Every year we have some mares come to us for breeding that failed to become pregnant the previous year. After an ultrasound to visually evaluate the reproductive tract and deal with any issues found there, we often suggest a culture of the inner lining of the uterus (the endometrium) to be sure there is not a population of harmful bacteria. Even bacteria not typically considered to be problematic, can prevent an embryo from maturing if the bacteria is present in a significant concentration.
We take a swab of the uterus, place it in growth media, and send it to a lab. The lab applies the swab to several different growth plates, and evaluates the growth every 12-24 hours for 72 hours. At the end of this time we hope for a result of “no growth”. Some cultures result in the isolation of one or more bacteria species. The lab will rate the bacterial growth as ‘scant’, ‘mild’, ‘moderate’, or ‘heavy’. This is necessary because a scant or mild growth of a bacteria not known to cause disease in the mare, may be discounted as a contaminant, or lab error.
Bacteria Reproduction
On the other hand, a heavy growth of the same bacteria might warrant treatment. Each bacteria is further exposed to antibiotics to evaluate if it is sensitive or resistant to each applied antibiotic. We evaluate these results and make an antibiotic selection based on several factors: if the antibiotic works in the uterus as well as it does in the lab (some do not), if the antibiotic is safe for use in the uterus, if the antibiotic is safe for use in the horse, if the cost is not prohibitive, if the individual mare has shown sensitivity/allergy to the antibiotic before, if we plan to use the antibiotic in the uterus or systemically.
Once we have an antibiotic choice, we have to know the preferred protocol. Is the pH of our antibiotic appropriate or does it need to be buffered or diluted more than usual? Is it effective after 3 days of treatment or does it need 5? Should it be consecutive days or intermittent? How many days after the last treatment can we safely breed her?
Breeding Soundness Exam
Once we answer all these questions and treat our mare, all is well and she should have no problem conceiving next breeding, right? That is surely what we hope for, but then we run into a few possible complications that can be caused by even treating a uterus in the first place: secondary bacterial or fungal infections may develop, there may be a significant endometrial reaction to the antibiotic itself, treatment for one bacteria may allow the proliferation of another bacteria, or the mare may simply fail to resolve the initial bacterial infection.
This is a quick look at the reproductive infection side of infertility. There are of course other causes: hormone issues, previous foaling trauma, anatomic abnormalities, timing issues… One of the smartest choices you can make if you decide to breed your mare, is to start with a breeding soundness exam first. It can let you know that some of these problems are not going to be a problem for you later, and help you approach the breeding season with more knowledge and less wasted time.
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