Q: I’m wondering if there is a bacterial component that can be present in stallion semen. We have a stallion that will settle mares easily one year and then not be able to settle mares the next. We have had his semen tested at Colorado State University, and they gave us recommendations about collecting him that we are following. But we still have this recurring problem, so I am wondering if it is possible for a stallion to infect a mare, causing fluid to build in her uterus and, thus, making it more difficult for the embryo to attach to the uterine wall.

Linda Conde, via e-mail

A: A short answer to your question is yes, bacteria can be present in semen that might result in decreased pregnancy rates. Several factors can contribute to the presence of bacteria in a breeding dose of semen. For instance, the semen in its raw form can contain bacteria, resulting from a bacterial infection of the stallion’s reproductive or urinary tracts. But the most common source of bacteria is from improper handling of semen during or after collection. Breeders and veterinarians must practice strict hygiene during every step of collecting, processing, shipping, and breeding.

Veterinarians can evaluate semen microscopically for bacteria as well as grossly (without a microscope) for the presence of purulent (consisting of pus) clumps or blood-tinged fluid. Cultures of the raw and processed semen can help identify the bacteria and an effective antibiotic that can be used in the semen extender as well as post-breeding in mares.

It is also critical to ensure the mare’s uterus is not the source of the bacteria, resulting in a post-breeding inflammatory reaction. Thus, a veterinarian should evaluate mares with culture and cytology before breeding. If he or she determines the mare to be clean even with fluid present after breeding, a post-breeding lavage (flushing) can be beneficial. Your veterinarian might also prescribe drugs for the mare such as the prostaglandin analogue cloprostenol or oxytocin, which both help clear fluid from the uterus after breeding. By determining the source of the bacteria, your veterinarian can help you establish a protocol that will result in more consistent pregnancy rates.