The
placenta: The transfer of oxygen, carbon dioxide, nutrients, and waste products
to and from the embryo is performed by the placenta. The placenta includes sites of intimate union of the chorion with
the maternal uterine epithelium. These sites of intimate contact are known
as placentomes.
There is no actual mixing of maternal and fetal
blood, but all of the nutrients, waste products and gases must diffuse through
the placenta. The number of layers of tissue in the placenta of various species
is shown in the adjacent Figure. The single layer of tissue between the
maternal and fetal vascular systems in rodents makes for a much more efficient
transport than is present in the larger domestic mammals (such as the horse and
cattle). The different types of placentas (based on the distribution of the
sites of exchange), are shown. In species with fewer layers of tissue in the
placenta, the area of attachment between the foetus and the uterus is usually
less.
Development of the placenta
in the horse is a slow process that takes about 150 days to complete.
Initially foetal fluid pressure holds the chorion, or outermost membrane
adjacent to the endometrium.
This begins at about 20
days after conception and is called fixation.
Later,
at about day 40, there is an attachment formed between finger-like projections,
or microvilli, between the chorion
and the surface of the endometrium. At about day 45, these villi form over the
entire surface of the chorion. It is only by day 150 that these villi have
formed all over the entire placental surface. This type of attachment in the
horse is called epitheliochorial
because the chorion of the foetus is in contact with the epithelium of the
mare’s uterus. Because the villi are scattered all over the surface of the
chorion, the attachment is described as diffuse. So the mare’s placental
attachment is more correctly described as diffuse
epitheliochorial. This diffuse type of attachment is shown diagrammatically
in the Figure. This contrasts with the more localized forms of contact in other
species such as the cow, bitch or human.
Foetal and Placental membranes in the Horse
The
placenta of the mare consists of foetal and maternal tissues that are in apposition
for purposes of physiological exchange.
Development of the Placenta in the Horse
The
blastocyst becomes converted into a two-walled (bilaminar) yolk sac from Day 10
onwards. The yolk sac does not contain
stored food material as in birds, but does convey nutritive material, to the
developing embryo.
·
By Day 14, the embryo has formed a globular-shaped blastocyst 1.3 cm in diameter.
The
transition between the yolk sac and a predominantly allantoic sac occurs at the
same time as the embryo proper develops.
The allantois, which will eventually assume the entire role of
physiological exchange emerges from the developing hind gut of the embryo
proper, at approximately Day 22. The
allantoic sac gradually becomes dominant over the yolk sac.
·
By Day 40 it has converted the yolk sac to a true
chorio-allantoic placenta. The front of
the chorio-allantois forms a band which advances across the surface of the
blastocyst and produces a specialized
girdle of cells on the outer surface of the placenta.
·
Close to Day 37 this band of cells separate and
rapidly invade the adjacent maternal endometrium to form the endometrial cups. These cells produce
the hormone eCG (PMSG).
The
placenta of the mare is of the diffuse
type with superficial villous attachment.
The term 'diffuse' implies that the placenta is attached throughout the
whole uterus except at the cervix
and at the end of the uterine horns.
The placenta is classified as epitheliochorial
since the uterine epithelium is in contact with the outer layer of the chorion.
This non-invasive type of placentation ensures that there is no loss of
maternal tissue at parturition.
The
placental membranes in the mare are the allantochorion and the amnion. The allantois emerges from the hind gut
at Day 22. By Day 25 the allantoic sac
is vascularised (has blood vessels) and quite large compared to the embryo
itself The allantois gradually forms a union with the chorion to form the
allantochorion or chorio-allantoic membrane.
The chorion comprises the outer surface of the allantochorion and is
covered with microvilli. The well
vascularised villi give the outer chorionic surface of the allantochorion a
red, velvety appearance. The inner surface
of the allantochorion is shiny and transparent. The larger veins and arteries which come from the umbilical
vessels can be seen through it.
Allantoic fluid
Allantoic
fluid is formed from the chorio-allantoic membrane and it receives urinary
fluid through the urachus.
During gestation there is a steady increase in total nitrogen and in
sodium, potassium, calcium, magnesium and phosphate ions. The volume increases steadily over the
course of gestation. While individuals and breeds vary, the following are good
approximate volumes:
·
from about 110 ml to 1300 ml between Days 45 and 60,
·
to 3000 ml by Day 128,
·
to 8500 ml by Day 310
The
allantoic fluid changes from a clear yellow in the first trimester to a turbid
brown or yellowish-brown by mid-term.
The fluid contains desquamated epithelium (shed epithelial cells), small
chorio-allantoic peduncles and a single hippomane.
The amnion,
which is formed by fusion of the allantois and amnion proper (this membrane
would more correctly be called the allantoamnion). It is an opaque white
membrane.
The
amnion is the innermost membrane in most intimate contact with the embryo
proper. The amniotic membrane separates the allantoic and amniotic cavities and
protects the embryo from the
urine-like wastes in the allantoic fluid.
It is an opaque white membrane containing many blood vessels. The amnion and allantochorion are completely
separate from each other and are only
attached indirectly via the umbilical cord.
Amnion
The
amnion develops about Day 17 and is complete by Day 21. It is closely allied to the embryo up to Day
56 and becomes separated as the quantity of amniotic fluid increases. At full
term the amnion weighs 2.4 kg (range 0.5-4 kg) and consists of a fine
transparent membrane interlaced by a network of blood vessels.
Amniotic fluid
The
electrolyte and salt content of amniotic fluids shows very little change during
gestation but there appear to be substantial changes in hormone levels and surfactant[1]
content. Its circulation and source
have not been established but its composition is approximately that of a serum
but without the larger proteins.
Amniotic
fluid is straw-coloured, slightly mucoid and sweet-smelling with a mean pH of
6.928 (range 6.75-7.05). The fluid
normally contains desquamated skin cells and some white blood cells. Volume
increases from about 400 mls in the first trimester to 3.5 litres at full
term.
|
The volume of amniotic
fluid and allantoic fluid at term in horses is 3 to 5 and 8 to 15 litres
respectively. |
Hippomane
Hippomanes
are soft putty-like aggregates of urinary calculus (deposits or stones) which
form throughout pregnancy and are present in all placentas in the allantoic cavity. Fragments can sometimes be found in the
urachus. They vary in colour and size
and have a layered appearance when cut. Occasionally there are accessory small
hippomanes either free in the fluid or attached to the chorio-allantoic
membrane.
The
hippomane is about 14 x 1.5 cm and contains high concentrations of nitrogen,
calcium, phosphorus, sodium, potassium and magnesium. The hippomane occurs singly
in the allantoic fluid and is passed during or after second-stage delivery
(this is the stage of delivery in which the foetus is expelled).
The
hippomane is first found in the allantoic fluid at about Day 85. The only
contribution from the foetal membranes is desquamated epithelium which provides
a nucleus of tissue debris for the subsequent formation of a soft allantoic
calculus[2].
The Umbilical Cord
The
umbilical cord crosses the allantoic cavity and consists of two portions
(amniotic and allantoic) which can be readily distinguished.
·
The allantoic portion of the cord contains the
prominent umbilical vessels (two arteries and two veins). The two major
arteries within the allantoic portion of the cord diverge so that one primarily
supplies the membranes in the gravid (pregnant) horn and cranial portion
of the uterine body, while the other supplies the membranes in the non-gravid
(non-pregnant) horn and remainder of the body.
The veins tend to accompany the arteries.
·
The umbilical cord attaches to the original
implantation site on the dorsal wall of the uterus.
·
At the level of the amnion, the two veins join to form
one vessel in the amniotic cavity. The
amniotic portion of the cord also contains the urachus. The urachus is a canal which connects the
fetal bladder to the allantoic cavity and serves to conduct fetal urine.
The
amnion and chorio-allantoic membranes are completely separate from each other,
and are only attached indirectly via the umbilical cord.
•Twisting
of the umbilical cord and/or dilations of the urachus are often associated with
abortion.
It
is strongly suspected that increased
cord length is associated with bad consequences for the foetus by:
(a) strangulation
causing death of the foetus between Days 212 and 287 or
(b) excessive
torsion resulting in urachal obstruction and bladder distension.
Excessive
twisting of the umbilical cord may lead to the obstruction of the blood vessels
followed by foetal death. In a series of 143 normal Thoroughbred foals the mean
umbilical cord length was 55 cm (95% of values were 36-83 cm).
Fetal growth
Day 30 most of the organs are
present in rudimentary form and the eyes, mouth and limb buds are visible.
Day 40 eyelids and auditory pinnae
have appeared.
Day 60 the lips, nostrils and feet
are developing
Day 90 hooves and mammae are
present. Day 120: The external genitalia, ergots and orbital areas are
visible and
Day 180, fine hair starts to appear,
on lips, orbital arch, nose and eyelids.
Day 240: Hair covering spreads
to include mane, tail, back and distal parts of extremities
Day 270 short, fine hairs cover the
entire body.
Day 320 the coat is complete and the
testes have descended through the inguinal ring.
Placenta and Nutrient Exchange
The
surface of the placenta is smooth in
early pregnancy but by Day 70 it becomes velvety
due to the development of minute villi.
Small
tufts of chorionic villi project into corresponding invaginations of the
endometrium to form small globular structures called microcotyledons.
Uterine
arteries enter the endometrium and pass to the deep surface of the endometrial
epithelium where they divide into branches which give rise to dense capillary
networks in the wall of the maternal crypts. The placental villi are supplied
by branches of the umbilical arteries and veins.
|
How Big is the Placenta in the Mare? At full term the
placenta weighs about 4 kg (range 1.5-8 kg); its surface area approximates
14000 cm2 (range 4500-18, 500 cm2) and it is 1 mm thick
(range 0.45-2.7 mm). The foal's birth-weight is directly proportional to
placental surface area, but no
correlation exists with weight or thickness of the membrane. |
Body Weight
Day 30-60: Body weight increases
100-fold from 0.2 g at Day 30 to 20 g at Day 60;
Day 60-120: 50-fold 20 to 2000 g
Day 120-180: five-fold 1 to 5 kg;
Day 180-240: three-fold 5 to 15
kg)
Day 240-300: two and a half times
15 to 37 kg
Day 300-Full Term: and
about one and a half times to full term at 340 Days (37 to 54 kg). The weight of the foetus varies according to
breed and normality of placental function.
Crown Rump Length Measurement of Prenatal
Growth
The
most widely used measurement of embryonic and foetal growth is called the
crown-rump or CR length. This measurement is a straight line taken from the
crown of the head to the base of the tail. The crown is the point midway
between the eye orbits. Growth in weight is usually more variable than CR
length, so CR length provides a more reliable estimate than weight when
attempting to estimate the age of a foetus.
Crown-to-rump (C/R) length increases
from:
Day 30: 1 cm
Day 60 about 7 cm
Day
120 about 20 cm
Day
180: 60 cm,
Day
240: 80 cm
Day
300: 120 cm
Day
340: 150 cm
Head to Tail Development
Embryonic
growth and development proceed in an anterior to posterior sequence (from head
to tail) so that the head reaches a relatively large size early in development.
During foetal development, the remainder of the body catches up to, and
overtakes the growth of the head. The head to body ratio, however, is rather
variable between animals. The feet and the tail develop last. This pattern of
differential growth is called allometric growth (change in body proportions).
UTERINE CHANGES
During
gestation the uterus grows to accommodate the developing foetus. The mechanisms which allow for the enormous
increase in size are unknown but are probably hormonal. Throughout most of pregnancy the connective
tissue resists stretch to an extent that permits slow distension of the body of
the uterus, where the proportion of connective tissue is low, but maintains
closure of the cervix, where the proportion is high. This property of uterine connective tissue is derived from
tightly packed bundles of collagen fibres. Changes in the matrix of the cervix
render it capable of dilating under endocrine control to allow passage of the
fetus at full term.
Another
aspect not yet fully explained is the uterine tolerance to the presence of
foetal tissue, tissue which is essentially foreign to the maternal
immunological system. The lack of
reaction to the placenta contrasts with the response to foetal cells which
invade the endometrium at the 'cups' and which are eventually rejected by the
maternal tissues.
Endometrial Cups
Endometrial cups are formed in the
late part of the first month of pregnancy.
They are a unique feature of the equine placenta. On or about day 28-36 of gestation, the
chorionic girdle begins to form at the junction of the yolk sac and allantoic
membranes. Specialised cells from the chorionic girdle invade the underlying
uterine epithelium. Once in the
endometrium, they enlarge and become clumped together to form the endometrial
cups. These are specialized areas which
develop at predetermined sites around the base of the pregnant horn and which first become apparent
macroscopically (are visible) about Day 40.
Ř
Endometrial cups are irregular in shape and vary
tremendously in size from 1 cm in diameter to as long as 5-10 cm. There is often a honey-like material present
in the depression of the cups. This
material is a mixture of debris and secretion of the endometrial glands and
cups.
Ř
The cups are arranged in a circle at the base of the
gravid (pregnant) uterine horn.
Ř
Endometrial cups produce eCG, which stimulates the
primary CL and causes induced secondary follicles to ovulate and/or
luteinise. Because progesterone
production remains high, mares do not show signs of oestrus at the time of
these secondary ovulations.
Ř
eCG production is independent of the presence of a
foetus and so once the endometrial cups are formed, eCG production will proceed
even in cases in which embryonic/foetal loss occurs.
Ř
A maternal immunological response ultimately rejects
the 'foreign' foetal tissue. This rejection starts at day 90 and is usually
complete by day 140. So eCG levels usually begin to decline at day 90 and are
not-detectable by day 140. The circle
formed by the cups at the base of the pregnant horn regresses during the rest
of pregnancy and the cups are not visible in the barren uterus; scarring may
remain on the placenta for some months after the cups have ceased to function.
Ř
By about day 140 sloughing of the cups is usually
complete. Normal cycling activity will not start again until sloughing is
complete.
Ř
If abortion occurs once the cups have become
established, i.e. after about 42 days gestation, the secretion of eCG continues
in the absence of the conceptus. In
these circumstances eCG may remain detectable in the blood up to 140 days after
conception. This means that in cases of embryonic loss after formation of the
endometrial cups, the delay in normal cycling being resumed usually results in
a decision not to mate the mare that season which could represent a serious
economic loss. Current thinking is that
the chances are very small that mares producing eCG can become pregnant. No practical therapy can shorten the
lifetime of the endometrial cups. Surgical removal including video-endoscopic
laser has been documented, but is not applicable in the clinical
situation. Neither is there a
satisfactory method for reducing levels of eCG (even though luteolysis can be
induced by repeated (3 - 5) daily injection of prostaglandin F2-alpha). ln
non-cycling mares after suspected embryonic loss, determination of blood eCG
levels may be advisable to confirm the existence of active endometrial cups.
Ř
As a result of the presence of eCG in the circulation
of pregnant mares, there is intense ovarian activity between Days 25 and 140
1. Days 1-40.
The ovaries have a CL and follicles of variable size.
2. Days 40-150. There is progressive activity including formation of follicles
and ovulation.
3. Day 150 to an undetermined date. There is a decrease in lutein tissue and
absence of follicles.
4. The period towards the end of
pregnancy. No CL is present. These changes are part of the
endocrinological mechanisms which maintain pregnancy.
Maternal recognition of pregnancy must
take place about day 16 to 17, or the uterus will produce prostaglandin F2a
which will regress the corpus luteum. If the corpus luteum regresses,
circulating progesterone levels decline and the embryo cannot implant. It is
now believed that embryo produces a protein or hormone-like substance which
signals the uterus of its presence and thereby either blocks secretion of
uterine prostaglandin F2a or renders the corpus luteum insensitive to it’s luteolytic
action. Significant embryo losses can occur around the time of maternal
recognition of pregnancy due to either failure of the embryo to produce the
signal or failure of the mother to recognise the signal from the embryo. lf the
embryo dies, or if the mare fails to recognize a pregnancy, the corpus luteum
will regress normally and the mare will return to oestrus at about 21 days
after mating.
Maternal
recognition of pregnancy is essentially the process whereby luteolysis is
prevented by the presence of the conceptus.
This prolongs progesterone secretion for the maintenance of
pregnancy. The mechanism by which the
equine conceptus signals its presence to the mare to prevent destruction of the
primary corpus luteum is unknown. The
primary corpus luteum of pregnancy can be visibly identified in the ovary for
up to 180-220 days. However, it is not known how much progesterone this primary
corpus luteum produces. We do know
however that the primary CL, alone, is unable to sustain pregnancy. The primary
CL regresses at approximately the same stage of gestation as the supplementary
CLs.
The
first step in the maternal recognition of pregnancy signalling mechanism must
involve an interaction with the endometrium.
Endometrial secretions are likely to be important in the maintenance of
early pregnancy in the mare. This is
supported by the finding that the mare has an unusually rich supply of tightly
coiled exocrine secretory glands in her endometrium which produce large amounts
of a protein-rich secretion during the luteal phase of the oestrous cycle and
early pregnancy. The constituents of
this early secretion have not been well characterised in the mare.
Movement
of the embryonic vesicle throughout the uterus is thought to be
important for the conceptus to signal to the dam that pregnancy has occurred,
thereby preventing luteolysis.
The critical time for
maternal recognition of pregnancy, in order to prevent luteolysis of the
primary CL and subsequent loss of the pregnancy, is thought to be 14 - 16 days
after ovulation in the mare. Conditions
which prevent the conceptus from migrating throughout the uterus (e.g. blocked
uterine horn) interfere with maternal recognition of pregnancy, resulting in
failure to prevent endometrial production and release of PGF2-alpha, and the
mare will return to oestrus in spite of conceiving.
Implantation
While
the embryo is undergoing cleavage and blastocyst formation, the uterus is also
undergoing changes which prepare the way for implantation. The embryo is said
to be implanted when it becomes
fixed in position and physical contact
with the mother is established. In the mare, the embryo remains in the uterine
cavity and whatever attachment it forms with the wall of the uterus before the
formation of the placenta is of an extremely loose nature.
Progesterone,
secreted by the corpus luteum of the ovary, acts to decrease muscular activity
of the uterus. In addition, progesterone increases blood supply to the uterus
and stimulates proliferation of the uterine epithelium, and an increase in
uterine milk secretions.
Changes occur in the female's
reproductive tract as pregnancy progresses. The foetal and maternal hormone
systems interact throughout pregnancy such that pregnancy not only is
maintained but that continued development and growth of the foetus is assured.
During pregnancy, cervical
secretions increase to produce a very viscid mucus, serving to seal the
cervical canal by the so-called mucous
plug of pregnancy. Before parturition, this seal breaks down and is
discharged. During pregnancy, the external opening of the cervix remains
tightly closed. A few days before the onset of labour, relaxin is released by the ovary and,
in conjunction with increasing oestrogen levels, acts to relax the cervix and
pelvis.
As pregnancy progresses, the uterus undergoes gradual enlargement to permit expansion of the foetus, but its muscular walls remain quiet to prevent premature expulsion. Three phases can be identified in the adaptation of the uterus to accommodate pregnancy:
·
proliferation, growth and stretching.
Uterine
proliferation occurs before blastocyst attachment and is promoted by
high progesterone levels. Characteristic changes of the inside lining of the
uterus initiated by progesterone are increased blood supply, growth and coiling
of the uterine glands, and leucocyte infiltration.
Uterine
growth starts after implantation. Uterine growth includes muscular
hypertrophy and an extensive increase in connective tissue. Modification of the
connective tissue is important both during uterine adaptation to the growing
foetus and during involution after foaling. The structural changes which take
place in the pregnant uterus are reversible but are restored at different rates
after parturition.
During
the period of uterine stretching (the last trimester), uterine growth
diminishes while its contents are growing at an accelerating rate.
Definitions
Embryonic Loss: Failure of pregnancy from fertilisation to 40 days.
Foetus: That part of the conceptus that gives rise to the foal, from Day 40 to
birth (end of second stage labour).
Abortion: Expulsion
of the foetus and its membranes before 300 days.
Stillbirth: Expulsion of the foetus and membranes from 300 days onwards.
Pregnancy loss: = Mares diagnosed Pregnant - Mares foaling Divided by Mares diagnosed pregnant.
Early Gestational Failure
Introduction
The
fertilisation rate is high in both the fertile (>90%) and subfertile mare
(80-90%) indicating that fertilization tends to be normal in both of these mare
classes. However, early embryonic death
(EED) in subfertile mares following fertilisation is much higher than fertile
mares (Table 1) indicating that early embryonic loss appears to be the primary
area of concern.
Early embryonic death (EED) in mares is
commonly divided into two classes based on the veterinarians ability to
diagnose pregnancy.
The
first class is early in gestation (<10-14 days), prior to the clinicians
ability to detect pregnancy with the ultrasound. This time period appears to be when subfertile mares lose the
majority (60-70%) of their pregnancies and also coincides with when the embryo
resides in the oviduct prior to its movement to the uterus at day 5-6
post-ovulation.
Mare Type EED<14 EED>14
Fertile 9% 3%
Subfertile 60-70% 3-5%
While this
tends to suggest that both the egg and sperm are competent enough to go through
the fertilisation process it does not rule-out defective integrity of either
the sperm or egg such that fertilisation can proceed, but death occurs at a
later date due to a defect in gene transcription contributed by either the male
or female gamete.
There
are many factors that contribute to the development and maintenance of the
early embryo:
Progesterone is critical for embryo
survival. The primary source early in
gestation is the corpus luteum (CL) that resulted from ovulation. Therefore, anything that could compromise
progesterone levels or the life span of the CL could result in the demise of
the embryo. Reduction in progesterone
levels could be caused by several factors.
Failure of maternal recognition is a defect in the
embryo causing failure of the mare to
a. recognise the pregnancy
or a failure of the mare to recognise the presence of a normal embryo.
b. Primary luteal insufficiency
c. Uterine induced luteolysis resulting in the release of PGF2a from the endometrium would
cause lysis of the primary CL.
d. Disease
induced release of PGF2 a is caused by a primary disease process or a stress, such as colic or a
surgery, sufficient to release enough PGF to lyse the primary CL.
All of these factors have the potential to cause lowered progesterone
levels resulting in pregnancy loss.
Clinically, it is difficult to diagnose "low" progesterone
levels because of the variability in laboratory results and the fact that there
is no exact level at which mares will lose the pregnancy. Some mares with "lower" levels
maintain pregnancies while other mares with "normal" levels lose
their pregnancies. Therefore, it is
difficult to determine when to administer progesterone.
2. Oestrogen levels
play a role in foetal viability. The
embryo secretes oestrogen as early as 6-12 days of gestation. This may contribute to early pregnancy
recognition, therefore, if an embryo is unable to secrete enough oestrogen. the
pregnancy may be terminated. Oestrogen
levels rise to detectable levels at 50-60 days of -gestation and can be used to
assess fetal viability at that time.
Seasonal
effects on oestrogen levels due to pasture conditions may influence tone and
degree of relaxation of the pregnant cervix.
It has been suggested that when some pastures are very lush they may
also have increased levels of oestrogen.
When mares are exposed to these pastures they have the potential to
cause early embryonic losses because of premature cervical relaxation. This can be manifest by an unexplained
decrease in fertility during the breeding season.
3. Oviductal pathology is probably more common than is recognised
because of its small size and clinical inaccessibility. Salpingitis (inflammation of the oviduct)
has been recognised on necropsy samples.
4. Uterine Condition - Once the embryo enters the uterus, the
uterine environment plays an important role in its maintenance, especially
prior to the initiation of placental formation at day 40 of gestation. Clinically, the uterus is more accessible
for diagnostic purposes than is the oviduct.
This allows for the use of the endometrial biopsy and culture as well as
cytologic techniques to determine the status of the uterine environment.
Culture of the endometrium can determine the presence of bacteria and
fungi. Normally, the uterine
environment should be sterile except immediately following breeding, when large
numbers of bacteria are introduced into the uterus. The normal mare is able to clear those contaminants in a
relatively short period of time while the " susceptible" mare is
not. If this inflammation persists,
especially up until the time when the embryo passes into the uterus, it will
have a direct effect on the health of the embryo. More older mares tend to fall into the susceptible category.
Increased age is also associated with an increased incidence of
periglandular fibrosis (PGF) (a symptom of endometriosis) around the
endometrial glands. This increased
incidence of PGF may compromise the ability of the glands to function and
support the embryo early in gestation as well as maintain pregnancy later in
gestation and if extensive enough, may limit the ability of the uterus to
physically move. Limiting the ability
of the uterus to move may result in the inability of the uterus to clear fluid
following mating, resulting in fluid retention. In addition, the uterus may not be able to facilitate uterine
migration of the embryo. It is
essential that the embryo migrate fully throughout the uterus prior to day 16
of gestation or embryonic loss will result.
Older mares in general, but mares of any age may have lesions such as
adhesions in the lumen of the uterus that limit the ability of the embryo to
migrate, resulting in EED. These
lesions may be due to caustic, inappropriate uterine therapy or previous
foaling trauma.
5. Foal heat breeding - There tends to be considerable variation
in clinical opinion regarding the practice of foal heat breeding. It has been suggested that breeding will
reduce pregnancy rate as well as result in an increased incidence of early
embryonic death in those mares that do get pregnant at that time. There are several factors that should be
taken into account when deciding whether to breed on foal heat:
i . When the mare is anticipated
to ovulate - if ovulation is anticipated to occur >10 days after foaling,
then the mare will be a better breeding candidate. There is a greater chance that by this time residual uterine
fluid will have been evacuated from the uterus thereby providing a more
suitable environment for the embryo.
2. Absence of
ultrasonographically detectable fluid in the uterus. The ultrasound is a
useful tool for detecting small amounts of fluid still present in the uterus
that cannot be identified by palpation alone. Detection of these small amounts of fluid may be important in
determining when or if to breed a mare during foal heat.
3. Availability of the
stallion - if the stallion is not heavily booked to mares and the above
criteria are present it may be worth breeding the mare. However, if the stallion is heavily booked,
the foal heat mare will likely be passed over for a more reproductively
desirable mare.
4.Time of year mare foaled - if mare foals late in the year (end
of May), breeding on foal heat may mean the difference between breeding the
mare in two heat cycles versus 1, assuming the breeding season ends June
30. Pregnancy is as much a game of odds
as anything else, which simply means that the more opportunities a mare has to
get pregnant (i.e. the more heat cycles bred), the greater the chance she will
get pregnant.
7. The stallion may play a
role in early embryonic loss if the genetic material that he contributes to the
embryo results in faulty gene expression during the embryonic period such
leading to early embryonic death. The
incidence of this phenomenon is difficult to measure, but there are clearly
differences in fertility between stallions.
While many of these differences may be accounted for by differences in
semen quality and management, some are likely due to faulty genetic material
and gene function following fertilisation.
Bacterial contamination from the penis due to pathogens, such as
Pselidomonas aeruginosa and Klebsiella pneuniotziae can lead to uterine
infections that persist until the embryo is passed into the uterus especially
in the susceptible mare. Until
relatively recently the incidence of stallion
transmitted infections to mares was considered to be more common than the
actual occurrence. This is due to the
fact that most isolates of Pseudomonas and Klebsiella species are not pathogens
in the mare. This indicates that while
the stallion may harbour these bacteria on his penis, transmission to the mare,
in most cases, is unlikely. Therefore,
the influence of these organisms may be overrated with respect to their effect
on EED.
Definitions
Embryonic Loss: Failure of
pregnancy from fertilisation to 40 days.
Foetus: That part of the conceptus
that gives rise to the foal, from Day 40 to birth (end of second stage labour).
Abortion: Expulsion of the
foetus and its membranes before 300 days.
Stillbirth: Expulsion of
the foetus and membranes from 300 days onwards.
Pregnancy loss: = Mares
diagnosed Pregnant - Mares foaling
Divided by Mares diagnosed pregnant.
Foetal Losses
Losses are thought to be greater during the first half of pregnancy in
general. but no "critical period" has been defined.
An overall pregnancy wastage of 10-12 1/2% is often cited. Bain (1965), in an extensive field study
based on manual pregnancy diagnosis, recorded that 75% of pregnancy losses
occurred by day 49.
Laboratory investigation of aborted foetuses begins at approx. day 50
when they are large enough to be evident.
General Approach To The Management Of An Aborting Mare
1. Implement the Code of
Practice.
2. Isolate the mare (and her
grazing companions) pending the results of investigations.
3. Obtain a good past and
present history of the mare (breeding history, movements, health, circumstances
of abortion) and the pregnancy (sire, scanning records).
4. Arrange to do a
postmortem examination, or to send the foetus and placental membranes to a
diagnostic laboratory.
5. Advise regarding the
future breeding history of the mare.
Diagnostic procedures Used by the Veterinarian
Record all details in writing as soon as possible.
1. External examination.
Record weight, sex and crown-rump length.
Check for state of freshness, congenital defects, birth trauma,
presence of meconium or other substances on skin, subcutaneous oedema.
- Mare's recent clinical history, drugs used
- Possible stress factors
to mare e.g. travel, weaning
- Mare's nutritional
status. or changes to diet.
- Unusual environemtal or
climatic factors, e.g. electrical storms,.
- Previous endometrial
disease.
Causes of Abortion and Stillbirth
Infective causes (15% of abortions examined)
1. EHV-1 Rhinopneumonitis
(rarely EHV-4)
Usually occurs between 5 months and term; foetus usually fresh; sudden
rapid abortion (no prelim. signs).
Typically has excess fluids, minute spots on liver, enlarged spleen,
soft thymus, peri-renal oedema, pneumonia, + exudate in lower airways. Chorion may be heavy with oedema, red side
out ruptured across body not star. Some
foals live up to 7 days post-natallv.
2. Equine Viral Arteritis
Abortion
3. Fungal/Bacterial
placentitis
Usually infection spreads from cervix and is associated with chronic
chorionitis and foetal growth retardation +- overlyin@o sticky exudate., in 12%
of cases lesions minimal and diagnosis requires histological assessment. Beta hacinolytic Sti-el-?toc.occi, E. coli
and Aspei-gilllis spl-) together accounted for half of cases seen. In only 5% of cases cervical
"star" unaffected - in these the route of infection is
uncertain. Danger of salpingitis if
horn tip involved. Bacterial infections
abort 5 months to term. Fungal
infections commonly abort at greater than 10 months and mares often run milk
with or without a vaginal discharge.
Some foals born alive but congenitally infected. Inflammatory foci present in foetal livers.
Some countries (c... USA) recognise Leptospiral abortions (6-9 months
gestation) associated with nonspecific diffuse placentitis and in 50% of cases,
epithelial hyperplasia. Affected
foetuses often have Leptospiral antibodies.
A form of non-cervical placentitis has been reported from Lexington
Kentucky, affecting the body of the chorion and the base of the horns. It is believed to be caused by a
Nocardia-like organism.
Non-infective causes (70 %of abortions examined)
1. Twinning
This used to be the commonest cause of losses in Thorout,
,hbreds. Rare in smaller
breeds. Three main patterns of
placental arrangei-nent. In-contact
area adherent to variable extent.
Foetuses show growth
retardation, although in type C (10% of cases) pregnancy continues long
enough for smaller twin to mummify and larger twin often survives and is of
normal size. Small vascular anastomoses
sometimes noted: 44% red cell chimaerisiii reported. Monoiiuclear cell inflammatory foci usually present in foetal
liver.
Eqiiiiie Stiitl Me(li(.iiie Coiti.@@,
150
2. Placental Disease
Damage to umbilical cord
vessels: Is now the commonest single diagnosed cause of abortion. It usually
occurs coiise(lueiii on iiiai-ked
twisting in @i long cord. Oedema and haemorrhage
and local urachal
clil@it@@tioii. Foetus
@itilolyseel. Calcification in some
choi-ionic villi and thi-oiiibosed vessels. particularly in
peripheral parts ol ilic clioi-ioii.
Necrosis of' chorion at cervix: Clear line of demarcation froi-n normal
choi.ion. sometimes leakage of 1'oet@il I'Iuids prior to abortion., associated
with Ion. cords; foetuses slightly ,rowtii retarded.
llody pregnancy: Horns of chorioii attenuated and/or shrivelled; foetus
carried in uterine body; cause uiikil(Wil.
3. Foetal disease
Severe anomalies: involvin. CNS or development of gut or body cavities
may be aborted; others may go
c
to term c... diaphralinatic hernia.
Many necessitate euthanasia at birth e.g. microphthalmia.
Foetal diarrhoea syndrome: Meconium diarrhoea and inhalation
pneumonia. Cause of foetal stress
unknown.
4. Maternal disease
c... pyrexia. malnutrition, stress. uterine abnormality.
c
5. Intra or post partum
stillbirth
llremature onset ot'labour
Delay during the birth process e.g. uterine inertia, pelvic deformity,
contracted foetus, foetal oversize,
iiialpresentation.
Premature placental separation
6. No diagnosis (13%)
(Many are decomposed or foetuses/placentae are incomplete).
NB: Remember to consider
"exotic" conditions, especially in imported mares.
Equipie Stud Medi(.iize Coiii.@@,
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