Changing prevalence of O-serogroups and antimicrobial susceptibility among STEC strains isolated from healthy dairy cows over a decade in Japan between 1998 and 2007.

The prevalence of STEC in Japan was examined using rectal stool samples taken from 932 healthy dairy cows from 123 farms in 11 prefectures between 2006 and 2007. Screening with stx-PCRs revealed the prevalence to be 30.4% (283 animals), and STEC strains were isolated from 111 animals. Although ten O-serogroups (O8, O22, O84, O103, O111, O113, O116, O136, O153 and O157) were the major O-serogroup in healthy dairy cows in Japan in 1998, half of the 118 selected STEC strains were serotyped as O2, O8, O26, O153, or O163 in this study. Twenty-eight of the 118 STEC strains (24%) showed resistance to some conventional drugs, such as dihydrostreptomycin, oxytetracycline and aminobenzylpenicillin. Although STEC prevalence in cows decreased from 17% to 12%, the antimicrobial resistance ratio increased from 8.7% to 24% in the past decade in Japan.