Comparison of serum amyloid A concentrations in mares with experimentally induced placentitis to normal, pregnant mares using a commercially available, stall-side device

Abstract

Ascending bacterial placentitis, a significant cause of pregnancy loss in mares, is challenging to diagnose. Diagnostic tools for ascending placentitis include detection of premature mammary development, vulvar discharge, and ultrasonographic changes to the caudal fetal membranes. These tools have limited sensitivity. Changes in serum biomarkers (ie, serum amyloid A, SAA) provide useful, adjunct information for diagnosing placentitis, but point-of-care assays are limited. The goal of this study was to evaluate a quantitative, stall-side test (Stablelab®, Zoetis, NJ, USA) to measure SAA concentrations in plasma from mares with experimentally induced placentitis compared to normal pregnant mares. We hypothesized that SAA concentrations in mares with placentitis would be higher than normal pregnant mares and that changes in SAA would be predictive of placentitis. Placentitis was induced in 12 mares (270-300 days gestation) with trans-cervical deposition of 1 × 107 Streptococcus zooepidemicus directly onto the chorioallantois. Infected mares were randomly assigned to: infected/untreated (UNTREATED; n=6); or infected/firocoxib (FIRO; n=6; administered firocoxib two days prior to infection until abortion/delivery). Gestationally age-matched, normal pregnant mares were assigned to NORM (n=6). Plasma samples were obtained from mares prior to the study (Day 0) and daily until abortion/delivery. Mares from NORM were sampled similarly to infected, gestation-matched mares. Duplicate plasma samples from post-infection Days 0-9, and peripartum, were evaluated for SAA concentrations using a Stablelab® system, permanufacturer's instructions. SAA concentrations from mares were compared between groups (UNTREATED, FIRO, NORM). To normalize distribution, data were log-transformed and groups compared (geometric means ± SE) using a two-sided t-test. Differences were not identified between infected mares (UNTREATED/FIRO), so SAA concentrations from all infected mares were grouped (INFECT; n=12) and compared to NORM (n=6). Significance was set at P < 0.05. SAA concentrations were significantly higher in INFECT than NORM by Day 4 post-infection (17.81µg/mL± 10.44 v. 0.76µg/mL± 0.62) and remained significantly higher through Day 9 post-infection (412.32µg/mL± 306.67 v. 0.40µg/mL± 0.04). Prior to foaling, SAA concentrations for INFECT were significantly greater than NORM at -48h (34.72 µg/mL± 23.07 v. 0.41µg/mL± 0.31); -24h (85.32 µg/mL± 48.09 v. 0.6µg/mL± 0.58); at foaling (130.19 µg/mL± 68.97 v. 0.41µg/mL± 0.31); and 24h after foaling (468.25 µg/mL± 224.15 v. 7.63 µg/mL± 4.95). These data showed that mares with experimentally induced placentitis had higher concentrations of SAA than normal foaling mares, early after infection and prior to foaling. Differences in SAA concentrations were detectable using a commercially available, stall-side product (Stablelab®, Zoetis). Monitoring SAA, with a point-of-care assay, may provide useful, adjunct information for diagnosing bacterial placentitis in mares. Funding: Grayson Jockey Club; Zoetis; Merial; Kentucky Thoroughbred Breeders, University of Florida