Pathogenesis of Escherichia coli O157 : H7 in weaned calves

Cattle are an important reservoir of Shiga toxin-producing Escherichia coli O157:H7 and other enterohemorrhagic E. coli (EHEC) that cause diarrhea, he morrhagic colitis, and hemorrhagic uremic syndrome in humans. One strategy for reducing human foodborne EHEC infections is to reduce the levels of EHE C in cattle. Bovine O157:H7 infection models will facilitate identification of virulence factors involved in bovine infections. O157:H7 cause severe d iarrhea and attaching and effacing (A/E) mucosal lesions in colostrum-depri ved neonatal (< 2h) calves, We hypothesized that O157:H7 also cause A/E les ions in older calves, but these were not detected in earlier studies becaus e intestinal levels of O157:H7 were too low (<10(6) CFU/g of tissue) for de tection of focally distributed microscopic lesions. Weaned 3- to 4-month-ol d calves were fasted 48 h, inoculated via stomach tube with 10(10) CFU of O 157:H7 or nonpathogenic E. coli, necropsied 4 d pi and examined histologica lly. Calves inoculated with O157:H7 had higher intestinal levels of inocula ted E. coli than control animals. The rectum was the major site of coloniza tion. A/E lesions were seen in the rectum and cecum of calves with high lev els of O157:H7. Weaned calves, like neonatal calves, are susceptible to int estinal damage induced by EHEC O157:H7. The rectum and cecum may be princip al sites of EHEC O157:H7 colonization during the carrier-shedder state in c attle.