Abstract
BackgroundWithin outbreaks of Escherichia coli O157 (E. coli O157), at least 10–15% of cases are thought to have been acquired by secondary transmission. However, there has been little systematic quantification or characterisation of secondary outbreak cases worldwide. The aim of this study was to characterise secondary outbreak cases, estimate the overall proportion of outbreak cases that were the result of secondary transmission and to analyse the relationships between primary and secondary outbreak cases by mode of transmission, country and median age.MethodsPublished data was obtained from 90 confirmed Escherichia coli O157 outbreaks in Great Britain, Ireland, Scandinavia, Canada, the United States and Japan, and the outbreaks were described in terms of modes of primary and secondary transmission, country, case numbers and median case age. Outbreaks were tested for statistically significant differences in the number of ill, confirmed, primary and secondary cases (analysis of variance and Kruskal-Wallis) and in the rate of secondary cases between these variables (Generalised Linear Models).ResultsThe outbreaks had a median of 13.5 confirmed cases, and mean proportion of 0.195 secondary cases. There were statistically significant differences in the numbers of ill, confirmed, primary and secondary cases between modes of primary transmission (p < 0.021), and in primary and secondary cases between median age categories (p < 0.039) and modes of secondary transmission (p < 0.001).Secondary case rates differed statistically significantly between modes of secondary and primary transmission and median age categories (all p < 0.001), but not between countries (p = 0.23). Statistically significantly higher rates of secondary transmission were found in outbreaks with a median age <6 years and those with secondary transmission via person to person spread in nurseries. No statistically significant interactions were found between country, mode of transmission and age category.ConclusionOur analyses indicated that ~20% of E. coli O157 outbreak cases were the result of secondary spread, and that this spread is significantly influenced by age and modes of primary and secondary transmission, but not country. In particular, the results provide further data emphasising the importance of simple but effective preventive strategies, such as handwashing, that can reduce the risk of secondary spread, particularly amongst young children in nurseries.
Highlights
Within outbreaks of Escherichia coli O157 (E. coli O157), at least 10–15% of cases are thought to have been acquired by secondary transmission
Our analyses indicated that ~20% of E. coli O157 outbreak cases were the result of secondary spread, and that this spread is significantly influenced by age and modes of primary and secondary transmission, but not country
Literature Search Medline®, the Web of Knowledge® and archives of publications issued by national public health organizations were searched for reports of E. coli O157 outbreaks occurring between 1st January 1982 and 31st December 2006 in Scotland, England and Wales, the United States, Canada, Japan, Ireland, Denmark, Finland, Norway and Sweden
Summary
Within outbreaks of Escherichia coli O157 (E. coli O157), at least 10–15% of cases are thought to have been acquired by secondary transmission. Escherichia coli O157 (E. coli O157) infections are a major contributor to severe infectious gastrointestinal illness in the developed world, resulting in 150–250 cases reported annually in Scotland [1] and 2500–4500 cases in the United States[2]. Water and person to person spread are long recognised modes of transmission [15], in more recent years, infection has been more directly linked to animal and environmental exposures[16,17]. Secondary transmission from an infected person, directly or indirectly[18] to another person, occurs [19,20] with sources suggesting between 10–20% of outbreak cases are acquired by secondary transmission [20,21,22,23]
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