Pregnancy Termination in Dogs and Cats

Accidents happen, and a large number of pets become pregnant despite their owner's wishes. In most cases, the best solution to this problem is to have the bitch or queen spayed, which will not only solve the current problem, but prevent it from happening in the future. Ovariohysterectomy during pregnancy, unless very close to term, does not pose significantly elevated risks to the animal over and above the standard surgery.

There are several situations in which it is desirable to terminate a pregnancy, yet retain the animal's ability to reproduce in the future. Examples would include a valuable purebred breeding bitch that is accidently bred by an "undesirable" dog, or a estrous bitch that the owner may want to breed in the future that is found cavorting with a neighborhood dog in the backyard!

Before discussing treatments for pregnancy termination, one point should be made: Just because an bitch in estrus is found together with a male doesn't mean they have already mated. In fact, it seems to be the case that a majority of bitches presented to veterinarians for "mismating" treatment are not pregnant.

One fairly reliable method of determining whether mating has indeed occurred is to examine a vaginal smear within a few hours, even up to a day, after the alleged liason occurred. If the dog was bred, one can almost always find sperm on the smear without much difficulty. Not finding sperm does not rule out mating, but suggests that it may well not have taken place, which can influence how the bitch is treated.

Estrogen Treatment for Mismating

A variety of estrogens have previously been recommended and used as treatments for mismating. Two of the most popular formulations have been diethylstilbestrol and estradiol cypionate (ECP). Interestingly, there is very little objective data to support either the safety or efficacy of these drugs for treating mismating in dogs, and essentially none in cats.

The few controlled studies that have been conducted indicate that estrogen therapy in bitches is associated with a high risk of inducing uterine disease such as pyometra and some risk of causing a lethal aplastic anemia. Additionally, the dosages of estrogen and timing of treatment that appear to minimize risk of these disease are poorly effective in preventing pregnancy. The impression that estrogen treatment is efficacious may be largely be due to the fact, mentioned above, that many bitches presented for mismating treatment have not actually been bred.

In summary, there is little doubt that estrogen therapy for mismating in bitches is not only unsafe, but often ineffective in preventing pregnancy.

Termination of Pregnancy with Prostaglandin F_2alpha

Prostaglandin F_2alpha (PGF, Lutalyse(R), dinoprost tromethamine) is a hormone that induces luteolysis in many species, including dogs. Because progesterone is necessary throughout gestation for maintenance of pregnancy, PGF-induced death of the corpus luteum leads to termination of pregnancy. PGF also has the ability to stimulate uterine contractions, which may contribute to its abortifacient activity.

The canine corpus luteum is essentially unresponsive to PGF prior to diestrus day 5, then becomes progressively more susceptible to luteolysis through gestation. As a consequence, lower doses of PGF are required to induce abortion later in gestation.

PGF is administered twice daily, either for 4 days (less than 4 weeks of gestation) or until abortion is complete (after 4 weeks). In the later, case, the bitch should be monitored daily by palpation or ultrasound to evaluate whether abortion has taken place.

PGF treatment has a number of unpleasant side effects in dogs, including vomition, panting, cramps excessive salivation and defecation. These effects can be ameliorated to some extent by walking the animal immediately after treatment. Because of these adverse effects of PGF, treatment should be conducted in a veterinary clinic.

Two important precautions should be recognized with respect to use of PGF:

• Women of childbearing age and people with asthma or other respiratory problems should use extreme caution in handling PGF solutions. This drug is readily absorbed through the skin and can cause uterine contractions and bronchospasm in exposed persons. This is another reason for conducting treatment of bitches in a clinic rather than by prescription.
• PGF analogs such as cloprostenol are not approved for termination of canine pregnancy. They are very much more potent than PGF, and using an analog at the same dosage as PGF_2alpha can be lethal.

PGF treatment is an effective treatment for termination of pregnancy in bitches. Properly administered, it is also safe and does not appear to have adverse effects on future reproductive performance of the bitch.

PGF can also be used to terminate pregnancy in cats, at least after day 33 of gestation.

Other Methods for Terminating Canine Pregnancy

Several other drugs have been investigated experimentally for terminating canine pregnancy, and while some show considerable promise, these treatments either are not currently available or cannot yet be recommended due to lack of data from clinical trials. Examples include:

Dopamine agonists: Prolactin is necessary to support function of the canine corpus luteum, and secretion is inhibited by dopamine. Drugs such as bromocryptine and cabergoline, which bind to dopamine receptors in the pituitary gland, suppress prolactin secretion and can terminate pregnancy in dogs by suppressing progesterone secretion from the corpus luteum.

Dopamine agonists are effective in terminating canine pregnancy only after about 25 days of gestation. Like PGF, they require repeated treatment and commonly induce vomition and inappetence.

Epostane: This drug inhibits steroid hormone synthesis by inhibiting the enzyme that converts pregnenolone to progesterone. It has been shown to terminate pregnancy in dogs after a 7 day treatment and appears to have this effect throughout gestation. Further, adverse side effects in dogs have not been been reported.

Mifepristone (RU486): This well-known drug acts as a progesterone antagonist. Small clinical trials have demonstrated that his drug is very effective in terminating canine pregnancy after 25-30 days of gestation, and without apparent adverse effects.

Clearly, several drugs appear to have promise as safe and effective abortifacients in dogs, and deserve additional research and marketing attention as aids for pet population control.

References and Reviews

• Bowen RA, Olson PN Behrendt MD et al. Efficacy and toxicity of estrogens commonly used to terminate canine pregnancy. J Amer Vet Med Assoc 186:783, 1985.
• Concannon PW, Weinstein R, Whaley S, et al. Suppression of luteal function in dogs by luteinizing hormone antiserum and bromocryptine. J Reprod Fert 81:175, 1987.
• Concannon PW, Yeager A, Frank D, et al. Termination of pregnancy and inductin of premature luteolysis by the antiprogestogen, mifepristone, in dogs. J Reprod Fert 88:99, 1990.
• Feldman EC, Davidson AP, Nelson RW, et al. Prosaglandin induction of abortion in pregnant bitches after misalliance. J Amer Vet Med Assoc 202:1855, 1993.
• Onelin K, Silva LDM, Donnay I, et al. Luteotrophic function of prolactin in dogs and the effects of a dopamine agonist, cabergoline. J Reprod Fert 47 (suppl):403, 1993.
• Sankai T, Endo T, Kanayama K et al. Antiprogesterone compound, RU486, administration to terminate pregnancy in dogs and cats. J Vet Med Sci 53:1069, 1991.
• Wichtel JJ, Whitacre MD, Yates DJ, et al. Comparison of the effects of PGF2a and bromocryptine in pregnant beagle bitches. Theriogenology 33:829, 1990.