Cover Story | Reproduction

Should You Use Antibiotics to Combat Breeding-Induced Endometritis?

By Marie Rosenthal MS

Flushing the uterus, rather than infusing antibiotics, is a better first response for a mare at risk for persistent breeding-induced endometritis, according to Tom Stout, PhD, ECAR, DRDVA, who spoke at the BEVA Congress 2025, in Birmingham, England.

“I try to avoid antibiotics,” said Dr. Stout, the Albert Clay Chair in Equine Reproduction at the Gluck Equine Research Center, and a professor at the University of Kentucky, in Lexington.

Breeding-induced endometritis and the associated inflammation are normal phenomena that occur after mating or artificial insemination (AI), he reminded. “You introduce semen into the uterus, and that induces a reaction. Bear in mind that the sperm cells induce this reaction, and if we don’t put sperm into the uterus, the mare’s not going to get pregnant.”

If working properly, the neutrophils that “come charging in” target dead or dying sperm, leaving the healthy live ones to fertilize the oocyte. Afterward—usually within 48 to 72 hours—the mare clears the inflammatory debris.

“The uterus will contract and will expel a lot of the inflammatory fluid and the sperm that are no longer necessary,” Dr. Stout said. The lymphatic system typically handles the rest, leaving a favorable uterine environment when the embryo comes in.

However, some mares do not expel the excess fluids, triggering a prolonged inflammatory response that becomes difficult to stop. “You get into the situation where you can’t get rid of the inflammatory fluid, and there is more cytokine generation going on for longer, which is not great when you want a clean uterus for when the embryo comes in.”

According to previous studies, about 10% to 15% of broodmares cannot clear their uterus within 48 hours. “If the mare still has more than 2 cm of free uterine fluid the day after breeding, it’s an indication that there’s an ongoing inflammation that we need to do something about,” Dr. Stout said.

The reasons for not expelling the fluid can vary, but the problem is more likely to occur in older mares and embryo donor mares than in young mares or foaling mares, he explained.
Fresh or frozen semen might also play a role. “In our hands, the frozen semen only really became a problem in the oldest mares—the 14-year-old plus [mares]. About a third of them retained fluid the day after insemination,” Dr. Stout said.

A history of this problem is a good indication that it will happen during subsequent inseminations. In those cases, early action is necessary, he said.

Some mares will have obvious predispositions. For example, excessive mucus in the uterus might be seen on a scan, or there could be cloudiness after a lavage with lactated Ringer’s. Acetyl cysteine can help thin mucus and help wash fluid from the uterus. If he sees a lot of mucus before breeding, he will treat with an acetyl cysteine infusion and lavage before breeding.

“You want to minimize the challenges, and the major challenge to the uterus, of course, is the insemination,” he said. For some mares, a dose of corticosteroids—he uses dexamethasone—just before or at the time of the AI is useful. “We would use it, but it’s not something you’re reaching for in every mare. Adjunctive therapies are available, but you’re not going to use them very often because there aren’t many mares that need them,” he said.

“In terms of treatment, where there’s an obvious predisposition like poor vulva conformation, you need to deal with it before you start. Thereafter, the main part of the treatment is assisting mechanical clearance—getting rid of sperm that no longer need to be there and the neutrophils, which are coming in,” he explained. Exercise, oxytocin and prostaglandin are all beneficial for enhancing uterine clearance. In Lexington, many veterinarians use prostaglandin in place of oxytocin because of its long duration of action, he said.

However, he would not use prostaglandin if the mare had already ovulated, he said.

“In old embryo donor mares, manual dilation of the cervix is one of the most helpful treatments,” he said. If the cervix is closed, no amount of uterine contraction is going to effectively expel excess fluid. “For lavage, we basically used lactated Ringer’s. We had 3-L bags, and we would generally put 1.5 L in, flush it out, infuse another 1.5 L, and wash the uterus out twice.”

Lavage can be done shortly after insemination, as soon as 4 to 6 hours afterward. With deep intra-uterine insemination of frozen semen, “you can flush as early as 1 hour after the insemination without reducing the pregnancy rates. And it just means you can start flushing out debris or inflammatory stimuli before the process really gets going,” Dr. Stout said.

However, he warned, you cannot keep “flushing and flushing and flushing.”

He explained: “We would normally try to stop at about 2 days after ovulation. If you go beyond that, the progesterone is rising, which means the mare’s innate resistance or the uterus’s innate resistance to anything else that you bring in is reduced. And at about day 6, the embryo is going to enter, and the uterus needs some time to recover, even from some something as simple as flushing with lactated Ringer’s.”

His “rule of thumb” is essentially that 2 days of fluid would require 2 days of oxytocin, and about 30% of those mares with 2 cm of fluid would require lavage. This appears to stop fluid accumulation from affecting the pregnancy rates—certainly in young mares.

Bacteria are a secondary issue, and are probably opportunistic, he added. Some bacteria will be introduced on the stallion’s penis or in the semen during insemination, and some are probably normal flora from lower in the reproductive tract that flourish when the inflammatory process is triggered.

The question is whether to treat them with antibiotics. A primary infectious endometritis detected before breeding should be treated, he said.

The evidence also suggests that prophylactic antibiotics “do something,” but they don’t seem to be as effective as lavage. “This 2 cms of fluid a day after AI or breeding—if you haven’t already intervened beforehand—is a useful indication that the mare requires some treatment. In our population, about 25% of mares retained fluid. In others, it may be less.”

It may be different in a naturally mated population, where the rate of fluid retention may be lower. He added most mares will respond well to simple treatments, like oxytocin and uterine lavage.

“What about antibiotics? Well, they do what it says on the packet; they kill bacteria, and they reduce neutrophil accumulation. However, they may be an irritant and seem to cause a little more edema.”

Most importantly, there’s no good evidence they are more effective than lavage. One must also consider the downstream effects on antimicrobial resistance, and an antibiotic’s effect on the horse’s microbiome, he said.

So, even if they help, there are still questions about the routine use of antibiotics and other more aggressive additional therapies, Dr. Stout explained. MeV