Transrectal ultrasonography and plasma progestin profiles identifies feto-placental compromise in mares with experimentally induced placentitis

Transrectal ultrasonography of the caudal uterus and a progestin profile were evaluated for accuracy in identifying mares with feto-placental compromise in a model of placentitis. Twenty-two pregnant ponies were divided into four groups: (1) control mares (n = 5); (2) instrumented controls (n = 2); (3) instrumented inoculated mares (n = 11); (4) inoculated mares (n = 4). Mares in Groups 3 and 4 were inoculated with Streptococcus equi subsp. zooepidemicus. Maternal plasma progestins, vulvar discharge, mammary gland development, combined thickness of the uterus and placenta (CTUP) and placental separation were evaluated weekly before instrumentation, inoculation or Day 320 (Groups 1 and 2) and, thereafter, either daily (first three measurements) or several times weekly (last two measurements). Plasma progestin profiles were plotted to identify pattern characteristics. An abbreviated profile was created, consisting of four progestin samples collected at 48-h intervals, with Sample 1 collected the day before inoculation or on Day 285 in controls. Profiles were considered abnormal if Samples 2, 3, or 4 increased or decreased by more than 50% of Sample 1. A CTUP > 1.0 cm or placental separation were considered abnormal. Placentitis was confirmed by histology of fetal membranes. Control mares had normal progestin profiles, transrectal ultrasonographic and clinical examinations. Control foals were born after Day 329; six were viable and one died after dystocia. All inoculated mares developed placentitis and foaled before Day 314. Thirteen of 15 foals were not viable. All inoculated mares had abnormal progestin profiles and 13 of the 15 were identified by the abbreviated progestin profile. Transrectal CTUP was affected by gestational age and increased after inoculation (P < 0.05). Nine of 15 inoculated mares had a CTUP > 1.0 cm by 5-day post-inoculation. By performing both tests, 20 of 22 mares were correctly identified with respect to pregnancy outcome. However, three inoculated mares exhibited minimal clinical signs and likely would not be examined in a clinical setting. These tests were diagnostic for identifying feto-placental compromise in the mare.