Equine fetal membranes are normally expelled within 3 hours after parturition; however, expulsion may be delayed for 8–12 hours or even longer without clinical signs of illness. The cause of retention of fetal membranes often is not known.
The incidence of retained fetal membranes in mares is low, with incidence rates ranging from 2%–10.6%. Risk factors include placentitis, abortion, short or prolonged gestation, uterine atony, dystocia, and advanced age of the mare. Mares that have retained their fetal membranes appear to be at increased risk of recurrence of the condition. Friesian mares are particularly predisposed.
Retention of just a portion of the fetal membranes entirely within the uterus (usually at the tip of the previously nongravid uterine horn) is less conspicuous; however, it is equally likely to result in complications. For this reason, the chorionic surface of the expelled membranes should be examined carefully to ensure they have been completely expelled.
Consequences of retained fetal membranes in the mare can be severe and potentially life-threatening and may include endometritis, metritis, peritonitis, laminitis, and endotoxemia. For these reasons, reactive removal of fetal membranes that have been retained 3 hours or longer should be considered.
The most common therapeutic regimen for mares at risk of developing retained fetal membranes is administration of oxytocin. Oxytocin may be administered as bolus injections (5–20 U, IV or IM, every 30 minutes to 2 hours for 6 hours after foaling or until the placenta is expelled) or as a slow IV infusion (60 to 100 U mixed in 1 L of lactated Ringer's solution or saline [0.9% NaCl] solution administered over 30 to 60 minutes). The Burns technique, wherein the chorioallantois is distended with dilute povidone-iodine or saline (0.9% NaCl) solution for 15 to 30 minutes, may facilitate release of microcotyledons from the endometrium and aid in removal of retained fetal membranes. Alternately, for fetal membranes retained < 8 hours, catheterizing an exposed umbilical vessel allows distention of fetal membrane vasculature, causing the chorioallantois to detach from the endometrium.
Manual proactive removal of fetal membranes is controversial and carries the risk of uterine damage, endometrial hemorrhage, tearing of the membranes causing retained fetal membranes, or uterine prolapse. Manual removal is not recommended beyond gentle tugging to displace already loosened membranes. After removing them, the mare should be thoroughly examined to ensure complete removal of the membranes and lack of uterine damage. Additional treatment should be considered, including uterine lavage and administration of oxytocin, antimicrobials, or anti-inflammatories as indicated. Because low serum calcium concentration has been associated with retained fetal membranes, particularly in Friesian mares, calcium supplementation may be beneficial.
In cases of prolonged retention of fetal membranes, antimicrobials should be administered prophylactically, along with other therapeutic strategies aimed at preventing laminitis Laminitis in Horses The horse hoof. Median section through the horse digit. Equine laminitis is a crippling disease in which there is a failure of attachment of the epidermal laminae connected to the hoof wall... read more . Mares that have recovered from retention of fetal membranes do not generally have lower fertility.