Q:Are there any new treatments on the horizon for "dirty mares"? I have a mare that is having trouble conceiving and was diagnosed with this problem.

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A:The expression "dirty mares" is used to describe mares that suffer from persistent or recurrent bacterial endometritis (inflammation of the endometrium, or the interior lining of the uterus). Bacteria most commonly isolated from the uterus of the mare are beta-hemolytic Streptococci (Streptococcus equi spp zooepidemicus), Escherichia coli, Pseudomonas aeruginosa, and Klebsiella pneumoniae. In contrast to true sexually transmitted diseases such as contagious equine metritis, persistent infectious endometritis often results from contamination of the uterus by the mare's fecal flora in combination with compromised uterine defense. P. aeruginosa, K. pneumoniae, and possibly S. equi spp zooepidemicus and E. coli can be sexually transmitted in horses, but the consequences of exposure to these microorganisms are determined by the particular bacterial strain involved and active participation of all facets of the mare's uterine defense mechanisms (the uterus' ability to eliminate bacteria, inflammatory debris, and fluid).

Veterinarians should direct treatment of mares with persistent uterine infections toward the underlying breakdown of the uterine defense and against the microbial agent. The first therapeutic concern should be to remove predisposing causes, such as a breakdown of external genital barriers. Persistent uterine infection frequently follows degenerative or traumatic anatomic changes and loss of integrity of the barriers of ascending infection. Therefore, Caslick's surgery (to close the upper part of the vulva), cervical damage and perineal laceration repair, and urovagina (urine pooling) correction should precede specific endometrial treatment. Veterinarians should minimize all potential contamination sources, including intrauterine passage of diagnostic and treatment implements. In some mares sexual rest alone can result in full recovery.

Veterinarians typically treat mares with persistent bacterial endometritis using antibiotics they administer either locally or systemically. Intraluminal fluid (in the lumen, or cavity, of the uterus) and inflammatory debris should be removed by uterine lavage (flushing) before local treatment. Veterinarians should treat mares during estrus, when natural defense is maximal, and use strict aseptic (sterile) techniques. Treatment should continue daily for four to six days during estrus. Repeated contamination could indicate an unsuccessfully resolved predisposing cause. The clitoral fossa (the folded lining near the clitoris) should always be considered as a location of bacterial growth in these cases. If culture results from the endometrium and the clitoral fossa match, the clitoral fossa should be cleaned and treated locally with antibiotics. Failure to respond to antibiotic treatment might be the result of bacterial resistance to the selected antibiotics, or an inability of the drug to reach the bacteria in the uterine tissue. It has been proposed that endometrial biofilms that appear to be nonresponsive to conventional treatment are responsible for chronic endometritis. A bacterial biofilm is a complex aggregation of microorganisms growing on a solid substrate, such as the uterine lining. The biofilms' complex structure greatly increases the resistance to antibiotic therapy. The presence of biofilms has been described in the mouth and in the bladder of people and has recently been suggested to occur in the uterus of the horse. This still needs to be documented, however. Researchers in Denmark have offered an alternative explanation for persistent bacterial endometritis. These veterinarians suggest that bacteria can persist in the endometrial glands and the uterine wall in a dormant state, which would increase their resistance to antibiotic therapy.

To overcome some of the obstacles for effective antibiotics therapy in the uterus, supplemental treatments have been suggested:

  • A 30% dimethyl sulfoxide (DMSO) solution and acetylcysteine in a 20% solution have been added to uterine lavage fluids in mares with suspected biofilms in the uterus to break down mucus or clear biofilms.
  • Ethylenediaminetetraacetic acid (EDTA, a chelating agent)-tris has been shown to be effective in decreasing the viability of P. aeruginosa from horses.
  • Veterinarians have infused a variety of other solutions to treat infectious endometritis such as iodine, chlorhexidine, hypertonic saline, kerosene, and hydrogen peroxide. Many of these are strong irritants and can damage the endometrium. Therefore, veterinarians should use these treatments with caution.
  • Treatment with immunostimulatory agents has been reported to improve pregnancy rates in mares with persistent endometritis, but the mechanism is not fully understood.

About the Author

Mats H.T. Troedsson, DVM, PhD, Dipl. ACT, ECAR

Mats H.T. Troedsson, DVM, PhD, Dipl. ACT, ECAR, is the Director of the Gluck Equine Research Center at the University of Kentucky.

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