Household outbreaks among culture-confirmed cases of bacterial gastrointestinal disease.

To examine the general frequency of household outbreaks, the authors performed a retrospective search among cases of the five most frequent gastrointestinal bacterial pathogens in Denmark, a country of 5.3 million inhabitants. This was done for 57,667 cases registered from 1991 to 2001 by finding all cases that shared addresses and became infected within 3 weeks of one another. The percentage of cases that were part of household outbreaks was found to be 3.2% for Campylobacter, 13.3% for Salmonella serotype Enteritidis, 5.6% for Salmonella serotype Typhimurium, 2.0% for Yersinia enterocolitica, and 10.4% for Shigella sonnei. The vast majority of the outbreaks had not previously been registered. The wide variation in the ability to cause household outbreaks among the different types of bacteria reflects differences in their epidemiology and most likely also mirrors their overall outbreak potential. Differences in the time occurring between infections of household members may also indicate differences in the importance of person-to-person transmission for the different types of bacteria. The fact that household outbreaks occur with a relatively high frequency may be utilized in future analyses of sources of infection, in particular of Campylobacter, for which more household outbreaks than expected were identified.