Strategies to Reduce Person-to-Person Transmission during E. coli Outbreak
Strategies to Reduce Person-to-Person Transmission during E. coli Outbreak
During the Escherichia coli O157:H7 outbreak in 2006 in the United States, the primary strategy to prevent illness was to advise consumers not to eat spinach. No widespread warnings were issued about preventing person-to-person (secondary) transmission. A disease transmission model, fitted to the current data, was used to investigate likely reductions in illnesses that could result from interventions to prevent secondary transmission. The model indicates that exposure to contaminated spinach occurred early in the outbreak and that secondary transmission was similar to that in previous E. coli outbreaks (≈12%). The model also suggests that even a modestly effective strategy to interrupt secondary transmission (prevention of only 2%–3% of secondary illnesses) could result in a reduction of ≈5%–11% of symptomatic cases. This analysis supports the use of widespread public health messages during outbreaks of E. coli O157:H7 with specific advice on how to interrupt secondary transmission.
Widespread distribution of contaminated spinach was implicated in an Escherichia coli O157:H7 (E. coli O157) outbreak in the United States in 2006. As of September 24, 2006, a total of 173 cases had been reported in 25 states; 88% of cases were reported over an 18-day period from August 19 through September 5, 2006. The outbreak strain was particularly virulent, resulting in 1 death, 53% of patients being hospitalized, and a 16% rate of hemolytic uremic syndrome. At the time of our analysis, the potential extent of the outbreak was unknown because new cases were still being reported. The Centers for Disease Control and Prevention (CDC) and the US Food and Drug Administration advised consumers not to eat spinach as the primary strategy for protecting against foodborne transmission of E. coli O157. No warnings, however, were issued regarding the prevention of person-to-person (secondary) transmission.
According to recent studies on the extent of secondary transmission for E. coli O157 and other pathogens, the initially reported foodborne illnesses in the outbreak may have represented only a small fraction of a larger outbreak that included asymptomatic infections and secondary infections among household members of infected persons and other close contacts. Specifically, the E. coli O157 literature indicates that a large proportion (72%) of infections are asymptomatic, exposure to low doses can result in infection, and reported secondary transmission rates are on the order of 4%–16%. Further, outbreaks of shigellosis, cryptosporidiosis, and giardiasis indicate that other highly infectious enteric pathogens can spread from person to person after being introduced into a community through water, food, or other sources. Recent adenovirus outbreak data indicate that persons with asymptomatic infection who are shedding virus can be a primary cause of continual transmission of infection. And in a prolonged giardiasis outbreak that occurred in late 2003 in a Boston, Massachusetts, suburb, 30 primary cases of giardiasis attributed to a children's swimming pool resulted in 105 secondary cases among persons from the same or socially related households. New cases of giardiasis continued to occur for up to 4 months after the pool was closed for the season.
Using an epidemiologically based disease transmission model, we investigated the potential for reducing the number of symptomatic infections (cases) of E. coli O157 by using interventions designed to reduce secondary transmission during the course of the 2006 E. coli O157 outbreak in the United States. We assumed that a combination of possible intervention strategies to interrupt secondary transmission would have a range of possible levels of effectiveness. These strategies would include strongly recommending handwashing, avoiding contact with persons with diarrhea (of any cause), meticulously preparing food, and avoiding work or school when ill with any gastrointestinal sign or symptom. Initiation of these strategies was assumed to occur at the same time as CDC's first press release on the outbreak on September 14, 2006, 1 week later, and 2 weeks later. We assumed that these strategies would reduce the transmission of infection to healthy persons from persons with both symptomatic and asymptomatic infections.
Abstract and Introduction
Abstract
During the Escherichia coli O157:H7 outbreak in 2006 in the United States, the primary strategy to prevent illness was to advise consumers not to eat spinach. No widespread warnings were issued about preventing person-to-person (secondary) transmission. A disease transmission model, fitted to the current data, was used to investigate likely reductions in illnesses that could result from interventions to prevent secondary transmission. The model indicates that exposure to contaminated spinach occurred early in the outbreak and that secondary transmission was similar to that in previous E. coli outbreaks (≈12%). The model also suggests that even a modestly effective strategy to interrupt secondary transmission (prevention of only 2%–3% of secondary illnesses) could result in a reduction of ≈5%–11% of symptomatic cases. This analysis supports the use of widespread public health messages during outbreaks of E. coli O157:H7 with specific advice on how to interrupt secondary transmission.
Introduction
Widespread distribution of contaminated spinach was implicated in an Escherichia coli O157:H7 (E. coli O157) outbreak in the United States in 2006. As of September 24, 2006, a total of 173 cases had been reported in 25 states; 88% of cases were reported over an 18-day period from August 19 through September 5, 2006. The outbreak strain was particularly virulent, resulting in 1 death, 53% of patients being hospitalized, and a 16% rate of hemolytic uremic syndrome. At the time of our analysis, the potential extent of the outbreak was unknown because new cases were still being reported. The Centers for Disease Control and Prevention (CDC) and the US Food and Drug Administration advised consumers not to eat spinach as the primary strategy for protecting against foodborne transmission of E. coli O157. No warnings, however, were issued regarding the prevention of person-to-person (secondary) transmission.
According to recent studies on the extent of secondary transmission for E. coli O157 and other pathogens, the initially reported foodborne illnesses in the outbreak may have represented only a small fraction of a larger outbreak that included asymptomatic infections and secondary infections among household members of infected persons and other close contacts. Specifically, the E. coli O157 literature indicates that a large proportion (72%) of infections are asymptomatic, exposure to low doses can result in infection, and reported secondary transmission rates are on the order of 4%–16%. Further, outbreaks of shigellosis, cryptosporidiosis, and giardiasis indicate that other highly infectious enteric pathogens can spread from person to person after being introduced into a community through water, food, or other sources. Recent adenovirus outbreak data indicate that persons with asymptomatic infection who are shedding virus can be a primary cause of continual transmission of infection. And in a prolonged giardiasis outbreak that occurred in late 2003 in a Boston, Massachusetts, suburb, 30 primary cases of giardiasis attributed to a children's swimming pool resulted in 105 secondary cases among persons from the same or socially related households. New cases of giardiasis continued to occur for up to 4 months after the pool was closed for the season.
Using an epidemiologically based disease transmission model, we investigated the potential for reducing the number of symptomatic infections (cases) of E. coli O157 by using interventions designed to reduce secondary transmission during the course of the 2006 E. coli O157 outbreak in the United States. We assumed that a combination of possible intervention strategies to interrupt secondary transmission would have a range of possible levels of effectiveness. These strategies would include strongly recommending handwashing, avoiding contact with persons with diarrhea (of any cause), meticulously preparing food, and avoiding work or school when ill with any gastrointestinal sign or symptom. Initiation of these strategies was assumed to occur at the same time as CDC's first press release on the outbreak on September 14, 2006, 1 week later, and 2 weeks later. We assumed that these strategies would reduce the transmission of infection to healthy persons from persons with both symptomatic and asymptomatic infections.
