Unusual placentation in a twin pregnancy. 

Keywords: placenta, equine, mare, inverted, twin, complication

Companion to an entry on twinning in mares in LORI.

There is still some uncertainty as to how and why twin equine pregnancies result in abortion. Sharing the available endometrial area is certainly a significant factor. Although typical inflammatory reactions at placental interfaces do not characterize twin abortions, immunological rejection of one co-twin by another may also be an important factor. Figure 1 shows one of numerous permutations of twin placentas apposed to one another within the uterus; placental sharing of the endometrial surface may be almost equal. In others, sharing is dramatically unequal. Figure is a schematic representation of such a case. Indeed, the situation in this pregnancy.

In Figure 1 red arrows indicate an area of apposition between the two chorionic surfaces. In these placentas there was little if any, macroscopic reaction between the two conceptuses despite the intimacy of placental apposition. Approximately 40% of the chorionic surface from the smaller co-twin was invaginated into the placenta of the larger twin.

Figure 1: Apposition of twin placentas and the path of delivery for both foals. Size available: 1626 x 2172px

The green arrows in Figure 1 show where the chorioallantois of the larger twin ruptured; in the region of the cervical star. The chorioallantois containing the smaller foal (yellow arrows) appeared within the allantoic cavity of the larger foal, invaginated within a pocket of the larger foal's placenta. The smaller foal was then delivered through a rupture of two layers of chorioallantois; that of the larger foal and that of its own placenta. The lower part of Figure 1 shows the path (grey arrow) that had to be taken by the smaller foal to be born; through the placenta of the larger foal.

Figure 2. A schematic representation of the placentation in this case. In some cases, the smaller co-twin may die and become a mummified attachment to the placenta of a normal foal. Image size available: 2553 x 1886px

Often, the placentas of twin foals will lie side by side within the uterus and each foal will be born in a conventional fashion, through the rupture of independent chorioallantoic membranes adjacent to the cervix.

Treatment of retained placentas

Keywords: equine, placenta, retained, Burn's, infertility

The incidence of retained placenta in mares has been reported to lie between 2 and 11 % of all foalings in most breeds. In other breeds, especially Friesians and draft horses, it can be far higher. Unlike the situation in cattle, retained placenta in mares is a potentially life-threatening disease.

If the placenta is still attached three or more hours after foaling, it should be considered retained. Although it has been stated (like the situation in cattle) that uterine atony does not contribute to retained placentas, the vast majority of retained placentas in mares are expelled after one or more injections of oxytocin; certainly not the case in cattle. In the author's experience, simple treatment with 10 iu of oxytocin i.m. every 30 minutes will dislodge the vast majority of retained placentas within five or six treatments. Treatment with continuous I.V. drips has been described but it is not clear if that method warrants its extra expense and inconvenience. As a nonapeptide, oxytocin is a small and stable molecule and is very rapidly absorbed after intramuscular injection. With little potential for abuse, it can be left with responsible horse owners together instructions for treating retained placentas. Placentas that are still retained after six treatments should probably receive intensive veterinary care.

A novel, effective, and apparently safe method of treating retained placenta in mares was described by Meijer et al in 2015. Briefly, this involves infusing water into any major umbilical blood vessel for several minutes until the placenta becomes dislodged. It has yet to become widely adopted.

An alternative treatment is to inflate the chorioallantois with fluid to loosen its attachment to the endometrium and presumably, to stimulate myometrial contraction. This is colloquially known as the Burns technique. The author has not found the Burn's technique to be reliable but has little experience with the procedure.

The collage of images below shows the typical appearance of a retained placenta and the application of the Burns technique in a mare. About 10 to 12 liters of saline are infused into the intact chorioallantois and the site of infusion is ligated with umbilical tape to retain the saline.

Image size: 1572 x 2046px

If the placenta has not been expelled within 30 minutes, the ligation is removed and the fluid is allowed to drain. Conventional care is then adopted. This care includes oxytocin, tetanus prophylaxis, antibiotics, non-steroidal anti-inflammatory drugs, and in the view of some, vasodilators to prevent laminitis.

Manual extraction of the placenta remains a contentious subject. In North America, the general belief is that the placenta should not be manually removed. Instead, a mare should be treated systemically (as described above) until her placenta has been expelled. This may take two or three days to occur. However, in Europe it is common to remove placentas manually if they have been retained for longer than three hours. Indeed in the Netherlands, it is actually considered to be malpractice if the placenta is not removed manually when it has been retained for longer than six hours post foaling. Interestingly, at least two peer reviewed studies have shown than manual removal of fetal membranes did not affect reproductive performance.


Selected references:

Canisso, I.F. et al 2013. A clinical approach to the diagnosis and treatment of retained fetal membranes with an emphasis placed on the critically ill mare. J.Equine.Vet Sci. 33: 570-579

Cuervo-Arango, J.et al. 2009 The Effect of manual removal of placenta immediately after foaling on subsequent fertility parameters in the mare. J. Equine.Vet Sci. 29:771-774

Gibbens, D. et al. 1972. The circulating levels of oxytocin following intravenous and intramuscular administration of syntometrine. British J. Ob.Gyn. 79:644-646

McKinnon, A.O. et al Eds. 2011, Equine reproduction. Second ed. Chapter 260. Wiley-Blackwell ISBN: 978-0-8138-1971-6

Meijer, M et al. 2015. How to use umbilical vessel water infusion to treat retained fetal membranes in mares. Proceedings AAEP 61:478-484

Paccamonti, D.L. et al 1999. PGFM response to exogenous oxytocin and determination of  the half-life of  oxytocin in nonpregnant mares. Equine Vet J. 31:285-288

Provencher, R. et al. 1998. Retained fetal membranes in the mare: A retrospective study
Can Vet J. 29: 903–910.

Rapacz, A. et al. 2012. Retained fetal membranes in heavy draft mares associated with histological abnormalities. J. Eq. Vet. Sci. 32: 38-44

Sevinga et al 2002. Reproductive performance of Friesian mares after retained placenta and manual removal of the placenta. Theriogenology.57: 923–930

Sevinga, M. et al 2002. Serum calcium and magnesium concentrations and the use of a calcium-magnesium borogluconate solution in the treatment of Friesian mares with retained placenta
Theriogenology 57: 941–947

Equine placentation overview

Keywords: placenta, diffuse, equine

This image shows the diffuse placentation typical of horses. The fetus is about two months old. Its placenta detached very easy from the endometrium in this fresh specimen.

Image size: 1800 x1350px

As shown in the inset, a close up image of a placenta at term, micro cotyledons (red arrows) eventually form an intimate attachment to the endometrium. These micro cotyledons are similar in structure to those of cattle but microscopic in size. The bond between the placenta and the endometrium becomes firmly established by about six or seven months of gestation. Only after this time, is the placenta likely to be retained in the case of abortion.

In the image below, the placenta has been opened to show the relationship between the fetus and the placental membranes.

Image size: 1182 x 660px

Most of the image is self-explanatory but some comment may be beneficial to the viewer.

The equine placenta has both intra- and extra-amnionic portions of the umbilical cord. This differs from ruminants, where there in only an intra-amnionic cord because the ruminant amnion is adhered to the inside of the chorion along much its length. In ruminants therefore, urine immediately escapes from the distal end of the  intra-amnionic cord into a "hollow belt" that connects the sections of the allantois that lie on either pole of the amnion. In equids by contrast, the amnion (strictly speaking, the amnio-allantois) is completely free from the chorion (the allantochorion). Therefore, the equine amnion can move and rotate freely within the allantoic cavity. This makes it possible for an equine fetus to change its presentation from cranial to caudal and vice versa until pregnancy is advanced, perhaps up to nine months or later. By contrast, the presentation of a bovine fetus has become fixed by approximately five months of gestation.

A distinct disadvantage of having an amnion that is free of the chorion is that it is possible for a foal to be born with an intact amnion covering its head. This can result in suffocation and perinatal death. In ruminants, that is practically impossible because the amnion is strongly adherent to the chorion and cannot follow the fetus after it has been born.

Note that the large vessels seen on the amnion here are temporary; they will have regressed and will be barely visible at the time of foaling.

Incidentally, the prepuce of the fetus can be seen just caudal to the attachment of the umbilical cord. It can easily be can be seen on ultrasound examination and is valuable in diagnosing the gender of a fetus.

Additional details on equine placental anatomy can be found elsewhere in LORI

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.

Image size: 887 x 574px

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.

Image size: 2592 x 1728px

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.

Image size: 702 x 475px

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

One should not confuse a prolapsed bladder with the chorioallantois.   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

Equine placental gross anatomy

Keywords: equine, mare, placenta, anatomy, yolk sac

The placenta of a normal equine term fetus. A clear plastic tube containing food dye has been inserted into the urachus. Note that the color of the dye is clearly visible through the wall of the umbilical cord. Although the urachus is lined with transitional epithelium (urothelium) that is continuous with the fetal bladder, it is little more than a space between the major vessels that occupy this portion of the cord. Therefore it cannot be isolated as a distinct vessel for surgical ligation when a neonate has a persistent urachus. Only if it is infected, inflamed and thickened will it attain substantial bulk.

Image size: 2093 x 1319px

The three major vessels adjacent to the fetus are the two umbilical arteries and the single umbilical vein. The more distal to the fetus, the more complex the vasculature. Close to the amnion there may be four or more vessels. In this specimen, four vessels can be seen at the point of emergence of the urachus into the allantois. Adjacent to the chorion, there are many more vessels as the arteries spreads out to perfuse the placenta (and the veins collect from their drainage points).

Although the histology of veins and arteries is usually different, this author finds it virtually impossible to discriminate between them in the umbilical cord adjacent to the fetus. Although the pressure gradients between the two systems may be different from those elsewhere, the reason for the similarity in their histology at this site is not obvious.

It is only the intra-amnionic portion of the cord that contains the urachus; the urine enters the cord from the foal's bladder (in place of the syringe in this image) and is expelled into the allantois where the tube emerges from the amnionic part of the cord. In contrast with other species, equids have long umbilical cords. The extra-amnionic cord is short and indistinct in carnivores and completely absent in ruminants. The presence of a extra-amnionic cord in equids allows the amnion to rotate freely within the allantoic cavity. In turn, this allows the equine fetus to change its presentation (anterior-posterior) in the uterus much later in gestation than in ruminants (~ 9 months vs. ~5 months). In other words, the presentation of an equine fetus is not fixed until about 9 months of gestation.

In this placenta one can see a golf-balled sized structure just above the extra-amnion cord in the image.  This a yolk sac remnant, a common finding in equine placentas. A close-up view is provided later in this entry. Sometimes these remnants are quite large and calcified. When opened, they contain dark  brown remnants of the yolk sac fluid. Perhaps obviously, these remnants are never found within the amnion. 

The image below shows the gross components, labeled. 

Image size: 1519 x 698px

Although is is common to refer to the allantoamnion as the amnion, it is incorrect to do so because the "amnion" is actually a double membrane that results from fusion between the outer surface of the amnion and the outer surface of the allantois. The same principle applies to the allantochorion where the outside of the allantois fuses with the chorion.  Despite common use of the term allantochorion, the term allantoamnion is never used. This convention may have arisen from anatomy in humans and sub-human primates where there is no functional allantois and therefore, no allantoamnion.

Interestingly, the terms allantochorion and chorioallantois appear to be interchangeable. In a general Google search (April 2014) the term allantochorion triggered about four times as many hits as chorioallantois. However, in Google Scholar (where far fewer hits for both terms were found) the opposite was true; the hits for chorioallantois far outnumbered hits for allantochorion. Of additional interest was the fact, that the terms were not exclusively confined to journals published in certain countries. For example, both chorioallantois and allantochorion were found in refereed Australian scientific journals. With regard to the words amnionic and amniotic, a Google search shows that the term amniotic is used in preference to the term amnionic at a ration of about 25:1. However, the perennial question must be asked : "Are those in the minority incorrect or are are those in the majority following blindly?"

The yellow ring indicates the entry point of urine (from the urachus) into the allantois. The yellow arrow points to the yolk sac remnant mentioned earlier.

A high-lited area in the image shows the point of attachment of the placenta to the allantochorion. This point of attachment is  normal ; at the base of one of the uterine horns. Occasionally the cord will be attached in the so-called "non-pregnant" horn or the body. Body attachments have been associated with abortion but abortion is not inevitable in all such cases. The terms "pregnant horn"  and "non-pregnant horn" are of course terms of convenience, not accuracy.

A macroscopic view of the micro-cotyledons in the body of this placenta. that characterize equine placentation. Although interdigitation between the maternal and fetal epithelia starts to become established as early as 40 days of gestation,  experience with fresh slaughter plant specimens shows that even at 60 days, the chorion can be separated from the endometrium with very little effort.

Image size: 4198 x 2636px; a large file. This can be viewed at full resolution to appreciate the appearance of the tufts of microcotyledons.  

Although the microcotyledonary tufts form an almost continuous carpet, the areas between each tuft are functionally important too. It is into those spaces that endometrial glands pour their secretions. These are called arcade spaces and they curve (hence the name "arcade" i.e. old English for "arch") towards the fetal side of the placenta, providing area for accumulation and absorption of fetotroph. The arcade spaces are also responsible for iron transport into the fetus via uteroferrin in gland secretions Arcades are also referred to as areola but the term areola is simply derived from the old English word for "area" so there is very little specificity to its use.

Below, see the external view of the chorion on the left, the internal view of the allantois, and the interposing tissue that fuses the two layers.

Image size: 1241 x 479px

As one would expect, the allantois is well vascularized because gaseous, nutritional and waste exchange between the fetus and the dam. In some areas (below) it is particularly well developed.

Image size: 1083 x 634px

A close-up view of the yolk sac remnant in this placenta. The casing of the remnant was firm and probably partially mineralized (as is common).

Image size: 2000 x 1333px

Image size: 2000 x 1885px Copyright Dr Lisa Metcalf. honahlee@imagina.com

This amalgum of images shows a large, multiloculated yolk sack remnant. The remnant was radiographed, revealing its highly mineralized character.

A similar case is shown below.

Image size: 1500 x 1324px 

In this image, the allantochorion has been turned inside-out, so that the bright red chorionic surface is not visible. In many cases this is how the placenta is found after foaling, perhaps because the foal has moved away from the expelled placenta, inverting it. Otherwise, it is expelled from the mare with the bright red chorionic surface still on the outside.

Acknowledgements:  The author wishes to thank Dr Metcalf for her images and Dr Don Schlafer for his editorial comments.

Reference:  Wisher, S. 2009. Review Article: Abnormal umbilical cord attachment sites in the mare: a review illustrated by three case reports. Eq. Vet J. 41:930-939