Perineal swabs reveal effect of super shedders on the transmission of Escherichia coli O157:H7 in commercial feedlots

Cattle that shed more than 10(4) cfu/g of Escherichia coli O157 in feces have been described as super shedders (SS) and are thought to have major impacts on prevalence and transmission of this organism. Two Southern Alberta commercial feedlots (feedlot X, 7 pens averaging 183 steers; feedlot Y, 5 pens averaging 153 steers) were sampled from May 2007 to January 2008. Background samples [fecal pat (FP) water, ropes] were taken weekly from each pen for 2 wk before collection of samples from individuals [fecal grab (FG); perineal swab] at 2 different times [during spring and summer (S1); immediately before slaughter during fall and winter (S2)]. Immunomagnetic separation and selective media were used for detecting E. coli O157: H7. Positive FG and FP were enumerated by direct plating onto sorbitol MacConkey agar supplemented with 2.5 mg/L of potassium tellurite and 0.05 mg/L of cefixime. Five sorbitol-negative colonies were agglutinated using an anti-O157 latex kit, and the proportion of positive colonies was adjusted for non-E. coli O157: H7. Overall, there were 153 (7.16%) and 10 (0.45%) SS at S1 and S2, respectively. In feedlot X, SS and penmates of SS during S1 were more likely (P < 0.01) to shed E. coli O157: H7 in their feces and have this organism on their perineum than cattle in a pen where no SS were identified. In feedlot Y, SS and penmates of SS during S1 were more likely (P < 0.01) to have E. coli O157: H7 on their perineum than those from a pen where only 1 SS was identified, but steers in only 1 pen with multiple SS were more likely (P < 0.01) to shed this organism in feces. Overall, E. coli O157: H7 was 1.85 times more likely (P < 0.01) to be detected in perineal swabs compared with FG and E. coli O157: H7 was more likely (P < 0.01) to be detected at S1 compared with S2 for all sample types. Super shedders were a larger proportion of shedding cattle in S1 than in S2, but the presence of SS increased (P < 0.01) prevalence of this organism on the perineum of cattle throughout the year. Even when SS did not increase fecal shedding of E. coli O157: H7, their presence increased contamination of hides, an outcome that could have important implications for contamination of carcasses at the abattoir.