PLE-3702

A 6-year-old develops watery diarrhea, which becomes bloody, frightening him. He is afebrile but becoming very ill. Macroscopic examination of the feces shows no areas of pus; microscopically, there was no excess of PMNs over that expected from peripheral blood. If his disease is a result of a trip to a Wisconsin dairy farm and petting cattle, what is most likely causing the bleeding?