Biomarkers for placental disease in mares

Placentitis is an important cause of abortion, stillbirth, and neonatal death in horses. The diagnosis of placentitis is based on occurrence of clinical signs (premature mammary gland development and vulvar discharge) and ultrasonography of the caudal placental pole. However, early and subtle cases can be missed. In the last few years, several studies have provided objective means of diagnosing placentitis in mares with single or serial measurements of blood markers. Among the markers evaluated the steroids produced by the fetoplacental unit have been shown to change in association with placentitis. Mares with chronic placentitis have an increase in peripheral progestogens; however, mares acutely infected will display a reduction in peripheral concentrations of progestogens. Estradiol-17β (free- and conjugated form) concentrations are drastically reduced in plasma of mares with placentitis. Acute-phase proteins, particularly serum amyloid A, are increased in plasma of mares suffering from placentitis, and this increase is due to endometrial and chorioallantoic secretions, and minimally from the fetus. Alpha-fetoprotein, a protein expressed in the fetoplacental unit, was shown to be increased in plasma of mares suffering from placentitis. A plephora of microRNA have been identified in plasma and tissues of mares undergoing experimentally induced placentitis, but have not been tested in spontaneous cases. Unique proteomic signatures were found in the fetal fluids of mares undergoing experimentally induced ascending placentitis, making the fetal fluids potentially useful to diagnose placentitis in mares. However, currently the lack of use of transabdominal fetal fluid sampling prevents wide use of the fetal fluids as diagnostic techniques. This manuscript aimed to discuss recent discoveries regarding biomarkers for placentitis in mares.