Mare Reproductive Loss Syndrome

Mare Reproductive Loss Syndrome (MRLS) has been diagnosed in mares of all breeds and ages. It was initially observed in Kentucky in a three week period around the first of May, 2001, when about 20% to 30% of Kentucky's pregnant mares suffered abortions. The total economic loss to Kentucky and the racing industry for the 2001 MRLS season was estimated at $500 million.

History

MRLS was first identified on April 26, 2001 by Dr. Thomas Riddle, who on that day observed an unusual number of equine in utero early fetal deaths in 60 day-old fetuses that he was ultrasound examining for sex determination. These early fetal losses were followed by a sequence of early and late fetal losses and, somewhat later, the coincident pericarditis, uveitis and encephalitis syndromes were identified.

During the three weeks around the first of May, 2001, about 20% to 30% of Kentucky's pregnant mares suffered abortions. Of foals conceived in spring 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). The total economic loss to Kentucky and the racing industry for the 2001 MRLS season has been estimated at $500 million.

During the spring of 2001 Kentucky experienced an unusually heavy infestation of Eastern tent caterpillars. 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 confirmed an association of MRLS with presence of the caterpillar. The exact mechanism by which the caterpillar caused MRLS is still under investigation. The condition was correlated spatially and temporally to the presence of Eastern tent caterpillars (ETCs), and ultimately linked to black cherry tree leaves,

Clinical signs

The Mare Reproductive Loss Syndrome has been diagnosed in mares of all breeds and ages.

Early and late fetal loss can be observed within the first and last trimester of pregnancy respectively. For Early Fetal Loss (EFL) clinical signs from the sick mares included puss-like discharge from the vulva, fetal membranes protruding from the vulva as the fetuses were located in either the vagina or vulva. One to three days prior to the EFL, 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 reveal either dead fetuses or live fetuses with a slow heart rate and lethargic movements. All the fetuses, both alive and dead, were surrounded by a cloudy amniotic fluid. ^[1]

Clinical signs for Late Fetal Loss (LFL) included explosive parturition, dystocia, foaling while standing, the placenta separating prematurely and foals either stillborn or born weak. The placenta was observed to carry a pale brown hue as opposed to its usual dark reddish brown color. The umbilical cord was thick, dull, yellowish and inflamed. The weak foals may be incapable of breathing on their own and require resuscitation. These foals were also observed to be dehydrated, hypothermic, having irregular heartbeat and respiration. The majority of the foals did not survive past four days. ^[1]

One finding observed exclusively in the Mare Reproductive Loss Syndrome was the unilateral uveitis; within the first 12 hours, mares expressed inflammation around the eye along with fluid accumulation on the cornea, anterior and posterior chambers due to the inflammation. The fluid in the anterior chamber exhibits a tan yellow hue and often accompanied by haemorrhaging around the iris’ surface. ^[1]

Causes

One of the main agents that appear to induce MRLS is the presence of Eastern tent caterpillars (ETCs) which are present temporally and spatially with the MRLS abortions. ^[1] One of the primary foods for ETCs are the back cherry tree leaves. An experimental study to prove the cause and effect relationship with ETCs and MRLS was confirmed when all mares exposed to ETCs (by being fed fresh black cherry tree leaves) aborted and none of the control mares had aborted. ^[1]

Prevention and Treatment

There appears to be no effective treatment for Mare Reproductive Loss Syndrome. Mares which aborted are treated with broad spectrum antibiotics to avoid bacterial infections. The foals born from mares infected with MRLS are given supportive care and supplied with medication to reduce inflammatory response and improve blood flow, but none of the treatments appear effective as the majority of the foals do not survive. Unilateral uvbetis where treated with antibiotics and anti-inflammatory drugs.^[1]

Preventive measure can include cutting down cherry trees, spraying ETC nests with insecticides, and preventing the mares from eating on the pastures (muzzling them).

For more information see A Toxicological Investigation of Mare Reproductive Loss Syndrome

References

1. ^ ^{a b c d e f} [1]

• . Manu, S. (2011). Chapter 85 - mare reproductive loss syndrome. In Ramesh C. Gupta (Ed.), Reproductive and developmental toxicology (pp. 1139-1144). San Diego: Academic Press. doi:10.1016/B978-0-12-382032-7.10085-2

External links

• Information about mare infertility and breeding