Horse-Breeding Issues: Bacterial Endometritis

See how inflammation inside your mare’s uterus can come between you and your horse-breeding plans.

Endometritis refers to inflammation of the tissue lining the lumen of the uterus and is often secondary to a bacterial infection. Bacterial endometritis causes decreased reproductive performance in mares. A majority of young mares rapidly eliminate bacterial contamination and are considered to be “resistant” to infection. Some older multiparous mares might be unable to spontaneously eliminate pathogenic organisms from their uterus and are considered to be “susceptible” to infection.

Factors predisposing mares to uterine infection include contamination at breeding, pooling of urine in the vagina and uterus, trauma from parturition or breeding and failure of natural uterine defense mechanisms. In addition, poor perineal conformation and decreased muscular tone of the vulva might lead to aspiration of air and fecal material into the reproductive tract. Physical barriers to infection of the reproductive tract of the mare are the vulva, vestibulovaginal fold and cervix. A compromise in the integrity of any of these barriers might predispose the mare to uterine infection.

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Uterine defense mechanisms include:

  • Elimination of bacteria by white blood cells.
  • Uterine antibody-mediated immunity.
  • Physical clearance of bacteria and inflammatory products from the uterus by uterine muscle contractions, which push the material out through the cervix.

Reproductively normal mares clear uterine fluid rapidly, whereas mares that are susceptible to endometritis also coincidentally exhibit delayed uterine clearance. The delay in uterine clearance might be due to abnormal myometrial contractions or the exhaustion of uterine smooth muscle.

There are three possible scenarios at work here:

  • Older mares often have a uterus that hangs lower than the pelvic brim and, consequently, the inflammatory cells and their debris accumulate and are not expelled by gravity flow.
  • Poor cervical dilation might reduce uterine clearance, while cervical damage or incompetence might predispose a mare to infection.
  • Susceptible mares might accumulate more fluid within their uterus because of a greater production of glandular secretions secondary to chronic inflammation.

The result is that fluid, inflammatory by-products and bacteria remain in the uterine lumen, inducing more inflammation, mucosal cell damage and fibrosis. Detection of a mare that is susceptible to chronic uterine infections is based on breeding history and results of a reproductive evaluation. Affected mares might have a history of short-cycling, repeated uterine infections and/or failure to conceive after having been bred to a fertile stallion.

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Examination of the mare's  uterus by transrectal ultrasonagraphy to detect the presence of intrauterine fluid is a practical method for identifying mares with a delay in physical clearance and possible susceptibility to infection. Cytologic examination of an endometrial swab might reveal the presence of white blood cells and a culture might detect the presence of a specific organism. An endometrial biopsy might be useful in predicting susceptibility, as mares with poor biopsy scores are more likely to be susceptible to infection.

The general principles of a treatment regimen are:

  • Remove the source of infection.
  • Aid in physical clearance of the uterus.
  • Eliminate pathogens by local infusion with antimicrobial agents.
  • Reduce future contamination by closer management.

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