A Caslick’s, also known as vulvoplasty, is a common surgical procedure veterinarians perform on mares. Kentucky practitioner E.A. Caslick, DVM, first described the operation in the 1930s, and it has since become a mainstay of equine reproductive practice. Caslick noted that certain mares would accumulate air in the vagina (windsucking), and those mares had trouble conceiving due to resulting bacterial infections of the uterus. Closing the upper portion of the vulva helped prevent this.

For a mare to become pregnant and carry a foal, her uterus must remain sterile, or free from infection. Pathogenic bacteria and accompanying inflammation inside the uterus make the environment too harsh for an embryo to survive. Three barriers in the mare’s anatomy, starting externally and moving internally, should keep this harmful bacteria from getting into the uterus: the vulva, the vestibulovaginal seal, and the cervix. Age, poor perineal conformation, weight loss, or injuries can cause these barriers to fail. A Caslick’s can be beneficial when the vulva or vestibulovaginal seal is inadequate.

The vulva A normal vulva should be perpendicular to the ground. Most of the vulva should be located below the mare’s pelvic brim (a bony shelf that can be felt just to either side of the vulva). When this conformation is faulty, bacteria from manure—which is obviously in close proximity—can enter the vulva and multiply, causing a uterine infection known as endometritis. Common vulvar conformation abnormalities include a tilt, or the appearance of a recessed anus, creating an area where the vulva is tipped almost to horizontal.

The vestibulovaginal seal This is a “curtain” of tissue located inside the vagina that should form a tight seal to prevent any contamination from inside the vulva from reaching the sterile uterus. If this seal is inefficient for any reason (old age, trauma from foaling, or simple faulty anatomy), any bacteria that gain entry to the vulva will have easy passage into the uterus itself.

The cervix This structure seals the uterus from the vagina. A mare’s cervix can be torn or injured during foaling, which can prevent her from becoming or staying pregnant. Cervical tears and injuries must be addressed or repaired, and this is one structure that a Caslick’s procedure won’t help.

Other reproductive problems, such as a uterus that “sags” inside the body or urine that collects inside the vagina (often due to trapped air), can also leave the uterus vulnerable to bacterial ­contamination.

To assess whether a mare needs a Caslick’s, a veterinarian evaluates the orientation of the vulva and tests the vestibulovaginal seal by parting the vulva and listening for the gurgle of air entry. He or she can also examine the area using an equine vaginal speculum.

Depending on the horse and on personal preference, the veterinarian might sedate the standing mare and inject a local anesthetic into the vulva before performing a Caslick’s. Then the veterinarian cuts away a small strip of tissue from each side of the vulva, then sutures the two sides together. The sides then heal together, forming a permanent seal that remains when the sutures are removed.

Enough of the opening must be maintained to allow normal urination, and in some cases extra space will need to be left open for breeding or uterine ­treatments.

If a mare is in foal, it’s very important to open the Caslick’s by cutting along the healed “seam” prior to foaling. Without an open vulva the foal’s passage can cause serious laceration of the surrounding tissues.