How to Manage Early Embryonic Death
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HOW TO MANAGE THE SUBFERTILE MARE How to Manage Early Embryonic Death Tom A.E. Stout, VetMB, PhD Author’s address: Utrecht University, Department of Equine Sciences, Yalelaan 114, Utrecht, The Netherlands; e-mail: [email protected]. © 2012 AAEP. 1. Introduction suggest that inadequate maintenance of CL function During the past 20 years, per-cycle and per-season is a common cause of EED. Recently, there have pregnancy rates have improved markedly in well- been attempts to determine factors other than pro- managed horse farms. By contrast, early embry- gesterone insufficiency that cause or predispose to onic death (EED) has remained a significant cause of EED. frustration and economic loss, with approximately 15% of pregnancies detected at day 15 after ovula- Causes of Early Embryonic Death tion failing to survive to term.1 Moreover, the ma- Factors contributing to EED can be broadly divided jority (Ͼ60%) of these losses occur before day 42 into embryonic abnormalities, inadequacy of the ma- after ovulation, a period when pregnancy mainte- ternal environment and “external” factors. Indeed, nance is critically dependent on progesterone pro- it is increasingly clear that abnormalities of the duced by the primary corpus luteum (CL), and when embryo per se account for a significant proportion of a number of essential developmental events take EEDs. In some cases there may even be ultrasono- place, including maternal recognition of pregnancy, graphically detectable signs that conceptus develop- embryogenesis and initial organogenesis, disinte- ment is not progressing normally, such as an gration of the blastocyst capsule, endometrial cup abnormally small vesicle or failure to develop an formation, and the onset of definitive (chorio-allan- embryo properly at the appropriate time.3 In addi- toic) placenta formation.2 Despite its economic im- tion, it appears that a high percentage of equine pact, surprisingly little is known about why EED embryos contain chromosomally abnormal cells, occurs and whether, how and to what extent EED mostly in insignificant quantities but occasionally in can be prevented. This is largely because EED is proportions certain to compromise embryo viabil- usually diagnosed retrospectively (ie, the conceptus ity.4 Although little is known about the true im- has already disappeared or is obviously dying), pact of chromosomal abnormalities on fertility and when it is no longer possible to reliably establish the EED in horses, it is likely that they contribute to the initiating cause. In addition, there has been a ten- increased incidence of EED characteristic of older dency to assume that the underlying problem is mares, even when oocyte or embryo transfer is used probably “progesterone insufficiency,” more because to negate uterine inadequacy,5 and in mares insem- this is a deficit that can be addressed pharmacolog- inated “too long” after ovulation.6 In addition, it ically than because there is any real evidence to has recently become apparent that repeated EED NOTES AAEP PROCEEDINGS ր Vol. 58 ր 2012 331 Orig. Op. OPERATOR: Session PROOF: PE’s: AA’s: COMMENTS ARTNO: 1st disk, 2nd beb spencers 8 3277 HOW TO MANAGE THE SUBFERTILE MARE can occur in mares that themselves carry a stable required for conceptus development. Age-related chromosomal abnormality (eg, a translocation) that endometrial degeneration can also markedly af- has remained undiagnosed because it causes no fect the supply of nutrients to a conceptus, and other obvious clinical signs.7 Similarly, it is likely although this classically leads to fetal compromise that in some of the stallions associated with above- later in gestation, EED can result if endometrial average rates of EED, the underlying problem is degeneration is severe or if large lymphatic cysts embryonic chromosome abnormalities arising either impede conceptus migration or nutrient uptake. from karyotypic abnormalities of the stallion or from Finally, there are reports that stress in the form of DNA damage or instability in his sperm.8 pain, systemic disease, weaning, transport, changes ”Deficiencies of the maternal environment” is a in group structure, poor nutrition, or extremes of broad concept encompassing diverse factors such as temperature can predispose to pregnancy loss.10,11 insufficient maternal progesterone; inadequate pro- Although the extent to which stress contributes to vision of nutrients from an aged, degenerate, or in- EED is not known, it is nevertheless prudent to adequately synchronized endometrium (eg, after minimize exposure to the potential stressors listed embryo transfer); and infection/inflammation as a above during early pregnancy. result of unresolved postbreeding endometritis, en- dometritis acquired subsequently (ie, ascending or Treatment or Prevention of Embryonic Death by “reactivation of dormant micro-organisms”) be- cause, for example, of inadequate closure of the cer- Many of the measures that can be taken to reduce vix, the vestibular-vaginal “sphincter,” and/or the the risk of EED are nonspecific and involve atten- vulva. Uterine infection can also be induced during tion to good breeding management (eg, prevent post- embryo transfer, either because the embryo was re- breeding endometritis and correct anatomical covered from an infected mare or because of the defects predisposing to pneumovagina or urova- accidental introduction of bacteria during transfer; gina), selection of mares (eg, retire mares with poor the limited ability of the diestrous uterus to combat endometrium quality or a badly damaged cervix), bacterial proliferation explains the relative ease minimizing stress, and minimizing the risk of intro- with which contamination at this time results in an ducing or spreading infectious disease among active infection. EED due to endometritis may broodmares. present as the accumulation of uterine fluid or wide- There are few pharmacological means of prevent- spread and marked uterine edema, despite the pres- ing EED, and not all of the underlying causes are ence of a conceptus and subsequent conceptus death amenable to treatment. Progestagen supplementa- will result either from direct infection of the embryo tion can avert EED when a mare shows clear signs of returning to estrus despite the presence of an or via luteolysis due to prostaglandin F (PGF)2␣ release from an inflamed endometrium. apparently normal conceptus in her uterus, has a Although it is tempting to assume that inadequate history of repeated EED associated with loss of the maternal progesterone is a major contributor to primary CL, is endotoxemic, or has undergone se- EED, there is little evidence to support this assump- vere acute stress likely to compromise CL mainte- tion.2 Indeed, even when maternal recognition of nance. In these cases, the logical approach is to pregnancy and maintenance of the CL do fail, the supplement with a suitable progestagen (eg, altreno- a failure is often paired with abnormal conceptus de- gest ) from as soon as CL failure is suspected or velopment (eg, “small for dates” vesicle) such that it systemic disease is identified, or, in the case of re- is impossible to determine cause and effect. Nev- peated EED, from before day 6 after ovulation. ertheless, luteal failure certainly does occur and can, When the incident takes place in early gestation, supplementation should continue until adequate for example, be induced by PGF2␣ release from or- gans other than the uterus, for example, in the case maternal progesterone production is certain, that is, of systemic disease involving endotoxemia.9 More- from some time after day 75 of gestation, by which over, it appears that the day 18 to day 35 pregnant time the placenta has assumed the role as major mare is particularly vulnerable to luteolysis because provider of the progestagens required to maintain the inhibition of endometrial PGF2␣ release that pregnancy. Alternatively, progestagen supplemen- underlies maternal recognition of pregnancy wanes tation can be discontinued from around day 45 if after the cessation of conceptus mobility on day 17.5 ultrasonographic examination demonstrates forma- Indeed, various manipulations (eg, twin aspiration) tion of equine chorionic gonadotropin (eCG)-induced and hormones (eg, oxytocin, estrogen, human chori- accessory CLs or endogenous plasma progesterone onic gonadotropin [hCG]) have been shown to elicit concentrations exceed 4 ng/mL. Although proges- endometrial PGF2␣ secretion during the day 18 to terone supplementation clearly has a role in protect- day 35 period. Although this PGF2␣ release rarely ing pregnancies threatened by maternal results in complete luteolysis, it is possible that progesterone deficiency or endotoxin- or inflamma- there is a mare-specific minimum threshold for pro- tion-induced PGF2␣ release, it is important to sub- gesterone concentrations below which pregnancy sequently monitor the pregnancy for viability may be endangered because, for example, the because many will fail despite the progestagen ther- uterus is no longer able to provide the nutrients apy (suggesting an alternative underlying cause), 332 2012 ր Vol. 58 ր AAEP PROCEEDINGS Orig. Op. OPERATOR: Session PROOF: PE’s: AA’s: COMMENTS ARTNO: 1st disk, 2nd beb spencers 8 3277 HOW TO MANAGE THE SUBFERTILE MARE and the progestagens will then prevent the mare 2. Allen WR. Luteal deficiency end embryo mortality in the returning to estrus. mare. Reprod Dom Anim 2001;36:121–131. An additional treatment that has been shown to 3. Vanderwall D, Squires EL, Brinsko SP, et al. Diagnosis and management of abnormal embryonic development character- improve pregnancy rates, presumably by preventing ized by formation of an embryonic vesicle without an embryo EED, is the administration of a single injection of 20 in mares. J Am Vet Med Assoc 2000;217:58–63. to 40 g buserelinb between days 8 and 12 after 4. Rambags BPB, Krijtenburg PJ, Drie HF, et al. Numerical ovulation. Although this treatment leads to a chromosomal abnormalities in equine embryos produced in fairly consistent 5% to 10% increase in pregnancy vivo and in vitro. Mol Reprod Dev 2005;72:77–87. rates,12 it is not known how it exerts its effects, 5.