Female horses, known as mares, possess a vagina as part of their reproductive system. This muscular canal connects the internal reproductive organs to the external environment, similar to other female mammals. The entire female equine reproductive tract lies in a horizontal position within the abdominal and pelvic cavities. This system is designed for the production of an egg, fertilization, gestation, and the expulsion of the foal during birth.
Anatomy of the Vagina and Vulva
The mare’s reproductive tract begins externally with the vulva, which acts as the first physical barrier protecting the inner organs from contamination. The vulva consists of the fleshy outer lips, known as the labia, and the clitoris, located at the lower junction of the labia. This external construction is important because it prevents air and environmental contaminants, such as feces, from entering the reproductive canal.
The vulva leads directly into the vagina, a muscular tube lined with a mucous membrane that ranges from six to eight inches long. This canal is highly elastic, allowing it to accommodate the stallion’s penis during copulation and stretch significantly during birth. The vagina also functions as the exit point for the urinary tract, as the opening of the urethra is located on its floor.
The junction between the vestibule, the final part of the vagina, and the vulva is secured by muscle tone and the vulva’s vertical position below the anus. Compromise to this seal, due to poor conformation, age, or injury, increases the risk of infection in the reproductive tract. The vulvar lips protect the inner structures against bacteria and other contaminants.
The Internal Pathway: Cervix, Uterus, and Ovaries
Moving inward from the vagina, the next structure is the cervix, which acts as a muscular sphincter between the vagina and the uterus. The cervix is approximately four inches long and appears as a circle of folded tissue that changes based on the mare’s hormonal cycle. During sexual receptivity, or estrus, the cervix becomes relaxed, swollen, and produces thin, watery mucus to facilitate the passage of semen.
The cervix opens into the uterus, a hollow, muscular organ composed of a body and two horns, which is the site of embryo development and gestation. The inner lining, called the endometrium, contains a rich supply of blood and glands crucial for nourishing the developing fetus. Inflammation of this lining, known as endometritis, is a major cause of infertility in mares.
Attached to the uterine horns are the oviducts, or Fallopian tubes, which are small, coiled tubes connecting the uterus to the ovaries. Fertilization occurs within the oviduct before the fertilized egg travels to the uterus. The ovaries are responsible for producing the egg and female sex hormones like estrogen and progesterone. The mare’s ovaries possess a unique wedge-shaped area called the ovulation fossa, which is the only point from which an egg can be released.
Common Reproductive Health Concerns
A common health concern for mares is pneumovagina, or “wind-sucking,” where air is aspirated into the vagina through a compromised vulvar seal. This aspiration draws in environmental contaminants, such as feces, which can irritate the vagina (vaginitis) and often leads to uterine infection (metritis). Mares that have foaled or are older are more susceptible due to changes in vulvar conformation.
Veterinarians commonly treat pneumovagina and associated infections with a procedure known as a Caslick’s operation. This surgical technique involves suturing the upper portion of the vulvar lips together to restore a proper seal and prevent the entry of air and debris. The bottom third of the vulva is left open to allow for normal urination.
The Caslick’s procedure is effective in reducing infection, but it must be opened before the mare is bred naturally or before she goes into labor to prevent severe tearing. Infections such as vaginitis and metritis can also be caused by specific bacteria, including those responsible for Contagious Equine Metritis (CEM). CEM, caused by Taylorella equigenitalis, is a venereal disease that leads to temporary infertility and a thick, often gray, vaginal discharge.