IN-DEPTH: CONDITIONS AND MANAGEMENT OF THE PERIPARTUM MARE
area for nutrient exchange.6 Following parturi- tion, the umbilical cord is ruptured, leading to col- lapse of the umbilical vasculature and smaller vessels throughout the fetal membranes, allowing contracture and detachment of the microvilli from endometrial crypts. Endogenous oxytocin release stimulates rhythmic contractions of the uterine myometrium to expel the free fetal membranes from the uterus.8 In the normal mare, the horns of the allantochorion invaginate as they are released and pass through the ruptured cervical star. This is encouraged from the dependent weight of the mem- branes (amnion) hanging down externally from the vulva. The membranes are expelled intact with the allantoic surface showing outermost, in most cases. The cause of retained fetal membranes in mares is
not well defined and is likely multifactorial. It is hypothesized that a multitude of impaired physio- logic changes result in an abnormal release of the microvilli from the endometrial crypts in suscepti- ble mares.2 Uterine inertia due to myometrial ex- haustion associated with dystocia or advanced age, overstretching of the myometrium following hydrops allantois, hormone imbalance associated with preterm delivery, or low calcium levels at the time of parturition along with specific placental characteristics may account for the higher inci- dence of retention.1,2,9,10 Allantochorion thick- ness, microvilli length, and degree of attachment at parturition along with an increased folding pattern of the non-gravid horn are potential factors in in- creased membrane retention.3 Fibrosis and adhe- sion formation have been reported in both the microcotyledons and stromal connective tissue in heavy draft breeds while low serum calcium concen- trations have been directly linked to incidence of retained fetal membranes in Friesian mares.10,11 No matter the cause, membrane retention longer
than 3 hours postpartum can lead to life-threatening sequelae in mares such as metritis, sepsis, and laminitis.1,12 Acute metritis can develop with au- tolysis of the retained allantochorion or microvilli within the endometrial crypts, providing a nidus for infection. The environment becomes ideal for bac- terial invasion and replication leading to inflamma- tion and intrauterine fluid accumulation; toxins from gram-negative bacteria may be systemically absorbed leading to endotoxemia and subsequent laminitis.1 For all of these reasons, prompt mem- brane removal and medical management is war- ranted. Proactive management of membrane removal (prior to 3 hours) may prove beneficial for mares with placentitis, abortion, or dystocia and for intensely managed broodmare operations or mares at distant locations.
3. Diagnosis
Clinical signs of retained fetal membranes in the mare may be obvious as portions of the membranes protruding externally from the vulva. However, mares whose foaling was unattended may have no
indication of retained fetal membranes other than a history of unknown or incomplete membrane expul- sion. In many cases, a portion of the allantochorion (i.e., tip of the non-gravid horn) can be retained and may go unnoticed if the fetal membranes have not been closely examined. Some mares with partial retention of the allantochorion can present 24–48 hours postpartum with clinical signs of metritis and/or endotoxemia to include fetid vaginal dis- charge, depression, pyrexia, injected mucous mem- branes, tachycardia, and increased digital pulses associated with the onset of laminitis.1 However, clinical signs of illness are not always present in mares with retained membranes, there- fore it is essential that the fetal membranes be examined carefully after delivery. The chorioallan- tois is laid out so that the full membranes can be examined. Placing the membranes in the shape of an F allows examination of the ruptured cervical star at the base of the membranes and each uterine horn (forming the arms of the F). The umbilicus is located at the base of the gravid horn and the am- nion is attached to the umbilicus. When the mem- branes are damaged or torn, sometimes they can be pieced together by following the vascular patterns.13 In any instance where retained fetal membranes are suspected, the mare should be examined further. Additional diagnostic procedures include trans-
rectal examination and bloodwork. Transrectal palpation of the uterus can reveal the degree of uterine tone/involution, which may be increased in some mares with retained fetal membranes (uterus actively contracting) or reduced with accumulation of intraluminal fluid that occurs in mares with me- tritis. Transrectal ultrasonography can further de- fine the amount and character of fluid in the uterus, and possibly, tags of retained chorioallantois. Fol- lowing cleansing of the perineal area, digital exam- ination of the uterine lumen is performed to attempt to locate retained membranes and degree of attach- ment evaluated. When endotoxemia is suspected, blood samples should be collected for complete blood count, biochemistry panel, serum amyloid A, and lactate.
4. Treatment
Multiple strategies are utilized by the veterinarian to achieve complete expulsion or removal of retained fetal membranes.2,5,10,14,15 The method selected for removal of retained fetal membranes is depen- dent on effectiveness, safety, cost, convenience, and experience of the veterinarian. Mares housed in close proximity to the veterinarian, or at large stud farms accustomed to postpartum management, can be closely monitored and conservatively evalu- ated with repeat veterinary visits. Mares housed in field conditions without ready access to veterinary care may require more rapid, but safe, management of retained fetal membranes. For veterinarians traveling long distances to treat mares with retained fetal membranes, prompt resolution of the
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