Mare Fertility Problems Caused by Endometritis

The following information on endometritis in broodmares is from Ed Squires, MS, PhD, Hon. Dipl. ACT, of the University of Kentucky’s Gluck Equine Research Center.

Endometritis, an inflammation of the interior lining of the uterus, continues to be a major cause of subfertility or infertility in the mare.

There are many diagnostic tools that have been developed to determine if a mare has endometritis. These include ultrasonography, examination of the cervix with a speculum, examining vaginal discharge, uterine culture, cytology of the uterus, and endometrial biopsy. The most common methods are culture and cytology.

Endometritis is generally divided into infectious and non-infectious causes. Infectious endometritis is the result of bacterial invasion into the uterus. Treatment is directed toward eliminating bacteria with suitable antibiotics, and most infections respond well to this treatment.

Recurrent infectious endometritis has been suggested to be caused by bacterial biofilms or an invasion of bacteria into the tissue. Bacteria can survive in a dormant state under these conditions for a long time without being sensitive to antibiotic treatment.

Non-infectious endometritis is usually in response to a mare being bred or artificially inseminated. This is called post-breeding endometritis. When sperm are placed in the uterus, within a few hours a small portion of the ejaculated or inseminated sperm reach the oviduct. However, the majority of the sperm is eliminated and fails to reach the oviduct. Mares that remove all products of breeding from the uterus within 24 hours are considered as having a normal reaction to semen, whereas mares that do not remove the products are considered susceptible mares to persistent post-breeding endometritis.

The elimination of sperm after breeding is a combination of immune response and mechanical clearance through uterine contraction. Sperm in a mare’s uterus trigger an immune response, which results in neutrophils (white blood cells) coming into the uterine lumen. These neutrophils bind with sperm and phagocytize (kill) the sperm. Also during the activation of the neutrophils, a hormone (prostaglandin F2 alpha) is released that causes the contraction of smooth muscles in the lining of the uterus. These uterine contractions assist in removing accumulated fluid and inflammatory products from the uterus.

In a normal, endometritis-resistant mare, once the products are removed from the uterus the inflammation subsides and the uterine environment returns to its normal state. In about 10-15% of mares, there is failure in the uterine defense mechanism to eliminate these inflammatory products and the mare is considered susceptible to endometritis.

Current Research

Elizabeth Woodward, a graduate student at the Gluck Equine Research Center, is examining the mechanism of impaired uterine clearance of susceptible mares after breeding. Previous studies have shown that impaired uterine clearance in susceptible mares was caused by reduced myometrial contraction in response to inflammation. However, the exact mechanism is not understood.

A recent study by the University of Kentucky Gluck Equine Research Center Director Mats Troedsson, DVM, PhD, Dipl. ACT, reported that susceptible mares had increased uterine accumulation of nitric oxide in the uterine lumen 13 hours after insemination. The nitric oxide causes smooth muscle relaxation and could possibly be the explanation for reduced uterine contraction in susceptible mares.

Woodward is conducting a follow-up study in which she is comparing nitric oxide levels in both resistant and susceptible mares after breeding. The goal is to identify 12 mares–six resistant and six susceptible–for use in this study. Each mare will then be challenged with insemination of dead sperm and evaluated at two, six, 12, and 24 hours after insemination. Measurements taken will include uterine fluid as measured by ultrasonography, number of neutrophils, and the level of nitric oxide in uterine secretions. Biopsies will be taken from the lining of the uterus and examined for nitric oxide synthase (iNOS) levels in the tissue.






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