Pulsed-field gel electrophoresis (PFGE) was used to investigate the epidemiology of
streptococcal mastitis in dairy cattle. The most prevalent streptococcal species, Streptococcus
uberis (60–80% of streptococcal isolates), was highly heterogeneous, with different cows only
rarely sharing the same pulsotype. S. agalactiae was rarely encountered, however all eight
isolates from one farm generated identical PFGE profiles, which differed from those of all
other isolates examined, confirming cow-to-cow transmission. Fifty-two isolates of S.
dysgalactiae from 27 cows on 5 farms generated 6 different profiles. However, on individual
farms, only one or two pulsotypes usually predominated. This species is generally regarded as
an environmental pathogen but our data suggest that cow-to-cow transmission of S.
dysgalactiae may occur. In spite of the variation in PFGE profiles of isolates from different
cows, persistent infections in individual cows were usually caused by the same pulsotype of S.
uberis or S. dysgalactiae.