Streptococcus equi subsp. zooepidemicus is one of the most commonly isolated pathogens from the uterus of mares with infectious endometritis. Its ability to cause chronic latent infection by residing deep within the endometrial tissue has previously been described. The aim of the study was to investigate whether clonal or genetically distinct S. zooepidemicus strains isolated from mares with endometritis were associated with mare risk factors and the outcome of natural cover.
Uterine swabs were obtained from mares with intrauterine fluid after natural cover (n=31) at thoroughbred stud farms in Australia. Fifty two percent of the mares (n=16) were diagnosed with infectious endometritis, and S.zooepidemicus was isolated in 81% (n=13) of these mares. Up to four S. zooepidemicus isolates were selected from each mare with growth of S. zooepidemicus and isolates from an additional five mares were further characterized by pulsed-field gel electrophoresis (PFGE). In total S. zooepidemicus isolates from 18 mares were analyzed. The isolates from 13 mares showed a high genetic relatedness within each individual mare, whereas two genetically distinct strains were isolated in five mares. A clonal S. zooepidemicus infection was associated with increased age, high parity and poor vulvar conformation. Mares with clonal infection had a low pregnancy rate (38%) compared with mares with two strains isolated (80%). In conclusion, the results indicate that clonal S. zooepidemicus endometritis is associated with increased age, parity and poor vulvar conformation in brood mares. Further studies investigating the host-pathogen interaction in clonal S. zooepidemicus endometritis are needed to support these preliminary results.