Original Research
Early Embryonic Loss in the Mare

https://doi.org/10.1016/j.jevs.2008.10.001Get rights and content

Introduction

Early embryonic loss in the mare is generally defined as pregnancy failure that occurs between fertilization and day 40 to 60 of gestation. The diagnosis of early embryonic loss and recognition of factors contributing to its occurrence have been dramatically improved by the routine use of transrectal ultrasonography for early pregnancy diagnosis. Under field conditions, transrectal ultrasonography is typically used for an initial pregnancy diagnosis as early as day 12 to 14 post-ovulation, whereas under experimental conditions it may be used as early as day 10 or 11; therefore, ultrasonography allows direct (and repeatable) assessment of the conceptus during approximately three quarters of the interval when early embryonic loss occurs. Prior to day 10, the conceptus is too small to be visualized with standard ultrasonographic equipment, therefore other techniques have been used to study embryonic loss during that interval. Specifically, clinical procedures such as embryo transfer and oocyte transfer and experimental techniques such as in vitro embryo culture and light/electron microscopy of oocytes/embryos have been used to study early embryonic loss prior to day 10.

Section snippets

Incidence

Because of the practicality and accuracy of using transrectal ultrasonography for the diagnosis of pregnancy as early as day 10, a considerable amount of data has been generated on the incidence of embryonic loss after that time. Although it can be difficult to compare data across experiments because of variables such as differences in the interval studied, and whether experiments were conducted under controlled conditions or under field conditions, based on serial examination with

Etiology

Factors that may contribute to the occurrence of embryonic loss in the mare have been classified as intrinsic, extrinsic, and embryonic.15, 16 Intrinsic factors include endometrial disease, progesterone insufficiency, maternal age, lactation, foal-heat breeding, time of insemination relative to ovulation, site of intrauterine fixation of the embryonic vesicle, and maternal chromosomal abnormalities. Extrinsic factors include stress; nutrition; season/climate; transrectal

Endometrial Disease

Endometrial disease is further classified as inflammatory or noninflammatory. Inflammatory forms of endometrial disease include acute and chronic endometritis, while noninflammatory forms include periglandular fibrosis and endometrial cysts. Acute endometritis is characterized by an influx of neutrophils into the stroma of the endometrium and the uterine lumen; the two primary pathological forms of acute endometritis are persistent mating-induced endometritis or infectious endometritis.

Stress

It is plausible that maternal stress could contribute to the occurrence of early embryonic loss because it has been demonstrated that stress associated with severe pain (ie, colic), infectious disease, and weaning resulted in a 30% to 50% decrease in circulating progesterone levels in pregnant mares.58 The adverse effect of stress on progesterone levels is apparently mediated through adrenal corticosteroids because administration of 150 mg of prednisolone to 2 pregnant mares resulted in a

Summary

A multitude of factors may contribute to the occurrence of early embryonic loss in a mare. It is clear that some factors, such as increasing maternal age, are directly associated with higher embryonic loss rates, while other factors such as breeding at foal heat are less clearly linked to pregnancy loss. In addition, there may be interactions among the various factors (eg, mare age, foal-heat, lactation, nutrition, etc) that may compound their effects on embryonic loss. As important as

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