A Caslick is a minor surgical procedure performed on mares in which the upper portion of the vulvar lips are sutured together to create a seal. Named after the veterinarian Dr. Edward Caslick who developed it, the procedure is one of the most common reproductive surgeries in horses and is used to prevent air, bacteria, and debris from entering the reproductive tract.
Why Mares Need a Caslick
The vulvar lips are the first physical barrier protecting a mare’s uterus from the outside environment. In an ideal conformation, the vulva sits vertically and the lips close tightly on their own, keeping contaminants out. Some mares lose that natural seal due to age, repeated foaling, weight loss, or simply the way they’re built. When the seal is compromised, air gets sucked into the vaginal cavity during movement, a condition called pneumovagina (sometimes called “windsucking,” though it has nothing to do with the oral habit of the same name).
Mares prone to pneumovagina typically have a vulva that tilts forward rather than sitting upright, along with a recessed anus. The lower opening of the vulva also sits abnormally high relative to the pelvic floor, which weakens the internal sphincter that normally keeps the vaginal canal closed off. Once air enters, bacteria follow, leading to inflammation and infection of the vaginal lining and potentially the uterus itself. This chain of events is a major cause of breeding failure in mares.
A Caslick is used to treat or prevent pneumovagina, urine pooling (where urine collects inside the vagina instead of draining normally), vulvar trauma, vaginal inflammation, and infertility. A thicker, deeper version of the procedure can also serve as a more permanent fix for mares at risk of vaginal prolapse.
How the Procedure Works
A Caslick is a standing procedure, meaning the mare stays on her feet. The veterinarian applies a local anesthetic to numb the edges of the vulvar lips, then trims a thin strip of tissue from each side to create fresh wound surfaces. These freshened edges are sutured together starting from the top of the vulva and extending downward, leaving only the lower portion open so the mare can urinate normally.
As the trimmed edges heal together over the following days, they form a permanent adhesion that recreates the seal the mare’s conformation couldn’t maintain on its own. The whole procedure typically takes only a few minutes and is well tolerated. Most mares show minimal discomfort afterward.
Impact on Fertility
For mares with poor vulvar conformation, a Caslick can meaningfully improve breeding outcomes. A large study examining over 9,000 mares found that all mares who received a Caslick had a significantly higher pregnancy rate than non-treated mares of similar age and conformation scores. Maiden mares that had been sutured during their racing careers also conceived at higher rates than maiden mares who had never been treated when they first entered a breeding program.
That said, a Caslick isn’t a guaranteed fix. One retrospective study found that fertility in operated mares was 51% compared to 67% in mares that never needed the procedure in the first place. The likely explanation is that by the time a mare needs a Caslick, she may already have a low-grade uterine infection (endometritis) from months or years of contamination. Closing the vulva stops new bacteria from entering, but it doesn’t resolve an infection that’s already established. This is why veterinarians often evaluate the uterus for infection and treat it alongside the surgery rather than relying on the Caslick alone.
Opening a Caslick for Breeding and Foaling
Because the upper vulva is sutured shut, a Caslick must be opened and resutured at specific points in the mare’s reproductive life. For natural cover breeding, the Caslick needs to be cut open before the stallion breeds the mare, then resutured afterward. With artificial insemination, only a small opening is sometimes needed, or the veterinarian can work through the existing lower opening depending on how much of the vulva was closed.
The most critical time to open a Caslick is before foaling. If a mare foals through an intact Caslick, the sutured tissue will tear in a ragged, uncontrolled way that damages far more tissue than a clean cut would. The standard recommendation is to have your veterinarian open the Caslick at least 30 days before the expected foaling date. This gives a comfortable safety margin in case the mare delivers early. After the foal is born and the mare has healed, the Caslick is typically resutured to protect her reproductive tract for the next breeding cycle.
This cycle of opening and reclosing is a routine part of managing broodmares. Many mares go through it every year for their entire breeding careers without issue.
What to Watch for After Surgery
Complications from a Caslick are uncommon, but the main risk is the sutures pulling apart before the tissue has fully healed. This can happen if the mare rubs against a fence or post, or if the area becomes swollen or infected. Keeping the surgical site clean and monitoring it daily for the first week or two helps catch problems early. Signs of trouble include swelling, discharge, or visible gaps where the suture line should be intact.
If sutures do fail, the procedure can simply be repeated once the tissue has calmed down. Because only a thin strip of tissue is removed each time, most mares can undergo multiple Caslick procedures over their lifetime without running out of usable tissue, though repeated surgeries can eventually leave scar tissue that makes each subsequent closure slightly more difficult.
Which Mares Typically Need One
Older broodmares are the most common candidates, since repeated pregnancies stretch the tissues around the vulva and pull the conformation forward. Mares that have lost significant body condition, whether from illness, hard training, or a difficult pregnancy, can also develop poor vulvar conformation even if they were fine before. Some young mares are simply built with a naturally tilted or sunken vulva and benefit from a Caslick as soon as they enter a breeding program.
Veterinarians assess the need for a Caslick using what’s called the Caslick Index, a measurement that combines the length of the vulva with its angle of declination from vertical. A score above 150 generally indicates the mare will benefit from the procedure. Thoroughbred and sport horse mares tend to need Caslicks more frequently than stock horse breeds, partly due to conformation differences and partly because they’re often kept leaner during competition, which worsens the anatomy.