CHAPTER 81 Induced Abortion and Parturition in the Goat
Elective induction of abortion or parturition is easily accomplished in goats. A functional corpus luteum (CL), the sole source of progesterone, is essential for maintenance of pregnancy throughout gestation in does. The caprine placenta produces little or no progesterone. Anything that interferes with proper function of the CL during gestation will result in termination of pregnancy. By far the most commonly used agent to achieve termination of pregnancy in goats is prostaglandin F2 alpha (PGF2α) or its analogues, but corticosteroids and estrogens have also been employed.
INDICATIONS FOR TERMINATION OF PREGNANCY
The typical reason for a client’s request for early termination of pregnancy is mismating. Bucks are clever individuals that are at times almost impossible to keep in their pens. If does are in estrus, bucks will attempt to reach them if at all possible. Mismated does may be too young or small for breeding, held in reserve for breeding at some future date, or scheduled to be bred by artificial insemination or to a different buck. Some older does may have been retired from breeding. Injury or disease that may compromise the life of the doe or the completion of pregnancy may also result in a request for termination of pregnancy.
Induction of parturition has the benefit of allowing prediction of the time of kidding. Many owners prefer this arrangement, as they are able to know in advance when a doe, or group of does, will kid and thus can be prepared for the event. Kiddings can take place when the owners have the time or personnel to attend or assist kiddings. Attended kiddings allow owners to remove the kids from their dams prior to suckling and ingestion of colostrum. This is important in some disease-control programs, especially caprine arthritis-encephalitis and mycoplasma infection, for which the prime mode of transmission to the new generation is through colostrum or milk. Other indications for induced parturition include injuries to the doe during late gestation and impending or early cases of pregnancy toxemia.
Events that lead to normal parturition in goats require functional maturation of the fetal adrenal cortex. Parturition is triggered by activation of the fetal pituitary-adrenal axis.1,2 Adrenocorticotropic hormone (ACTH) is released by the fetal pituitary, which stimulates release of corticosteroids by the fetal adrenal glands. An increase in fetal corticosteroids stimulates placental estrogen biosynthesis, which in turn stimulates the synthesis and release of PGF2α from the placenta and endometrium. The cascade continues and PGF2α causes luteolysis, which results in a decrease in progesterone. An increase in estrogen and decrease in progesterone stimulate myometrial activity, which is further enhanced by the effects of PGF2α, causing a direct effect on the myometrium and stimulating oxytocin release. By mimicking some of these events, abortion or parturition can be artificially induced.
Both ACTH1,2 and corticosteroids3,4 have been used for early termination of pregnancy in goats. Infusion of 10 μg ACTH per hour into the fetus on day 126 of gestation resulted in live births on day 131, with placentas expelled within 18 hours after delivery. Daily doses of cortisol acetate (100 mg IM) to does before day 112 and after day 136 resulted in delivery at normal term, but cortisol acetate given on days 113 to 120 resulted in abortion by day 125.3 Administration of methylprednisolone acetate to does prior to day 84 caused no early termination of pregnancy, but doses of 240 to 270 mg IM given on day 111 or day 125 resulted in abortion in 6 days.3 In another experiment,4 nine does were given 16 mg dexamethasone IM on day 144 and all delivered live kids in a mean of 119 ± 29.8 hours, and only one experienced retained fetal membranes 24 hours after delivery.
Exogenous estradiol-17β causes the release of PGF2α and thus induces luteolysis.5 When infused intravenously into goats pregnant 124 to 142 days, it causes the delivery of fetuses within 54 to 81 hours. All the fetuses died of respiratory failure within 1 hour of delivery. The administration of estradiol benzoate (12 mg IM) to goats at 126 to 138 days of gestation resulted in delivery of live, nonviable fetuses in 58 to 87 hours.5 In another study, goats given estradiol benzoate (15–25 mg IM) on days 147 and 148 of gestation delivered kids on day 149.6
During the breeding season, PGF2α is effective in inducing luteolysis in goats. Luteolysis will occur if the doe is on day 4 to 17 of the estrous cycle when PGF2α is administered. Because the doe is CL-dependent throughout gestation, PGF2α is effective in terminating pregnancy from day 4 until term. Doses as low as 1.25 mg PGF2α have been shown to be effective7 for luteolysis, as has 0.0385 mg/kg (1.75 mg PGF2α/100 lb).8 One study reported that 15 mg PGF2α given on day 30 or 65 of gestation resulted in abortion at 34 to 54 hours and at 54.5 to 75.5 hours, respectively, after injection.9 Many does induced to abort with 5 mg or 15 mg PGF2α during the breeding season experienced one or more consecutive interestrous intervals of 2 to 15 days.10,11 Progesterone concentrations associated with these short cycles suggest failure to form a functional CL or the formation of an atypical CL, either of which was accompanied by a luteinizing hormone surge of decreased magnitude.10
The drug of choice for induction of parturition in goats is PGF2α or one of its analogues. When administered IM at doses of 2.5 mg,12 5 mg,12 15 mg,9 or 20 mg4 on days 140 to 144 of gestation, PGF2α will induce parturition and delivery of live kids 29 to 57 hours (average, 43 ± 11.8 hours), 28 to 48 hours (average, 35 ± 8.6 hours), 36 to 56 hours, and 31.5 ± 1.1 hours, respectively, after injection. The higher doses of PGF2α result in a more predictable time of parturition. Treatments are followed by a dramatic decrease in plasma progesterone concentration 24 hours after injection.4,12 Few cases of retained fetal membranes have been reported with PGF2α-induced parturition. Induction of parturition on day 144 did not result in the birth of kids that were smaller than the species average.12 Owners have reported that some does produce less colostrum when parturition is induced.