The probable identification of monozygous twin embryos in mares

Twin management in the mare: A review

Twin gestation in the mare is undesirable and can have disastrous consequences. As in many cases, the key to success in twin management lies in a thorough follow-up and accurate recording of clinical findings in the pre-breeding examination. A pregnancy diagnosis in the mobility phase is imperative for a good outcome in the event of twin reduction. If a twin gestation is not diagnosed during this early pregnancy stage, several other procedures exist for managing post-fixation twins (>16 days) with varying degrees of success. Most twin pregnancies are the result of multiple ovulations (dizygotic twins). However, monozygotic twins are also sporadically diagnosed, due to the increasing number of transferred in vitro produced equine embryos. In these cases, the most optimal treatment strategy still needs to be determined. This review provides an overview of the various twin reduction techniques described with the expected prognosis as well as of some less reported techniques with their results. In addition, physiological events and the reduction techniques are demonstrated to the user in virtual 3-dimensional illustrations.

Manual reduction of multiple embryos in the mare: the effect on subsequent pregnancy outcome

To determine whether manually reduced multiple pregnancies (MPs) are at a greater risk of pregnancy loss than single pregnancy (SP) in mares, and to examine if a difference exists in the timing of pregnancy loss between manually reduced MPs and SPs, 1916 Thoroughbred mares were ultrasonically monitored every 2 days during oestrus to confirm ovulation, and up to Day 40 post-ovulation to confirm pregnancy. Ultimate pregnancy outcome was ascertained from the General Thoroughbred Stud book and classified as live foal (LF), early abortion (EA; Days 40-150), slipped foal (SF; Days 150-term), and barren (B; Time of pregnancy loss unknown). Significantly (P<0.05) more SPs failed (17.23%; 226/1312) than manually reduced MPs (13.41%; 81/604). Both SPs and MPs were at greatest risk of being lost as EA (72.16%; 127/176 and 61.67%; 37/60, respectively) compared to SF (27.84%; 49/176 and 38.33%; 23/60). There was no significant difference in the time of greatest risk of pregnancy loss between manually reduced MPs and SPs. It was concluded that owners and veterinarians can be assured that manual reduction of MPs does not increase the risk of pregnancy loss above those associated with SPs.

The effect of age on multiple ovulation rates, multiple pregnancy rates and embryonic vesicle diameter in the mare

Numerous and conflicting reports exist regarding factors that may effect mare reproductive performance, in particular multiple ovulation (MO) and its consequences. Sequential ultrasonic examination was used to monitor 3075 ovulations in 1581 mainly Thoroughbred mares to ascertain: whether increasing age is associated with an increase in MO; whether this is counteracted by an increase in embryo mortality (EM) prior to Day 13; and whether this embryonic loss may be associated with small-for-age embryonic vesicles (Days 13/14). Overall ovulation rate was 1.31, MO occurring in 29.3% of cycles. MO incidence significantly (p<0.05) increased with age (20.7% in 2-4-year olds compared to 35.6% in 17-19-year olds). 25.2% of MO were apparent as multiple pregnancies (MP) (40.0% of all pregnancies arising from MO) and 37.8% as single pregnancies (SP) at Days 13/14. Older mares demonstrated significantly (p<0.001) lower pregnancy rates and of those pregnant, significantly (p<0.01) fewer were MP than younger mares. Observation of 1442 embryonic vesicles failed to demonstrate any consistent significant association between age and vesicle size in single ovulating (SO) or MO mares on Days 13/14. We conclude that: (i) increasing age was significantly (p<0.05) associated with increasing incidence of MO; (ii) increasing age was significantly associated with a decreasing incidence of pregnancy/ovulation (p<0.001), and MP (p<0.01), at Days 13/14; (iii) there was no consistent significant association between mare age and vesicle size.