Uterine defense mechanisms in the mare

External genital barriers together with local uterine defense mechanisms protect the mares' uterus from infection. As a result of intrauterine ejaculation, a transient uterine inflammation occurs during mating in horses. This is possibly a necessary process to clear the uterus from seminal contamination. While most young nulliparous mares are fully capable of clearing an inflammation from the uterus within 36 hours, many older, multiparous mares, fail to clear the uterus before the descent of the embryo at day five after conception. Failure to clear the uterus from an inflammation in mares that are susceptible to persistent endometritis is caused by an impaired myometrial function as a response to inflammation. Dysfunctional myometrial contractility results in an accumulation of fluid and inflammatory products in the uterine lumen, providing a uterine environment that negatively affects phagocytosis by PMNs and is incompatible with embryonal survival.