Transmission
Infection with E. coli O157:H7 follows ingestion of contaminated food or water, or oral contact with contaminated surfaces. It is highly virulent, with a low infectious dose: an inoculation of fewer than 10 to 100 CFU of E. coli O157:H7 is sufficient to cause infection, compared to over one-million CFU for other pathogenic E. coli strains.
A main source of infection is undercooked ground beef; other sources include consumption of unpasteurized milk and juice, raw produce and salami, and contact with infected live animals. Waterborne transmission occurs through swimming in contaminated lakes, pools, or drinking inadequately treated water. The organism is easily transmitted from person to person and has been difficult to control in child day-care centers.
E. coli O157:H7 is found on cattle farms and can live in the intestines of healthy cattle. The toxin requires highly specific receptors on the cells' surface in order to attach and enter the cell; species such as cattle, swine, and deer which do not carry these receptors may harbor toxigenic bacteria without any ill effect, shedding them in their feces, from which they may be spread to humans. Flesh can become contaminated during slaughter and butchering, and organisms can be thoroughly mixed into beef when it is ground into hamburger. Bacteria present on the cow's udders or on equipment may get into raw milk. Contaminated foods look, smell, and taste the same as their normal counterparts.
Another potential vector of E. coli O157:H7 is filth flies (which includes house flies, Musca domestica). Filth flies have been shown to be vectors of E. coli O157:H7 using PCR.
United States food advocates have unsuccessfully attempted to control the spread of this illness by promoting the so-called "Kevin's Law". This law would give the United States Department of Agriculture power to shut down food processing plants that fail multiple inspections. The food processing industry vigorously opposes this proposal.
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