Cattle are an important reservoir of Escherichia coli O157:H7 leading to contamination of food and water, and subsequent human disease. This pathogen colonizes its hosts by producing several proteins such as Tir and EspA that are secreted by a type III secretion system. These proteins play a role in colonization of the intestine, suggesting that they might be useful targets for the development of a vaccine to reduce levels of this organism in cattle. Vaccination of cattle with proteins secreted by E. coli O157:H7 significantly reduced the numbers of bacteria shed in feces, the numbers of animals that shed, and the duration of shedding in an experimental challenge model. Vaccination of cattle also significantly (P=0.04) reduced the prevalence of E. coli O157:H7 in a clinical trial conducted in a typical feedlot setting. This strategy suggests it is possible to vaccinate cattle to decrease the level of E. coli O157:H7 shedding for the purpose of reducing the risk of human disease.