Endometritis, a Common Cause of Infertility

Endometritis, the inflammation of the endometrium (lining of a mare’s uterus), affects up to 15% of broodmares and is a major cause of infertility. Any time a contaminant (urine, feces, bacteria from environmental sources) makes its way past the physical barriers that guard the uterus, this organ quickly begins an inflammatory response that signals the uterus to contract repeatedly, clearing the foreign material within a few days. This is a natural reaction that also takes place after natural breeding or artificial insemination.

Some mares have a delayed inflammatory response that allows contaminants to remain in the uterus for three or four days, setting up infection and chronic inflammation that does not resolve. Fluid accumulating in the uterus weakens the ability of white blood cells to attack bacteria, and the hairlike cilia that sweep microbes out of the organ do not function well.

Older mares that have never foaled, mares that have had a difficult delivery, and mares that have had a number of foals are most susceptible, sometimes due to anatomical changes or injuries to the reproductive tract. Many affected mares show no signs except the inability to become pregnant.

Ultrasonography is a helpful tool in determining the presence and characteristics of fluid in the uterus. Biopsy of the endometrium, endometrial swabs, and flushing of the uterus are also useful, but diagnosis may still be uncertain. Veterinarians use breeding history, laboratory test results, and a physical examination in diagnosing endometritis.

Treatments vary, but most center around curing infection and restoring proper contractility to the uterus. Without a normal pattern of contractions, the uterus can’t stay clear of fluid and foreign material. There is some evidence that older broodmares that have delivered several foals may have poor uterine blood flow that leads to less effective uterine contractility. Various techniques can be used to remove or heal inflamed tissue and induce contractions. These treatments should be performed by a veterinarian, as scarring or secondary infections can be caused by improper techniques.