Most of these losses occur during the embryonic period of gestation. This period Ž
extends from fertilization to the completion of the differentiation stage an embryo is .
considered a fetus when mineralization begins . Furthermore, most of the embryonic losses occur during the first days after fertilization and during the process of implanta-
Ž .
tion Wathes, 1992 . The adhesion stage of the implantation process in domestic animals starts at day 14 in sows, days 15–16 in ewes, day 16 in bitches, days 18–20 in goats,
Ž .
days 21–22 in cows, and days 36–40 in mares Gandolfi et al., 1992; Guillomot, 1995 . Ž
EM has been estimated to be about 20–40 in cows Lopez-Gatius et al., 1996; Hanzen
´
. Ž
. et al., 1999 ; 10–40 in sows Lambert et al., 1991; Gordon, 1997 , 10–30 in goats
Ž .
and 15–60 in mares Allen, 1992; Bergfelt and Ginther, 1992 . Fetal death has been Ž
. Ž
estimated to be about 5 Lambert et al., 1991 , but may exceed 10 Lopez-Gatius et
´
. al., 1996 .
Prenatal losses can be caused by infections and by non-infectious factors. Primary attention has often been directed to infections but non-infectious causes probably
Ž .
account for 70 or more of the cases Christianson, 1992 . Non-infectious causes are often multifactorial and are difficult to diagnose.
This paper aims to give a review of the causes of EM in domestic animals, and to focus upon some species-specific causes. Particular focus is concentrated on embryo–
pathogen interactions in the discussion.
2. Infectious causes
Viral, bacterial, protozoal and, possibly, mycoplasmal infections can result in embry- onic death, indirectly by systemic effects via septicemias, viremias, or toxemias on the
dam, or directly by affecting the embryo or contaminating its environment. EM caused by systemic pathogens is usually related to fever during the infection.
High fever present in the first stage of pregnancy can lead to early embryonic death as a result of denaturation of embryonic proteins. Prostaglandines, which may be elevated in
febrile states, can cause luteolysis and subsequent loss of pregnancy. Furthermore, the stress condition present when an animal is febrile may indirectly lead to loss of
pregnancy through the elevated steroids by themselves and through a lowered immune
Ž .
response to other organisms that can cause EM Christianson, 1992 . Direct infection of the embryonic environment and of the embryo is another cause for
EM. 2.1. Infection of the embryonic enÕironment
Ž .
Infection of the embryonic enÕironment oviduct and uterus can be caused by
specific and non-specific uterine pathogens. Specific uterine infections are caused by a number of viruses, bacteria and protozoa. These pathogens enter the uterus by the
Ž .
haematogenous route e.g., primary infection of the female with Toxoplasma gondii or Ž
. Ž
via the vagina at natural service e.g., Campylobacter fetus or at insemination e.g., Ž
.. contamination of semen with bovine viral diarrhea virus
BVDV . Non-specific
pathogens are mainly bacteria that enter the uterus by ascending infection or at insemination. Sometimes they cause endometritis. The infection and the resulting
inflammatory products must be eliminated before the embryo descends into the uterus. Ž
. Particularly in older animals e.g., the mare , this uterine clearance can be impaired.
Acute endometritis, after mating or artificial insemination, has a direct effect on the embryonic environment and is, in severe cases, accompanied by the production of
Ž .
luteolytic substances such as prostaglandines De Winter et al., 1995 . Bacterial uterine infections cause mostly a diffuse and severe purulent inflammation. Viral infections are,
in most cases, characterized by a necrotizing endometritis, causing diffuse and total lymphocytic and plasmacytic changes in the endometrium.
Chronic endometritis involves a range of morphological and functional changes in the uterus besides inflammation. The deposition of layers of fibrous tissue around
endometrial glands results in a deficiency of functional glands. This deficiency will deprive the embryo of the protein-rich exocrine secretion.
2.2. Infections of the embryo Infections of the embryo proper can take place at two important phases in the
embryonic development: before hatching and after hatching. 2.2.1. Before hatching zona pellucida-intact embryos
Of all pathogens, viruses are the most insidious and dangerous type of infection for the early-stage embryo. Viral infection of the zona pellucida-intact embryo can already
have taken place before fertilization during maturation of the oocyte. Viruses, e.g. Ž
. bovine herpes virus 1 BHV-1 and BVDV, might be present in follicular fluid or
Ž .
granulosa cells of bovine oocytes Bielanski et al., 1993 , and can also contaminate the embryos by adhering to the glycoprotein layer, which surrounds the oocyte, the so-called
zona pellucida. In persistently-infected cattle, BVDV antigen has even been detected Ž
. inside the oocyte Brownlie et al., 1997 . Virus adhering to the zona pellucida or to the
fertilizing spermatozoon might be introduced into the oocyte by the sperm track in the Ž
. zona pellucida created at the time of fertilization Bowen, 1979 . Passive migration of
virus through the meshes in the zona pellucida is highly unlikely to occur, since particles Ž
. with a diameter of 40 and 200 mm comparable size as BVDV and BHV1 remain stuck
Ž .
in the peripheral part of the zona pellucida Vanroose, 1999 . Only one report has ever shown that one of the smallest viruses, the porcine parvovirus, could pass the zona
Ž .
pellucida in pigs Bolin et al., 1983 . After fertilization, the zona pellucida can be Ž
considered as an effective barrier for virus penetration Stringfellow et al., 1991;
. Vanroose et al., 1999a . However, at these early embryonic stages, death of a zona
pellucida-intact embryo can occur because of a hostile uterine environment. 2.2.2. After hatching
Embryos hatch from the zona pellucida at 8–9 days in cattle, 7–8 days in sheep and 6–7 days in pigs. In the horse, a non-cellular membrane surrounding the early equine
conceptus resembling the zona pellucida is gone by day 20. After hatching or after removal of the zona pellucida, the embryonic cells are susceptible to some infectious
Ž .
agents Wrathall and Sutmoller, 1998 . For example, zona pellucida-free bovine morulae
¨
Ž and blastocysts are susceptible to bovine herpesvirus-1 Bowen et al., 1985; Bielanski et
. Ž
al., 1987; Vanroose et al., 1997 and BVDV Brock and Stringfellow, 1993; Vanroose et .
Ž .
al., 1998 , but not to bovine parvovirus Bowen, 1979 . Recent research on embryo– pathogen interactions has mainly been performed in cattle, by exposing in vivo derived
Ž or in vitro produced embryos to specific pathogens in vitro Wrathall and Sutmoller,
¨
. 1998; Vanroose, 1999 .
Besides these in vitro studies, earlier in vivo studies have shown that viral uterine infections can result in extensive viral replication in embryonic cells after hatching and
implantation. In addition, cells undergoing rapid division, as occurs in embryos, are Ž
. particularly susceptible to the replication of certain viruses Bowen, 1979 . The outcome
of such an infection can either be cytolytic or non-cytolytic. Both can result in EM, but a non-cytolytic infection can also cause chromosomal damage and induce embryonic cells
to divide more slowly. This retardation in cell division occurs during the critical phase of organogenesis. Consequently, viral infection of embryos may result in the develop-
ment of congenital malformations.
After implantation, the haematogenous route of uterine infection becomes more important, since both endometritis and a direct cytolytic effect on the embryo are
possible.
3. Non-infectious causes
