Page 460 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Reproductive system: 2.1 The female reproductive tr act 435

VetBooks.ir tubercle, develops on the body surface of the cease to function at around day 120. Blood concen-
trations of eCG, often measured by ELISA tests, are
fetus, between the hindlimbs, after 60 days and
is identifiable via transrectal ultrasonography as
up to day 120. False negatives will be given if sam-
a highly echogenic, bilobed structure resembling useful between days 45 and 90 and in some mares
an equal sign (=). Over the next 5 days the ples are taken outside these time limits or with mule
tubercle migrates caudally in the female to a fetuses. False positives can occur with embryonic
position under the tail, while in the male it moves death after day 35.
cranially onto the abdominal wall behind the
umbilicus. Diagnosis requires a scanner of 5–7.5 Oestrogen
MHz frequency with good resolution at 12 cm ELISA tests are available for determining the con-
depth. Considerable experience and practice are centration of oestrogen in urine, blood or faeces
needed to become accurate in this technique. in pregnant mares after day 60 and up until full
Abnormalities of the fetus and pregnancy can term. Peak conjugated oestrone sulphate concen-
also be identified (e.g. lack of fetal heart beat, trations increase until days 180–240 of pregnancy
cloudy echogenicity or reduction of fetal fluids (peak days 150–160) and slowly decline thereafter.
and, rarely, morphological abnormalities). Oestrogen levels are a good indicator of fetal viabil-
• Day 180 onwards. Transabdominal ity, as the fetus and placenta both contribute to their
ultrasonographic examination is possible at this production.
stage and this allows assessment of the fetus and
its membranes and fluids. TECHNIQUES FOR ASSESSING
FETOPLACENTAL HEALTH
Hormonal tests
Indirect testing of pregnancy can be undertaken by In the well-managed pregnant mare there is inten-
measuring a variety of hormones in the mare’s blood sive monitoring during early pregnancy to assess the
or milk. Unfortunately, they can be unreliable if development of the singleton embryo/fetus. This
used on their own. continues until day 45 when, unless fetal sexing is
performed at 65 days, the mare is usually left to nego-
Progesterone tiate the rest of her pregnancy without any veteri-
Progesterone levels can be measured in blood (or nary interference. It is only if a problem is suspected
milk), and in the stud farm or practice laboratory or detected by the owner that a mare will present
situation they are often determined using an ELISA during mid to late gestation. Such a problem may
kit. Pregnant mares 17–24 days post ovulation should involve mare illness, with a concern for the effect
have progesterone levels >6.36 nmol/l (2 ng/ml) and on the fetus, or it may involve signs of premature
often as high as 12.7–31.8 nmol/l (4–10 ng/ml), but parturition suggestive of placental dysfunction or
considerable individual variation exists. A retained pregnancy failure. Mares are classed as low or high
CL or early embryonic death will also lead to ele- risk depending on their previous breeding history.
vated (false-positive) levels. Concentrations con- The well-being of the fetus is of great importance
tinue to rise after day 21 and remain elevated up for welfare, emotional and financial reasons and can
until days 150–200 before declining, sometimes to pose a diagnostic challenge to the attending veteri-
<6.36 nmol/l (2 ng/ml). nary surgeon. Identifying the ‘high-risk’ pregnant
mare is essential and may include mares with:
Equine chorionic gonadotropin
Following the invasion of the fetal trophoblast cells • History of abnormal pregnancy previously or
into the maternal endometrium and the formation during this gestation.
of the endometrial cups, detectable levels of eCG • Premature mammary development and lactation.
are produced at around days 35–38 post ovulation. • History of previous production of premature,
Maternal rejection starts at day 60 and the cup cells dysmature or septic foals.