3.3.4. Age of dam Older animals have lower follicular activity and lower oocyte quality resulting in a
decrease of the developmental competence of embryos. Furthermore, the quality of the endometrium is deteriorating with increasing age.
3.3.5. Inbreeding Ž
. Inbreeding has been reported as a cause of EM Hanzen et al., 1999 . It has also been
proven that the EM differs from breed to breed.
4. Species-specific causes of embryonic death
4.1. Cattle 4.1.1. Infectious causes
Many viruses, bacteria and protozoa can be associated with EM and early fetal loss. Bluetongue virus, BVDV and BHV-1 are the most important viral agents. From in
vivo experiments with in vivo-derived embryos, it is known that BHV-1 causes infection Ž
. in hatched embryos that result in EM Miller, 1991 . Furthermore, it is known that,
particularly in BHV-1-seronegative cattle, artificial insemination with BHV-1-con- taminated semen can result in markedly reduced conception rates and endometritis. The
outcome of infection depends on the amount of virus per straw and the properties of the virus strain. Natural breeding with bulls shedding BHV-1 in their semen does not appear
Ž .
to affect fertility van Oirschot, 1995 . The in vitro infection of zona pellucida-intact in vivo-derived embryos has no effect on the embryonic development. However, the
exposure of hatched in vivo-derived embryos to BHV-1 was rapidly embryocidal Ž
. Bowen et al., 1985; Bielanski et al., 1987; Wrathall and Sutmoller, 1998 .
¨
Infections of BVDV-seronegative heifers with BVDV resulted in a lower pregnancy rate. Artificial insemination or natural breeding with BVDV-contaminated semen can
result in fertilization failure or embryonic death. The incubation of zona pellucida-intact in vivo derived embryos with BVDV has no effect on embryonic survival, but the
exposure of hatched embryos to cytopathic BVDV causes EM.
Recently, the interactions of BHV-1 and BVDV with in vitro-produced embryos were investigated by experimental in vitro infections. It was demonstrated that embryonic
cells of early zona pellucida-free oocytes and zygotes were refractory to an infection Ž
. with BHV-1 and BVDV Vanroose et al., 1997, 1998 . In more advanced stages, such as
zona pellucida-free 8-cell stage embryos, zona pellucida-free morulae and hatched blastocysts, newly produced BHV-1 or BVDV was detected. It was observed that the
exposure of zona pellucida-free embryos to BHV-1 and to a cytopathic BVDV strain results in destruction of the embryonic cells. Furthermore, both viruses could replicate in
Ž oviductal cells, resulting in cell lysis and elimination of their biological functions such
. as secretion of embryotrophic factors supporting embryonic development . The presence
of BHV-1 and BVDV in an in vitro embryo production system has clear adverse effects Ž
on fertilization and embryonic development Guerin et al., 1992; Bielanski and Dubuc,
´
. 1994, 1995; Allieta et al., 1995; Booth et al., 1998; Vanroose et al., 1999b . It was also
demonstrated that the zona pellucida of in vitro-produced embryos was an effective barrier against viral infections, but the outer pores of the zona were large enough to
Ž allow entry of BHV-1 and BVDV in the outer layers of the zona pellucida Vanroose,
. 1999 . Consequently, the embryo can become infected when it hatches out of a
virus-contaminated zona pellucida. The importance of the zona pellucida was also Ž
demonstrated for in vivo-derived embryos Singh et al., 1982a, 1982b; Potter et al., .
1984; Bielanski and Hare, 1988; Gillespie et al., 1990; Stringfellow et al., 1991 . Some bacterial and protozoal infections, such as trichomoniasis and campylobacterio-
sis, which are venereal diseases, are characterized by endometritis resulting in infertility and EM. Other infections such as brucellosis, Arcanobacter pyogenes infection, can-
didiasis, leptospirosis, neosporosis, fungal infection, listeriosis, and Haemophilus som- Ž
nus infection are more associated with late EM, fetal death and abortion Larsson et al., .
1994; Sekoni, 1994; McGowan and Kirkland, 1995; Moen et al., 1998 . 4.1.2. Hormonal imbalances
A number of studies have demonstrated a relationship between low maternal proges- terone levels and early pregnancy failure. Both a late post-ovulatory progesterone rise
and low luteal phase concentrations are associated with poor embryo development and the production of insufficient interferon-t to prevent luteal regression. The post-ovula-
tory rise of progesterone is of particular interest as it maintains the synchrony between embryo and uterus. An overall analysis of studies concerning progesterone supplementa-
tion revealed a significant improvement in the pregnancy rate of 5 following proges-
Ž .
terone supplementation Mann et al., 1998 . 4.1.3. Trauma
Trauma after pregnancy diagnosis by rectal palpation or by using an ultrasound scanner can result in pregnancy loss. In cattle, early pregnancy diagnosis is generally
performed between 35 and 50 days of gestation, which is the period of the completion of the differentiation. Therefore, the risk exists that the embryo or the fetus becomes
Ž .
damaged. However, Vaillancourt et al. 1979 found no indication that embryonic loss at the time of or shortly after early pregnancy examination was increased. Recently, Baxter
Ž .
and Ward 1997 reported that ultrasound examination has no detrimental effect on the fetus. Rectal palpation is also a safe procedure when performed correctly.
4.1.4. NegatiÕe energy balance A severe negative energy balance of high yielding dairy cows after calving may
Ž affect oocyte quality and may enhance EM once the cow has been inseminated Butler
. and Smith, 1989 .
4.2. Horse 4.2.1. Infectious causes
Equine herpesvirus and equine viral arteritis virus are both associated with reproduc- tive failure in mares. However, the majority of infections with these viruses result in late
fetal death and abortion.
A large number of bacteria can infect the endometrium resulting in failure of fertilization or EM. They are often opportunist pathogens that can be isolated from the
genital tract of normal mares, i.e. E. coli and Streptococcus spp. Others are considered to be venereal pathogens, i.e. Pseudomonas spp., Klebsiella spp. and Taylorella
Ž .
equigenitalis contagious equine metritis . 4.2.2. Persistent endometritis, chronic degeneratiÕe endometritis and endometriosis
Persistent endometritis, chronic degenerative endometritis and endometriosis are major causes of reduced fertility in brood mares. These are age- and parity-related. A
persistent inflammation of the uterus results in premature luteolysis and EM in response to increased PGF
concentrations. The inflammation can also interfere with the survival
2 a
of an embryo. After fertilization has taken place the embryo remains in the oviduct for 5–6 days. The embryo then descends into the uterine lumen where the presence of fluid,
bacteria, and inflammatory products is incompatible with its survival. Chronic degenera- tive endometritis and endometriosis involve a range of morphological and functional
changes in the uterus such as periglandular fibrosis of the uterine glands and a deficiency of functional glands. The uterine glands produce ‘uterine milk,’ which is,
until day 40, the only source of nutrition available to the still unimplanted conceptus. Thus, deficiency in nutrients will affect the fetus, right up to the point of starvation and
Ž .
death Allen, 1992 . Endometriosis is also associated with the development of large lymph-filled endometrial cysts that protrude in the uterine lumen. Cysts have no
microcotyledons, and therefore, there is a proportional reduction in the nutritional sustenance of the embryo. In addition, large cysts or a large quantity of small ones can
prevent the migration of the conceptus during the critical period between days 12 and 16
Ž after ovulation resulting in a failure of the maternal recognition of pregnancy Thatcher
. et al., 1997 . Finally, chronic degenerative endometritis and endometriosis may cause
Ž early EM by the loss of myometrial tone Nikolakopoulos and Watson, 1999; Troedsson,
. 1999 . These features result in inadequate expulsion of endometrial gland secretions of
oestrus, together with seminal fluid, penile smegma, and bacteria before the cervix contracts following ovulation. Consequently, when the embryo reaches the uterus, it
Ž .
enters a mixture of debris and it quickly succumbs Allen, 1992 . 4.2.3. Chromosomal abnormalities
Chromosomal abnormalities are a major cause of early pregnancy failure in horses. These failures may start very early but the majority were seen to occur between 20 and
30 days of gestation, which is the period when the embryo is undergoing all the changes Ž
. associated with organogenesis Allen, 1992 .
4.2.4. Hormonal deficiencies and imbalance Ž
Progesterone is critical for the maintenance of pregnancy in mares Daels et al., .
1991 . The only source of progesterone during the embryonic period is the primary Ž
corpus luteum. Primary luteal insufficiency is a cause of early embryonic death Pycock .
and Newcombe, 1996 , but is of rare occurrence. 4.2.5. Twin pregnancy
Twin pregnancy is another cause of EM. Embryonic losses in twin pregnancies are greater than for individual embryos.
4.2.6. Stress Stress due to malnutrition, transport and severe pain has been implicated as a cause of
EM. However, the relationship between malnutrition and embryonic death has not been proven. Also, transport of pregnant mares did not result in a lower pregnancy rate. On
the other hand, severe pain, for example caused by colic can result in luteolysis and thus Ž
. EM Daels et al., 1991 .
4.3. Swine 4.3.1. Infectious causes
Infections play an important role in prenatal losses in swine and can be categorised based on whether the agents exert systemic effects, e.g. swine influenza virus, or infect
the embryo directly, e.g. pseudo-rabies virus. Infections before 35 days lead to embry- Ž
onic resorption or early abortion. Viral infections e.g., porcine enteroviruses, porcine .
parvovirus, pseudo-rabies virus, or classical swine fever after 35 days will often result Ž
. in mummified fetuses Christianson, 1992 .
Many ubiquitous bacteria can cause endometritis and as a result embryonic death, for example E. coli, Erysipelothrix rhusiopathiae, Listeria spp. and Staphylococcus spp.
Ž .
De Winter et al. 1995 have demonstrated that the syndrome of endometritis post-in- semination can be caused by ascending infections with facultatively pathogenic bacteria
present in the vagina or in the semen. Such infections do not impair fertilization but disturb the embryo–maternal interactions or disrupt the process of implantation of the
embryos. This results in vaginal discharge 14–25 days after insemination. When the uterus has already been infected before service, e.g. because of a chronic infection after
a previous insemination the uterine environment has changed very much. As a conse- quence, fertilization will not take place or early embryonic development is disturbed
Ž .
resulting in EM before day 11 De Winter, 1995 . The sow’s endometrium has the best resistance to these uterine infections during
oestrus, but is already susceptible to bacterial infections at the end of oestrus. 4.3.2. Number of embryos
Ž .
In swine, at least four embryos two in each horn are needed at the time of Ž
. implantation for maintenance of pregnancy Christianson, 1992 .
4.3.3. Variation in deÕelopment Twelve days after insemination, there is considerable variation in morphological
Ž .
development between littermates Lambert et al., 1991 . It has been postulated that the more-developed embryos within the litter advance uterine secretions by synthesizing
more oestradiol than their lesser-developed littermates. As a result, the lesser-developed embryos probably become more susceptible to this asynchronous environment and
consequently die.
4.3.4. Stress Stress due to malnutrition and transport of pregnant sows has been implicated as a
Ž .
cause of EM Gordon, 1997 .
4.3.5. High plane of nutrition The high plane of nutrition early in pregnancy can have an adverse effect on embryo
survival. A very high level of dietary protein induces a greater activity of enzymes, resulting in an increased rate of metabolism causing a reduction of progesterone that is
Ž .
needed for the embryonic development Dziuk, 1992 . 4.3.6. Season infertility
A reduction in the fertility in pigs in the summer and early autumn has been reported in many countries and appears to manifest as a range of problems from silent oestrus
and ovarian cysts to EM. 4.4. Dog and cat
4.4.1. Infectious causes The premature termination of gestation by embryonic or early fetal death is uncom-
mon in the bitch. Minute virus of canines may cause transplacental infections with Ž
. embryo resorptions Carmichael et al., 1991 . In cats, feline leukaemia virus can cause
embryonic resorption. Other viruses causing EM and fetal death are feline panleucopenia virus, feline infectious peritonitis virus and feline herpesvirus 1.
4.4.2. Habitual foetal death Some bitches and queens have a true luteal insufficiency resulting in habitual
Ž .
resorption of the conceptuses Okkens et al., 1992; Roth et al., 1995 . Particularly in the bitch, EM can result in the diagnosis of pseudo-pregnancy. In the queen, approximately
30 of all ovulated oocytes are either not fertilized or undergo pre-implantation EM Ž
. Swanson et al., 1994 .
4.4.3. Cystic endometrial hyperplasia in the bitch and queen Cystic endometrial hyperplasia, which precedes pyometra, may also result in concep-
Ž tion failure, failure of implantation and embryonic resorption Okkens et al., 1992; Roth
. et al., 1995 .
5. Conclusions
