The vulva is the visible exterior part of the mare's reproductive tract. The vagina is the muscular tube that leads from the vulva forward to the cervix; it is about 20-30cm in most mares. A short distance from the vulva, on the floor of the vagina, is the urethral opening, through which urine empties.

In a normal healthy mare, the uterus is protected from the outside world by three barriers: a tight cervix, the collapsed walls of the vagina, and a tight vulvar seal.

In the normal mare, urine flows naturally the short distance along the floor of the vagina and out the vulva. As mares age, the reproductive tract gradually sags. The vulva becomes tilted, the uterus sinks and the angle of the tube becomes more steep. These changes in orientation encourage urine to pool at the front of the vagina, against the cervix, and leak into the uterus. Urine pooling may also occur in mares that have had foaling injuries, or simply have poor vulvar/vaginal conformation.

Urine pooling (and associated uterine inflammation) can result in difficult breeding, early pregnancy loss and can cause irregular estrus cycles. The urine is toxic to sperm, and is irritating to the lining of the vagina, the cervix and the uterus.

DIAGNOSIS is made primarily through the use of a vaginal speculum, which allows visualization of the floor of the vagina. At times, urine can be seen pooling on the floor, near the cervix. In some mares, urine may only pool during estrus (when the vagina and vulva are maximally soft and relaxed), so it can be missed if the examination does not take place when the mare is in heat.

TREATMENT is primarily surgical: procedures that encourage urine to flow from the urethral opening forward out the vulva. There are several different standing surgical procedures that achieve this different ways. One involves the creation of a tunnel that leads outward toward the vulva.