One of the most common causes of infertility in mares is an infection in the uterus. However, many mares are treated for infections even though they might not actually be infected. Just because a culture of the uterus reveals an infectious organism does not mean this organism is a real pathogen causing a problem. Consequently, it is important to also look at the uterine cells under a microscope to make sure there is actual inflammation and the mare really is infected.
For uterine infections, it is common to infuse antibiotics in the uterus; this is the method that has been used for many years for treatment. However, it is important to know that a lot of these mares have material in their uterus that will inactivate the antibiotics, so it is always a good idea to flush the uterus with large volumes of sterile solutions before infusing. Most mares need antibiotics infused in the uterus for 3 to 7 days, depending on the infection. Also, systemic use of antibiotics has been shown to reach high concentrations in the uterus and can be effective for treating uterine infections. Systemic antibiotic treatment means that you are giving antibiotics orally or by injection rather than placing them directly in the uterus. The advantage of treating with systemic antibiotics is you do not have to physically enter the uterus daily and potentially contaminant the uterus. A disadvantage is that a longer treatment period is required and only certain antibiotics are effective. If the uterine infection is due to an anatomic abnormality of the genital area, no infection will be cured long term until the abnormality is surgically repaired.