What do we know about susceptibility of mares to endometritis?

Breeding of mares is followed by uterine contamination of bacteria and inflammation. The inflammation is due to polymorphonuclear leukocytes (PMN) which are induced by spermatozoa. Normal mares eliminate bacteria, sperm and inflammatory by-products rapidly, in 24-48 hours, but some mares are not capable of doing this. These mares are called susceptible to uterine infections, because the normally transient inflammation becomes persistent endometritis. This review focuses on uterine defence mechanisms on post-breeding endometritis particularly and on the predisposing factors that lead to persistent uterine infection. The defence mechanisms of the uterus against foreign invaders comprise complex interactions between different elements. The defence is largely modified by steroid hormones. The uterus is part of the mucosal immune system which consists of humoral and cellular defence. All aspects of PMN function are vital in the defence against bacterial invaders: migration in sufficient numbers into the desired site in response to chemotaxis, coating of bacteria by opsonins, phagocytosis and intracellular killing. There is no conclusive evidence, however, that any of these are deficient in mares susceptible to endometritis. Instead, increasing evidence shows that differences in the mechanical drainage of the uterus provide the best explanation for susceptibility to uterine infections. Adequate cervical dilation, effective myometrial contractions and lymphatic drainage are required for the maintenance of uterine health. Delayed uterine clearance results in intrauterine fluid accumulation and in subsequent endometritis. The anatomical barriers of the mare have to be intact: their disruption severely predisposes the mare to endometritis.