CHAPTER 27 Induced Abortion
In mares, elective termination of pregnancy is performed for several reasons, including mismating, change in ownership, age or health of dam, abnormal gestation, and twin pregnancy. Many methods may be used to induce abortion, and care should be taken to select a procedure that is safe and effective and that minimizes damage to the mare’s reproductive tract and future breeding health. When terminating pregnancy, the clinician should consider the following factors: stage of gestation, presence of endometrial cups, expected time of return to estrus, presence of twin fetuses, and physical condition of the mare. In every case of elective abortion, the patient should be re-examined at an appropriate time after the procedure to ensure that pregnancy has been effectively terminated.
The earlier pregnancy termination is attempted, the more likely it is to be safe and successful; however, no method has been shown to reliably terminate pregnancy before day 5 after ovulation.1,2 After the corpus luteum (CL) is fully functional (days 5–6 after ovulation), elective abortion is easily accomplished by causing luteolysis. The simplest method is intramuscular injection of prostaglandin F2α (PGF2α) or a PGF2α analogue.1–4 The two products commonly used in mares, dinoprost tromethamine and cloprostenol, have similar efficacy, but the common side effects of sweating and mild colic are avoided with the administration of cloprostenol. Before maternal recognition of pregnancy (approximately by days 12–14) a single injection of either product (10 mg of dinoprost or 500 μg of cloprostenol) has been shown to cause lysis of the CL and effectively terminate pregnancy. After pregnancy recognition, two or more consecutive injections may be necessary to lyse diestrual, or secondary, corpora lutea.3 Mares can be expected to return to estrus within 3 to 5 days.
Intrauterine infusion or lavage performed after day 6 also terminates pregnancy in mares. Abortion is likely caused by embryotoxic effects or release of endogenous PGF2α as a result of cervical and uterine manipulations.1 Sterile saline (2–3 L divided into 500- to 1000-ml aliquots) is the preferred solution for uterine lavage because it is relatively nonirritating, but infusions of Lugol’s solution, dilute povidone-iodine, or nitrofurazone have been used successfully. These antiseptic solutions are potentially irritating to the genital tract and should be used with care. Chlorhexidine solution should not be used for intrauterine infusion.5 Any technique that necessitates invasion of the cervix can result in bacterial contamination and endometritis.
Manual crushing of the conceptus can be performed easily between days 16 and 25 after ovulation. After day 25, this technique is more difficult and less efficacious. Transvaginal ultrasound-guided pregnancy reduction has been successful in terminating pregnancies between days 20 and 45.6 Ovariectomy, although not a practical technique, consistently results in abortion during this period.3