Uterine Clearance

Delayed physical clearance of intrauterine fluids, bacteria and particulate matter is a major contributor to persistent endometritis in both pluriparous and older maiden mares. In addition to gross physical clearance, cellular and humoral mechanisms are active. A number of theories exist as to why pluriparous and older maiden mares are unable to clear their uterine lumen.
In normal mares, uterine clearance depends on a transient local neutrophilic inflammatory reaction and coordinated myometrial activity clearing the uterus of residual semen, bacteria and particulate contaminants introduced during breeding. Inflammatory mediators produced in response to endometrial bacteria and spermatozoa increase vascular permeability allowing entry of neutrophils and serum proteins into the uterine lumen. Antibody mediated opsonization, phagocytosis and intracellular killing of bacteria results. Most uterine fluid is then eliminated by uterine contractions until cervical closure, when lymphatic drainage removes the remainder of the fluid, inflammatory mediators and immune cells.
The majority of mares effectively clear the uterus before descent of the embryo 5 to 6 days post ovulation, however some pluriparous and older maiden mares (susceptible mares) develop a persistent post breeding endometritis that reduces fertility. These susceptible mares have delayed uterine clearance that allows accumulation of fluid within the uterine lumen following breeding. This fluid provides an unfavorable environment for immunological clearance, with resultant inflammation depressing fertility, impairing contractility and damaging the endometrial surface allowing enhanced bacterial adherence. A cycle of persistent endometrial inflammation is enabled.[…]