β-hemolytic streptococci, particularly Streptococcus equi subsp. zooepidemicus, are a frequent cause of reproductive tract infections in mares. While some information is available from uterine isolates from the non-pregnant mare, , less has been reported on the role of streptococci in placentitis and abortion.
The objective of this study was to characterize, phenotypically and genotypically, isolates of β-hemolytic streptococci obtained from spontaneous cases of placentitis and/or aborted fetuses.
A total 43 bacterial isolates from 24 spontaneous cases of placentitis/abortion in broodmares from Kentucky were investigated. All 43 isolates agglutinated with Lancefields group C antibodies. Genotypical identification using a PCR targeting the S. zooepidemicus specific SodA gene resulted in an amplicon from 37 out of the 43 isolates. The six unidentified isolates were subjected to fermentation tests including glucose, lactose, sorbitole, trehalose and ribose. Three isolates had a fermentation pattern corresponding to S. zooepidemicus, whereas the remaining three corresponded to S. dysgalactiae subsp. equisimilis. The identity of the six isolates was confirmed by 16S rDNA sequencing.
Up to three bacterial isolates per case were then characterized by pulsed-field gel electrophoresis to allow assessment of the within-mare and between-mare relatedness.
All isolates were identical in cases where more than one isolate was obtained suggesting that each mare/fetus was affected by a single bacterial clone. None of the S. zooepidemicus isolates affected more than one mare supporting previous results indicating a considerable diversity within the S. zooepidemicus population1.
In summary we show that among β-hemolytic streptococci causing placentitis and/or abortion S. zooepidemicus appear to be the dominating subspecies. The infection in the individual mare appears to be clonal, but there is a substantial genetic variation between strains causing placentitis in different mares. Our findings clearly indicate that at least some strains of S. zooepidemicus have a pathogenic potential in relation to late pregnant mares.