BACKGROUND: Clinical laboratories are rapidly adopting culture-independent diagnostic tests (CIDTs) for the detection of Shiga toxin-producing Escherichia coli(STEC), an important cause of foodborne illness. STEC incidence and clinical laboratory testing practices over an 11 year period were analyzed to describe the evolution and impact of CIDT adoption.
METHODS: The Tennessee Department of Health (TDH) conducted active surveillance for STEC through the Foodborne Diseases Active Surveillance Network. STEC-positive specimens were serogrouped to characterize O157 and non-O157 STEC. Surveys of clinical laboratory practices were conducted with laboratories reporting to TDH in 2007, 2012, and 2014 - 2016. Data on clinical laboratory practices and serogroup of reported STEC cases identified from 2006 - 2016 were compared temporally.
RESULTS: During 2006 - 2016, the number of laboratory-positive STEC cases steadily increased, ranging from 88 cases (2009) to 257 cases (2016). The number of cases diagnosed only by CIDT without culture confirmation (CIDT-only) nearly tripled from 2006 (38 cases) to 2016 (107). The proportion of CIDT-only cases varied from 24% to 42% annually, with 31% of cases tested only by CIDT in 2015. Among culture-confirmed cases, the proportion of non-O157 infections increased from 12% (2006) to 60% (2015). In 2015, 18 distinct non-O157 serogroups were identified, increasing from 4 serogroups in 2006. The most common non-O157 serogroups in 2015 were O103 (29 cases), O111 (20), O26 (20), O145 (6) and O118 (6). The number of STEC testing laboratories reporting any CIDT testing increased from 9 (16%) in 2007 to 44 (90%) in 2016. The percentages of laboratories using CIDT-only methods for testing were 1%, 10%, 14%, 15% and 22% in 2007, 2012, and 2014 - 2016, respectively.
CONCLUSIONS: In Tennessee, increasing adoption of CIDTs over the past decade has mirrored an increasing incidence of reported STEC infections, most notably in non-O157 serogroups and cases identified by CIDTs only. Increases in non-O157 STEC infections and uncommon O serogroups have highlighted the importance of non-O157 STEC, which have overtaken O157 infections. Understanding the risk factors for non-O157 infections is critical, especially as an increasing number of cases are identified without culture confirmation. Approximately one third of STEC cases annually were not culture-confirmed, preventing serotyping and molecular fingerprinting, which are instrumental in outbreak investigation. With 90% of STEC testing laboratories reporting CIDT use in 2016, it is crucial to stress the importance of culture confirmation for surveillance and outbreak investigation and to enhance understanding of non-O157 risk factors.