bacterial infections in the uterus can cause reduce fertility in the mare. Bacteria gain access to the uterus through the cervix, due to poor conformation or during breeding or foaling. Healthy young mares clear bacteria rapidly. In mares with compromised uterine defense mechanisms bacteria can establish an infection in the uterus. If the infection is unnoticed or left untreated, the infection may become chronic.
The most commonly isolated bacteria from such conditions are Streptococcus equi subspecies zooepidemicus (S. zoo). In the chronically infected mare S. zoo can establish an infection deep within the uterine lining (endometrium). Strep zoo appear to lower its metabolic rate during chronic infection, thus complicating diagnostics and antimicrobial treatment dependent of active bacterial growth.