Margo L. Macpherson
Placentitis
Pregnancy losses from placental infection represent an important cause of abortion and neonatal death in horses. Bacteria are the most common cause of equine placentitis. Streptococcus equi subspecies zooepidemicus, Escherichia coli, Klebsiella spp, Pseudomonas spp, and Staphylococcus aureus are the organisms most commonly implicated in placentitis. Less commonly, fungal organisms (Candida and Aspergillus spp) cause placentitis. Placental infection usually arises in the last trimester of gestation. Bacteria invade the caudal part of the reproductive tract and migrate through the cervix to the placenta. Infective organisms colonize the caudal portion of the placenta at the cervical star and disrupt the intimate contact between the allantochorion and endometrium. Infection, with secondary inflammation and placental compromise, generally results in premature delivery of a nonviable foal. In some cases, foals undergo precocious maturation and are delivered alive. This scenario is particularly true in mares infected with Nocardia-type organisms (actinomycete bacteria such as Crossiella equi and Amycolatopsis spp).
The most common clinical signs of placentitis are premature udder development and purulent vulvar discharge. However, the severity of clinical signs does not necessarily determine the outcome of the pregnancy. Some mares will have scant to no vulvar discharge and minimal udder development. These mares may have subclinical infection, which will result in death of the fetus. Alternatively, some mares have extensive placental separation but still deliver viable foals. Mares with nocardioform placentitis often have a marked degree of placental compromise at the juncture of the uterine body and pregnant uterine horn, but the fetus is not infected. Precocious maturation is induced in many of these cases, and the foal survives early delivery.
Diagnostic Tools to Differentiate Mares with Placentitis
History and Physical Examination
Historical information can be useful in determining whether a mare has placentitis because certain facts can provide clues that will aid in the diagnostic process. For instance, information regarding the method of pregnancy diagnosis (i.e., rectal palpation alone vs. use of transrectal ultrasound) used early in gestation can help distinguish mares with twins from those with an ascending placental infection. Both types of mares may have precocious mammary development, but mares with twins would not be likely to have a vulvar discharge. A mare with poor perineal conformation would be predisposed to developing ascending bacterial infection. Fresh vulvar discharge is often wiped away by the mare’s tail and can be missed if the mare’s hindquarters and tail are not examined regularly. The mare’s systemic health would rarely be compromised with either twin pregnancy or placentitis. Blood counts, serum chemistry values, and blood lactate concentration generally are within normal ranges in mares with either condition.
Transrectal Palpation and Ultrasonographic Examination
Transrectal ultrasonography in late gestation is a useful tool for evaluating placental integrity (at the cervical star), fetal activity, and fetal fluid character. Fetal orbit diameter can be measured to estimate fetal age. Diagnosis of twins in late gestation is not reliable with transrectal ultrasonographic examination.
The caudal aspect of the allantochorion at the cervical star is the most frequently affected area in mares with placentitis from the ascending route of infection. Thorough examination of this area is critical in diagnosis of placentitis. In the normal pregnant mare, the width of the combined uterine and placental (chorioallantoic) unit is measured. Values for the combined thickness of the uterus and placenta (CTUP) for mares with normal pregnancies have been established (Table 168-1). Mares with placental infection or inflammation will have an increase in CTUP measurements or separation of the membranes with purulent material.
TABLE 168-1
Normal Values for Ultrasound Measurement of the Combined Thickness of the Uterus and Placenta
| Gestational Day | CTUP (mm) |
| Days 271-300 | <8 mm |
| Days 301-330 | <10 mm |
| After day 330 | <12 mm |
Transabdominal Ultrasonographic Examination of the Reproductive Tract
Transabdominal ultrasonography is an excellent tool for evaluating the fetus and placenta in mares. Fetal well-being can be assessed through transabdominal ultrasonographic measures of fetal heart rate, body tone, musculoskeletal activity, and size. The average heart rate in a fetus older than 300 days’ gestation has been reported as 75 ± 7 beats/minute, but significant individual variation in heart rates may be observed. Fetal heart rate slows by approximately 10 beats/minute at greater than 330 days’ gestation. Activity level can affect fetal heart rate throughout late gestation. Consistently low (<55 beats/minute) or high (>120 beats/minute) fetal heart rates are associated with fetal stress and warrant reexamination.
Transabdominal ultrasonography is the most accurate method for detecting twins in late gestation. Confirmation of twins is generally made by identifying two fetal thoraces or beating hearts. Measurements of fetal thoraces can be used to confirm the presence of twins, if thoracic size differs between fetuses. Additionally, the orientation of the thorax can be used to verify twin fetuses.
Fetal activity level and musculoskeletal tone are easily determined when the fetus is being evaluated for heart rate. Fetal activity can vary during the examination period because fetuses have periods of sleep and wakefulness. In response to the ultrasound beam, the normal fetus commonly becomes very active during the examination period. Fetal tone is a subjective term used to describe the viability of the fetus. A live fetus has excellent tone in that it is active and flexes and extends the torso, neck, and limbs. An atonic fetus is flaccid and lies passively in the uterus and may be folded in upon itself. Clear identification of an atonic fetus can be difficult because traditional landmarks, such as the heartbeat, may be obscured by the limbs of the flaccid fetus.
Serial ultrasound examinations should be performed to verify fetal well-being or distress. Once-daily transabdominal ultrasonographic assessments are commonly performed in high-risk mares. Fetuses with signs of distress are often evaluated several times a day to assess heart rate and activity level.
Hormonal Assays and Biomarkers
Progesterone Assays
Several progestagens are synthesized by the fetoplacental unit to support pregnancy during the last two thirds of gestation. These progestagens are metabolites of progesterone (P4) and pregnenolone (P5). Peripheral concentrations rise gradually in mare serum as pregnancy advances. Premature increases in progestagen production are seen in mares with placental pathology or as a result of fetal stress; therefore, measurement of progestagens can be a useful tool for monitoring fetoplacental health. Direct measurement of progestagens requires sophisticated mass spectrometry equipment. However, many P4 immunoassays cross-react with progestagens produced by the fetoplacental unit. To aid in detection of placental pathology or monitor response to therapy, it is recommended that serum progesterone concentrations be monitored, serially, three or four times at 1- to 2-day intervals. These values can be assessed for trends, including a premature, gradual rise (before day 300 of gestation) or a rapid decrease (impending delivery) that may be useful for directing treatment.
Estrogens and Relaxin
Estrogens are also broadly produced by the fetoplacental unit during pregnancy in mares. The fetal gonads produce estrogen precursors, which are used by the placenta to produce a variety of estrogens, including estrone, estradiol-17α and -17β, equilin, and equilenin. Estrogen production increases from about day 80 of gestation, plateaus, and then gradually decreases before delivery. High maternal serum estrogen concentrations (usually measured as estrone sulfate or total estrogens) are indicative of a functional fetoplacental unit and are a strong indicator of fetal viability, although the usefulness of this tool for detecting and monitoring pregnancy health has been less well defined than measurement of serum progestagens. It has been suggested that measurement of total estrogens in mare serum between 100 and 300 days of gestation may be useful for assessing pregnancy health.
Relaxin production by the placenta begins at about day 80 of pregnancy and continues until expulsion of the placenta after parturition. Maternal serum concentrations of relaxin have been correlated to pregnancy health, but commercial assay of this hormone is not available at present.
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