Embryonic loss in mares. Incidence, possible causes, and diagnostic considerations

Fertilization rates were similar for normal and subfertile mares, and much of the difference in fertility between normal and subfertile mares was due to embryonic loss. Fertilization rate estimates for mares ranged from 71 to 96 per cent. The incidence of embryonic loss detected by ultrasonography between Days 11 and 50 was approximately 9 per cent for normal mares, and the estimated incidence of embryonic loss before Day 14 was also 9 per cent. Therefore, the estimated incidence of embryonic loss in normal mares between fertilization and Day 50 is approximately 18 per cent (Fig. 1). In subfertile mares, the corresponding estimate for embryonic loss between fertilization and Day 50 is 80 per cent, with most embryonic losses occurring before Day 14 in subfertile mares (Fig. 1). The high rate of early embryonic loss in subfertile mares could be related to embryonic defects, oviductal environment, or uterine environment. Oviductal embryos from subfertile mares were less viable than embryos from normal mares; therefore, embryonic defects were important in early embryonic losses in subfertile mares. These defects might be inherent within the embryo or might arise from the early oviductal environment. The uterine environment of subfertile mares was adequate to support normal embryos in early gestation; however, the relationship between the uterine environment and the increased metabolic demands of the conceptus in the late embryonic or early fetal periods requires further study. The uterine environment is also altered in mares with endometritis; therefore, endometritis may also be an important factor in embryonic loss in some mares. Uterine-induced luteolysis, as well as the effect of the pathogen or the resulting inflammation, may lead to embryonic loss. An increased susceptibility of some subfertile mares to endometritis could result in embryonic loss secondary to a postcoital endometritis that persists until the embryo reaches the uterus at Days 5 or 6 postovulation. Although progesterone is critical to embryonic survival, the cause-and-effect relationship between progesterone and spontaneous embryonic loss remains unclear. Reduced progesterone concentrations could be related to endometritis, failure of maternal pregnancy recognition, or luteal insufficiency. Progesterone supplementation may be indicated for some mares, but the value of exogenous progesterone for prevention of spontaneous embryonic loss has not been critically tested. A number of other factors have been associated with embryonic loss in mares.(ABSTRACT TRUNCATED AT 400 WORDS)