Premature lactation

Keywords: lactation, abortion, premature, equine

In humans and primates, a common sign of impending abortion is vaginal hemorrhage. In carnivores, it is a greenish (canids) or brownish (felids) vaginal discharge. In ruminants, there may be a mucous discharge or vulvar swelling.  By contrast, in a mare that is about to abort, there is little or no vulva swelling and no vaginal discharge. Owners become alarmed when blood is seen to drip from the vulva lips of a heavily pregnant mare but this is not a harbinger of abortion. Instead, the blood is probably originating from varicose veins in the vestibule.

The first sign of impending abortion is mares is premature lactation. 

When an owner first notices a sudden increase in the size of a mare's mammary gland, it may only be possible to express small volumes of milk from the teats as shown in this image. In other cases, milk may stream from the teats, drying on the inner surface of the hind limbs. 

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The nature of the mammary secretion probably differs according to the stage of gestation at which abortion occurs but the presence of casein in the fluid appears to be constant, giving it the appearance of milk. If abortion occurs in the first half of gestation, premature lactation is unlikely.

The author considers premature lactation to be the production of a milk-like secretion before the number of days at after which most newborn foals can survive i.e. about 290 days of gestation. This being said, one also has to consider the fact that mares have gestation lengths that are highly variable; varying as much as two months, even within a single breed. Normal gestation may be as short as 290 days or as long as as 419 days within this species. However, significant positive correlations have been demonstrated between the durations of sequential pregnancies within individual mares. Therefore, for example, the onset of lactation may be be significantly early than usual if it occurs at 310 days in a mare that usually foals after 350 days. It may be a harbinger of abortion or premature birth in that mare but not in other mares. 

In most cases there is no uncertainty as to whether lactation is premature or not. Indeed, lactation before 290 days should always be viewed with alarm. Even if there is no abortion in some of these cases, there is loss of colostrum and failure of passive transfer is likely.

It is impossible to predict when a mare will abort after premature lactation has been first been noticed. Sometimes abortion will follow in a matter of days, in other cases, the mare may lactate for several weeks before aborting. 

In all cases of premature lactation, one should examine placental thickness by transrectal ultrasonography (CTUP) to determine if placentitis is present. To prepare owners for almost inevitable abortion in a mare, one should also attempt to detect twins by transabdominal ultrasonography.

References:

Coutinho da Silva M.A. et al 2013 Serum amyloid A concentration in healthy periparturient mares and mares with ascending placentitis. Equine Vet J. 45:619-24.

Davies Morel, M.C.G et al 2002  Factors affecting gestation length in the Thoroughbred mare. Anim. Reprod. Sci. 74: 175–185

Takahiro Aoki, T. et al. 2013 Factors Affecting Gestation Length in Heavy Draft Mares. J. Equine Vet. Sci. 33:437-440

Valera, M. et al. 2006. Genetic study of gestation length in Andalusian and Arabian mares. Anim. Reprod. Sci. 95:75–96

Walkowicz, E. et al. 2014. The effect of selected factors on the length of gestation period in Silesian mares. Anim. Sci. Papers and Reports 32: 55-64

Immature (dysmature or premature) Jennet

Keywords: donkey, fetus, premature, equine

Jennys (also spelled "Jennet" from French, but pronounced Jenny) have gestations that are somewhat longer than those in mares. As in mares, gestation length is highly variable but in donkeys it can be as long as 14 months.

This donkey was born dead, in a slightly immature state.

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If the gestational age can be shown to be significantly shorter than the mean for the species, the condition is correctly referred to as "prematurity". However, if the gestation length is consistent for that species yet the neonate still appears to be immature, the correct term for the condition is "dysmaturity". Because the breeding date was not available it was not possible to determine if this fetus was  dysmature.

The signs of immaturity in this foal are indicated by arrows or circles and include its soft, floppy ears, a downy hair coat and its flaccid joints in the distal extremities, allowing hyperextension at the fetlock joints. In equids, the testicles have usually descended by birth. In this cases however, the testicles were still within the inguinal canal, the scrotum being filled predominantly by the gubernaculum.

Note: the image also shows the amnion with both intra- and extra-amnionic sections of the umbilical cord

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.  

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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.

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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