their production of cyanide. Cyanide conveyed to mares via the caterpillars was initially thought to be the causative agent. Administration of both cyanide and mandelonitrile (a cyanide-containing chemical in cherry leaves) to horses demonstrated cyanide does not cause MRLS.
When the caterpillars returned to central Kentucky in late April 2002, they were readily shown to cause both the early and late fetal losses.
In 2002, investigators' attention was drawn to the highly toxic Brazilian Lonomia obliqua caterpillar setae and their enzymatic fibrinolysin toxin. A caterpillar defense mechanism based on specific setal toxins made more biological sense than a defense based on intestinal cyanide content. More importantly, if the ETC abortions were due to an enzyme or toxin, it should be possible to vaccinate horses against the toxin, an approach already explored with reference to the Lonomia toxin. In June 2002, conception of this ETC enzymatic setal toxin hypothesis was recorded as an intellectual property (IP) disclosure with the University of Kentucky IP office in which creation of an anti-ETC abortifacient toxin vaccine was proposed.
Following these initiatives, when ETCs again became available in 2002, a number of experiments on pregnant mice seeking to evaluate the use of pregnant mice as a laboratory model of MRLS and to develop experimental support for a filterable protein ETC toxin. These experiments showed intact ETC setae appeared to cause fetal resorption in mice, but frozen ETC setae and ETC setal filtrates failed to produce fetal resorptions in mice.
Mechanism of action
The septic penetrating setal emboli (SPSE) hypothesis is the most probable mechanism of action for ETC exposure leading to MRLS. SPSE is believed to be a completely novel hypothesis, without precedent in biology or medicine.
Reviewing the speed with which ETC dosing produced LFLs in controlled experiments and the barbed nature of ETC hairs (setae), and thereafter) intestinal blood vessel penetration by barbed ETC setal fragments, followed by their systemic distribution following cardiac output (SPSE) and then distant tissue penetration by these distributed fragments to tissues with reduced immune responses, i.e., the early- and late-term fetus, the eye, and the heart/pericardial space/fluid, followed by bacterial proliferation in the reduced immune response tissue (fetus, eye, pericardial fluid) would both quantitatively account for and mechanistically link all four MRLS syndromes, including most particularly the unique and unprecedented single-eye lesions.
In this probabilistic model, the number of actual distributing setal fragments is assumed to be small, on the order of tens per day, but the probability of a clinically observed adverse event in a poorly immune-protected tissue penetrated by a setal fragment was assumed to be close to 1.0, leading to the numerous EFL and LFL events and the rare but essentially unique single-eye events of MRLS. A poor immune response in affected tissues is an integral part of the SPSE hypothesis. ETC setal fragments distributing bacteria to immune competent tissues, as occurred in all horses in central Kentucky during the ETC/MRLS do not appear to produce any observable clinical responses. However, the fetus is well known to be poorly immune protected, as is the eye, and extracellular tissue fluids such as the pericardial fluid are also less well immune protected, i.e., the four tissues in which clinical lesions of MRLS were observed.
The pericardial and single-eye events, occurring at the same time as the reproductive loss events, but in horses of all ages and sexes, meant the MRLS factor "went through" essentially all central Kentucky horses at the time of the MRLS reproductive events. However, only in pregnant mares were the most numerous, dramatic and commercially significant events observed.