Variation in the prevalence of non-O157 Shiga toxin-producing Escherichia coli in four sheep flocks during a 12-month longitudinal study

Shiga toxin-producing Escherichia coli (STEC) are an important group of emerging pathogens, with ruminants recognised as their main natural reservoir. The aim of this longitudinal study was to provide information on the prevalence and existence of seasonal variation in the occurrence of non-O157 STEC in four sheep flocks over a 12-month period. A total of 504 faecal samples from 48 adult sheep in four flocks were collected and examined for STEC using both phenotypic and genotypic methods. STEC were isolated from 407 (80.8%) faecal samples representing all the animals sampled. The overall monthly prevalence of STEC varied between 71.1 and 94.4% and no seasonal variation in the occurrence of STEC could be observed over the study period. A total of 521 STEC isolates were characterised. The PCR procedure indicated that 275 (52.8%) isolates carried the stx1 gene, 44 (8.4%) carried the stx2 gene and 202 (38.8%) contained both of these genes. The eae and ehxA genes were detected in 4 (0.8%) and 368 (70.6%) isolates, respectively. The isolates belonged to 28 O serogroups, although 72.4% were restricted to only 10 serogroups (O5, O6, O76, O87, O91, O123, O128, O146, O166 and O176). None of the isolates belonged to the O157 STEC serogroup. STEC isolates of serogroups O33, O53, O105 and O162 have not previously been reported in sheep. This is the first study to report the maintenance of high frequencies of non-O157 STEC infection in sheep flocks over long time periods.