After breeding, most mares effectively clear the uterus of excess semen and inflammatory products before the descent of an
embryo from the oviduct five to six days after ovulation. Other mares develop a persistent post-mating endometritis that results
in reduced fertility.1,2
This condition is a major clinical problem, resulting in great economic loss to the equine industry. Two equine surgeons—Palle
Brink, DVM, Dipl. ECVS, Jägersro Equine ATG Clinic, Malmö, Sweden, and James Schumacher, DVM, Dipl. ACVS, College of Veterinary
Medicine, University of Tennessee—have devised a surgery that may improve the fertility of mares that are barren because of
Why does post-mating endometritis occur?
The etiology of persistent post-mating endometritis is unknown, but strong evidence indicates that mares susceptible to this
condition have reduced uterine clearance associated with reduced uterine contractility. The condition is especially seen in
older mares. It has been suggested that since the uterus maybe be located more ventrally in the abdomen, these mares may be
more predisposed to retention of intrauterine fluid after breeding compared with reproductively normal mares.3
Another study concluded that several risk factors such as age, foaling status, a history of dystocia and the stallion to which
a mare is bred predispose mares to impaired uterine clearance after breeding.4 They found that about 15 percent of a normal population of Thoroughbred mares retained intrauterine fluid the day after
Clinically, mares with persistent breeding-induced endometritis accumulate fluid (semen and bacteria) within the uterine lumen
after breeding.2 Delayed uterine clearance of intrauterine fluids after mating is most commonly seen in pluriparous mares more than 14 years
of age.3 Increasing age and parity in the mare coincides with a lengthening of the vulva and an increase in the vulvar angle of declination,
or a cranial tilting of the vulva. These changes are likely a consequence of repeated pregnancies, loss of body condition
and genetics. Loss of the structural support of the caudal reproductive tract and stretching of the broad ligament from repeated
pregnancies may result in the uterus tilting caudally and ventrally in the abdomen.
A study revealed that the position of the uterus within the abdomen may affect a mare's ability to rapidly clear the uterine
lumen of contamination.3
It was shown that the uterus' position, determined from left and right flank scintigrams from 44 mares (24 reproductively
normal; 20 that exhibited a delay in uterine clearance), had a variance in uterine conformation. The uterine-cervical angle
relative to horizontal was found to be more ventral in mares with delay in uterine clearance and was more horizontal in reproductively
normal mares. The researchers concluded that a uterus that tilts ventrally in relation to the pelvic brim may contribute to
delay in uterine clearance and an inability of mares to rapidly clear their uterine lumen of contamination.
"In mares, the uterus gets heavy during each pregnancy, and it drops with each successive foaling," says Chris Johnson, DVM,
Dipl. ACVS, Woodford Equine Hospital, Versailles, Ky. "With age and the effects of having multiple foals, the uterus, instead
of being tucked up near the back bone like it should be—or at least horizontal with the ground, allowing fluid to run downhill—tips
into the abdomen so it actually points down, just hanging like an old curtain," Johnson says.
These mares have poor perineal conformation. In normal perineal conformation, the anus and the vulva are in a vertical line.
In problem mares, the anus starts tipping toward the head, so they have a sloped perineal conformation. This goes along with
this delayed uterine clearance syndrome. "Those mares stay infected because they retain fluid in their uterus and they can't
clear it out," Johnson says. "Uterine tone suffers as well."
"I don't think anybody knows why this occurs," Johnson says. "It's kind of a 'chicken-and-egg' thing. We don't know whether
the uterus becomes heavy and they subsequently retain fluid and their uterine tone decreases, or whether their uterine tone
decreases and they subsequently drop and retain fluid. I'm sure there are as many theories as there are veterinarians that
think and write about it."