During pregnancy the equine placenta and uterus are tightly associated through highly vascularized (filled with blood vessels) infolded structures called microvilli, which allow nutrient and waste exchange between the mare and foal. Once the foal is born, the placenta is no longer needed and the mare must deliver all of it. The umbilical cord breaks, and blood vessels within the placenta lose pressure and collapse, causing the microvilli to recede from the uterine wall. Concurrent rhythmic uterine contractions help expel the placenta.
Most mares pass these fetal membranes shortly after foaling. If it hasn’t happened within three hours postpartum, it’s considered a retained placenta. This retention may be complete, involving the entire placental surface area, or partial, which usually includes the tips of one or both placental horns.
Foaling complications, abortion, and infection all increase the mare’s risk of retaining a placenta, but the condition can also occur in apparently normal foalings. Draft mares, particularly Friesians, are especially susceptible.
Because the placenta is no longer a viable organ postpartum, it begins to degenerate within the uterus, causing extensive bacterial overgrowth and toxin production. These bacteria and toxins can enter the mare’s bloodstream through the vascularized postpartum uterus and quickly cause sepsis (a body-wide infection) or endotoxemia (the potentially fatal presence of endotoxin in the bloodstream, released when bacterial cells disintegrate), sometimes as soon as 12 hours after foaling. This condition might be life-threatening and can also precipitate laminitis and founder, the latter of which involves sinking of the coffin bone. Mares with sepsis or endotoxemia could become lethargic and develop a fever.
In most cases diagnosis is fairly straightforward, with the placenta visibly retained in the mare. Because partial retention can occur, it is important to lay out the membranes after they pass and examine them for completeness. Check both horns carefully to ensure they have passed in their entirety, including the tips.
In cases where you cannot locate the placenta or determine with certainty if it’s complete, the veterinarian will examine the mare’s uterus manually. In most cases he or she will be able to find any placental remnants, although occasionally they’re beyond reach. If your veterinarian suspects a retained placenta but can’t verify it, he or she can start treatment until making a definitive diagnosis.
Prior to eight hours postpartum, treatment for a retained placenta is usually conservative and includes administering oxytocin (a hormone that causes uterine contractions) at two-hour intervals and tying the placenta at hock level as it passes, so the mare doesn’t trample it and make later visual assessment difficult. If the hanging portion of the placenta has already torn off, the veterinarian can attach a small weight to the remaining portion to provide gentle traction. Avoid attaching heavy weights or using forceful manual traction, as these can cause uterine hemorrhage or prolapse.
If the mare has not passed the placenta within eight hours, it’s time to start more aggressive therapy. Your veterinarian can use several techniques to facilitate expulsion of the placenta if it is still intact. With one, called the Burn’s technique, the veterinarian infuses the inside of the placenta with fluid while holding it closed at its opening. The cavity expands, causing oxytocin release and abdominal contractions, which facilitate the placenta’s release and passage. A more recently developed technique involves infusing water directly into the umbilical cord vessels. This causes the vascular system, including the microvilli, to expand and stretch, allowing the placenta to release from the uterus in most cases.
If the placenta is partially retained, neither of these techniques will be effective. In these cases the veterinarian lavages (flushes) the uterus once or twice daily to remove bacteria and toxins. He or she might also gently twist the retained piece of placenta to encourage its release.
If the mare has retained the placenta for more than 12 hours, usually the veterinarian will treat her to prevent potential complications. This includes administering antibiotics and anti-inflammatories and starting laminitis prevention methods such as standing her in ice boots.
The overall prognosis for cases treated early and appropriately is generally very good, with most mares regaining their reproductive health by their 30-day heat cycle. Cases with more serious complications come with a guarded prognosis, especially when laminitis or founder have developed.