The equine uterus is central to the mare’s reproductive success, serving as the site of fertilization, early embryonic development, and gestation. Its specialized structure and dynamic functions are precisely regulated to support a single pregnancy over nearly a year. Understanding this organ is foundational for managing mare fertility, especially in the breeding industry where reproductive efficiency is a constant focus. Uterine health directly influences a mare’s ability to conceive and carry a foal to term, making its assessment and care a high priority in veterinary practice.
Physical Structure of the Equine Uterus
The mare’s uterus is bicornuate, possessing two horns that merge into a single, large uterine body. The equine uterine body is prominent, while the horns are comparatively short, typically measuring 20 to 25 centimeters long. The uterus is suspended by the broad ligaments within the abdominal cavity.
The uterine wall has three layers. The perimetrium is the thin, outermost serosal covering. The myometrium is a thick muscular layer responsible for contractions needed to expel the fetus during birth and clear fluid during the estrous cycle.
The innermost layer is the endometrium, a mucosal membrane rich in glands and blood vessels that secretes nutrients for the early embryo. Caudal to the uterine body is the cervix, a thick, muscular structure about 5 to 7 centimeters long. The cervix acts as a physical barrier whose tone changes with the hormonal environment, protecting the sterile uterine environment.
The Uterus and Pregnancy Maintenance
Once fertilization occurs, the developing embryo enters the uterus approximately 5.5 to 6 days after ovulation. A unique feature of equine pregnancy is the prolonged period of embryo mobility, traveling throughout the uterine lumen between days 6 and 17 post-ovulation. This migration is essential for maternal recognition of pregnancy.
The embryo’s movement signals its presence to the uterine lining, preventing the release of the hormone that would normally terminate the pregnancy. Mobility ceases around day 16 or 17, known as fixation, when the conceptus anchors at the base of a uterine horn. It is protected during this mobile phase by a tough, glycoprotein capsule.
Implantation, the attachment of fetal membranes to the uterine wall, is delayed, occurring around day 40 to 42 of gestation. The equine placenta is a diffuse, epitheliochorial type, covering nearly the entire uterine lining. This non-invasive placenta uses microvilli to form microcotyledons, facilitating nutrient and gas exchange throughout the pregnancy.
Cyclical Changes and Hormonal Signaling
The uterine lining undergoes changes throughout the mare’s estrous cycle, responding to ovarian hormones. During estrus, when estrogen levels are high, the uterus becomes soft and flaccid due to decreased muscular tone. The endometrium develops pronounced edema, visualized on ultrasound as a characteristic “cartwheel” or “sliced-orange” pattern.
When the mare transitions into diestrus, following ovulation and corpus luteum formation, the uterus increases its muscular tone. Under progesterone influence, the uterine wall becomes firm and turgid, and the endometrial edema subsides. The cervix also tightens and constricts during this phase.
If the mare is not pregnant, the corpus luteum begins to regress around day 14 post-ovulation, a process called luteolysis. This regression is triggered by the release of Prostaglandin F2 alpha (PGF2 alpha) from the endometrium. This causes the destruction of the corpus luteum, leading to a drop in progesterone and allowing the mare to return to the estrous phase.
Common Reproductive Health Impairments
A common cause of subfertility in mares is endometritis, inflammation of the inner uterine lining. This includes infectious endometritis (bacterial or fungal invasion) and persistent mating-induced endometritis (PMIE). Infectious endometritis results from the failure of natural defenses to eliminate contaminants introduced during breeding.
PMIE is the failure of the uterus to clear inflammatory products, such as semen and fluid, within 24 to 48 hours post-breeding. Mares susceptible to PMIE often have poor uterine clearance mechanisms, like a pendulous uterus or inadequate myometrial contractions. This leads to fluid accumulation toxic to the embryo, preventing pregnancy establishment.
Endometrosis is an age-related, degenerative condition characterized by fibrosis, or scar tissue, in the endometrial layer. This fibrosis interferes with uterine gland function, compromising the uterus’s ability to nourish the embryo and maintain the pregnancy. Uterine fluid accumulation or large cysts can also reduce the available surface area for placental attachment.
Veterinary Diagnostics and Management
A Breeding Soundness Examination (BSE) assesses the health of the mare’s uterus. Transrectal palpation evaluates the size and tone of the uterus and cervix. This is complemented by transrectal ultrasonography, which provides a detailed view of the uterine wall, detects intrauterine fluid or cysts, and monitors cyclical changes like endometrial edema.
To diagnose endometritis, veterinarians rely on uterine culture and cytology. The culture identifies infectious agents, while cytology examines cells collected from the uterine lining for inflammatory cells, such as neutrophils. These tests determine if a bacterial infection or a non-infectious inflammatory response is present.
The most prognostic diagnostic tool is the endometrial biopsy, where a tissue sample is collected and analyzed microscopically. The biopsy is assigned a grade using the Kenney-Doig system, which assesses the degree of inflammation and fibrosis. This system offers an estimate of the mare’s probability of carrying a foal to term.
Management of uterine issues often involves:
- Treatment with intrauterine antibiotics for infectious endometritis.
- Uterine lavage, or flushing, to remove fluid and debris.
- Medications like oxytocin to stimulate myometrial contractions and enhance mechanical clearance.