Conditions of the External Genitalia of Mares

CHAPTER 168 Conditions of the External Genitalia of Mares


Perineal conformation is one of the most important influences on a mare’s fertility. Poor perineal conformation increases the risk of developing pneumovagina, urovagina, and uterine fluid accumulation, all of which can substantially impair fertility. With increasing age, number of foals produced, and particularly following foaling trauma, the integrity of the barriers protecting the mare’s uterus from external contamination becomes compromised.

Evaluation of the mare should include thorough inspection of the perineum (anus, vulva, and skin under the tail) and buttocks for signs of vulvar discharge, ulcerative lesions, tumors, and perineal conformation. Overall pelvic conformation is an important consideration because many mares with reproductive problems have a flat croup with a high tailhead and downward slope to the pelvis. A sunken anus is associated with shelf formation of the dorsal aspect of the vulva, leading to fecal contamination of the vagina (Figure 168-1). Mares with poor body condition or fillies that are in fit racing condition and lacking perineal fat often have poor perineal conformation and are predisposed to developing pneumovagina.

Three important barriers protect the reproductive tract of the mare from external contamination: the vulvar seal, vestibulovaginal sphincter, and cervix. Functional defects in any of these barriers have the potential to impair fertility. Each of these barriers should be assessed for proper form and function. The vulvar seal is formed by a combination of factors. The labia should be full and firm and should lie in apposition to one another. Normal integrity of the constrictor vulvae muscles prevents inrushing of air into the vagina when the vulvar labia are parted. The dorsal commissure of the vulva should be less than 4 cm above the pelvic floor at the level of the ischial tuberosities (Figure 168-2) such that less than one third of the vulvar length lies above the level of the pelvic floor and at least two thirds of the length lies below. The vulvar labia should be on a vertical or near-vertical plane. Deviation of more than 10 degrees beyond vertical is associated with reduction in fertility.


The Caslick index is a score determined as a result of objective measurement of the effective vulva length (length of vulva measured from the dorsal commissure to the level of the pelvic brim) multiplied by the number of degrees of deviation from vertical. The score is used to assist the clinician in determining which mares would benefit from a Caslick suture of the vulva. A score of 100 or less is considered normal and is associated with good fertility. A score of 150 or higher is associated with reduced fertility. For example, a mare with an effective vulvar length of 6 cm and an angle of 40 degrees would have a Caslick index of 240. Mares with a Caslick index greater than 100 should receive a Caslick suture.

The intact vestibulovaginal sphincter is a substantial barrier against bacterial and fecal contamination of the reproductive tract. The vestibule is the caudal part of the vagina between the vulvar labia and the area of the hymen formed by the transverse fold. This barrier is formed by the caudal portion of the vagina, the internal floor of the pelvis, the hymen, and the constrictor vestibuli muscles that encircle the vestibule. When the vulvar and vestibular seals are functioning properly, the vagina is a potential space and the cavity is obliterated by apposition of the vaginal walls. When these seals are not functioning properly, a windsucking noise is often heard during urination or locomotion, particularly at the trot.

An additional consideration is the integrity of the perineal body, which here is defined as the area of tissue lying between the rectum and vagina. Defects of the perineal body can be the result of multiple foalings or traumatic injury during foaling. The perineal body is assessed by placing a gloved first finger into the rectum and the thumb just inside the dorsal aspect of the vulva (Figure 168-3). In mares with normal conformation, there should be at least 3 to 4 cm of firm tissue in the perineal body.


When abnormal perineal conformation is discovered, the clinician should decide on the best course of action. In many instances, placement of a Caslick suture is sufficient to eliminate the air and fecal contamination and restore fertility. I prefer to partially suture mares before artificial insemination (AI), leaving enough room to introduce the arm into the vagina for AI or uterine lavage. The Caslick suture is completed at the time the mare is confirmed to be in foal, at 14 days of pregnancy. In cases of more severe anatomic defects, perineal reconstructive surgery or a perineal body transection may be necessary.

To perform a Caslick surgery:

May 28, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Conditions of the External Genitalia of Mares
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