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Conclusions
During 2005 through 2010, the number of laboratories in Washington State that tested for Stx increased from 2 (4%) in 2005 to 19 (33%) in 2010, and the incidence of reported non-O157 STEC infections increased from 8 cases (0.13/100,000 population) in 2005 to 76 cases (1.13/100,000) in 2010. The most dramatic increase in reported non-O157 STEC infections occurred between 2008 and 2010, during which time incidence increased nearly 3-fold, from 26 cases (0.39/100,000) in 2008 to 76 cases (1.13/100,000) in 2010. This increase in reported incidence occurred at the same time during which most laboratories that test for Stx (11, or 58%) implemented testing (Figure 1). This suggests the increase in the reported incidence of non-O157 STEC is likely caused by changes in testing practice.
Despite the increased use of Stx testing, 37 (65%) of the laboratories in Washington State that processed nearly half (47%) of the stool cultures in the state during 2010 cultured for O157 STEC exclusively and, therefore, could not detect non-O157 STEC. Had all specimens been tested for both O157 and non-O157 STEC, we estimate that the incidence of non-O157 STEC would have been 2.12/100,000 population (60% of all reported STEC infections) in 2010, rather than the reported 1.13/100,000 (40% of all reported STEC infections).
Enhanced detection and reporting of STEC infections will likely increase workloads for local communicable disease investigators and public health laboratories during a time when funding for public health is limited. At the local level, every reported STEC infection requires an epidemiologic investigation, while additional detection of Stx at clinical laboratories will increase submission volume at public health laboratories.
National studies have found that non-O157 STEC infections are clinically indistinguishable from O157 STEC infections, with comparable hemolytic uremic syndrome attack rates. The potential virulence of non-O157 STEC infections underscores the need for enhanced laboratory testing and epidemiologic research. To encourage adherence to STEC testing recommendations, healthcare providers should request Stx testing if it is not routinely performed at their laboratory. Public health professionals and epidemiologists are encouraged to assess STEC testing practices to correctly interpret incidence trends and make clinical comparisons.
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