Histological differences in the placenta of three mares treated for ascending bacterial placentitis compared to physiological pregnancies.

Ascending bacterial placentitis is a frequent cause of late pregnancy loss in mares. The histology of placentitis has been described but mainly with a focus on inflammatory changes. In this study, we assessed several aspects beyond inflammatory processes between mares foaling close to term during treatment for placentitis and physiological pregnancies. We included placental tissue from physiological term pregnancies (group PP; n = 8) and clinical cases with confirmed ascending bacterial placentitis (group PL; n = 3). Samples were collected from several locations of the allantochorion (cervical star, base, and tip of both uterine horns). Immunostaining was performed for caspase 3 (apoptosis), Ki67 (proliferation), and MAC387 (macrophages); histochemical staining was made with PAS (Periodic acid Schiff, blood vessels) and Kossa stain (microcalcification). Allantochorionic cell proliferation was increased (p < 0.05) in PL mares compared to PP mares at several locations; no group differences in caspase 3 indicating apoptotic activity were detected. Whereas the blood vessel area in the allantochorion was increased in PP mares in some locations, a larger number of blood vessels (p < 0.05) was determined at the base of the pregnant horn in PL mares. In conclusion, bacterial ascending placentitis was characterized by tissue proliferation and vascularization. This might suggest compensatory mechanisms that were induced because of tissue function loss due to inflammation.