The draft’s discussions of the baseline number of E. coli O157:H7 infections and adjustments for underdiagnosis and underreporting are scientifically sound. The logic followed is clear, and the epidemiologic data are used in a reasonable and plausible way. However, by focusing solely on the O157:H7 serotype of enterohemorrhagic E. coli and on direct contamination, the draft underestimates the overall burden of disease caused by this category of pathogen and the benefit that could derive from interventions.
Strains of the O157:H7 serotype of Escherichia coli isolated since the early 1980s typically carry a set of virulence factors encoded by chromosomal, plasmid, and phage genes that allow them to cause a spectrum of clinical illness in humans ranging from watery diarrhea and hemorrhagic colitis to hemolytic uremic syndrome (HUS) and thrombotic thrombocytopenic purpura (TTP). It is the last two severe clinical syndromes, particularly HUS, that make O157:H7 an important public health problem in the United States because they may result in hospitalization, chronic disease, and death.
The virulence properties that allow O157:H7 to cause hemorrhagic colitis, HUS, and TTP are common to a category of diarrheogenic E. coli often called enterohemorrhagic E. coli, or EHEC. It is important to recognize that a number of serotypes of E. coli other than O157:H7 also possess these properties. The common virulence factors carried by EHEC include a chromosomal pathogenicity island that encodes proteins allowing the bacteria to cause attaching and effacing lesions of the intestinal mucosa, an approximately 60 megadalton plasmid that encodes attachment factors and an enterohemolyin, and bacteriophages that encode Shiga toxins 1, 2, or both. That array of virulence properties stably carried by some E. coli strains makes them “EHEC” and renders them capable of causing the severe diseases that stimulate the demand for interventions.
It should be emphasized that the vast majority of E. coli strains associated with HUS and hemorrhagic colitis carry the full array of virulence traits. Most of, although not all, those strains are in a known set of O:H serotypes of which O157:H7 is the most common. Others include O111:H8, O111:NM, O26:H11, O145:H25, and O103:H21. In contrast, E. coli strains that produce only Shiga toxin but do not have other virulence properties