Elective, attended foalings are advantageous to monitor mares that have experienced dystocia or premature placental separation in previous deliveries. Mares with gestational abnormalities such as rupture of the prepubic tendon or hydroallantois may require assistance during delivery. However, induction of parturition in itself can be associated with side effects such as dystocia, premature placental separation, fetal hypoxia, and dysmaturity. Thus careful case selection before induction of parturition is critical for successful delivery.
Methods Of Induction
Oxytocin is generally considered to be the drug of choice for induction of parturition in the mare. Oxytocin has a rapid effect and results in delivery within 15 to 90 minutes after administration. The progress of parturition is consistent with oxytocin and few adverse effects are noted in the term foal. Various methods and doses of oxytocin induction have been described including a bolus injection of 2.5 to 120 units oxytocin, via the intramuscular (IM) or intravenous (IV) route; IM or subcutaneous (SQ) injection of 2.5 to 20 units oxytocin at 15 minute intervals; and IV administration of 60 to 120 units oxytocin in 1 L of saline delivered at a rate of 1 unit/minute. Method of delivery of oxytocin (bolus injection, repeated incremental injections or IV drip) does not appear to affect neonatal outcome in induced deliveries. Logistically, administration of oxytocin by injection allows the mare to move about freely in a stall or paddock without human intervention. When administering oxytocin through an IV drip an individual must stand at the head of the mare at all times. Alternatively, an elaborate tubing system must be constructed to prevent the mare from becoming tangled in the drip lines as she lays down to roll or push.
The dose of oxytocin is an important consideration when parturition is induced in the mare. Initial reported doses of oxytocin ranged between 75 and 120IU. Few untoward effects on the mare or foal were reported with these high doses of oxytocin; however, one must consider the possibility of uterine hyperstimulation with higher doses. More recently, doses of 15 to 20 units oxytocin (IV, IM, or SQ) have been used. Injections are administered at 15- to 30-minute intervals until the chorioallantois ruptures. Before further injections, a vaginal examination and evaluation of the progress and position of the fetus are critical. First-stage labor is abbreviated in induced parturition, so a higher likelihood exists that the fetus will be abnormally positioned/postured. Correction of fetal limbs should be made before administering additional oxytocin, because the expulsive effects of the mare may make it difficult to perform manipulations on the fetus later on. In all cases of induced foaling, the veterinarian should remain present through the delivery of the foal.
Recently, it has been shown that doses of oxytocin as low as 2.5 IU IV are effective in inducing the term mare. In one study, mares that had 8 mmol/L Ca++ in mammary secretions were considered to be near foaling, and 2.5 IU oxytocin were administered. Mares that did not foal within 1 hour of oxytocin administration were judged not ready to foal, and they received a second dose of oxytocin (2.5 IU, IV), daily, until foaling occurred. Fourteen of 17 mares (14/17, 82%) foaled after the first treatment; one mare foaled after oxytocin administration on day 2, and 2 mares foaled after oxytocin on day 3. The investigators concluded that a single, low-dose injection of oxytocin (2.5 IU, IV) was effective to induce parturition in mares.
Furthermore, the researchers suggested that this induction scheme would work only in mares with a mature fetus, and that mares foaling on days subsequent to the initial treatment did not respond to oxytocin because the fetus was not fully mature. This method of induction appears to be a safer but more labor-intensive approach for a field practitioner.
In summary, several factors affect the success of induced parturition in the mare. Fetal readiness for birth is paramount to survival of the foal after birth. Critical evaluation of mammary secretion electrolytes, cervical relaxation, and gestational length facilitates proper mare selection and neonatal survivability with induced parturition. Oxytocin is the current agent of choice to induce parturition in the mare. The method of oxytocin administration does not impact neonatal adaptability after induced birth. Because a low-dose oxytocin protocol is effective for inducing parturition in the mare, higher doses of oxytocin are unnecessary and may be inappropriate.