Enterohemorrhagic Escherichia coli O157:H7 (EHEC) is a significant human pathogen that resides in healthy cattle. It is thought that a reduction in the prevalence and numbers of EHEC in cattle will reduce the load of EHEC entering the food chain. To this end, an intervention strategy involving the addition of chitosan microparticles (CM) to feed in order to reduce the carriage of this pathogen in cattle was evaluated. Experiments with individual Holstein calves and a crossover study found that the addition of CM to feed decreased E. coli O157:H7 shedding. In the crossover study, CM resulted in statistically significant reductions in the numbers recovered from rectal swab samples (P < 0.05) and the duration of shedding (P < 0.05). The effects of feeding CM to calves differed, indicating that the optimal levels of CM may differ between animals or that other factors are involved in the interaction between CM and E. coli O157:H7. In vitro studies demonstrated that E. coli O157:H7 binds to CM, suggesting that the reduction in shedding may result at least in part from the binding of positively charged CM to negatively charged E. coli cells. Additional studies are needed to determine the impact of CM feeding on animal production, but the results from this study indicate that supplementing feed with CM reduces the shedding of E. coli O157:H7 in cattle.Escherichia coli O157:H7 remains a significant cause of food recalls and human illness despite the implementation of government regulations and process interventions to reduce transmission by contaminated foods. Ground beef remains a primary vehicle of food-borne dissemination, but a variety of foods have been involved in outbreaks (21). Cattle are considered a primary source in outbreaks involving nonbeef foods which become contaminated by environmental or waterborne E. coli O157:H7. The low infectious dose of this pathogen (12, 28) requires new or additional intervention strategies to further reduce its prevalence and numbers entering the food chain.Practices to prevent or reduce E. coli O157:H7 contamination of beef carcasses are primarily applied at the processing level, with the most common treatments being hide washes, trimming of contaminated carcass parts, steam vacuuming, hot water and acid washes, and steam treatment (1, 2, 4, 5). Despite the implementation of these practices, there continue to be significant numbers of recalls and beef-linked illness caused by this pathogen. The prevalence of cattle shedding E. coli O157:H7 prior to processing ranges from 2% to 42%, which correlates with the frequency of carcass contamination (7). One of the challenges to the development of preharvest interventions is the transmission of E. coli O157:H7 between animals and by environmental sources, like contaminated water (8, 27). Competitive exclusion and vaccination have been evaluated as preharvest interventions, and both have been reported to reduce but not eliminate E. coli O157:H7 from cattle (3,9,16,19).Chitosan has been used to make microparticles (i.e., chitosan micropa...