Endometritis is an invisible disease. It affects the delicate lining of a mare's uterus–the endometrium–which can become inflamed and create a hostile environment for sperm, as well as any resulting embryo, to live. Thus, often the only clinical sign of endometritis is not what you do see but what you don't see: a pregnancy. Endometritis is a major cause of female infertility, affecting up to 15% of broodmares. But because it frequently lacks clear clinical signs, it often goes undiagnosed.

Inflammatory Responses of the Uterus

The mare's uterus is well-protected physically, with the cervix, vestibular-vaginal sphincter (hymen), and vulva lips all serving as effective barriers against contaminants such as feces, urine, and bacteria. However, these contaminants can still enter the uterus during mating or artificial insemination, as well as during estrus or veterinary procedures. To clear contaminants and dead sperm after breeding, the uterus sets off a natural inflammatory response. This acute form of endometritis is a healthy, effective cleaning system that summons an army of white blood cells (leukocytes) to attack and kill bacteria and rid the mare of dead sperm. The debris is then expelled from the uterus, which returns to its normal, uninflamed state. In healthy, fertile mares this process takes less than two days, says Irwin Liu, DVM, PhD, professor of equine reproduction and theriogenology at the University of California, Davis