Mare reproductive loss syndrome (MRLS) is an enigmatic condition first described in Kentucky in 2001, when an estimated 3,500 foals were either aborted, stillborn, or died shortly after birth.
Research revealed that abortions and foal death was closely associated with exposure to the Eastern Tent caterpillar, which were particularly abundant that year due to environmental conditions triggering a massive hatching event. [1]
Further investigation suggested that ingestion of the Eastern Tent caterpillar by pregnant mares allowed their spiny exoskeleton to penetrate the mare’s intestinal tract, allowing bacteria or toxins into the uterine environment.
Abortions from MRLS typically occur in the first 35 to 100 days of pregnancy, or during the last trimester. Mares typically show no clinical signs prior to abortion.
When the foal is born alive, they are highly susceptible to neonatal hypoxia and often die shortly after birth despite intensive care. Symptoms in live foals include severe weakness, difficulty breathing, inability to stand, and difficulty nursing.
Diagnosis of MRLS is challenging as the precise causative agent has not been identified. There is no known treatment for mares with MRLS to maintain pregnancy, as most fetuses have died by the time of diagnosis.
Prevention of MRLS primarily focuses on reducing exposure of mares to Eastern Tent caterpillars. Pesticide applications are effective in controlling caterpillar populations, but other strategies such as limiting pasture turnout, using grazing muzzles, or relocating mares away from cherry trees may also be preventative.
Mare Reproductive Loss Syndrome
Mare reproduction loss syndrome (MRLS) was first described in April and May of 2001, when there was an unprecedented number of abortions, stillbirths and weak-born foals in Kentucky, USA. [1] A similar scenario occurred the following year, but seemingly resolved in 2003.
MRLS resulted in a 17% decrease in the number of Thoroughbred foals born in 2001, with 3,500 foals affected. [1][2] Estimates suggest that the combined economic losses between 2001 and 2002 totaled $500 million. [2]
Since its initial description, sporadic cases of MRLS have been identified in Florida, Kentucky, and New Jersey. [1] Historical records also reveal similar cases of reproductive loss in 1890 – 1891, 1906 – 1907, and 1980 – 1981. [3]
Subsequent research determined that exposure to the Eastern Tent Caterpillar (ETC) was the main contributing factor to pregnancy loss in these horses. In 2001 and 2002, there was a significant increase in ETC populations attributed to unusually warm weather, which in turn led to a massive hatching event. [2]
Causes
The initial outbreak of MRLS in Kentucky was an enigma for reproduction specialists, as there was no obvious causative agent to explain the surge in pregnancy losses.
Researchers subsequently conducted an epidemiological study and determined that the most severely affected farms had high numbers of Eastern Tent Caterpillars (ETC) infesting trees in and around horse pastures. [1] Eastern Tent Caterpillars are native to the eastern United States and primarily feed on black cherry trees.
To confirm ETC as the causative agent of mare reproduction loss syndrome, researchers fed pregnant mares with ETC and identified abortions similar to MRLS.
Studies identified that the exoskeleton, or cuticle, of the caterpillar administered to pregnant mares caused abortions, suggesting that a component of the exoskeleton or something on the surface of the exoskeleton was causing MRLS.
Eastern Tent Caterpillar Populations
The Eastern Tent Caterpillar population follows a cyclical pattern, experiencing sharp increases approximately every 10-20 years before undergoing a significant decline. [3]
Surges in the ETC population are believed to be the main catalysts for MRLS outbreaks, as evidenced by the 2001-2002 outbreak.
Environmental Factors
Recent research on MRLS has pinpointed five distinct environmental conditions necessary for its emergence. These factors include: [3]
- Increased population of black cherry trees
- Increased population of ETC
- Abnormally high temperatures for a prolonged period
- Migration of ETCs from black cherry trees to the ground in search of alternative food sources
- Dry conditions allowing potential toxins within ETCs to remain in the environment.
Prolonged periods of hot weather appear particularly critical for the development of MRLS. These environmental conditions encourage large numbers of ETC to hatch in a very short period, resulting in a massive increase in young caterpillars. [3]
Pathogenesis
Initial studies suggested that the setae (exoskeleton hairs) of the caterpillar embedded themselves in the intestinal tract. [1]
As the setae penetrate the intestinal wall, they may allow bacteria or toxins from the caterpillar or gastrointestinal tract to enter the bloodstream. The toxin or bacterial agents can affect the uterus and cause abortion. [1]
At present, the migration of caterpillar setae is believed to be limited to the wall of the gastrointestinal tract. [3] However, future research might uncover their potential to penetrate deeper tissues.
Actinobacillus Bacteria
The frequent detection of the bacterium Actinobacillus in MRLS-related abortion cases supports the theory that setae from Eastern Tent Caterpillars contribute to reproductive loss.
Setae from ETCs are believed to carry the Actinobacillus bacteria from the horse’s gastrointestinal tract to deeper tissues, including the uterus.
Actinobacillus is part of the normal gastrointestinal flora of horses, and is a well-described cause of equine abortion. Bacterial infection involving Actinobacillus result in similar findings at necropsy to the reported findings of MRLS. [4]
During the 2001 – 2002 MRLS outbreak, the detection rate of Actinobacillus in equine abortion samples was notably higher than before the outbreak. This observation further underscores the potential connection between Actinobacillus bacteria and MRLS. [4]
Mandelonitrile Benzoate
Some researchers propose that Eastern Tent Caterpillars cause equine abortion by producing a toxin, since not all cases of MRLS have Actinobacillus or other bacteria identified on bacterial culture.
These caterpillars feed on cherry tree leaves that contain the compound prunasin, which the caterpillars convert into mandelonitrile benzoate (MB). This compound is highly irritating to mammalian tissues, and some researchers believe MB or its metabolites may be the true cause of MRLS. [3][5]
However, the definitive cause of MRLS remain unknown, as mandelonitrile benzoate does not cause MRLS when administered to mares on its own. [5] Further research is required to fully understand the cause of MRLS and how Eastern Tent Caterpillars contribute to the development of disease.
Equine Amnionitis and Fetal Loss
A similar condition called Equine Amnionitis and Fetal Loss (EAFL) syndrome was described in Australia in 2004. [6] Much like MRLS, mares exposed to processionary caterpillars experienced higher rates of fetal loss and abortion. [6]
The setae of processionary caterpillars were found to induce equine abortions under experimental conditions, mirroring the findings of MRLS. [7] EAFL abortions are also associated with bacterial infections involving normal flora of the equine gastrointestinal tract. [7]
Interestingly, the setae of the processionary caterpillar have fine barbs along their length, which facilitate migration into deeper tissues. [3] While the setae of ETC have only been observed in the walls of the intestinal tract, those of processionary caterpillars have been found in the uterus. [3][7]
EAFL is considered less severe than MRLS, despite the more aggressive nature of the processionary caterpillar’s setae. [6] Future research might determine that EAFL and MRLS are variants of the same condition with analogous developmental paths.
Clinical Signs
When MRLS was first identified in Kentucky, it was categorized into two types according to the timing of abortions. [1] Early fetal loss (EFL) occurs within 35 to 100 days of breeding, while late fetal loss (LFL) occurs during the last trimester of pregnancy.
Early Fetal Loss
Symptoms of early fetal loss in the mare are minimal. In most cases, the only observed sign is expulsion of fetal membranes and fluids. Some mares show vulvar discharge prior to abortion or developed mild colic signs. [1][2]
Mares that experienced early fetal loss were unable to conceive again in the breeding season, as the fetus developed sufficiently to form endometrial cups. These endometrial cups produce hormones in the uterus that prevent the mare from returning to estrus after pregnancy loss, preventing subsequent conception during the same breeding season. [2]
However, affected mares were successfully able to carry foals to term the following year. [2]
Late Fetal Loss
The symptoms associated with late fetal loss are similar, with mares showing few to no signs prior to abortion. During the abortion, premature placental separation is common, resulting in a “red bag delivery”.
Dystocia (difficult birth) is common due to malpositioning of the foal, as the sudden onset of delivery does not give the foal time to move into proper birthing position. [1] Without immediate intervention, dystocia may result in severe reproductive tract damage or even death of the mare.
Rarely, foals born from LFL are born alive. Clinical signs of mare reproductive loss syndrome in foals include: [1]
- Severe weakness
- Dehydration
- Difficulty breathing
- Inability to stand
- Difficulty nursing
- Disinterest in the mare
In cases of LFL, the placenta is typically edematous (swollen due to fluid accumulation) and heavy with numerous hemorrhages along the umbilical cord.
Other Syndromes
During the initial MRLS outbreak in Kentucky, two other syndromes were reported around the same time and were also linked to exposure to Eastern Tent Caterpillars.
Fibrinous Pericarditis
Fibrinous pericarditis, the more severe of the two syndromes, has an approximately 50% fatality rate. Fibrinous pericarditis refers to inflammation of the sac around the heart and eventually results in heart failure, fluid accumulation in the abdomen, and edema in and around the lungs.
Symptoms of fibrinous pericarditis in horses include:
- Fever
- Severe lethargy
- Rapid breathing or difficulty breathing
- Rapid heart rate
- Weight loss
- Colic signs
Endophthalmitis
Endophthalmitis is a condition characterized by inflammation of the eye’s inner structures. During the MRLS outbreak, cases of endophthalmitis were also identified at higher than expected rates. These cases affected only one eye.
Symptoms of endophthalmitis in horses include:
- Swelling
- Discharge from the eye
- Redness
- Accumulation of pus-like material within the eye
Diagnosis
Because the precise cause of mare reproduction loss syndrome is unknown, confirming a diagnosis is challenging. Currently, most diagnosticians rely on a history of exposure to the Eastern Tent caterpillar and black cherry trees with subsequent fetal loss to make a diagnosis. [4]
Cases of MRLS most commonly occur in the spring during periods of hot weather, which may provide additional clues. At necropsy, identification of Actinobacillus on bacterial culture may also support MRLS, due to previous studies showing a high incidence of this bacteria in MRLS cases. [4]
Treatment
Unfortunately, treatment for MRLS has proved ineffective in preventing abortion or premature delivery, as in most cases the fetus has died by the time the condition is diagnosed.
Foals born alive require intensive nursing care immediately after birth. Treatment may include:
- Antibiotics
- Plasma transfusions
- Oxygen therapy
- Anti-inflammatory medications
- Tube feeding
- Vitamin E supplementation
Foals have a high risk of developing neonatal asphyxia (“dummy foal syndrome“) due to premature separation of the placenta reducing their oxygen supply while in utero. During the initial Kentucky outbreak, many foals born alive died quickly after birth, despite intensive care.
Prevention
Prevention of MRLS focuses on reducing mare exposure to Eastern Tent Caterpillars through insecticide treatments. Effective insecticide treatments include: [1][8]
- Treatment of egg masses during winter with bifenthrin or permethrin
- Foliage sprays with bifenthrin and spinosad
- Microinjection of trees with dictorophos
- Barrier treatments of permethrin applied around the edges of pastures, fences, and cherry trees
Insecticide treatments require careful application to prevent spread to neighbouring farms or residential areas. Additionally, many insecticides come with grazing restrictions that must be followed to avoid causing illness in horses grazing nearby pastures. [1]
Consultation with your veterinarian is recommended prior to using any insecticide product on or around your farm, particularly in areas accessible to horses.
Management
You can also reduce the risk of MRLS by keeping your mare off pastures infested with Eastern Tent Caterpillars during outbreaks. Strategies may include: [3]
- Housing mares indoors during peak ETC periods
- Using grazing muzzles in mares turned out to pasture to reduce exposure
- Changing pasture or paddock locations to areas with no cherry trees present
With further research, additional preventative measures may become available for mare reproduction loss syndrome.
References
- McKinnon. A. O. et al. Equine Reproduction . Wiley-Blackwell, 2010.
- Sebastian. M. M. et al. Review Paper: Mare Reproductive Loss Syndrome. Vet Pathol. 2008. View Summary
- Burns. S. J. et al. Environmental Influences on Mare Reproductive Loss Syndrome: Do They Fit with a Toxin as the Causative Agent?. Journal of Equine Veterinary Science. 2022.View Summary
- Donahue. J. M. et al. Classification of Actinobacillus Spp Isolates from Horses Involved in Mare Reproductive Loss Syndrome. American Journal of Veterinary Research. 2006. View Summary
- An Unidentified Toxin Associated with Eastern Tent Caterpillars Causes Mare Reproductive Loss Syndrome (MRLS). Animal Reproduction Science. 2010. View Summary
- Todhunter. K. et al. Equine Amnionitis and Fetal Loss: The Case Definition for an Unrecognised Cause of Abortion in Mares. Australian Veterinary Journal. 2009. View Summary
- Cawdell-Smith. A. J. et al. Equine Amnionitis and Fetal Loss: Mare Abortion Following Experimental Exposure to Processionary Caterpillars (Ochrogaster Lunifer): Caterpillars and Fetal Loss. Equine Veterinary Journal. 2012.
- Potter. D. A. et al. Managing Eastern Tent Caterpillars Malacosoma Americanum (F) on Horse Farms to Reduce Risk of Mare Reproductive Loss Syndrome. Pest Management Science. 2005. View Summary
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