THE PLACENTA

THE NORMAL PLACENTA

The placenta comprises the chorioallantois, the amnion, and the placental vasculature that constitutes the umbilical cord. All of these tissues are of fetal origin, and, although there are very significant effects upon the mare consequent upon failure to expel all or part of the placental membranes, examination of all the placental tissues is vital for the foal. Indeed, it is suggested that examination of the placenta provides as much information about the pregnancy and the foal itself as almost any other procedure. A considerable amount of information can also be gleaned about the status of the endometrium, which might have implications for subsequent pregnancies.

The equine placenta can be defined as diffuse, microcotyledenary and epitheliochorionic in character. The structure of the placenta and its relationship with the maternal endometrium are illustrated in Fig. 9.1.

Fig. 9.1 (A) The macroscopic structure of the mare’s placenta, illustrating microcotyledonary structures. (B) The mature equine placenta, showing the nature of the cotyledons. (C) The vascular arrangement of an equine placental microcotyledon.

This placental arrangement has a number of significant implications:

• Placental transfer of nutrients to the foal is relatively poor; only one foal can be fully supported.

• The entire endometrial surface is required to provide adequate nutrition and gas exchange for a single fetus.

• Twins are therefore always dysmature to some degree at least (see p. 242).

• Areas of scarring on the maternal endometrium do not have the normal structure that allows placental attachment and transfers, and are avillous. The result of endometrial scarring or other avillous areas is the loss of effective transfer capacity. The extent of the deficits is reflected in deficiencies of growth or maturation in the foal. Minor placental inadequacy as a result of endometrial scars or cysts may have no detectable effect on the foal’s well-being, but more extensive ones will have an increasing influence. Thus, a foal born to an affected mare may be less developed than it should be.

• If twin pregnancies are present, the problem of avillous areas of endometrium is relatively minor. The problem here is that the two placentas will abut each other and so the placental surfaces will reflect avillous areas that do not correspond with endometrial damage (Fig. 9.2). This may perhaps explain the significant number of abortions or stillborn foals and the widely held belief that twins are usually disastrous. However, it is also important to realize that a very small percentage (<1%) of twins are born relatively normal and can also grow well and mature into normal adults. More commonly one foal is better than the other and the large majority of twin births result in poorly grown foals that do not adapt well to extrauterine life. A mare that is known to be carrying twins or has previously had twins is therefore classified as high-risk (see p. 252). Proper management of a twin pregnancy at an early stage can reduce the risks of twinning (see p. 242).

Fig. 9.2 Various types of twin placentation. (A) One twin occupies the uterine body and one horn and possesses 70% of the functional surface area. Where the two chorions abut there is usually a degree of invagination of the smaller into the other. (B) The chorion of one twin almost totally excludes the other, occupying the uterine body, one horn and most of the other. (C) Equal division of surface area with no invagination of one chorion by the other. (D) One twin undergoing mummification.

• Placental transfer of large protein molecules is not possible. There is no effective/significant acquisition of immunoglobulins before birth. Consequently, the newborn foal is best regarded as immunologically naive.

• There is little chance for transfer of infective organisms to the foal without concurrent placentitis, and abortion will commonly ensue in such cases. Even minor placental inflammation can result in rapid abortion or significant compromise in the development of the foal. Therefore the health of the pregnant mare is critical for the delivery of a healthy foal at full term. Serious infections carry a significant danger of early termination of the pregnancy.

• Endometrial bleeding, such as is seen in the human hemochorionic placentation, is not possible. Significant blood loss at parturition usually means maternal (cervical, uterine or vaginal) trauma or premature rupture of the umbilical cord. Fresh bleeding after delivery of the foal (and the placenta) is usually a result of uterine, cervical, vaginal or vulvar damage.

• Premature separation of the umbilical cord is a relatively frequent event in mares foaling under disturbed conditions. In this situation, the foal can be deprived of up to 1.5 liters of blood.¹ This can result in considerable problems with metabolic acidosis and failure to adapt quickly to the free-living state.² There is debate about the significance of the early separation of the cord and the importance of the blood remaining in the placenta.³ However, it seems reasonable that a foal will be slower to adapt and may be more susceptible to infection if it is deprived of the maximum amount of blood.

• Any factor that adversely affects the intimate relationship of the placenta with the endometrium has the potential to cause serious problems, such as abortion, fetal reabsorption, deformity, or neonatal infection or compromise.

Abnormalities of the placenta are extremely important and allow the attending clinician to make a rational and often life-saving assessment of the viability of the foal before any clinical abnormality becomes apparent.

Note:

• Examination of the placenta (fetal membranes) is a vital part of the assessment of the newborn foal. It is important to recognize the normal placenta and the normal variations that have no significance on fetal viability. The clinical appearance of the placenta changes quickly after delivery and these normal changes may easily be over-interpreted.

• A careful placental examination should be used to investigate failed pregnancies and abortions and may in fact be more useful than examination of many of the other fetal tissues.

• Recent research suggests that placentitis is a common feature with mares showing (heavy) prepartum lactation.

The problems associated with placental retention in the mare are described on p. 313.

EXAMINATION OF THE PLACENTA

• Early examination of the placenta provides the clinician with valuable and potentially life-saving information for the foal (as well as the mare). It should never be ignored in any foaling.

The stud manager/owner should place the unwashed placenta carefully into a plastic bag immediately it is passed, and store it in a cool place (not a freezer) until examination. The placenta should be examined within 12–24 hours; usually this will conveniently coincide with the primary (2–6-hour) examination or the second routine examination of the foal (at 24 hours).

The stud manager or groom should be asked to write down the findings as the clinician dictates them. It is unwise to rely on memory. An examination form can be useful in ensuring that nothing is omitted and that the results are recorded correctly. (An example of a suitable form is shown in Fig. 9.25 at the end of this chapter.) In an increasingly litigious society, a meticulous record of events should be kept.

Fig. 9.25 Placental examination form.

Step 1: Prepare by appropriate dress with protective coveralls, gloves, and boots

i. Remember that the placenta may carry infective organisms.

• Do not be responsible for the transmission of infective agents from stud to stud.

ii. Leave protective boots and overalls on the stud to be washed.

iii. Perform normal hygiene precautions (hand-washing and changing of overalls, etc.) before and after examining the placenta.

iv. Always examine the healthy prepartum mares first, then the foals and other mares, and finally the placenta.

Step 2: Prepare all equipment required

• Disposable gloves/apron/mask.

• Scissors.

• Scalpel handle and blade (preferably disposable).

• Sterile swabs with bacterial and viral transport medium.

• Sampling bottles:

10% formal saline.

Michelle’s medium.

Plain (Carnoy’s) tubes.

• Disinfectant for washing up afterwards.

• Notepad and paper; record findings as they are identified (it is often useful to dictate to the stud groom as the procedure is performed or use a tape recorder). A suitable protocol is shown at the end of this chapter (pp. 340–342).

• A refrigerator and freezer are also useful, but potentially infected specimens must never be stored in a freezer or fridge that also contains other material such as colostrum or plasma.

• If there is a possibility that viral agents are involved, a pack of dry ice is often useful as many specimens deteriorate markedly unless cooled quickly.

Step 3: Weigh the placenta

It is useful to encourage stud managers to weigh the placenta and record the weight routinely before storing it prior to examination. This limits errors arising from drying, predation, contamination, etc.

• The normal fresh (wet) weight is about 10–11% of the foal’s body weight ⁴ (e.g. the placenta of a foal born to a multiparous Thoroughbred mare should weigh 4.8–5.5 kg).

• A placenta that is heavier than expected may signify edema or inflammatory changes, or the presence of an abnormal amount of fetal blood (suggestive of possible early cord separation).

• A placenta that is lighter than normal is usually immature, incomplete or has significant avillous areas.

Step 4: Examine the placenta

It is essential to examine the placenta thoroughly from both sides (chorionic and allantoic) and to examine the amnion and the cord. Failure to do this could result in failure to recognize vital clues that might impact on the foal and/or the mare.⁵

i. Place the placenta on a clean dry surface (e.g. concrete floor) well away from any area where other horses might make contact with it (it might carry infectious agents).

ii. Lay the placenta out in the shape of an ‘F’, the way in which it was delivered (see Fig. 9.3). The smooth gray, shiny allantoic surface of the allantochorion is exposed (this is the inside of the chorioallantois; it should not be confused with the thinner, tough, smaller amnion, which should be identifiable as a separate entity).