Induced Abortion, Pregnancy Prevention and Termination, and Mismating



Unwanted pregnancies frequently occur in the bitch, and veterinarians may be asked by owners to terminate pregnancies early in gestation in an effort to avoid all the potential difficulties encountered with a pregnant bitch or an unwanted litter of puppies. The problem of an unwanted breeding may arise because dog owners are not aware that their pet is “in heat.” Alternatively, the owner may be aware of estrus but may underestimate the will of a stud or bitch that wants to encounter the opposite sex. Even the best-educated and most careful owners may still find that their bitch has been or may have been bred (mismated, misalliance).

Reasons for having a bitch aborted vary, in addition to the obvious mating of two dogs not intended for breeding. This might be the bitch’s first estrus; there may be pelvic abnormalities in the bitch that would prevent vaginal delivery of pups; or disproportionate size of the male and female may be a concern. The health of the bitch may be a factor, or an owner may simply not wish to have their bitch pregnant at this time.

Contraceptive medications are available that prevent a bitch from cycling by delaying the onset of proestrus. These drugs are approved for use in the bitch and are successful. However, such agents have limitations that prevent their widespread use in bitches. They are not to be used prior to the first estrus; they cannot or should not be used for prolonged periods; they may change behavior or activity levels; and they are not recommended for use in brood bitches (Von Berky and Townsend, 1993).


A thorough history is valuable prior to treatment of the mismated bitch. Did the owner actually see a tie? What evidence is there of estrus? When was the dog last in heat? Is the bitch spayed? Although a tie is not necessary for conception, seeing a tie is the best evidence for realizing that the bitch is in standing heat. It is always possible that a bitch was not bred unless the owner actually saw the dog breeding.

A vaginal smear is the best diagnostic tool for evaluating mismated dogs. A large majority of bitches have sperm or sperm heads in the vaginal smears for 24 to 36 hours after breeding (Fig. 22-2). Forty-eight hours after breeding, 50% of bitches still have sperm or sperm heads on vaginal cytology (Olson, 1989). The presence of sperm or sperm heads confirms that breeding took place. However, lack of sperm does not prove that a fertile breeding did not occur.

The vaginal smear may also indicate the stage of the cycle. Bitches in diestrus, anestrus, or early to middle proestrus (majority of vaginal cells are parabasal and/or intermediate type) are unlikely to be fertile, and the owners need not be concerned about an unwanted pregnancy. However, if the bitch is in estrus, she must be assumed to be fertile and may have bred. Greater than 80% superficial cells on vaginal cytology is strong evidence of estrus (standing heat) with or without the presence of sperm. However, greater than 80% superficial cells with a concurrent plasma progesterone concentration less than 2 ng/ml would indicate that the bitch is most likely in late proestrus, not willing to breed, and not fertile.


Pregnancy “Trimesters”

Abortion Prior to Implantation: Estrogens (Not Recommended)


Estradiol cypionate (estradiol 17β-cyclopentyl propionate) was an estrogen compound commonly used by veterinarians in the management of mismating (Jackson and Johnston, 1980). Several intramuscular dosage schedules were used and/or recommended. These protocols include one-time doses administered within 3 days of mismating: 0.25 to 1 mg total dose; 0.02 mg/kg, never to exceed 1 mg; and 0.04 mg/kg, never to exceed 1 mg (Burke, 1986). More so than DES, ECP has been associated with serious potential side effects. At 0.02 mg/kg, the drug was successful in preventing pregnancy in only 50% of bitches during proestrus or estrus and in 75% of bitches demonstrated to be in diestrus. The 0.04 mg/kg dose was 100% successful in preventing pregnancy in bitches receiving that dosage during estrus or diestrus (Olson et al, 1984; Bowen et al, 1985).

Despite the potentially favorable information on successful termination of pregnancy, ECP has major side effects that limit or eliminate its usefulness. Pyometra developed in two of eight dogs receiving ECP during diestrus (Bowen et al, 1985). Furthermore, ECP is well recognized as having the potential for causing permanent bone marrow suppression or destruction (or both) in dogs, resulting in severe anemia, leukopenia, thrombocytopenia, and death. This life-threatening syndrome may be caused even when ECP is given at proper dosages (Olson et al, 1984). Bone marrow suppression has been diagnosed 2 to 8 weeks after administration. For these reasons, the manufacturer has not approved ECP for use in dogs for any purpose, and we do not recommend its use.


Estrogen preparations (DES and ECP) are thought to increase the incidence of pyometra. Estrogen predisposes bitches to pyometra by sensitizing progesterone receptors and enhancing the binding of progesterone to the endometrium (Niskanen and Thrusfield, 1998). There do appear to be two distinct pyometra syndromes in the dog. One is a uterine infection seen in older bitches that have an acquired uterine condition called cystic endometrial hyperplasia (CEH). Pyometra is a potential sequela to the chronic presence of CEH. The second, more common and more worrisome pyometra syndrome is uterine infection of young bitches secondary to estrogen administration. Pyometra, open or closed cervix, can occur 1 to 10 weeks after mismating therapy.

The pathogenesis of estrogen-induced pyometra is not well understood. The exogenous estrogen used in the management of misalliance is invariably administered when endogenous estrogen concentrations are decreasing (estrus) or low (diestrus). Thus the bitch is exposed to estrogen concentrations that are not physiologic and that are completely atypical for that phase of her ovarian cycle. Estrogens may stimulate synthesis of progesterone receptors in the endometrium, potentiating the effects of luteal progesterone. Chronic CEH is not a sequela of estrogen therapy.

Our continuing clinical trials provide evidence against long-term damage to the endometrium as a result of exogenous estrogen therapy. However, the exogenous estrogen causes transient uterine disease (pyometra); this condition is not only life threatening, it also is treated by ovariohysterectomy in most hospitals. Bitches with pyometra that are successfully treated with prostaglandins have an excellent prognosis for carrying pregnancies to term in subsequent cycles (Nelson et al, 1982). Thus a chronic endometrial disorder is not present. However, some bitches treated with estrogen do have recurrences of pyometra. Pyometra has been diagnosed in young bitches that never received estrogens at any time. However, pyometra as a naturally occurring disease in the young bitch (less than 6 years of age) is not nearly as common as it is in the estrogen-treated bitch. For these reasons (pyometra and bone marrow destruction), the use of estrogen compounds in the management of misalliance is strongly discouraged.


Excesses in the serum estrogen concentration caused by exogenous estrogen treatment always have the potential to destroy the bone marrow in dogs (Verstegen et al, 1981). This bone marrow sensitivity to estrogen is not well understood. Bone marrow destruction may occur after exposure to abnormal increases in the serum estrogen concentration from endogenous sources (e.g., Sertoli and interstitial cell tumors in males) (Suess et al, 1992), as well as from exogenous sources. The most common and dangerous exogenous source of excess estrogen is administration of ECP.

Iatrogenic bone marrow destruction is frequently fatal. The pancytopenia results in immunosuppression, chronic bleeding, and anemia. Treatment traditionally has been supportive, with repeated transfusions to maintain life. This therapy usually provides transient improvement. Expense, time, frustration, and the bleak prognosis result in most owners having their pets euthanized.

Lithium carbonate (11 mg/kg PO bid for 6 weeks) has been reported as a possible treatment for estrogen-induced bone marrow hypoplasia (Hall, 1992). Lithium treatment, however, needs to be evaluated in a larger group of dogs to assess efficacy. Side effects of this drug include tremors, hypothyroidism, renal tubular nephropathy, edema, and cardiac “sick sinus” syndrome. Close patient monitoring, therefore, is critically important. Administration of recombinant canine granulocyte colony-stimulating factors may be more effective and safer than lithium therapy.

The iatrogenic syndrome is inexcusable and completely avoidable: Do not administer ECP to any dog for any reason.

Prostaglandins in General


Pregnancy in the bitch depends on progesterone secretion from corpora lutea. Ovariectomy (which would result in immediate loss of all progesterone) or inhibition of progesterone synthesis at any stage of gestation results in fetal resorption or abortion. Administration of prostaglandin F (PGF) or its analogs has been repeatedly demonstrated by researchers to successfully terminate pregnancy in the bitch. Abortion with prostaglandins is induced through three different mechanisms: (1) by inducing vasoconstriction, reducing blood flow to the corpora lutea and causing cellular degeneration; (2) by interfering with progesterone synthesis through binding to specific receptors; and (3) by acting directly on the myometrium to cause smooth muscle contractions.* The myometrial contractions are associated with natural prostaglandins, not the synthetic analogs. Most of these studies have been performed on laboratory dogs using a variety of protocols. Several clinical reports have also suggested that PGF could be used by veterinary practitioners to terminate pregnancy in the bitch (Lein, 1983; Johnston, 1990; Feldman et al, 1993).

Currently, abortifacient therapy with prostaglandin has few limitations. As became apparent in our studies with bitches that have pyometra, canine corpora lutea may be relatively resistant to the luteolytic effects of prostaglandins during the first 14 to 28 days of pregnancy. Resistance after this time is still present but to a lesser degree. Thus the “mismated” bitch is not as consistently responsive to prostaglandin administration in the first few weeks of pregnancy as she is in the second half of gestation (beginning at approximately 30 days after breeding).

Natural Prostaglandins


Abdominal ultrasound is recommended approximately 30 days after the last (or unwanted) breeding or 30 days after the first day of diestrus as confirmed with vaginal cytology. After pregnancy has been confirmed, abortion should be discussed with the owner. In our opinion, the use of prostaglandins at this time of gestation is effective and safe. Treatment prior to day 30 is not recommended. No evidence of straining or dystocia has been observed in any bitch. It is assumed, in part, that difficulty in delivery of fetuses is unlikely because each fetus is so small that it can be passed through the birth canal even if presented in an abnormal posture.

Commercially available PGF is not licensed for use in dogs, and veterinarians may wish to obtain owner consent prior to its use. In our initial trials, three treatment protocols with PGF were evaluated: (1) 0.1 mg/kg given subcutaneously every 8 hours; (2) 0.25 mg/kg given subcutaneously every 12 hours; and (3) 0.1 mg/kg given subcutaneously every 8 hours for 2 days, and then 0.2 mg/kg given subcutaneously every 8 hours thereafter. All the bitches in our studies were treated with natural PGFdaily until complete abortion was confirmed with abdominal ultrasonography. It is strongly recommended, as will be explained, that any bitch being treated with prostaglandins to induce abortion be hospitalized throughout the treatment period.

Our current recommendation is the protocol that acted most rapidly while resulting in the fewest and/or least severe side effects: natural PGF at a dosage of 0.1 mg/kg given subcutaneously every 8 hours for 2 days, and then 0.2 mg/kg given subcutaneously every 8 hours daily until abortion has been completed, as confirmed by abdominal ultrasonography. This protocol resulted in the least severe side effects and the fewest number of days to complete the abortion process (Feldman et al, 1993).


Abdominal ultrasound was performed every 48 hours until abortion was completed. We now recommend that abdominal ultrasonography be performed within 6 to 12 hours and prior to the next scheduled administration of PGF if abdominal contractions, fetuses, or bloody or dark vaginal discharge are observed. A physical examination, including abdominal palpation, should be performed at least twice daily. After each injection of PGF, the dog should be taken for a 20- to 30-minute walk, which allows close observation of the animal. The dog should be fed 1 to 2 hours after prostaglandin injection so as to avoid vomiting with feeding prior to treatment. The feeding period can also be used as an observation period (Feldman et al, 1993).

Amniotic vesicles are palpable in most 30- to 35-day pregnant bitches prior to therapy. Each dog we have treated has been demonstrated to have at least two viable fetuses on abdominal ultrasound examination, although exact litter size was not always known. It is important to emphasize that once PGF treatment is initiated, abdominal palpation is no longer reliable in determining when abortion has begun or when and if it has been completed. Abdominal ultrasonography, however, is reliable in confirming whether abortion has been completed.

The need for repeated abdominal ultrasound examinations to confirm that abortion had been completed was disappointing because of the expense and the facilities required. However, ultrasonography has been the only reliable means of assessing pregnancy status in each bitch. The ultrasound examinations were easily performed, and the results were known immediately. Abdominal palpation was not considered reliable or acceptable as the sole means of determining which bitches were pregnant or when the abortion process was complete. Hospitalization is recommended to allow observation of the bitch and to spare owners from witnessing an abortion process they may not wish to see.


It has been suggested that plasma progesterone concentrations less than 2.0 ng/ml result in pregnancy termination in bitches (Olson et al, 1986). The pretreatment plasma progesterone concentrations were greater than 6.0 ng/ml in all bitches we treated. As shown in Fig. 22-4, the mean plasma progesterone concentration from all the bitches decreased after the start of PGFadministration. The plasma progesterone concentration decreased in all dogs approximately 50% from the pretreatment concentration after only 24 hours of PGF treatment. After 72 hours and at each subsequent blood sampling time, all treated bitches had plasma progesterone concentrations less than 2.0 ng/ml. No bitch aborted any fetal material until at least 24 hours after the plasma progesterone concentration was less than 2.0 ng/ml (Feldman et al, 1993). Although monitoring of the plasma progesterone concentration is not necessary, these results demonstrate the physiologic effect of prostaglandin administration, as well as the speed with which it acts.

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Jul 10, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Induced Abortion, Pregnancy Prevention and Termination, and Mismating
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