Cleaning Up “Dirty” Mares

A successful pregnancy often can be obtained by cleaning up a "dirty" mare through diagnosis of underlying issues and proper treatment.
portrait of a mare and foal. Cleaning up dirty mares can help them become pregnant.
“It only takes one problem mare each season to feel like all you have done is ‘clean up’ dirty mares,” said Dr. Sandy Tasse. Arnd Bronkhorst Photography

Chances are you successfully breed dozens of mares each spring. “It only takes one problem mare each season to feel like all you’ve done is clean up ‘dirty’ mares,” commented Sandy Tasse, DVM, an ambulatory veterinarian at Rood & Riddle Equine Hospital in Saratoga, New York. On average, she breeds 30-40 horses each season. She says only about 10%–20% are “dirty” and require extra effort to get bred. Sometimes, regardless of the work, the mare won’t conceive or hold the pregnancy.

Tasse recently had a maiden mare that was about 7 years old, theoretically an “ideal” candidate. But the mare retained a large volume of fluid in her uterus before they even attempted breeding.

“We tried lavaging, antibiotics, oxytocin and manual dilation of her cervix to get the fluid out of there. It just always comes back,” Tasse said. “So, it was a lost cause. If you can’t get rid of the fluid, and it is present when the embryo comes down, it’s like the embryo is jumping into a swimming pool. It’s going to die.”

In cases like these, sometimes you have to call it quits. But often, cleaning up “dirty” mares is possible. A successful pregnancy requires being aware of client goals, knowing the underlying issue(s), and following strict hygiene practices any time you enter the cervix for culture, lavage or insemination.

Cleaning up mares might require culture and lavage to ensure the mare can carry a pregnancy.
Cleaning up mares might require culture and lavage to ensure the mare can carry a pregnancy. Arnd Bronkhorst Photography

Understand Client Goals and Budget

Horse owners breed for a myriad of reasons. It sounds basic, but knowing the purpose behind the breeding is essential in determining the time and finances they can invest in breeding a mare.

“Ask the client about their goals,” said Tasse. “Find out if they have a budget cap and what they might be willing to try or invest in.”

Live Cover vs. Cooled or Frozen Semen

Tasse suggested finding out whether the mare will be bred live cover or with cooled or frozen semen. Artificial insemination might increase the chances of creating a “dirty mare” in certain cases.

“Some mares are more reactive to cooled semen,” she said. “The extender used to keep the semen alive until insemination is the perfect medium for bacteria to grow because it is the feeder system for the sperm.”

Tasse experienced this with an older maiden mare. Theoretically, the horse should have been clean. Admittedly, 16 years old was not the ideal time to breed a maiden mare. However, she had no signs indicating that she could not conceive or carry a foal.

Tasse inseminated the mare with cooled shipped semen. She found no signs of excessive inflammation or fluid at the ovulation check. However, she found a uterus full of fluids rather than an embryo at the two-week ultrasound for pregnancy.

One challenge: The mare’s cervix was barely opening on its own. Tasse believed that played a part in preventing the fluids from exiting post-breeding.


Tasse said a culture revealed pseudomonas, and the mare was lavaged and treated with antibiotics. It was close to mid-summer, so they elected to stop efforts until the following year. The mare was cultured in advance of the subsequent breeding, and there were no sign of fluid or bacteria. It took three cycles, but the mare got pregnant and had no issues that year.

“We were more aggressive with dilating her cervix, and I think that helped,” Tasse said. “This can be a common problem with breeding older maiden mares. Their cervix may not dilate normally when in estrus, and their uterus may not have effective enough contractions to expel the fluid post-breeding.”

Breeding with cooled or frozen semen can require more maintenance post-breeding, such as a lavage or antibiotics, thus increasing the cost per breeding cycle.

Bacteria Causing ‘Dirty’ Mares 

Mares that experience chronic uterine infections can develop scar tissue and other degenerative changes. E. coli, strep and pseudomonas are common bacteria in “dirty” mares. Identifying the bacteria informs the next steps the practitioner takes.

Tasse noted that strep is a common gram positive bacteria in mares, but it is generally easy to treat. Gram negative bacteria can be more difficult to address. The bacteria can produce a biofilm that is like a slime incasing it. This makes it hard for the antibiotics to get to the bacteria to kill them. Generally, a treatment that gets rid of the biofilm—such as N-aceytylcystine, DMSO or Tris-EDTA—is used intra-uterine first. Then, the mare can be infused with antibiotics.

Keep in mind that a swab culture might not provide a full picture of what’s going on. Even though swabs are double- and triple-guarded, there is a contamination risk. Additionally, it is a small sample from only the beginning of the uterus. Tess recommends a small-volume lavage, which uses 250 mL of fluid such as saline or lactated ringer, to take back to the lab and spin down for a more accurate picture.

“The goal is to pick up any bacteria that might be in there, so just going into the cervix with a swab means you could miss something,” she said. “The lavage is a more complete way to get a culture and hopefully not miss something, especially on a mare that has been problematic to breed.”

Benefits of Cytology for Cleaning Up Dirty Mares

Cytology can provide a more thorough picture of what is happening in the mare. If white blood cells are visible on the slide, it’s nearly guaranteed that the mare has an infection.

Results don’t rule out breeding. For example, a positive culture and positive cytology can lower the changes of pregnancy to 10%-20%. With a positive culture and negative cytology, the mare has a 40%-50% chance of breeding. A mare with a negative culture and negative cytology has the best likelihood of getting pregnant at 60%-70% if no other negative variables (such as low stallion fertility and timing of breeding is close to ovulation) are present, according to Tasse.

In some situations, a mare might be bred before a clean culture. Most live covers require a clean culture, whereas a farm shipping cooled semen might not. Stud farms often want the culture to be taken on the same cycle as you are breeding. That means the culture must occur the first few days of estrus with results taking 24-48 hours.

On post-foaling Thoroughbred mares, some stud farms breed one to two cycles without a culture. If the mare does not conceive, then the farm requires a clean culture before rebreeding. When a mare is bred on foal heat (nine to 12 days post-foaling), it is often expected that she will not be clean.

However, the ultrasound exam should be normal, uterine involution should have occurred, and she should have
a uterine lavage that looks clean even if there is no time to culture it before breeding. This helps ensure that the mare and stallion owners are not wasting their time breeding the mare if the chances of conception are low.

“It is ultimately the veterinarian’s decision on if a culture is needed, even if the stud farm is not requiring one,” Tasse said. “A mare should still have a normal ultrasound exam with no signs of inflammation or infection before being sent to be covered by the stallion.”

Timing the Culture

Timelines can make it tricky to clean up a “dirty” mare. Sometimes, you can use a cycle when the mare is in heat and her cervix is open to clean her up. Hopefully, the next cycle she’ll be clean, and you can breed her.

But some stallion farms don’t breed after July 1, and there might not be time to skip a cycle. In those situations, Tasse has the mare owner order the cooled semen and begins cleaning the mare with a lavage and infused antibiotics. Then she breeds the mare and performs a post-breeding lavage.

“You have up to 48 hours after breeding to go into the uterus; then you should not go in there anymore or risk interfering with the embryo, or trying to fight a closed cervix if ovulation has occurred,” she said.

Conformation Contributes to “Dirty” Mares

Suturing the top of a mare's vulva closed (a Caslick's procedure) can help reduce reproductive tract contamination and infection.
Suturing the top of a mare’s vulva closed (a Caslick’s procedure) can help reduce reproductive tract contamination and infection. Arnd Bronkhorst Photography

“Dirty” is a term used broadly to describe the infections found in mares that cycle but do not conceive. But it’s essential to recognize that the conformation of the mare’s reproductive organs significantly impact treatment and subsequent breeding.

“Some mares have a longer vulva, others have a shorter one, and both of those can lead to infections if the vulvar lips don’t form a proper seal to keep out contaminants,” Tasse said. “Older mares’ rectums can be sunken in so that the vulva sticks out farther than the rectum, so the manure falls onto the vulva.”

Suturing the top of the vulva closed in a Caslick’s procedure can help prevent this. Mares can also suck air in through their vulva and into their vaginal vault, which also leads to contamination, so a Caslick’s procedure is used to prevent mares from sucking air.

“Sometimes mares need the Caslick’s stitches removed for the stallion to breed them,” she said. “Problem mares may need to have the Caslick’s stitches put back in right after breeding to pre-vent contamination.”

The cervix can also influence a mare’s likelihood of developing uterine infections. The cervix can be too constricted during estrus or too open during diestrus. Post-foaling and older mares that have a tilt to their uterus can pool urine in their vaginal vault. This is very irritating to the cervix, causing inflammation and contamination. Again, a Caslick’s procedure can help prevent urine pooling, but if it fails to, a urethral extension surgery can be performed.

Keeping It Clean

A mare and foal pressing their heads together.
Veterinarians need to take into consideration whether a mare will be bred live cover or with cooled or frozen semen. There are challenges that can be addressed based on breeding type.

Mother Nature and conformation can make it a challenge to clean up a “dirty” mare and to keep the uterus free from infection. Tasse stressed the importance of using multiple modalities of ultrasound findings, uterine culture and uterine cytology in order to develop a plan of action to clean up a “dirty” mare.

It is also important to take note of the conformation of the entire reproductive tract in order to best assess problems and determine the chances that the mare will get in foal.

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