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Early Embryonic Loss & Abortion
Abortion and Early Embryonic Death in Mares, an Overview
Introduction
The bad news is abortion is a fairly common event in horses, particularly early in gestation. The good news is that most late gestational horse abortions are preventable. This article discusses the causes, diagnosis and treatment of embryonic loss, abortion of the late term fetus, and Mare Reproductive Loss Syndrome and ways to differentiate them. Links are then available to more specific articles on abortion and early embryonic loss.
Early Embryonic Death (EED)
Early embryonic loss is usually defined as a loss of pregnancy after day 15 but before day 40. This seems to be a process designed to eliminate imperfect embryos, but exact causes in some cases remain speculative. Besides damaged embryos, inflammation of the uterus, malnutrition, and severe systemic disease can result in early embryonic death. In cases of EED you should also consider a misdiagnosis of pregnancy. Intrauterine cysts are easily mistaken for early embryonic vesicles on a single exam.
In one study 75% occurred before day 26. In normal horses without any risk factors the rate of EED was almost 10% so it always behooves you recheck pregnancies at day 40 or later. A large study of TB mares yielded these risk factors for EED:
- Barren Mares: 17%
- Age greater than fifteen: 15%
- More than 10 past pregnancies: 18%
- Breeding early in the year: 18%
- Breeding on foal heat: 24%
- Fluid remaining in the uterus after breeding: 18%
- Twin vesicles: 24%
Luteal Insufficiency: Low Progesterone?
Speculative causes have been primary luteal insufficiency and hypothyroidism but experimental work shows if these are a problem they are rare and hard to demonstrate. Primary luteal insufficiency would result in low blood progesterone levels and this is the hormone that helps maintain pregnancy. While the use of progesterone injections or oral Regu-Mate (altrenogest), to maintain pregnancy for systemically ill pregnant mares is justified, the use for a primary luteal insufficiency s controversial, ...more. Effort has been put into finding a beneficial effect, but currently research has not found a pregnancy preserving effect of exogenous progesterone or Regu-mate. In spite of this it remains a common practice.
More InformationLuteal deficiency and embryo mortality in the mare. Reprod Domest Anim 2001 Aug;36(3-4): 121-31 Allen WR. Four separate components combine to produce the progesterone and biologically active five alpha-reduced pregnanes needed to maintain pregnancy in the mare. The primary corpus luteum (CL) is prolonged beyond its cyclical lifespan by the down-regulation of endometrial oxytocin receptors to prevent activation of the luteolytic pathway and its waning progesterone production is supplemented from day 40 of gestation by the formation of a series of accessory CL which develop in the maternal ovaries as a result of the gonadotropic actions of pituitary FSH and the equine chorionic gonadotropin (ECG). From around day 100 the allantochorion secretes progesterone and progestagens directly to the endometrium and underlying myometrium and, in the last month of gestation, the enlarging foetal adrenal gland secretes appreciable quantities of pregnenelone which is also utilized by the placenta to synthesize progestagens. Between 10 and 15% of mares undergo foetal death and abortion at some time in gestation and the majority of these losses occur during the first 40 days of gestation when the primary CL is the sole source of progesterone. Yet, all the available evidence suggests that untoward luteolysis is not common in this period and the losses that do occur have other underlying causes. Beyond day 40 the secondary CL receive powerful luteotrophic support from ech and from day 80-100 until term the supply organ (placenta) and target tissues (endometrium and myometrium) are in direct contact with each other over their entire surface. In the face of this interlocking and failsafe system for progestagen production throughout pregnancy, and despite a paucity of evidence that a deficiency of progesterone production is a cause of pregnancy loss in the mare, it is surprising, and worrying, that annually many thousands of pregnant mares throughout the world are given exogenous progestagen therapy during part or all of their gestation as a form of preventative insurance against the possibility of pregnancy failure. Basic investigative research is required urgently to validate or debunk the practice. |
Diagnosis and Treatment of EED
With recurrent EED, uterine cytology or biopsy is a reasonable diagnostic procedure, looking for signs of endometritis. If found it should be treated and low contamination insemination techniques used, ...more. In some cases of persistent EED, embryo transfer can be tried but these mares also have a lower rate of successful transfers, suggesting the problem may be with the embryos themselves. With recurrent EED monitoring progesterone levels seems logical and if levels fall below 1 ng/ml supplementation with Regu-Mate should be attempted.
Late Gestational Abortion
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