IN-DEPTH: CONDITIONS AND MANAGEMENT OF THE PERIPARTUM MARE


appear hyperechoic. In hydramnios, the amniotic membrane may appear taut rather than its more usual undulating appearance. Transabdominal ul- trasound examination is also very useful when dis- tinguishing between conditions contributing to abdominal enlargement in the pregnant mare. In a normal pregnancy, the volume of allantoic fetal fluid ranges from 8 to 18 l and the volume of amni- otic fetal fluid from 3 to 7 l.39–41 Mares suffering from hydrallantois or hydramnios can accumulate massive quantities of fluid within either the allan- toic or amniotic compartments or a combination of both with up to 230 l reported for a case of hydral- lantois and up to 96l reported for one case of hy- dramnios.41,42 Determining fluid depth in the amniotic and allantoic compartments can help dis- tinguish between the hydropic conditions and also monitoring how fluid depth changes over time. It is reported that allantoic fetal fluid depth aver- ages a maximal vertical depth of 13.4  4.4 cm and 7.9  3.5 cm for amniotic fluid.15 Cases of hydrops may have depths of more than 30 cm usually the maximum that can be measured ultrasonographi- cally. Using transabdominal ultrasound guidance, amniocentesis and allantocentesis of fluid samples can be sampled from the respective compartments and analyzed for biochemical constituents to confirm the origin of the enlarged compartment. This pro- cedure does carry a significant risk of abortion and should be performed by experienced veterinarians.43 Amniotic fluid has higher concentrations of sodium, chloride, and total carbon dioxide and alkaline phos- phatase activity and lower concentrations of glucose, potassium, magnesium, phosphorus, total protein, creatinine, total bilirubin and -glutamyltrans- ferase activity than allantoic fluid.37 Calcium, CK, and urea were greater only in allantoic fluid in the few weeks before parturition.44 Fetal biophysical parameters may reveal fetal distress with abnor- malities in FHR, evidence of growth restriction, an increased CTUP and thickening of the chorioallan- tois and amnion from edema.


9. Other Causes of Abdominal Enlargement


Abdominal wall rupture and/or rupture of the pre- pubic tendon can also cause abdominal enlargement in pregnant mares. Abdominal wall rupture may be due to traumatic injury to the abdominal muscles or can be idiopathic in origin. Abdominal wall rup- ture can occur bilaterally or unilaterally, or along the ventral midline. Unilateral muscle wall rup- tures are likely to be traumatic in origin. Idio- pathic abdominal wall ruptures are thought to be more common in draft breeds and older brood- mares.45 Researchers measured normal abdominal muscle thicknesses, with ultrasound, in pregnant mares and reported the results in a study.46 The muscle measurements in heavy-type mares were rectus abdominis [18.0 1.4 (15.6–21.0)] [mean (mm)  SD (range)], pectoralis [11.2  0.7 (10.5– 11.8)], and cutaneous trunci [9.8  0.4 (9.1–10.0)].


They noted in 2 mares prior to muscle wall rupture that although muscle thicknesses were within the normal range there were echotextural changes in- cluding increased linear areas of intramuscular edema and increased echogenicity from hemorrhage identified in the cutaneous trunci and internal ab- dominal oblique muscles. The investigators also noted that serum CK, but not AST, levels were elevated.46 Prepubic tendon rupture is another cause of ab- dominal enlargement. Ruptured prepubic tendons can be partial or complete and may be present in association with abdominal wall muscle ruptures. Differentiating clinically between partial prepubic tendon ruptures and caudoventral muscle ruptures may be difficult. With complete rupture of the pre- pubic tendon, the udder will drop and rotate crani- ally and hemorrhage will be visible in the mammary secretions. The mare often develops widespread edema in the ventral abdomen and mammary gland. Mares with a prepubic tendon rupture exhibit lor- dosis or a “saw-horse stance.”


10. Other Placental Conditions Affecting Late Pregnancy


Premature placental separation secondary to other placental conditions may be detected during ultra- sound scans assessing fetal wellbeing although the primary cause can remain unidentified. Rapid pla- cental separation is a feature of acute herpes virus infection secondary to uteroplacental vasculitis. Separation may also occur with fescue toxicosis, mare reproductive loss syndrome (MRLS) placenti- tis and ischemic necrosis of the cervical pole. Ex- tensive placental separation will result in fetal death and abortion or preterm delivery; however, partial separation may be compatible with the preg- nancy continuing toward term. Fetal compromise as a result of placental compromise occurs from re- duced placental contact resulting in hypoxia. Body pregnancy is a rare condition that can also result in placental compromise. Body pregnancy occurs when the vesicle fixates in the uterine body and the developing chorioallantois does not extend normally into the pregnant horn. Caudal body pregnancies are usually lost early in gestation but cranial uter- ine body pregnancies can develop to term.47 Fi- nally, villous atrophy or hypoplasia can be a cause of fetal growth restriction and late fetal loss.48 Vil- lous atrophy may correlate with focal or diffuse chronic degenerative endometrial disease. Mares with significant chronic degenerative endometrial disease detected prior to becoming pregnant are suitable candidates for late pregnancy monitoring to detect the occurrence of any fetal compromise early on.


11. Fetal Deformities


A variety of congenital defects occur in foals, some of thesemaybe true malformations and have a genetic or toxic etiology; however, some of the more common con-


AAEP PROCEEDINGS  Vol. 65  2019 137


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