RISK CATEGORY OF THE FOAL

ASSESSMENT OF FETAL HEALTH AND PREPARATION FOR BIRTH

Care of the foal starts at conception and maximizing the true fertility of a mare involves regular monitoring of the developing fetus and the construction of strategies that take account of problems that develop during gestation and after birth.

Perinatal death accounts for a significant proportion of foal mortality and a significant number of deaths occur during what may appear otherwise to be a normal pregnancy. Noteworthy advances have been made in the achievement of pregnancy and considerable interest has been focused on parturition and the early adaptive period of the foal. Although these events are clearly vital, they actually occupy a trivial proportion of the total gestational length. There is considerable scope for problems during the time that up to now has been largely ignored. Only recently has any significant interest been focused on the gestational development of the foal. With the improved success in fertility and in treating sick neonates, the importance of fetal assessment has also increased and there is now an increasing awareness of the potential benefits in monitoring fetal health. Prepartum abnormalities in the mare are relatively common and cause considerable concern amongst owners; this is justifiable because of the intimate dependence of the foal upon the mare. The placental unit provides a barrier between the circulation of the mare and that of the foal and the foal relies totally on this.

Although there have been improvements in the science of neonatal intensive care, many foals still die. Often this outcome is the more disappointing because the original condition might not have been incurable had it been recognized early and treated appropriately. Pre-emptive measures including attended deliveries and, in turn, early diagnosis of a problem provide the most effective opportunities for success. Early diagnosis of alterations in the state of the placenta and of the foal is essential since delays may seriously endanger the life of both the fetus and the mare. The maxim ‘forewarned is forearmed’ applies to almost all aspects of foal medicine and prevention is obviously better than cure, particularly so because cures are usually difficult to achieve when disease is advanced on presentation.

Disease processes and developmental abnormalities may affect the equine fetus in much the same way as the neonate and adult horse. However, because of its hidden location it is difficult to examine the developing foal completely and to assess its well-being. These limitations also severely limit the therapeutic options. Information gained during antepartum assessment is important for the management of both the mare in late pregnancy and the newborn foal. Information gleaned from regular fetal and maternal clinical assessments provides essential information for the attending staff.

1. ‘Should the birth be attended by a veterinary surgeon and if so is an induced parturition an option?’

2. ‘Would a cesarean section be a likely outcome and if so should the mare be admitted to hospital so that emergency transport is not needed?’

3. ‘Will the foal require intensive care and if so is this likely to need the facilities of a hospital or are the facilities and personnel likely to be able to manage “on farm”?’

PREPARTUM ABNORMALITIES IN THE MARE

Although many pregnancies are uneventful, others have detectable abnormalities that could affect the outcome and will certainly affect the management of the mare.

Apart from the possibility/justification for routine regular pregnancy assessment of both the mare and the developing foal, specific indications for fetal assessment include:

• Vulval/vaginal discharge (Fig. 2.1): this can be a very important clinical sign. The origin of the discharge should be established by careful vaginoscopic examination. Where a purulent discharge originates from the external cervical os a detailed ultrasonographic examination is always required. This may identify a focal (centered around the cervical star) or diffuse placentitis. The foal will be threatened by either state. Even a generalized vaginitis can be a serious problem with a major risk of ascending infection.

Figure 2.1 Vaginal discharge.

• Premature mammary development and premature lactation: this is an important sign that may indicate the presence of placentitis or twins. A detailed clinical and ultrasonographic investigation is indicated.

• Maternal colic: maternal intestinal colic is common in pregnant mares but colic signs may be indistinguishable from those caused by uterine torsion or retroflexion. Impending abortion often causes abdominal pain. This possibility must be considered in all pregnant mares presented for colic investigation.

• Maternal malnutrition and concurrent disease: the nutritional and concurrent health status of the mare will have an inevitable consequence on the well-being of the foal. However, in spite of severe malnutrition it is remarkable how bright a foal can be at birth. Even if the foal is in poor physical condition at birth it can still survive if the mare has a strong maternal instinct and reasonable milk supply. Where the mare has concurrent illness the threat to the foal may be much greater. In some cases total intervention has to be provided if the foal is to have any chance of survival.

• Prolonged gestation: there is a wide variation in the gestational length in mares but inordinately prolonged gestation is a serious event that warrants careful investigation. In some cases this is simply a matter of embryonic diapause where development of the fetus is delayed. The outcome in the latter case may be completely satisfactory and this can be predicted to some extent if a regular thorough fetal and maternal assessment is made. In some cases, however, the prolongation may be due to fetal mummification. The maintenance and termination of pregnancy are not controlled by ovarian activity after 45 days and so, without regular assessment, there may be no indication of an abnormality.

• Prior history of twins, abortion, stillbirth, prematurity, or dysmaturity: the mare’s breeding history may be a significant reason for fetal assessment. Recognition of abnormal fetal development or placental thickness, etc. can be identified and, even though the therapeutic options are limited, extra care can be given to minimize further risks.

MONITORING FETAL HEALTH DURING GESTATION

The value of foals and the importance of gestational health have encouraged the development of in utero monitoring of fetal health. Technological advances have provided the facilities to obtain significant information at all stages of gestation and as more data is accumulated the value of the procedures is becoming clearer.

A full fetal assessment protocol would be as follows:

Mare’s history

General clinical and breeding history

Previous difficulties or abnormal foals could signify that problems are more likely. However, mares may have one episode of problems during a long breeding life. Many have no problems at all whereas others have repeated difficulties.

Many mares show the same traits, for example behavioral patterns, with each pregnancy and these may be characteristic to a particular mare.

Previous pregnancies

Although it is important to know the stage of gestation, there are significant variations in the duration. Many foals will be delivered normally at shorter or longer gestational age whereas some that are ‘at normal term’ may be developmentally abnormal.

The gestational length ranges are:

• pony breeds: 320–345 days

• thoroughbred: 320–360 days

• donkeys: 360–380 days.

The whole period of uterine development may be regarded as preparation for birth, involving fetal and mare changes. ‘Readiness for birth’ infers maturation of the fetus consistent with postbirth life.

Note: The fetus determines the length of gestation in the horse, but as the majority of equine births are at night this suggests that there may also be a maternal influence.

Physical examination

• White flecks are often seen on the teats themselves a few days before the mare ‘waxes up’. Many mares ‘wax up’ 24 hours before birth, a term given to the clotting of a bead of colostrum at the end of the teat (Fig. 2.2). The lack of wax is not indicative that parturition is a long time off, because these plugs may be dislodged, especially in active mares turned out. Similarly, most mares that prematurely deliver or abort will not “wax-up”.

Figure 2.2 A mare ‘waxing up’ before birth.

Mammary secretion

• Color and viscosity:

Typically mammary secretions change from straw-colored water-like secretions becoming cloudier in the weeks preceding parturition, ending up as white-yellowish and viscous (colostrum) close to delivery (Fig. 2.3).

Figure 2.3 Milk changes before delivery. From left to right: –12, –9, –3, 0, +3 hours.

It can be normal for mares to ‘run milk’ (Fig. 2.4) for hours, days or even weeks before parturition.

Figure 2.4 A mare running milk.

If a mare runs milk it may be a good idea to milk her out and collect and store the colostrum.

IgG: use colostrometer or retractometer; ideally the specific gravity should be over 1.060 (equivalent to a protein concentration of over 70 g/L (20%)).

• Electrolytes:

Changes in electrolyte composition of mammary secretions during late pregnancy (Fig. 2.5) have been used to determine fetal readiness for birth.¹

Figure 2.5 Graph of changes in electrolyte composition of mammary secretions during pregnancy.

Immediate prefoaling changes are an increase in calcium (> 10 mmol/L) and potassium (> 35 mmol/L), and a decrease in sodium (< 30 mmol/L).

Field test kits are available.

A scoring system involving all the electrolytes together has been used but has little advantage and requires a more complex assay facility.

Note:

• Calcium concentration is the most reliable test.

• Sodium and potassium are usually not dependable in predicting impending parturition in maiden mares.

• It is also emphasized that the values quoted may provide a useful guide to the imminence of parturition, but do not necessarily relate to the maturity of the fetus.

Abdominal size

As the fetus increases in size, the abdomen expands, becoming large and pendulous in late pregnancy – excessive size in mid/late gestation may indicate twins or hydrops.

Fetal limb movements can be seen in normal mares in the lower flank area from approximately 9 months’ gestation, but especially when close to foaling.

Perineal changes

These changes occur to allow passage of the foal during delivery.

• Relaxation of pelvic ligaments (i.e. sacroiliac and sacrosciatic) and softening either side of tail head several days before parturition. A ‘hollowness’ may start to appear either side of the tail head and, if the area either side of the tail head is felt daily in the last 3 to 4 weeks of pregnancy, it may be possible to detect this change as the muscle tone relaxes.

• Relaxation and lengthening of the vulva several days before birth.

• Relaxation and softening of the cervix shortly before birth.

Vulval discharge

Immediately before delivery there may be a variable mucoid discharge. Rupture of the chorioallantois results in a rush of yellowish fluid. Usually the latter will occur within minutes of delivery.

Rectal examination

Rectal examinations are seldom used as a means of assessing the readiness for birth but the position of the foal and possibly some movement can be felt. Abnormal positioning will mean that a greater degree of monitoring will be required and of course a vaginal examination will be indicated in all such cases.

Fetal imaging

Transrectal ultrasonography (see also p. 379)

• In early gestation, transrectal imaging has revolutionized the detection of pregnancy, identification of multiple conceptions, and determination of fetal sex.

• Throughout pregnancy it allows examination of the placenta, although from mid to late gestation only the cervical pole can be visualized fully. Therefore, detection of ascending placentitis is possible. There are published values for expected placental thickness, etc., throughout the last trimester.²

• The normal combined thickness of the uterus and placenta (CTUP) at the ventral aspect of the uterine body and near the cervical–placental junction is reported to be < 7 mm before 300 days’ gestation.

• Mares at risk of placental failure have CTUPs > 8 mm at 271–300 days and > 10 mm at 301–330 days.

Transabdominal ultrasonography

• Ultrasonography is now used routinely to evaluate the equine fetus.^3,⁴ The fetal thorax is usually the most easily identified structure; the fetal heart can be readily identified contracting in the cranial thorax with the major blood vessels passing caudally, the aorta lying next to the spinal vertebrae. The diaphragm can be recognized separating the thoracic and abdominal cavities and fetal breathing movements may be noted. In the fetal abdomen, the fluid-filled (anechoic) stomach, kidneys and liver are all easily identified.

• In mid and late gestation it allows maximal visualization of the pregnancy (placenta, fetal fluids and fetus), enabling assessment of fetal health and development.

• The ultrasound scanners used for routine broodmare examinations are suitable for fetal imaging; however a 3.0 or 3.5 MHz probe is ideally needed because of its extra penetration. Linear array transducers give the widest field of view with an easily interpretable image, although sector transducers can also be used.

Technique:

Good mare preparation will improve image quality and reduce artifacts.

• The mare’s ventral abdomen should be free of dirt and closely clipped with a fine blade from the sternum to the udder either side of the midline.

• Removal of all grease in the skin by using a soapy wash and a spirit final wipe.

• Generous application of high viscosity ultrasonic coupling gel is important.

• The probe is orientated longitudinally, approximately over the mare’s midline and then the fetus is located by scanning from side to side.

Note: Sedation of the mare should be avoided because the effects on fetal activity and heart rate will invalidate fetal assessment.

Interpretation:

1. Placental assessment:

• Careful evaluation of the placenta is best performed with a 5 or 7.5 MHz transducer. The average combined uteroplacental thickness in late gestation is 1.15–1.26 ± 0.33 cm.^3,⁴

Note: Uteroplacental thickness does not correlate with postpartum placental weight.

• A mare with a placental thickness of greater than 2 cm is likely to be suffering from placentitis (Fig. 2.6).

Figure 2.6 A transabdominal ultrasound scan of a pregnant Irish-cross mare at 310 days gestation showing (A) a normal area of placentation and (B) an area showing marked placentitis as indicated by placental thickness (bracket) and separation (arrow).

Note:

• Absence of a detectably abnormal uteroplacental thickness does not preclude a diagnosis of placentitis, because not all the placenta can be visualized.

• Hyperechoic foci within the placenta have been noted owing to calcification and subsequently related to chronic degenerative changes in the placenta following abortion.

2. Fetal fluids:

• Both the allantoic and amniotic fluid compartments can be imaged. In normal pregnancies of more than 300 days the mean depth of allantoic fluid is 13.4–19 ± 4.4 cm.^3,⁴ The maximal depth of amniotic fluid is 7.9 ± 3.5 cm. Normal amniotic fluid is always ultrasonographically clearer than normal allantoic fluid. Amniotic fluid has significantly fewer echogenic particles;⁴ hyperechoic particles in the amniotic cavity may be associated with the presence of meconium. A single or few larger hyperechoic particles (hippomanae) can be observed in allantoic fluid as early as 36 days prepartum and in all mares at 10 days prepartum. Excessive amounts may indicate the presence of abnormal quantities of blood.