Equine Subclinical Endometritis Caused by Dormant Beta-Hemolytic Streptococci

Bacterial endometritis as a cause of sub/infertility in the mare has been known for almost a century.1 Over the years reproductive efficiency has increased due to improved management and treatment protocols and in well-managed breeding farms the number of open mares by the end of the breeding season is expected to be less than 10%.2 Despite these improvements, and in some settings routine use of antibiotics, infectious endometritis maintains to be a significant problem in the mare.

Beta-hemolytic streptococci (beta strep), predominantly Streptococcus equi subspecies zooepidemicus (Strep zoo) and E. coli are the most commonly isolated pathogens from the uterus of the mare, accounting for approximately 80% of cases.3-5 Despite no reports of bacterial resistance and high in vitro sensitivity to the most commonly used intra-uterine antibiotics, beta-strep infections have been identified as the uterine infection correlating with the lowest fertility in well-managed Thoroughbred mares.6

Inspired by the work of Jesper Nielsen demonstrating improved diagnostic sensitivity and specificity when using the uterine biopsy as a diagnostic tool compared to the swab,4 we decided to investigate the localization of Strep zoo in the endometrium of different types of infected mares including experimentally infected and chronically infected mares. In the experimentally infected mares (young healthy research mares, which all cultured negative prior bacterial challenge), Strep zoo could be found only on the endometrial surface following uterine infusion, whereas no bacteria could be re-isolated 48 h after bacterial challenge. In the chronically infected mares (mares with a history of repeated uterine infections) Strep zoo was consistently found to have a multifocal distribution, located within the endometrium, thus markedly different from the experimentally infected mares. In some mares, Strep zoo could be visualized several millimeters into the endometrium. These findings support previous notions by Nielsen who found a significantly higher proportion of endometrial biopsies culture positive when compared to endometrial swabs from the same mare4 . […]