Unplanned and unwanted mating 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 (natural hormone) at 0.1 mg/kg, SC, three times a day for 48 hours followed by 0.2 mg/kg, SC, three times a day to effect (until all fetuses are evacuated as confirmed by ultrasonography). Treatment times can reach 14 days. In dogs, treatment time can be reduced (usually by 48 hours) by the concurrent administration of prostaglandin E (misoprostol) intravaginally at 1–3 mcg/kg/day. 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. Synthetic prostaglandins (cloprostenol 1–3 mcg/kg every 12–24 hours to effect) more specifically target the myometrium, causing fewer systemic adverse effects, and are currently preferred.
Pregnancy can also be reliably terminated in female dogs by administration of dexamethasone at 0.2 mg/kg, PO, bid to effect. The owner should be informed of the adverse effects of corticosteroid administration (eg, panting, polyuria, polydipsia).
Combination drug protocols (cabergoline 5 mcg/kg, PO, divided every 24 hours for as long as 10 days, cloprostenol 1 mcg/kg given SC twice at ~28 and 32 days after the LH surge) 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.
Antiprogestins (aglepristone) are safe and effective abortifacients; availability remains problematic.