By C. Scott Bailey, DVM, Dipl. ACT
Although twinning in the horse is a well-described condition academically, it remains a topic for which most horse-owners and many veterinarians feel inadequately prepared. Historically, twinning has been the most common cause of mid- to late-term pregnancy loss in mares; this is because the equine placenta requires a large surface area (the entire uterus) to supply sufficient oxygen and nutrients to the developing fetus in late gestation. Even a relatively small decrease in surface area will result in growth- retardation and a smaller and less thrifty foal at birth.
Physiologically, twins are most often the product of 2 asynchronous ovulations in a single cycle, which may be up to 5 days apart in the mare. For the first 16 days both developing embryos will be propelled around the uterus either adjacent to each other or separately, and they will then become fixed by muscular contraction between 16 and 17 days. In 80% of cases, the embryos are fixed together at the base of one horn, and in 20% of cases, they are each fixed in separate horns. In the latter case, there is no competition for nutrients in early gestation and the vast majority of cases result in late-term abortion. However, if the embryos are adjacent to each other, they may undergo spontaneous regression (70%) by day 40. Overall, in one study 64% of naturally occurring twins were found to undergo spontaneous regression, most before day 26. The likelihood of spontaneous reduction decreased thereafter until day 40. After day 40, 90% of twins will result in late-term pregnancy failure, either in the form of abortion of both fetuses, stillbirth and stunting of term foals or dystocia.
Despite the high rate of pregnancy failure in mares carrying twins, the incidence of double-ovulations is high (20%) is some breeds. The very act of reducing twins results in increased numbers of horses that carry the genetic propensity for twinning. Likewise, inducing ovulation with exogenous hormones also increases the likelihood of double-ovulations. As a result, the use of ultrasound for early pregnancy diagnosis has become routine for many equine practitioners. At this stage, reduction of pregnancy by “pinching” one embryonic vesicle can be performed reliably and safely. This procedure is generally performed around 14-15 days of gestation – a time at which embryonic vesicles are readily detectable and are still mobile in the uterus, making it possible to move one embryo to the tip of a horn by manual trans-rectal manipulation. Once the vesicle is separated adequately from the remaining embryo, and is at the tip of a horn it can be “pinched” manually or with the ultrasound. This procedure has a 90-95% success-rate when performed by a skilled practitioner and can be performed early in pregnancy, at a time in which re-breeding the mare is possible if the procedure results in the loss of both embryos.
After this critical window, success-rates for various reduction methods are low. Between 17 and 26 days natural reduction with or without the benefit of energy deprivation by eliminating grain-based feed may be the most effective means of eliminating one twin. However, if twins are still identified after day 30, another means of reduction should be selected. Allowing the pregnancy to continue beyond day 35 will result in the development of endometrial cups and a loss of the remaining-breeding season if pregnancy is lost subsequently. Therefore, trans-vaginal ultrasound-guided twin reduction may be attempted at this time. A 5 - 7.5 MHz vaginal ultrasound probe with a needle-guide is used to visualize one embryo, which is held adjacent to the probe with one hand in the rectum. The needle is then punctured through the vaginal wall and uterus, and the fetal fluids are aspirated aggressively. This technique has been reported with a 30% success-rate for the production of one live foal in the case of unilateral twins, and up to 70% success-rate for the production of one live foal in the case of bilateral twins when performed between 30 and 36 days.
After 35-36 days of pregnancy, two procedures have been described for selective reduction of one twin.
Most recently, cranio-cervical dislocation of one fetus has been described by a practitioner in central
Alternatively, ultrasound-guided trans-abdominal cardiac puncture has been well-described in the literature, with a success-rate around 50%. For this procedure, the fetuses are identified by trans-abdominal ultrasound after sedation of the mare and one fetus is injected with KCl or procaine penicillin by intra-cardiac puncture using a spinal needle. The success of this procedure is operator-dependent, but is less invasive than the surgical procedure described above and requires little equipment other than a 3.5 MHz curvilinear probe. It is best performed between 115 and 130 days of gestation and as for the procedure above, both location and fetal gender may be used as criteria for selection. It should be noted that this procedure may result in the birth of a small and unthrifty foal. This is likely due to the fact that the placenta is largely developed by 120 days and the area of placenta which opposes that of the reduced twin remains unavailable for nutrient transfer for the remainder of gestation.
When all other procedures fail to achieve the goal of one developing fetus, abortion of both conceptuses is the single remaining option. Due to the risk of dystocia and periparturient complications to the mare and the low chance of achieving two live foals (1- 2%), allowing the pregnancy to be maintained naturally cannot be recommended. Chemical abortion can easily be achieved before formation of the endometrial cups (36-40 days) with a single dose of prostaglandin or prostaglandin analog, but may require multiple doses of a prostaglandin/ prostaglandin analog and/or oxytocin in later gestation. At this time, abortion should be performed in a hospital-setting, where the mare can be monitored carefully and where parturition can be attended.
Thus, while twins undoubtedly constitute a significant risk to equine pregnancy, there are many methods of managing them successfully. Indeed, the common use of ultrasound for early pregnancy diagnosis has been so successful that many practitioners now feel that double-ovulations are actually of benefit, because they increase the likelihood of fertilization and because any twins can readily be managed early in gestation.
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