An interesting case of early pregnancy loss in a mare with persistent endometrial cups

Abstract

A recent extensive survey of efficiency on Thoroughbred studfarms throughout England revealed overall pregnancy loss rates of around 8% between Days 15 and 40 after ovulation, 4% between Day 40 and the traditional 1st October pregnancy check around Day 150, and a further 2% between October and term (Allen et al. 2007). Possible noninfective causes of the preimplantation Days 15–40 losses include accidental misalignment of chromosomes at the time of fertilisation, failure of maternal recognition of pregnancy, perhaps due to obstruction of embryonic mobility by the presence of endometrial cysts (Bracher et al. 1992), persisting bacterial or fungal endometritis (Asbury and Lyle 1993), or inadequate production of histotroph for nidation of the embryo due to age-related degenerative changes in the endometrial glands (Kenney and Doig 1986). Possible causes of mid-gestation losses between Days 40 and 150 might include lethal karyotypic abnormalities inhibiting embryogenesis and fetal development, a failure of fetomaternal interdigitation and placental development (Allen 1982), or inadequate primary and/or secondary luteal development and function (Squires and Ginther 1975). Excessive torsion of the umbilical cord causing a vascular crisis (Whitwell 1975), or ascending bacterial or fungal placentitis (LeBlanc et al. 2002), are common causes of late stage abortion. A major managemental problem associated with midgestation (i.e. Days 40–150) pregnancy loss is persistence of the equine chorionic gonadotrophin (eCG)-secreting fetal endometrial cups in the mare’s uterus for their normal 60–100 day lifespan despite the loss of the pregnancy. The continuing high levels of eCG in her blood tend either to prolong and support luteal function in the ovaries and thereby suppress oestrous behaviour and the chance of remating or, in the absence of luteal tissue, may completely suppress follicular and luteal development, thereby creating a type of ‘eCGinduced pregnancy-related’ anoestrus (Allen 2001). Recently, as a consequence of the very high rate of pregnancy loss between Days 40 and 90 of gestation in mares on studfarms in central Kentucky in April/May 2001, following their ingestion of eastern tent caterpillar (ETC) larvae during a plague-like infestation of the grazing land and water troughs (Cohen et al. 2003; Dwyer et al. 2003; Zent 2003; Webb et al. 2004), Steiner et al. (2006) discovered that, following loss of the conceptus after Day 40, the endometrial cups in some mares lived much longer than normal, often for some months after their expected time of degeneration and sloughing from the endometrium around Day 120–140 of gestation. Such persistence of endometrial cup function after pregnancy loss, whether for the normal (60–80 days), or an extended (>100 days) period, usually prevents the affected mare from returning to oestrus and being able to be remated within that same breeding season (Mitchell and Betteridge 1972; Steiner et al. 2006). The present report describes a Thoroughbred mare that spontaneously underwent fetal death and abortion between 71 and 75 days of gestation. Serial blood sampling and ultrasound examinations, and occasional hysteroscopic examinations, for 148 days after abortion revealed abnormal prolongation of the endometrial cups, persistence of eCG in the serum and abnormal activity of the mare’s ovaries.