Premature placental separation (placenta previa) and prolapse of the bladder
Keywords: dystocia, premature, placenta, separation, equine
The term chorioallantois is used more than three times as common as allantochorion in academic citations. Therefore it is used in preference to allantochorion in this entry. The chorioallantois is the result of fusion of the chorion (the outermost fetal membrane in all mammals) and the outer surface of the allantois.
At the onset of second stage parturition in all domestic animals, the chorioallantois normally ruptures where it overlies the internal cervical os. Allantoic fluid is released and the amnion appears at the vulva lips.
During foaling, the bright red villous surface of the chorioallantois should not be seen until after the birth of the foal, when the placenta is expelled during third stage parturition. If it is seen at any time before the foal is born, the surfaces for gaseous exchange between fetus and dam (the endometrium and chorioallantois) are being separated, oxygen tension is lowered in the fetus and it is in danger of dying. A similar situation in humans is referred to as placenta previa (Latin praevius meaning previous) where the placenta lies over the cervix and precedes the birth of the fetus. In humans however, the primary threat is life threatening hemorrhage, then anoxia. In horses the condition is commonly referred to as premature placental separation or colloquially, as "red bag" for obvious reasons. This author believes that the term placenta previa is also justified in mares because the placenta is being born before the fetus i.e. its appearance precedes that of the foal.
The cervical canal is has no glands and does not participate in gaseous exchange. This may explain the avillous nature of the chorionic surface of the chorioallantois at this site i.e. the so-called "cervical star". The avillous rays may develop from folding of the chorioallatois over the internal cervical os.
The image below is that of a five month pregnancy, removed from the uterus. The cervical star, covering the head of the foal, is particularly obvious.
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In the image below, the cervix has been opened at post mortem in a term equine pregnancy. The cervical canal is at the lower right of the image. A finger points to the white, avillous rays of the surface of the allantochorion that lie over the cervix.
Although the cervical star is the most common site of chorioallatoic rupture, in normal foalings it may also rupture a short distance away.
Image size: 1616 x 1080px
The image below is that of a placenta from a term pregnancy. As is commonly the case, the placenta was found in the stall inside-out, with the allantoic surface of the chorioallantois showing and the chorionic surface inside the placenta. For purposes of demonstration, the placenta was restored to its normal in-utero orientation, with the chorionic surface on the outside. A finger points to the site of rupture; in the center of the cervical star. The extra-amnionic umbilical cord lies within the site of rupture and the amnion is seen under the pointing hand.
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As mentioned, premature placental separation is cause for an obstetrical emergency. If the chorioallantois protrudes from the vulva lips before the birth of the foal (as shown below) it should be transected immediately. This will release allantoic fluid. The translucent amnion will then be seen; the foal within. That membrane in turn should be transected and the foal delivered immediately.
Images such as this are rare because the condition demands immediate attention, seldom allowing time for photography. The author of the image and holder of copyright is Dr Dale Paccamonti. pacc@lsu.edu
 In the case pictured at left, a Shire mare prolapsed her bladder before foaling. It is the bright, smooth and glistening structure protruding from her vulva lips. By contrast, the chorioallantois is dull and velvet-like. This is a rare condition. The white membrane adjacent to the bladder is part of the amnion. Clenbuterol was used to suppress foaling and epidural anesthesia was used to facilitate replacement of the bladder. Then, using conventional methods, a live foal was delivered. Image size: 865 x 481px |
