Pregnancy Loss

Mats H.T. Troedsson

Pregnancy loss refers to the failure of a conceptus to be main­tained successfully to term. Pregnancy loss may be classified as early embryonic death (EED), abortion, or stillbirth, depending on the gestational stage when the pregnancy loss occurred.

Early Embryonic Death

The exact incidence of EED in any species is difficult to determine because most losses occur before pregnancy can be routinely diagnosed. Embryonic death early in pregnancy usually results in reabsorption of the embryonic tissues and fluids. Consequently, EED cannot be distinguished from failure of fertilization in most instances. However, despite the lack of clinical evidence, EED probably accounts for the largest percentage of reproductive wastage in large animals. The incidence of EED has been estimated at 5% to 24% in mares,⁶⁶ 30% to 35% in cows,⁶⁷ and 20% to 30% in ewes.⁶⁸ The rate of EED is generally higher in subfertile or repeat breeder females and in older females.

Loss of the embryo before maternal endocrine recognition of the pregnancy (i.e., days 11 to 14 in the mare, days 15 to 17 in the cow, and day 12 in the ewe and doe) results in return to estrus at the normal time. Embryonic loss after this critical period may result in persistence of the CL in horses, pseudo­pregnancy in goats, or irregular returns to estrus in cattle.

Chromosomal and genetic defects of the oocyte, sperm, or embryo; a poor oviductal or uterine environment; endocrine dysfunction; and maternal stress are all considered important factors in the pathogenesis of EED.^66,69 Infectious agents such as T.

Abortion

The rate of abortion after pregnancy diagnosis at 60 days of gestation has been estimated to be approximately 10% in horses⁷³ and 3% to 4% in cattle.⁷⁴ Fetal death may result in abortion (expulsion of the fetus from the uterus) or retention of the fetus in the uterine lumen, with subsequent fetal macera­tion or mummification. In animals with a CL-dependent pregnancy for all or most of gestation (cows, goats, llamas), death of the fetus usually results in the abortion of an autolyzed fetus because of a delay between the time of fetal death and lysis of the CL. In species that do not depend on a CL for maintenance of pregnancy for most of gestation (e.g., mares and ewes), fetal death causes an immediate decrease in placental progestogen production and rapid expulsion of a relatively nonautolyzed fetus. Mummification is characterized by fluid reabsorption from a fetus retained in a sterile uterine environ­ment. Fetal mummification is most common in multiparous species with a CL-dependent pregnancy (e.g., sows) and is rare in uniparous species that are CL independent for most of gestation (e.g., mares). Maceration refers to the degenerative changes that occur in a fetus after retention in a nonsterile uterine environment. Fetal maceration may be associated with significant maternal endometrial damage.

HORSES. Equine abortions may be characterized as infec­tious or noninfectious in origin.

Consumption of eastern tent caterpillars (larva of Malacosoma americanum) by mares causes abortion. The syndrome has been named mare reproductive loss syndrome (MRLS). Pregnancy loss before 150 days of gestation is characterized by hyperechoic amniotic and allantoic fluids on ultrasonographic examination along with a dead or dying fetus.^76,77 Near-term abortions are characterized by histologic lesions in the placenta and umbili­cal cord. In many cases, endometrial cultures are positive for non-beta-hemolytic streptococci and Actinobacillus species. Diagnosis of MRLS is based on four variable factors: placental lesions, culture of characteristic bacteria from fetal tissues, conformation of increased caterpillar exposure, and diagnostic elimination of other known causes of abortion (John Roberts, personal communication). The microscopic lesions observed in natural late-term MRLS cases are primarily the result of host response to bacterial infection, with the most common isolates being non-beta-hemolytic streptococci and actinobacilli.

During an outbreak in central Kentucky in 2001 and 2002, a variety of nonspecific lesions were reported that included placental edema, fetal pneumonia, and hemorrhages in the heart and placenta. During this outbreak, inflammation of the umbilical cord (funisitis), specifically the amniotic segment, was observed in 78% of cases.⁷⁸ Funisitis was also observed in necropsied cases during a 2006 Florida outbreak.⁷⁹ The lesion is initiated as a suppurative to pyogranulomatous response between the outer circumferential stroma and the outer tunica adventitia of major umbilical vessels and progresses outward to involve or ulcerate the amniotic surface of the umbilical cord.^79,80 Other causes of equine abortion such as noninfectious umbilical cord lesions, leptospirosis, and EHV should be ruled out by appropriate diagnostic tests.

Crosiella equi, Amycolatopsis spp., and Streptomyces spp. have been associated with chronic placentitis and subsequent late- term abortion, stillbirth, and premature birth.^83,84 The condition is often termed nocardioform placentitis due to similarities of the causative agents to the Nocardia species. The pathogenesis, route of infection, or risk factors have not been identified for the disease. Lesions are typically located in the ventral aspect of the uterine body and base of the uterine horns. Gross lesions are easily recognized as a brownish, thick, and tenacious exudate on the chorionic surface. The causative bacteria have been isolated only from clinical cases and not from the environment.⁸⁵

Leptospira species have also been identified as a significant cause of equine abortion in Kentucky.⁸⁶ Leptospira interrogans serovar Pomna type Kennewicki is most commonly causing abortion in horses in North America, but other serovars such as Grippotyphosa and Hardjo have also been implemented in equine abortions.⁸',⁸⁸ Environmental wet conditions are considered important to dissemination of Leptospira from wildlife to horses.

Bacterial and fungal abortions in mares are primarily caused by infections that ascend through the cervix, causing placentitis and subsequent fetal infection. The bacterial organisms most commonly cultured from aborted fetuses include Streptococcus species, Escherichia coli, Pseudomonas species, Klebsiella species, and Staphylococcus species. The most frequently recovered fungi are Aspergillus species.

Ihe most common noninfectious cause of equine abortion is twin pregnancy.⁷³ Inability of the uterus to support two fetuses to term because of insufficient placental support may result in abortion at any stage of gestation but is most common after 7 months.

RUMINANTS. Infectious bovine rhinotracheitis-infectious pustular vulvovaginitis (IBR-IPV) virus and bovine virus diarrhea-mucosal disease (BVD-MD) virus are two of the most common viral causes of abortion in cattle.⁸⁹ Bacterial abortions caused by Brucella abortus, Arcanobacterium (Actinomyces) pyogenes, Bacillus species, Listeria monocytogenes, E. coli, Leptospira species, and Pasteurella haemolytica and fungal abortions caused by Aspergillus species and Mucor species usually result from hema­togenous spread and localization in the placenta.⁹⁰ Protozoal abortion, caused by Neospora organisms, has been recognized as a significant cause of abortion in cattle worldwide.⁹¹ Epizootic bovine abortion (EBA) is a common cause of third-trimester abortion in susceptible heifers and cows inhabiting the foothills of the Sierra Nevada mountain range of California, Nevada, and Oregon.⁹² The Pajaroello tick, Ornithodoros coriaceus, is responsible for transmitting the causative agent, which has been identified as a delta proteobacteria, unofficially named Pajaroellobacter abortibovis when it feeds on pregnant cows.^93,94

Campylobacteriosis (vibriosis), caused by C. fetus and C. fetus subsp. jejuni, and enzootic abortion of ewes, caused by Chlamydia psittaci, are the most common infectious causes of abortion in sheep.⁹⁵ They are characterized by abortion in the last 4 to 6 weeks of gestation, premature births, stillbirths, and birth of weak, infected lambs. C. psittaci is also the most common cause of infectious abortion in goats in the United States.⁹⁶

Noninfectious causes of large animal abortion include genetic or chromosomal factors, maternal stress, inadequate nutrition, vitamin or mineral deficiencies, ingestion of poisonous plants or other toxins, hormonal factors, environmental factors, physical factors, and certain medications.

Approach to Diagnosis of Abortion

A definitive diagnosis is reached in 20% to 40% of bovine abortions,⁶⁵ 50% to 60% of equine abortions,⁷³ and 30% to 40% of sheep abortions.⁹⁷ The generally low diagnostic success is a result of the complexity of the condition (Boxes 12.10 and 12.11). Abortion involves disease in the maternal, placental, and fetal compartments individually or together, and all these compartments have to be examined thoroughly. In addition, a “triad” of determinants for animal disease has to be considered: 1) the presence of a pathogenic organism, 2) the environment in which a host lives, and 3) the susceptibility of the host to the disease.⁹⁸ To enhance diagnostic success, information and samples must be collected from the fetus, placenta, dam, and herd. A thorough history should be obtained, including the gestational age of the fetus; reproductive, medical, and vaccina­tion history of the dam and other individuals in the herd; previous abortions and diagnoses; new arrivals to the herd and contacts of the animal with other herds; potential causes of maternal stress; possible access to toxins and poisonous plants; and sources of water and nutrition.

A physical examination that includes all body systems should be performed on the aborting dam. Examination of the repro­ductive system should include palpation or ultrasonography of the reproductive tract per rectum, speculum examination of the vagina, and digital examination of the cervix. Samples should be collected from the vagina, uterus, or both, for culture and cytologic studies. Paired serum samples from the dam and other females in the herd (10 animals or 10% of the herd, whichever is greater) may also help demonstrate an immunologic response to an infectious agent. Maternal serologic testing is generally most useful if paired samples are submitted, combined with accurate information about the animal's vaccination history. However, postabortion titers from cows that aborted can be compared with titers from unaffected cows in the herd at a similar stage of lactation. Demonstration of a fourfold rise in titer between acute and convalescent serum samples suggests recent exposure to an agent, but the presence of antibodies does not necessarily indicate that the agent caused the abortion. An exception can be made for brucellosis, for which a high titer from a single sample can be diagnostic.

For optimal diagnostic efficiency, the entire aborted fetus and placenta should be submitted to a diagnostic laboratory for necropsy. If this cannot be done, a prompt necropsy should be performed and collections of fetal, placental, and maternal samples should be submitted to a diagnostic laboratory (Table 12.1).

A systematic necropsy must be performed on the aborted fetus. Fetal age and development may be assessed by measuring crown-rump length, hair patterns, and color. Meconium staining of the skin suggests uterine fetal distress. The condition of the fetus, including the degree of autolysis, should be noted. A careful examination for fetal anomalies (e.g., cerebellar hypoplasia, hydrocephalus, cleft palate, cardiac anomalies) should be performed. Histopathologic samples should be immersed in a volume of 10% buffered formalin (or Bouin fixative) equivalent to 10 times the volume of tissue. Samples for culture, virus isolation, and fluorescent antibody tests should be submitted on ice in separate sterile containers. A sample of abomasum and stomach contents should be aseptically collected for culture. Fetal heart blood or thoracic fluid may be collected for serologic evaluation. A late-term fetus is immunologically competent, and high titers may indicate activity of a pathogenic agent. Serologic testing of fetal fluids can be useful both in detecting a nonspecific active fetal immune response (total immunoglobulin [IgG]) and for titers against a specific antigen.

The fetal membranes should be examined for size, weight, degree of autolysis, condition, and completeness. Samples of

■ BOX 12.10

Causes of Pregnancy Loss in Mares

Common Causes

Impaired ovιductal and uterine environment (EED)

Chronic endometritis (EED, Ab)

Embryonic defects (EED)

Endometrial fibrosis (EED, Ab)

Twinning (Ab)

Equine herpesvirus type 1 (EHV-1) (Ab)

Bacterial placentitis (Streptococcus species, Escherichia coli, Pseudomonas species, Klebsiella species, Staphylococcus species, Crosiella equi, Amycolatopsis species, Streptomyces species) (Ab)

Fungal placentitis (Ab)

Umbilical cord abnormalities (Ab)

Less Common Causes

Endotoxemia (EED, Ab)

Leptospirosis (Ab)

Mare reproductive loss syndrome (MRLS) (EED, Ab)

Fetal anomalies (Ab)

Maternal stress, other disease (EED, Ab)

Chromosomal abnormalities (Ab, EED)

Fescue toxicity (EED, Ab)

Advanced maternal age (EED)

Equine viral arteritis (Ab)

Equine infectious anemia (Ab)

Uterine torsion (Ab)

Endocrine factors (EED)

Malnutrition (EED, Ab)

Drug-induced causes (EED, Ab)

Premature separation of the placenta (Ab)

Fetal asphyxia (Ab)

Placental insufficiency (Ab)

Uncommon Causes

Ehrlichia risticii (Potomac horse fever) (Ab)

Uterine body pregnancy (Ab)

Endometrial adhesions (EED)

Taylorella equigenitalis (contagious equine metritis) (EED)

Uterine lymphatic lacunae, cysts (EED, Ab)

Hyperlipemia (Ab)

Lymphosarcoma (Ab)

Iatrogenic causes (EED, Ab)

Fetal diarrhea syndrome (Ab)

Ergot toxicity (EED, Ab)

Brucellosis (Ab)

Mycobacterium infection (Ab)

Corynebacterium pseudotuberculosis (Ab)

Rhodococcus equi (Ab)

Mycoplasma infection (Ab)

Coccidioidomycosis (Ab)

Histoplasmosis (Ab)

Babesiosis (Ab)

Vitamin A deficiency (Ab)

Iodine deficiency (EED, Ab)

Granulosa theca cell tumors (Ab)

Cryptococcosis (Ab)

Sorghum, Sudan grass (Ab)

Locoweed (Astragalus) species

Hoary alyssum poisoning (Ab)

Salmonella abortus equi (Ab)

Gonadal stromal tumors (Ab)

Trauma (Ab)

Ab, Abortion; EED, early embryonic death. placental tissue, especially areas with lesions, should be collected for histologic examination, impression smears, bacterial culture, virus isolation, and fluorescent antibody tests. The equine placenta should be examined for integrity, lesions, and distribu­tion of chorionic villi. The normal equine placenta is everted after expulsion, with the allantoic surface presented outward and chorioallantois ruptured at the site of the cervical star. Blood may be collected from the free end of the cord. The allantoic surface should be examined for abnormalities such as multiple allantoic pouches that may indicate compromised fetal circulation.⁹⁹ The chorionic surface of the placenta should be examined for lesions and distribution of chorionic villi. Areas of avillous chorion are normally observed in association with the cervical star, narrow folds over large vessels, and areas opposing endometrial cups. Absence of chorionic villi over a circumscribed area is characteristic of twins and represents the region where two placentas were in contact. The region of the placenta adjacent to the cervix should be examined for loss of chorionic villi and the presence of inflammatory exudate, a hallmark of ascending infection. In cases of nocardioform placentitis, a characteristic thick and sticky exudate often covers an avillous area of the chorion at the ventral part of the site of the uterine body and base of the horn.

The cotyledons and intercotyledonary spaces of the ruminant placenta should be carefully examined for lesions. Autolytic changes of the placenta may be difficult to interpret. Some normal features of the bovine placenta must be kept in mind.⁷³ Amniotic plaques are present on the inner surface of the amnion and on the umbilical cord. They are most prominent at 3 to 7 months of gestation. Necrotic areas of the chorioallantois in the tips of the uterine horns are also normal and caused by insufficient vascularization to that area. Mineralization of the placenta is normal during the first months of gestation but may reflect placental injury associated with infection at the end of gestation.

All aborted fetuses and placental tissues should be handled with care, and tissues not submitted to a diagnostic laboratory should be burned or buried. Dams that have aborted should be isolated from the remainder of the herd.