Escherichia coli O157 Exposure in Wyoming and Seattle
We tested the hypothesis that rural populations have increased exposure to Escherichia coli O157:H7. We measured circulating antibodies against the O157 lipopolysaccharide in rural Wyoming residents and in blood donors from Casper, Wyoming, and Seattle, Washington, by enzyme immunoassay (EIA). EIA readings were compared by analysis of variance and the least squares difference multiple comparison procedure. Rural Wyoming residents had higher antibody levels to O157 LPS than did Casper donors, who, in turn, had higher levels than did Seattle donors (respective least squares means: 0.356, 0.328, and 0.310; p<0.05, Seattle vs. Casper, p<0.001, rural Wyoming vs. either city). Lower age was significantly correlated with EIA scores; gender; and, in rural Wyoming, history of bloody diarrhea, town, duration of residence, and use of nontreated water at home were not significantly correlated. These data suggest that rural populations are more exposed to E. coli O157:H7 than urban populations.

Escherichia coli O157:H7 is an important human pathogen. This organism can affect humans in a variety of ways, ranging from asymptomatic carriage to diarrhea, bloody diarrhea (the most common manifestation of illness in culture-proven cases), and the postdiarrheal thrombotic microangiopathy, hemolytic uremic syndrome (HUS). Infections with E. coli O157:H7 in the Pacific Northwest of the United States have been endemic and epidemic. Vehicles transmitting this pathogen include unpasteurized milk and juice, undercooked beef, drinking water, and contact with infected persons.

Data from the Centers for Disease Control and Prevention (CDC) demonstrate higher incidences of E. coli O157:H7 infections in rural counties in the United States than in urban (Paul Mead, unpub. data). Worldwide, rural populations have been postulated to be at greater risk for exposure to E. coli O157:H7 by virtue of increased exposure to animals or their excreta in Scotland; dairy farm visits have been implicated as a source for infection in Finland and the United States; and animal contacts are a risk factor for the development of HUS in Switzerland. Serologic studies from Canada demonstrated higher frequencies of antibodies to the O157 lipopolysaccharide (LPS) side chain among residents of rural areas compared to residents of urban areas, and in Wisconsin children, manure and sheep contact were recently demonstrated to be risk factors for O157 seropositivity. Taken together, these data suggest more intense or more frequent human exposure to E. coli O157:H7 in nonurban areas.

Populations in the Pacific Northwest and Rocky Mountain states provide an opportunity to assess the frequency of exposure to E. coli O157:H7 through serologic studies. Antibodies to the O157 LPS follow natural infection with E. coli O157:H7 and are believed to be quite specific because they are rarely found in healthy people. Thus, circulating antibodies to the O157 LPS are potential markers of population exposure to E. coli O157:H7. We therefore attempted to assess the distribution of antibodies to this antigen in three different populations, encompassing a gradient of population density.