Molecular epidemiology of Streptococcus uberis isolates from dairy cows with mastitis

Pulsed-field gel electrophoresis and antimicrobial sensitivity testing were used as tools to investigate the epidemiology of Streptococcus uberis mast itis in dairy cows. A total of 62 different strains were found among 138 is olates from the four herds investigated, and between 10 and 26 different st rains were found in each herd. There was no strain common to all four herds . Identical strains of S. uberis were detected from different quarters of i ndividual cows and from cows within the same herd, suggesting that transmis sion from quarter to quarter and cow to cow had occurred. Despite the great variation in S. uberis strains, persistent infection with the same strain within a lactation was observed in most cows. Predominant strains were pres ent in two herds. Preliminary investigations could not clarify why these pa rticular strains might predominate, but in one herd there was a significant difference between the prevalence of clinical mastitis in quarters infecte d with the predominant strain and that in quarters infected with other stra ins, suggesting the greater,virulence of the predominant strain. The wide v ariety of S. uberis strains found is consistent with an environmental sourc e of S. uberis. However, evidence of direct transmission, the persistence o f infection, and the predominance of particular strains in some herds indic ate that S. uberis infections are epidemiologically complex and that the re lative importance of these factors in the occurrence of mastitis may differ between herds.