Reasons for performing the study: Ascending placentitis results in premature birth and high foal mortality. By understanding how placentitis induces premature delivery, it may be possible to develop diagnostic markers and to delay premature delivery pharmacologically, thereby decreasing perinatal foal mortality.
Objective: To identify relationships between bacterial infection, inflammation and premature parturition in mares with experimentally induced placentitis.
Materials and methods: Experiment 1: Concentrations of allantoic fluid prostaglandins (PGs) F2α and E2 were measured in 8 mares after intracervical inoculation with Streptococcus equi ssp. zooepidemicus (at Days 285–291 of gestation) until parturition and compared with controls (n = 4). Experiment 2: mRNA expression of interleukin (IL)-1β, IL-6, tumour necrosis factor (TNF)-α and IL-8 in the chorioallantois from inoculated mares in Experiment 1 were compared with 7 mares that foaled normally.
Results: Bacterial inoculation resulted in 7 aborted fetuses and birth of one premature, viable foal. Infection was associated with inflammation of the chorioallantois in the region of the cervical star, isolation of bacteria and high concentrations of PGE2 and PGF2α in allantoic fluid obtained within 48 h of delivery (P = 0.04). Chorioallantois from all mares expressed mRNA for IL-8, TNF-α, IL-6 and IL-1β. Experimentally infected mares expressed more mRNA for IL-6 (P = 0.003) and IL-8 (P = 0.009) in the cervical star region and more mRNA for IL-6 (P = 0.004) in tissues from placental horns than control mares.
Conclusions and clinical relevance: Bacterial placentitis may result in liberation of cytokines from the chorioallantois and prostaglandin formation leading to abortion or birth of a precociously mature foal.