In the past, endometritis was considered to be caused primarily by bacteria or fungi introduced to the uterus during breeding. Although this is certainly a cause of infection, it is reported that inflammation by the sperm can cause enough irritation to prevent pregnancy. Semen from natural service and fresh cooled semen contains seminal plasma that normally suppresses inflammation to sperm in the uterus after breeding. However, seminal plasma is removed from frozen semen, and this may be one reason the reproductive rates on breeding mares with frozen semen are less. Regardless, sperm-induced inflammation has been shown to be a major cause of infertility in mares.

Also, some mares are more susceptible to infection while others are resistant. Those susceptible to infection usually have a delay in clearance of organisms due to a decrease in uterine muscle function or may have poor conformation of the anal and vulvar area, allowing the entrance of bacteria.

This inflammation can be documented with uterine cytology by taking a swab and looking at cells of the uterine lining. Also, excessive uterine fluid has been shown to correlate with uterine inflammation after breeding. This fluid must be removed for a pregnancy to occur. Flushing the uterus with sterile fluids and administering oxytocin and prostaglandins can aid in fluid removal in mares that are difficult to get pregnant.

Uterine cytology along with a culture is critical to determine if infection or inflammation is present. A uterine biopsy is also important in not only determining inflammation but also can give you a prognosis on your mare becoming pregnant and carrying a foal to term. If you have a mare that has difficulty becoming pregnant or carrying a foal to term, call us to schedule a breeding soundness exam.