Unplanned and unwanted breeding of cats and dogs is a common concern. Pregnancy can be completely prevented or terminated by ovariohysterectomy. Sixty percent of misbred female dogs do not conceive, so confirmation of an undesired pregnancy is advised before proceeding with termination, unless neutering is agreed upon.
Postcoital douches are of no value in preventing unwanted pregnancy. Although injectable estrogens, when administered appropriately, can prevent pregnancy, their use involves great risk of serious adverse effects, including pyometra and potentially fatal bone marrow suppression, and they are not advised. Oral estrogens given during diestrus greatly increase the risk of pyometra, are unreliable in terminating pregnancy, and are also not advised.
Safe and effective termination of pregnancy is possible in both dogs and cats by administration of prostaglandin F2alpha (dinoprost; 0.1 mg/kg, SC, every 8 hours for 48 hours followed by 0.2 mg/kg, SC, every 8 hours, to effect [until all fetuses are evacuated as confirmed by ultrasonography]).
Treatment times can reach 14 days. In dogs, treatment time can be decreased (usually by 48 hours) by the concurrent administration of prostaglandin E, misoprostol (1–3 mcg/kg, intravaginally, every 24 hours).
The adverse effects of prostaglandins at this dosage (panting, trembling, nausea, and diarrhea) are mild and transient.
The therapeutic window for prostaglandins is narrow, and doses must be calculated carefully.
The synthetic prostaglandin cloprostenol (1–2.5 mcg/kg, SC, every 12–24 hours to effect) more specifically targets the myometrium, causing fewer systemic adverse effects, and is currently preferred.
Combination drug protocols such as cabergoline (5 mcg/kg, PO, divided into 2 doses every 24 hours) and cloprostenol (2.5 mcg/kg, SC, once) have been reported to terminate pregnancy reliably with minimal adverse effects, shortest treatment times, and greatest success rates; their cost and need for compounding are disadvantages.
Pregnancy can also be reliably terminated in female dogs by administration of dexamethasone (0.2 mg/kg, PO, every 12 hours, to effect). The owner should be informed of the adverse effects of corticosteroid administration (eg, panting, polyuria, polydipsia, immunosuppression).
Rarely, an ecbolic agent such as cloprostenol (1–2.5 mcg/kg, SC, every 24 hours) is necessary to evacuate fetuses.
The antiprogestin aglepristone (10 mg/kg, SC, repeated once in 24 hours) is a safe and effective abortifacient. Preimplantation embryonic death is likely in early gestation (< 24 days after ovulation), with embryonic resorption if administered between days 25 and 35 after ovulation and fetal expulsion if administered after day 35 after ovulation. Availability remains problematic in some countries.
