Prevention or Termination of Pregnancy in Small Animals

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 bitches do not conceive, so confirmation of an undesired pregnancy is advised before proceeding. 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. If used, they must be administered soon after copulation, before potentially fertilized ova reach the uterus. Oral estrogens given during diestrus greatly increase the risk of pyometra, are unreliable in terminating pregnancy, and are not advised.

Safe and effective termination of pregnancy is possible in both the bitch and queen by administration of prostaglandin F_2α (natural hormone) at 0.1 mg/kg, SC, tid for 48 hr followed by 0.2 mg/kg, SC, tid to effect (until all fetuses are evacuated as confirmed by ultrasonography). Treatment times can reach 14 days. In the bitch, treatment time can be reduced (usually by 48 hr) 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 should be calculated carefully. Synthetic prostaglandins (cloprostenol 1–3 mcg/kg every 12–24 hr to effect) more specifically target the myometrium, causing fewer systemic adverse effects, and are currently preferred.

Pregnancy can also be reliably terminated in the bitch 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 hr 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 sucess rates; their cost and need for compounding are disadvantages.

Antiprogestins (aglepristone) are safe and effective abortifacients; availability remains problematic.