Protocols

Prostaglandin F_2α (PGF_2α) induces lysis of the corpora lutea. In dogs, the natural PGF_2α (Lutalyse) given at a dosage of 0.1 to 0.25 mg/kg q8-12h SC is effective at terminating pregnancy after pregnancy confirmation. One protocol reported to have few side effects is administration of PGF_2α at 0.1 mg/kg q8h SC for 2 days and then 0.2 mg/kg q8h SC until abortion is complete (Feldman et al, 1993). Abortion usually is complete within 9 days, but some dams still have live fetuses after 9 days. It is extremely important to continue the treatments until abortion is complete. In queens, natural PGF_2α is most effective after 40 days of gestation. Beginning at 45 days of gestation, PGF_2α (0.2 mg/kg q12h SC first day, 0.5 mg/kg q12h SC for up to 5 days) caused abortion in 75% of queens.

Synthetic PGF_2α analogs are more potent and have fewer side effects than natural PGF_2α. Cloprostenol (Estrumate) at a dosage of 1 to 2.5 µg/kg q24h SC for 4 to 5 days was 100% effective at inducing abortion (Verstegen, 2000). Once-per-day treatments with synthetic PGF_2α analogs provide an advantage over the three-times-per-day treatments required with natural PGF_2α. Hospitalization or frequency of injections adds greatly to client cost when using PGF_2α as an abortifacient.

To shorten the treatment period required to induce abortion with PGF_2α, PGF_2α (0.1 mg/kg q8h SC for 2 days and then 0.2 mg/kg q8h SC to effect) can be combined with PGE₁ (misoprostol, Cytotec) at a dosage of 1 to 3 µg/kg q24h deposited into the cranial vaginal vault. The mean time to complete abortion using this combination was 2 days shorter compared with PGF_2α alone (5 days vs. 7 days, respectively) (Davidson et al, 1997). Similar results have been reported using PGE₁ with the synthetic PGF_2α, alfaprostenol (Agaoglu et al, 2011).

Adverse Effects

Adverse effects of natural PGF_2α are more common in dogs than cats and include vomiting, diarrhea, and possibly circulatory collapse. The adverse effects usually subside within 20 minutes, but the bitch should be monitored carefully during this time. To minimize caloric loss, bitches should be fed at least 1 hour after treatment. Minimal to no adverse effects are observed when using PGF_2α analogs at a 1 µg/kg q24h SC dosage.

Drugs used to decrease the adverse effects of natural PGF_2α include a combination of atropine sulfate (0.025 mg/kg), prifinium bromide (0.1 ml/kg), and metopimazine (0.5 mg/kg) SC 15 minutes before PGF_2α administration. Lower doses of natural PGF_2α also decrease adverse effects. Complete pregnancy termination 5 days after starting treatment with minimal adverse effects (tachypnea lasting less than 15 minutes) was reported after administration of natural PGF_2α at a dose of 0.012 mg/kg q6h SC at 30 days of gestation (Len et al, 2011). Clients must be able to administer safely multiple injections to their animals, or the animals must be hospitalized. The safety and efficacy of this protocol makes it the author’s primary choice as an abortifacient in the bitch.

Prolactin Inhibitors in Combination with Prostaglandin F_2α Analogs

Combining prolactin inhibitors with PGF_2α increases the efficacy of pregnancy termination and reduces the side effects of treatment. Cloprostenol (1 or 2.5 µg/kg q24h SC once) in combination with cabergoline (1.65 µg/kg q24h SC for 5 days) from midgestation induced abortion with no adverse side effects (Onclin et al, 1995). After an average of 9 days, cabergoline (5 µg/kg q24h PO) 1 hour after cloprostenol (1 µg/kg q48h SC) caused fetal death with no side effects when started at 25 days of gestation peak (Onclin and Verstegen, 1996). Treatment with cabergoline (5 µg/kg q24h PO) for 10 days combined with either one dose (2.5 µg/kg SC) or two doses (1 µg/kg SC repeated 4 days later) of cloprostenol at the start of the treatment was successful at inducing pregnancy termination. Similarly, bromocriptine (30 µg/kg q8h PO) for 10 days with either one dose (2.5 µg/kg SC) or two doses (1 µg/kg SC repeated 4 days later) of cloprostenol at the start of the treatment was successful at inducing pregnancy termination (Onclin and Verstegen, 1999). All bitches in the aforementioned study became pregnant during the subsequent estrous cycle, which occurred sooner than normally anticipated. As with PGF_2α alone, using PGE₁ with the cabergoline combined with a synthetic PGF_2α decreased the time to abortion compared with those not receiving PGE₁. Oral cabergoline (5 µg/kg q24h PO) combined with cloprostenol (5 µg/kg q48h SC) starting at the thirtieth day after coitus caused abortion in 100% of queens after an average of 9 days of treatments.