RETAINED PLACENTA

The fetal membranes are usually expelled 30 minutes to 3 hours after parturition and are considered to be retained if they are not expelled within 3 hours after birth of the foal. The percentage of postparturient mares with retained fetal membranes is reported to range from 2% to 10%.^1,²

The probability of retained placenta increases after dystocia, probably as a result of trauma to the uterus and myometrial exhaustion.^2,³ Disturbance of the normal uterine contractions at parturition⁴ also might make retained placenta more likely. Retention has been reported to be more likely if severe placentitis was present, particularly if adhesion formation has occurred between the endometrium and chorion, or if infection has extended into the myometrium causing myometrial degeneration.¹ Placental retention is more common in the nongravid uterine horn, perhaps because of a progressive increase in the degree of placental folding and attachment from the gravid horn to the nongravid horn.² Retention of a portion of the placenta is also more likely to occur in the nongravid than gravid uterine horn because the chorioallantoic membrane is thinner, which predisposes to easier tearing.

Disturbed uterine contractions might result from fetomaternal endocrine dysfunction, inadequate release of oxytocin, or inadequate response of the myometrium to oxytocin. The role of oxytocin in placental expulsion is attested to by prompt placental expulsion after oxytocin-induced parturition and by the failure of some mares with retained placenta to exhibit the characteristic abdominal discomfort typically associated with uterine contraction and placental expulsion in the early postpartum period.^2,⁵

With retained fetal membranes, a variable portion of the placenta may be exposed through the vulvar opening. The veterinarian may also be alerted to the possibility of retained placenta when no placenta is found after foaling. Aseptic intrauterine examination may reveal the presence of the placenta within the uterine cavity. Alternatively, part of the placenta may remain in the uterus and continue to initiate mild straining or colic after most of the placenta has been removed. For this reason the fetal membranes should be examined to ensure that they are complete and that no portion remains in the uterus. Retention of portions of the placenta other than just the tip in the previously nongravid horn is also possible, so thorough examination of both surfaces of the expelled chorioallantois is necessary to detect less obvious missing portions. To facilitate examination, the expelled placenta can be filled with water. If the placenta is trampled, it can be impossible to determine whether remnants remain in the uterus.⁶

Sequelae of retained placenta vary from none ⁷ (particularly if mares are well managed and promptly treated) to the development of metritis, septicemia, toxemia, laminitis, and death.^2,⁶ Uterine involution is often delayed even if mares do not develop these sequelae.^8,⁹ Retained placenta with serious sequela is reportedly more common in Draft horses than in light breeds of mares such as Thoroughbreds or Standardbreds.^10,¹¹ Mares with retained placenta after dystocia are at greater risk of developing toxic metritis and laminitis.³ Severe toxic metritis and laminitis after dystocia are believed to result from delayed uterine involution, increased autolysis of the placenta, and severe bacterial infection.¹ In patients with septic/toxic metritis after dystocia and retained placenta, the uterine wall becomes thin and friable or even necrotic. Absorption of bacteria and bacterial toxins probably follows loss of endometrial integrity and precipitates the peripheral vascular changes that lead to laminitis.^12,¹³

Various treatments for retained placenta in mares have been advocated. Oxytocin therapy (alone or in conjunction with other treatments) is the most common and apparently the most beneficial form of management.^2,⁷ Recommended doses range from 10 to 60 units (smaller doses are typically given intravenously, whereas larger doses are given subcutaneously or intramuscularly) and can be repeated every few hours if the placenta is not passed.¹ The authors prefer to inject 5 to 20 units intramuscularly to promote more physiologic (peristaltic-like) uterine contractions. Clinical signs of abdominal discomfort occur within a few minutes of injection and are usually followed by straining. Discomfort is more pronounced with large doses and might result from intense and perhaps spasmodic uterine contractions. If abdominal discomfort is pronounced yet repeated dosing is deemed necessary, reducing the dose administered to as few as 5 units injected intramuscularly may likewise reduce abdominal discomfort to a more manageable level. A less intense response may occur during slow intravenous drip of 30 to 60 units of oxytocin in 1 or 2 L of normal saline over a 30 to 60 minute period.² The main disadvantage of administering a slow intravenous drip is the time involved and the need to administer the oxytocin using an intravenous catheter and fluid line. Anecdotal reports from many practitioners would suggest success rates achieved with one treatment of oxytocin may be superior with this method of administration. One precautionary note is that uterine prolapse can occasionally occur following oxytocin therapy, so care should be taken to observe for this potential complication.

If not expelled shortly after injection of oxytocin, the placenta sometimes is expelled 1 or 2 hours later. The placenta can sometimes be extracted by applying gentle traction to the portion protruding from the patient’s vulva. Vandeplassche ² reports that vigorous manual attempts to remove the placenta can damage the uterus, including causing hemorrhage within the myometrium, so attempts to manually separate the placenta must be gentle. Twisting the portion of placenta that extends beyond the vulvar lips with one hand (some practitioners will wrap the protruding membranes around a sweat scraper to facilitate twisting), while simultaneously working the fingers of the other hand (within the uterus) between the placenta and uterine wall, is a technique used by some practitioners for manual separation. The authors caution that if the placenta seems to be tightly attached, manual attempts should cease and medical treatments relied on.

Mares that have undergone severe dystocia or that have aborted are less likely to respond to oxytocin therapy alone.⁶ If the chorioallantois is intact, distension of the chorioallantoic cavity with 9 to 12 L of warm water or saline solution is an effective treatment of retained placenta. The opening of the chorioallantois is closed to contain the fluid. Stretch receptors are activated when the chorioallantois and uterus are distended, followed by endogenous release of oxytocin and separation of the chorionic villi from the endometrial crypts. Escaping fluid is forced back into the retained portion of the placenta until separation is complete and the placenta is expelled (usually 5 to 30 minutes). This treatment protocol reportedly results in more complete villous and microcarcuncular separation and can be used in conjunction with exogenous oxytocin therapy.¹⁴

Mares undergoing cesarean section frequently retain their placentas.¹ Retained fetal membranes have been reported to occur in approximately 30% of mares following cesarean section if the foal is dead and in 50% of cases when the foal is alive.¹⁵ Furthermore, it is not uncommon for mares to retain their placentas for up to 3 to 5 days or more following cesarean section or severe dystocia.³ Mares that retain their placentas for periods greater than 12 hours, including those following cesarean section, require aggressive therapeutic measures to prevent life-threatening sequelae such as septic metritis and laminitis.

Other treatments that can be combined with oxytocin therapy include systemic and local antibiotics, fluid therapy, uterine lavage, exercise (unless laminitis is present), and institution of prophylactic measures to prevent laminitis (e.g., systemic administration of cyclooxygenase inhibitors, application of foot pads). Continuing intrauterine treatment after the fetal membranes have passed is controversial; such treatment is particularly questionable if contamination of the reproductive tract was minimal and placental passage after treatment was prompt.⁶ However, for placental retention beyond 24 hours post partum, the authors generally continue treatment (including daily uterine lavage) for 2 to 3 days after the placenta is expelled to ensure placental remnants are removed and any uterine infection is brought under control.

The rationale for uterine lavage in mares that are at risk of developing metritis is to remove debris and bacteria from the uterus in order to reduce contamination and create a less favorable environment for bacterial growth. Purulent material and cellular debris that bind to and inactivate many antibiotics are also removed by uterine lavage. After disinfection of the perineal and vulvar areas, a sterilized or disinfected nasogastric tube is passed into the uterus. The hand is cupped around the end of the tube to prevent the uterus and any remaining placenta from being siphoned into its end. The uterus is gently lavaged with warm (40° to 42°C), sterile, physiologic saline (administered via a sterile or disinfected stomach pump) in 3- to 6-L flushes until the effluent is relatively clear. For on-farm treatments, sufficient volume of sterile fluids may not be available. When this is the case, the authors commonly prepare 2 to 3 gallons of lavage solution in a disinfected bucket by adding 34 g NaCl and 20 ml povidone-iodine solution to each gallon of warm tap water. As discussed above, uterine lavage can be repeated on successive days until the initial effluent is free of purulent material and tissue debris.⁶

If there is concern that the patient might develop septic or toxic metritis and laminitis, systemic and intrauterine antimicrobial therapy are indicated. Intrauterine antimicrobials are administered after evacuation of lavage fluid. Because antimicrobial agents infused into the uterus seldom achieve acceptable levels anywhere except in the uterine lumen and endometrium, relatively high antibiotic doses are administered systemically to prevent or control the development of septicemia from uterine infection that might involve tissue deeper than the endometrium. The agents chosen should be compatible and should have broad-spectrum activity because a wide variety of organisms has been recovered from the postpartum uterus. The antimicrobial regimen must be effective against anaerobic bacteria (e.g., Bacteroides spp., Clostridia spp.) and endotoxin-producing organisms (e.g., Escherichia coli).³ Antimicrobial agents effective against aerobic bacteria that are commonly employed include trimethoprim sulfadiazine (15 to 30 mg/kg PO twice daily) or penicillin G (22,000 mg/kg IV 4 times daily) plus gentamicin (6.6 mg/kg IV once daily). Metronidazole (15 to 25 mg/kg PO 4 times daily) is typically the drug of choice when treating anaerobic infections.

If there is evidence of toxemia (e.g., neutropenia with toxic neutrophils in the peripheral circulation, elevated heart and respiratory rates, altered mucous membrane perfusion, or other circulatory disturbance), cyclooxygenase inhibitors such as flunixin meglumine should be administered. Cyclooxygenase inhibitors have been shown to attenuate or prevent the circulatory disturbances associated with experimentally induced endotoxemia. Flunixin meglumine therapy is continued until there is no more danger of endotoxemia. The agent is usually given intravenously at a reduced dosage (0.025 mg/kg 3 times daily) in order to avoid potential adverse side effects.⁶ Other drugs advocated for this purpose include phenylbutazone (2 to 4 mg/kg PO or IV twice daily), dimethyl sulfoxide (1 g/kg IV; 10% solution in 5% dextrose), hyperimmune plasma (2 to 10 ml/kg IV), and polymyxin B (6000 units/kg IV twice daily).¹⁶

Acute laminitis is a medical emergency, and treatment should commence as soon as possible. Many therapeutic regimens have been recommended for the treatment of laminitis. Current equine medicine textbooks should be referred to for discussion of various treatments for laminitis.

UTERINE PROLAPSE

The uterus of the mare rarely prolapses. Uterine prolapse is more likely to occur immediately after parturition but sometimes occurs several days later. Conditions causing strong tenesmus (e.g., vaginal trauma) combined with uterine atony will predispose to uterine prolapse.^1,^17,¹⁸ The condition has been reported to be more common after dystocia when fetal membranes are retained.¹⁹

Only one uterine horn may be prolapsed, or the uterine body may constitute the major portion of the exposed uterus. Uterine prolapse may be complicated by rupture of internal uterine vessels, shock, or incarceration and ischemia of viscera, leading to death.^1,^20,²¹ In addition, damage to the prolapsed uterus may predispose to development of tetanus.

Treatment of uterine prolapse is first directed at controlling straining, either by administration of sedatives, caudal epidural anesthetic, or general anesthetic.¹ If no clinical signs are present that suggest compromised circulatory capacity, a tranquilizer or sedative can be administered. If the mare is tractable and not showing signs of severe abdominal pain or shock, the authors also recommend administering an epidural anesthetic. A local anesthetic, such as 2% lidocaine (1.0 to 1.25 ml/100 kg), can be slowly administered into the epidural (first coccygeal) space to reduce abdominal straining.²² An overdose may cause the mare to become ataxic or even fall. Alternatively, 0.17 mg/kg xylazine, as much as is required to 10 ml with 0.9% sterile NaCl, can be administered into the epidural space. Epidural injection with xylazine has been recommended to provide analgesia without hindlimb ataxia.²³ The authors typically use a combination of xylazine (0.6 ml, 30 mg), Carbocaine (2.4 ml), and sterile 0.9% NaCl (5 ml) for epidural injection in most light breeds of mares (body weight approximately 454 to 545 kg). This combination appears to provide good analgesia without causing ataxia or hindlimb weakness. If the mare is intractable yet shows no signs of circulatory shock, general anesthesia can be induced. When inhalation anesthetic is not available, short-acting intravenous anesthetic agents can be used (e.g., 1.1 mg/kg xylazine IV, followed by 2.2 mg/kg ketamine hydrochloride IV—once sedative effects of xylazine are apparent). Duration of anesthesia can usually be safely extended for short periods with intravenous administration of 1 L of 5% guaifenesin containing 1000 mg ketamine hydrochloride and 500 mg xylazine, which is dripped at a rate of 2.75 ml/kg/hr, or slowly to effect. When general anesthetic is used, it is advantageous to place the mare on a slight incline (30 degrees from horizontal) with its head downward; the incline should not be so great as to compromise respiratory function. With the mare in such a position, the uterus should be relatively easy to replace.²²

In mares kept standing (after wrapping the tail and cleansing the perineal area), the uterus is lifted to the pelvic level in an attempt to restore circulation, reduce congestion, and decrease traction on ovarian and uterine ligaments that cause pain. Elevating the uterus also permits the bladder to resume its normal position, facilitates release of intestine incarcerated within the prolapsed uterus, and reduces the chance vessels in the broad ligaments will rupture.¹ If bowel is entrapped in the prolapsed uterus, ischemic damage to the bowel may have occurred that could warrant evaluation and treatment after performing a ventral midline celiotomy.²¹ If the urinary bladder is distended, catheterization may be required before uterine replacement.²² If the bladder cannot be catheterized, urine can be aspirated from the bladder through a large-bore needle.²⁴ The uterus is gently cleaned with a disinfectant soap, carefully removing any loosely attached placenta. If placenta is firmly attached, excess free placenta is removed to better enable replacement of the uterus and to decrease likelihood of persistent traction from the heavy placenta that might cause recurrence of the prolapse. Bleeding vessels should be clamped and ligated. Complete uterine tears should be sutured, bringing serosal surfaces into contact. Application of petrolatum jelly to the endometrial surface is useful for protection against lacerations during massage and replacement.²² Placing the uterus inside two or three plastic garbage bags has been advocated to reduce risk of puncturing or lacerating the uterus during replacement. The garbage bags are removed as the uterus is pushed inside the vagina.²⁵

To avoid reprolapse, the uterus must be completely replaced. The uterus, including both uterine horns, is gently kneaded per vaginam into its normal position. Gently filling the uterus with warm water may be helpful in ensuring complete replacement of uterine horns. Excess fluid is then siphoned off through a stomach tube.²² Walking or jogging the mare down a gentle slope has been advocated to aid in full repositioning of the uterus.²⁶ Following replacement of the uterus, palpation of the genital tract per rectum is performed to ensure the uterus and ovaries have been returned to their proper position. Intrauterine antibiotics are given to control infection. Small doses (10 to 20 units IM q2-4h) of oxytocin are administered in an attempt to stimulate uterine contractions and thus reduce the chance of recurrence of the prolapse. Mares with uterine atony due to hypocalcemia (i.e., serum ionized calcium below 5.0 mg/dl) may benefit from slow intravenous administration of 100 to 300 ml of 20% calcium borogluconate mixed in 5 L lactated Ringer’s solution. The heart should be auscultated periodically as calcium is given, and administration of calcium should cease if the heart rate or rhythm changes. Placing the mare in cross ties for 1 or 2 days has been advocated to reduce the chance of recurrence. A soothing, emollient salve can be placed in the vagina if vaginal lacerations are present. Suturing the vulva will also prevent pneumovagina and speed resolution of vaginal irritation, which might stimulate further straining. Broad-spectrum antibiotics are administered systemically to control infection. Other treatment is as described for metritis, and tetanus prophylaxis is required.²² The mare should be monitored for evidence of internal hemorrhage, which can occur after uterine replacement.²⁴

INVAGINATION OF UTERINE HORN

An invaginated uterine horn is suspected when a mare has mild colic unresponsive to analgesics. The invagination is sometimes associated with a placenta that remains attached to the tip of the uterine horn, causing partial inversion as a result of traction. Palpation per rectum usually reveals a short, blunted uterine horn and tense mesovarium. Intrauterine examination will reveal the dome-shaped, inverted tip of the horn projecting into the uterine lumen. The placenta may be incarcerated in the intussuscepted horn.²² Rarely, in advanced cases, a reddish-black discharge will be associated with necrosis of the inverted uterus,¹ which might lead to peritonitis if unattended.²⁴

Treatment involves replacement of the uterine horn to its normal position, which may require manual removal of the placenta. The mare is sedated to control straining. To manually separate the placenta, the placenta is carefully twisted at the vulvar opening while simultaneously attempting to manually separate the chorion from the endometrium.²² This manipulation must be gentle to prevent damaging the uterus, which causes placental tags to remain attached to the endometrium.²⁷ The portion of the placenta protruding from the vagina should be severed if manual separation cannot be accomplished safely. Removing the free placenta reduces the weight pulling on the tip of the uterine horn, permitting it to return to its normal position. The invaginated portion of the uterine horn is gently kneaded inward to its normal position. If the examiner’s arms are too short to reach or fully correct the invagination, a clean bottle can be inserted into the affected horn and used to gently push against the invaginated horn to aid in its reduction. Infusion of 1 to 2 gallons of warm, sterile saline solution (or other suitable lavage solution) has also been used to facilitate complete replacement of the inverted uterine horn. Aftercare is as described for uterine prolapse and metritis. If a portion of the placenta was left within the uterus, daily uterine lavage and intrauterine treatment is necessary until the portion of the placenta left within the uterus is expelled.²²

On occasion, a mildly invaginated uterine horn will be noted in a mare with no untoward clinical signs. Such invaginations may not require treatment, but may persist for a few days postpartum. When an invaginated uterine horn is noted incidentally in a postpartum mare that has expelled her placenta and is not showing signs of discomfort, monitoring the extent of the invagination by palpation per rectum at daily intervals until the condition corrects itself may be all that is required.