Increased summertime prevalence of cattle carriage of enterohemorrhagic Shiga toxin-producing Escherichia coli O157:H7 (STEC O157) is associated with increased summertime incidence of human infection. The mechanism driving the seasonality of STEC O157 carriage among cattle is unknown. We conducted experimental challenge trials to distinguish whether factors extrinsic or intrinsic to cattle underlie the seasonality of STEC O157 colonization. Holstein steers (N = 20) exposed to ambient environmental conditions were challenged with a standardized pool of STEC O157 strains four times at 6-month intervals. Density and duration of recto-anal junction mucosa (RAJ) colonization with STEC O157 was compared by season (winter vs summer), dose (10(9) CFU vs 10(7) CFU), and route of challenge (oral vs rectal). Following summer challenges, RAJ STEC O157 colonization density was significantly lower (P = 0.016) and duration was shorter (P = 0.052) compared to winter challenges, a seasonal pattern opposite to that observed naturally. Colonization was unaffected by challenge route, indicating that passage through the gastrointestinal microbiome did not significantly affect the infectious dose to the RAJ. A 2-log reduction of the challenge doses in the second year trials was accompanied by similarly reduced RAJ colonization in both seasons (P < 0.001). These results refute the hypothesis that cattle are predisposed to STEC O157 colonization during summer months, either due to intrinsic factors or indirectly due to gastrointestinal tract microbiome effects. Instead, the data support the hypothesis that increased summertime STEC O157 colonization results from increased seasonal oral exposure to this pathogen.
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