Hydrops is a rare condition in the mare, with hydroallantois occurring more commonly than hydramnios. Hydroallantois causes rapid abdominal enlargement during the last trimester of pregnancy (), and a sudden increase in the volume of allantoic fluid during a period of 10 to 14 days. The pathophysiology of hydroallantois in the mare remains unknown. Some authors have suggested that the increase in fluid is a placental problem caused either by increased production of fluid or decreased transplacental absorption. Others have proposed that the etiology is related to placentitis and heritability. In these authors’ experience, examination of the fetus and fetal membranes have rarely demonstrated any consistent abnormality. One mare had diffuse mild placentitis (with leptospirosis) and another had histologic evidence of vasculitis when an endometrial biopsy was taken within 2 days of delivery of the fetus.
Mares may present with anorexia, tachycardia, severe ventral edema/plaques, abdominal discomfort, and labored breathing caused by pressure on the diaphragm. They typically have difficulty walking and often become recumbent. Uterine rupture may occur in advanced cases. Other complications associated with the excessive weight of the uterine contents include prepubic tendon rupture and development of abdominal wall and inguinal hernias.
Rectal palpation is diagnostic and reveals a huge, taut, fluid-filled uterus. The fetus cannot be balloted. Transrectal ultrasound imaging shows hyperechogenic allantoic fluid. The fetus is seldom observed as a result of the depth of the enlarged uterus. Transabdominal ultrasound will confirm the presence of excessive echogenic allantoic fluid and this approach does permit evaluation of fetal viability (movement and a heart rate of 80-100 bpm).
Treatment of Placental Hydrops
During the last few years the medical facility at these authors’ clinic has seen an increase in the number of hydroallantois cases presented to the clinic. Once the diagnosis of hydroallantois is confirmed, fetal viability is determined, and udder development and the milk electrolytes are assessed to estimate the level of fetal maturity. Those mares that present early in gestation may undergo elective termination of pregnancy by IM injection of cloprostenol (500 μg, 2 ml IM ql2h until delivery). Cases that occur later in gestation, or those with profound abdominal enlargement, may have large volumes of fluid within the uterus and require controlled drainage of the fluid before expulsion of the fetus. The reason for the controlled drainage is that excessive uterine distention alters total body fluid balance and venous return to the right heart. Sudden loss of this large volume of fluid may result in hypovolemic shock. The effect of the fluid loss is exacerbated by the sudden expansion of the abdominal venous circulation once the uterine weight is reduced. In the short term, abdominal support (i.e., belly band), IV fluids, steroids, broad-spectrum antibiotics, and antiinflammatory medication will provide systemic support for the mare. Slow siphoning of the allantoic fluid is then attempted. Once the size of the distended uterus has been reduced, the authors have used oxytocin (20 IU IV given repeatedly or 50 IU in a saline drip) or cloprostenol (two doses of 500 μg, 30 minutes apart) to promote fetal expulsion. In these authors’ experience, cloprostenol has provided a smooth progression of labor, with stage 2 occurring 30 to 60 minutes after the second dose. Mares that present within the last 2 to 4 weeks of pregnancy may be managed by partial drainage. The aim in these cases is to maintain the pregnancy for as long as possible in order for additional fetal maturation to occur.
The technique for drainage involves several considerations. Location (stocks or stall) is determined by individual preference and the condition of the mare. The process takes 2 to 3 hours, so comfort is a factor. The clinician should initiate supportive care by placing an IV catheter and administering a slow infusion of a crystalloid fluid. A tail wrap and sterile surgical preparation of the mare’s perineum is essential. The equipment includes a 24- to 32-French sharp thoracic trocar catheter, a two-way plastic adapter, sterile plastic tubing, a sterile sleeve, and buckets to collect the allantoic fluid. Smaller-sized catheters will take longer for fluid removal. The technique involves sterile passage of the catheter through the vagina and cervix, and sharp puncture of the chorioallantois. The sharp trocar is removed and the two-way adapter is used to connect the catheter to the tubing. The catheter is held in place by the clinician’s arm within the vagina. Controlled gradual drainage can then be performed into the buckets. Some pericervical separation of the placenta is common.
Several mares have been successfully treated in these authors’ hospital with this technique. In a few cases (6) that were within 2 to 4 weeks of term, maintenance of the pregnancy has been attempted after partial drainage of the allantoic compartment. These mares were treated with additional antimicrobial therapy, antiinflammatory medications (flunixin meglumine, pentoxifylline), agents with possible tocolytic activity (isoxsuprine, clenbuterol, albuterol), and altrenogest. In cases where partial drainage is attempted, fetal death may occur as a result of fetal asphyxia that results from varying degrees of placental separation. Iatrogenic fetal infection, secondary to contamination of the placental fluids during drainage, is also a problem. In most cases attempted to date, the fetus has become infected with Escherkhia coli and subsequently died. One mare died after 72 hours as a result of rupture of a uterine artery. The fetus in that mare had remained alive and exhibited normal parameters when monitored by transabdominal ultrasound.
Owners should be advised that the fetus is usually lost in mares with hydroallantois. However, early recognition of the problem, and prompt intervention, provides a good prognosis for the mare both physically and reproductively. Complications that should be anticipated when managing a mare with hydroallantois include hypovolemic shock, dystocia, and retention of the fetal membranes. The hypovolemia requires rapid volume expansion with use of large volume crystalloid infusion (as high as 40 ml/kg) alone, or in combination with hypertonic saline (4 ml/kg). The use of colloid fluids such as hetastarch (10 ml/kg) might also be beneficial. Dystocia may be associated with incomplete cervical dilation and uterine inertia. Malpositioning and malpostures are common. Therefore manual assistance to deliver the foal is necessary. Management of retained fetal membranes is discussed elsewhere in this text (see “Retained Fetal Membranes”). Because it is possible that a heritable component to this condition exists, breeding to a different stallion may be prudent.