An investigation on the carriage and characteristics of Shiga-toxin producing Escherichia coli in the recto-anal junction of sheep presented for slaughter in Ireland

Shiga-toxin producing Escherichia coli (STEC) are a diverse group of pathogenic bacteria capable of causing serious human illness and serogroups O157 and O26 are frequently implicated in human disease. Ruminant hosts are the primary STEC reservoir and small ruminants are important contributors to STEC transmission. The aim of this study was to investigate the prevalence, serotypes and shedding dynamics of Shiga-toxin producing Escherichia coli, including the super-shedding of serogroups O157 and O26, in Irish sheep. Recto-anal mucosal swab samples (N=840) were collected over 24 months from two ovine slaughtering facilities. Samples were quantitatively and qualitatively assessed via real-time (RT) PCR for Shiga-toxin prevalence and serogroup. A subset of STEC isolates (N=199) were selected for whole-genome sequencing and analysed in silico.In total, 704/840 (83.8%) swab samples were Shiga-toxin positive following RT-PCR screening, and 363/704 (51.6%) animals were subsequently culture positive for STEC. Five animals were shedding STEC O157 and three of these were identified as super-shedders. No STEC O26 was isolated. Post-hoc statistical analysis observed that younger animals are more likely to harbour STEC and STEC carriage is most prevalent during the summer months. Following sequencing, 178/199 genomes were confirmed as STEC. Thirty-five different serotypes were identified, and fifteen serotypes have been isolated for the first time in sheep. Serotype O91:H14 was the most frequently reported. Eight Shiga-toxin gene variants were reported, two stx1 and six stx2. Variant stx1c was the most prevalent, while many strains also harboured stx2b. three novel Shiga-toxin subunit combinations are reported for the first time.