Prevalence of Non-O157 Shiga Toxin-producing Escherichia coli Shed by Beef Calves before and after Weaning

Introduction: Cattle are primary reservoirs of Shiga toxin-producing Escherichia coli (STEC) and are considered to be “super-shedders” if they shed more than 10⁴ CFU/g of STEC. In addition to E. coli O157:H7, six non-O157 STEC serogroups were added to the zero-tolerance adulterant list in June 2012. Limited information is available about the prevalence of non-O157 serogroups in beef cattle during the pre-harvest stages.

Purpose: The purpose of this study was to determine the prevalence of non-O157 STEC in beef calves and identify the main serogroups associated with the “super-shedders” before and after weaning.

Methods: At 6 and 9.5 months of age, fecal samples were taken from 110 calves and the concentrations of STEC were enumerated by plating samples onto CHROMagar™ STEC agar. Three suspect non-O157 colonies were picked from the plates of “super-shedder” samples, confirmed to be non-O157 by using CHROMagar™ O157 plates, and then were further tested by a multiplex PCR assay targeting stx1, stx2, and eaeA genes. Isolates that were positive for at least one gene were sent for serotyping and an antibiotic resistance study was completed for the O26 isolates obtained from the farm.

Results: Based on the CHROMagar™ STEC enumeration results, 9 out of 110 calves were super-shedders; together with the super-shedding calves, the shedding levels of an additional 20 calves which had approximately 10² CFU/g STEC did not change before and after weaning while two calves changed from high STEC shedders to non-shedders. Among the suspect non-O157 “super-shedder” isolates, 56% of them were positive for at least one target gene. Serotyping results showed that O26 was the dominant serogroup.

Significance: The results from this study show that calves can start STEC shedding early in their lives and the weaning step does not generate a big impact on STEC shedding levels.