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]

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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 condition