The term "dirty mare" can be very complicated, and one that applies to as many as 85% of broodmares that have problems conceiving. It consists of more than one specific complication; in fact there are three broad categories of problem mares.
The first group consists of mares that have an acute infection, which means bacteria can be cultured on the first or second day of heat. This mare is relatively easy to diagnose--many have a purulent discharge and when her uterus is cultured bacteria can be isolated. Once your veterinarian determines that your mare falls into this category, treatment consists of antibiotics to specifically kill the bacteria that has been cultured.
The mare should be evaluated at the time the infection is noted because she may re-infect herself if she has poor reproductive conformation. A Caslick procedure may be needed to stop re-infection. This involves stitching up the vulva from the top of the vulva to the brim of the pelvis so air and bacteria can no longer enter the uterus, but the mare can urinate.
Treatment consists of antibiotics in the uterus for three to four days, during which time she is not to be bred. The mare may be bred during the next heat if there are no signs of infection. While there are no firm or fast rules on the treatment of this mare, the infection should be eliminated early in the year so time remains for her to be bred later in the season.
The prognosis for this group becoming pregnant is good, with 60-70% of the mares conceiving after the bacterial infection is cleared up.
The second group consists of mares that have a long-standing, severe infection. These infections usually are caused by bacteria such as a klebsiella, pseudamonas, or a yeast infection. Many different treatments have been tried, including antibiotics, uterine lavage, saline, and DMSO. There does not seem to be a correlation between treatment and pregnancy. At this point, if no progress is being made, the best thing for the mare may be rest. Clinical impressions are that a lot of mares tend to clear up the infections on their own if they are allowed to rest in the fall and winter and are not bred. For the second group the prognosis is slim, with 10-25% of the mares conceiving.
The third group usually does not culture any bacteria from the uterus prior to breeding. The problem with these mares is that they cannot physically clear their uteri of the by-products of breeding. When a mare is bred, she responds to the semen that is deposited into her uterus by having a tremendous inflammatory response. Reproductively healthy mares clear the inflammation through their cervix within 24 to 48 hours. Mares that fall into the third group do not clear the fluids for 72 to 96 hours. If the inflammation of the uterine tissue lasts for five days, the embryo may enter a hostile environment and possibly die.
This condition is called persistent mating-induced endometritis or a delay in the uterine clearance. There are a number of predisposing factors that contribute to this disease. The uterine muscles may have lost their ability to contract, or the broad ligaments that hold the uterus in place may have stretched from the mare having many foals causing the uterus to fall lower in the abdomen. Reproductively healthy mares clear the fluids of breeding by gravity flow because their uterus is located high in the abdomen. Mares with persistent endometritis can not clear the fluids because the uterus is slung low in the abdomen.
Unfortunately, it is expensive to accurately diagnose this condition. An owner may take the mare to a university or private veterinary clinic that has scintigraphy equipment. When the mare is in heat a radioactive substance can be infused into her uterus and the amount of radioactivity that remains in the uterus after two hours can be measured. If the mare is normal, she will clear 50% of the radioactive substance. If she has a delay in her ability to clear the uterus, she will clear virtually nothing. The veterinarian can ultrasound the mare's uterus 24 hours after she has been bred to determine if she is retaining fluid. If there is fluid, treatment should be performed at that time. However, the mare may not conceive because the inflammation has been present for too long.
Treatment is aimed at making the uterus contract so that the mare pushes the fluids out. This can be accomplished with drugs such as oxytocin and prostaglandins. Prior to injection of the drug, the veterinarian may want to lavage the uterus with fluids to remove the pus. Treatments are conducted early after breeding--four to eight hours--so that the pus does not remain in contact with the uterine lining for a long time. The longer pus remains in the uterus, the more inflammation it causes and the longer it may take the uterus to heal. The veterinarian waits four hours so that the sperm have time to reach the oviduct.
A second examination with ultrasound is conducted 24 hours after breeding to determine if there is any fluid left in the uterus. If there is, a second injection of oxytocin is administered.
Prognosis for pregnancy is extremely variable for this group. For example, if you breed and treat a mare properly the first time, her chance of foaling is greater than with a mare which has been bred four cycles prior to treatment. This mare has had more insult during that breeding season, so by the time the veterinarian sees her, her chances of conceiving are poor.
For any of the three categories of "dirty mares," the best prevention is to have your mare checked by a veterinarian in the fall, then bred using good management techniques and treated properly early in the year.