Clinical signs include those observed in mares with pending abortion. Udder development, premature lactation, and cervical softening are often seen before the mare aborts. Vaginal discharge may or may not proceed abortions. Once clinical signs develop, the disease has reached an advanced stage and treatment may not always be successful.
Evaluation of the equine placenta should routinely be performed after abortion or parturition. In aborting mares with an ascending placentitis, the pathologic lesions are characteristic. An area of the chorion adjacent to the cervical star is depleted of chorionic villi and is thickened, discolored, and covered by fibronecrotic exudate (). Placentitis caused by N. actinomycete causes characteristic lesions at the ventral aspect of the base of the gravid horn and nongravid horn at the junction between the body and the horn of the placenta. The chorionic surface is thickened and covered with brown-red, thick, mudlike material. Placentitis caused by a hematogenous route of infection shows less characteristic multifocal lesions of the chorionic surface of the placenta. A thorough inspection of the placenta is necessary to ensure that any existing lesions are found. Examination of the placenta postpartum provides excellent information on disease processes or dysfunctions that could have affected the well being of an aborted fetus, or that may potentially cause illness in the neonatal foal. However, this examination does not aid the clinician in decisions that are aimed to prevent abortion or neonatal diseases of the foal. Evaluation of the placenta in the pregnant mare must be performed by the use of ultrasonography and endocrine tests.
Ultrasonographic Evaluation of the Equine Placenta
Ultrasonographic examinations of the placenta in mares that are considered to be at risk for abortion during late gestation can be performed by a transabdominal approach (Figure 5.24-2). With a 5- or 7.5-MHz sector scanner, four quadrants of the placenta should be examined; right cranial, right caudal, left cranial, and left caudal. With this technique, mares with normal pregnancies should have a rrunimal combined thickness of the uterus and the placenta (CTUP) of 7.1 +/-1.6 mm, and a maximal CTUP of 11.5 +/-2.4 mm. Pregnancies with an increased CTUP have been associated with the delivery of abnormal foals. The caudal portion of the allantochorion cannot be imaged with transabdominal ultrasonography, which prevents the clinician from diagnosing ascending placentitis in its early stages. However, placental thickening and partial separation of the allantochorion from the endometrium may be observed with transabdominal ultrasonography in mares that have placentitis originating from a hematogenous infection. In addition, a pocket of hyperechoic fluid can be seen at the base of the lowest area of the gravid uterus in mares with placentitis caused by N. actinomycete. Mares that graze on endophyte-infected fescue often experience retained placenta, premature separation of the allantochorion, and increased allantochorion weight and thickness. A significant increase in uteroplacental thickness and premature separation of the allantochorion has been found on transabdominal ultrasonographic examination of endophyte-infected mares. However, the thickness was not observed until an average of 8 hours before the onset of labor.
Transrectal ultrasonography of the caudal allantochorion in late gestational mares provides an excellent image of the placenta close to the cervical star (). A 5-MHz linear transducer should be positioned 1 to 2 inches cranial of the cervical-placental junction, and then moved laterally until the middle branch of the uterine artery is visible at the ventral aspect of the uterine body. The CTUP should then be measured between the middle branch of the uterine artery and the allantoic fluid (). The clinician has to make sure that the amniotic membrane is not adjacent to the allantochorion, because this may result in a falsely increased CTUP. The CTUP should be measured in the ventral part of the uterine body. The CTUP in the dorsal part of the uterus is often thicker than in the ventral part of the uterus. In addition, placental parts of the dorsal uterus have often been found to be edematous in normal pregnant mares during the last month of gestation (). Normal values for CTUP are illustrated in Table Normal Upper Limits for the Combined Thickness of the Uterus and the Placenta during Late Gestation. An abnormal thickness and partial separation of the allantochorion from the endometrium has been observed on ultrasonographic examination in mares with clinical signs of ascending placentitis (). A CTUP greater than 8 mm between day 271 and 300, less than 10 mm between day 301 and 330, and greater than 12 mm after day 330 has been associated with placental failure and pending abortion. In advanced stages, the space between the uterus and the placenta is filled with hyperechoic fluid.
Table Normal Upper Limits for the Combined Thickness of the Uterus and the Placenta during Late Gestation
|151-270 days||<5 mm|
|271-300 days||<7 mm|
|301-330 days||<9 mm|
|331 +||<12 mm|
Although transrectal and transabdominal ultrasonographic examination of the placenta is useful to detect early signs of some placental pathology, the clinician should keep in mind that placental changes resulting in periparturient problems can be subtle and not readily detected on ultrasonographic examination.
Endocrine Monitoring of the Placenta
The equine placenta is part of an endocrine fetal-placental interaction that synthesizes and metabolizes progestogens. This endocrine function of the placenta is important for maintenance of pregnancy after the endometrial cups and the secondary corpora lutea disappear at approximately day 150 of gestation. Mares with advanced stages of placentitis or placental separation may have increased plasma concentrations of progestogens as a result of stress to the fetal placental unit. In contrast, circulating progestogens have been reported to fall below normal after fetal hypoxia and infection with equine herpesvirus. Although increased concentrations of plasma progesterone during mid and late gestation would suggest placentitis, therapeutic decisions should not be made on the basis of one sample. Serial blood samples need to be obtained from an individual mare in order to detect a clinically useful trend in progesterone concentrations. Fetal-placental progesterone is rapidly metabolized to 5-pregnanes, and the metabolites may not be recognized by commercial progesterone assays. Therefore maternal serum progesterone concentrations in late pregnant mares do not accurately reflect the conditions in the uterus. Monthly blood sampling of mares at risk of abortion showed no differences in plasma progesterone concentrations in mares with impending abortion and mares with normal pregnancies.
Both estradiol and conjugated estrogen (estrone sulfate) are elevated during late pregnancy in mares. Estrone sulfate in maternal serum is thought to be a marker of fetal well-being. However estrogens have not been useful to detect early signs of placentitis.
Relaxin is produced by the equine placenta and can be detected in peripheral blood plasma from day 80 of gestation and throughout the pregnancy. The role of relaxin during pregnancy is not fully understood, but some evidence exists that placental relaxin production is compromised in mares at risk of aborting their fetuses. No commercial test for equine relaxin is currently available, and more research needs to be performed to evaluate the usefulness of plasma relaxin as a clinical tool to diagnose placentitis and to monitor the efficacy of treatment strategies.