An “dirty mare” is defined as any mare who has been ridden during her anestrus or diestrual phases of reproductive cycle, when her uterus is flaccid, thin-walled and quiescent. Only during follicular and luteal stages can her uterus begin to respond (heat). As the follicle expands in size and surrounding ovarian tissue changes; its secretions then cause progesterone secretion from corpus luteum secreting progesterone that causes her uterus’ response receptivity which then ultimately ends in prostaglandin lysis allowing diestrus.
The vulva serves as the initial line of defense against environmental contaminants entering the uterus. A healthy mare’s vulvar lips should meet evenly in the midline and 80% or more of its opening should fall below her pelvic brim; any deviation can allow air and fecal bacteria contamination into her anal tract and lead to potentially life-threatening infection that could even prove fatal.
The anal canal is protected by a circular sphincter located within the vestibulo-vaginal fold inside of the urethra and assessed through manual parting of lips of vulva. A mare with easily opening lips indicates low muscle tone that could result in air and bacteria entering her anal tract and lead to pneumovagina and therefore require Caslick surgery – ensuring conformation evaluation in all mares, especially those predisposed to postpartum uterine infections.