infections before or during pregnancy have an increased occurrence of
retained placentas.
The general opinion today is that areas of allantochorion near the tip of
the non-pregnant horn most often fail to separate. This is thought to be
due to the chorioallantoic membrane being thinner in this area, allowing
it to tear more easily when the weight of the dettaching placenta is
pulling on it. Uterine involution also occurs at a slower rate in the nongravid than in the gravid horn.
The increased occurrence of retained placentas following a dystocia is
probably related to the trauma of the endometrial tissue and fatigue of
the myometrium The failure of sufficient oxytocin release at the
appropriate time could also add to the occurrence of retained placenta.
Many authors believe that retained fetal membranes are primarily due
to uterine inertia and hormonal imbalance, because affected mares
typically do not exhibit the mild signs of colic associated with
postpartum uterine contractions. Mares that normally pass the placenta
during stage 3 of parturition show mild colic due to uterine contractions.
Mares with retained placentas infrequently demonstrate this activity
and, when present, it is diminished
Uterine inertia can be caused by low blood calcium, overstretching of
the myometrium, myometrial degeneration and myometrial exhaustion
after dystocia.
Retained foetal membranes provide an environment in which bacteria
can rapidly multiply, resulting in local inflammation and systemic
absorption of uterine toxins. Sequelae of retained fetal membranes in
the mare may vary from none to metritis, endotoxaemia, laminitis, or
death.
A retained placenta protruding from the vulva and touching the hocks
should be tied up away from the hocks below the vulva. This procedure
will reduce the probability that the mare will kick at the placenta and in
doing so possibly injure the foal. The mare will also be less likely to
step on the placenta, causing it to tear or causing the tip of the uterine
horn to invaginate within itself or to evert through the vulva.
The most conservative treatment is the use of oxytocin. Oxytocin can
be administered in slow intravenous drip form (30–60 units of oxytocin
in 1–2 liters of saline given over 1 hour). It can also be given as an
intravenous or intramuscular bolus. Disadvantage of the bolus form of
administration potentially include intense and spasmodic contractions
which may be unproductive. Dosages can be repeated at 2-hour
intervals until passage of the placenta or up to 24 hours postpartum.
Systemic and intra-uterine antibiotics are used in the therapy of
retained fetal membranes by most clinicians. It has been recommended
that administration of antibiotics should commence at 8 hours following
delivery. Antibiotics are thought to be an important part of retained
placenta treatment owing to their ability to control numbers of
15-‐18
February
2016
East
London
Convention
Centre,
East
London,
South
Africa
80