LB-16. Adverse Impact of Changing Clinical Diagnostics on Tracking Progress in Reducing Shiga Toxin-Producing Escherichia coli (STEC) Infections—FoodNet, 2008-2010
Session: Poster Abstract Session: Late Breaker Posters
Saturday, October 22, 2011
Room: Poster Hall B1
Background: STEC surveillance is culture-based, allowing differentiation of O157 and non-O157 strains.  Increasing use of Shiga toxin (Stx) antigen detection (StxAD) tests in clinical laboratories combined with characterization in public health laboratories (PHL) will detect more non-O157 STEC.  We assessed impacts of StxAD testing without subsequent STEC isolation on surveillance.

Method: FoodNet conducts population-based surveillance for culture-confirmed (CC) STEC infections in 10 states; in 2008–2010, it piloted surveillance for non-culture-confirmed (NCC) StxAD-positive illnesses.  We calculated incidence rates and used negative binomial regression to assess changes in incidence between 2008 and 2010 for CC STEC infections with and without including NCC StxAD-positive illnesses confirmed as Stx-positive at a PHL.  We assumed, using published data, that 36% of NCC StxAD-positive illnesses are caused by STEC O157.

Result: In 2010, rates of CC infection were 1.0 per 100,000 persons for both O157 and non-O157.  Compared with 2008, O157 incidence decreased by 19% (95% CI=3–32%), non-O157 increased by 94% (CI=52–148%). Only 15–45% of NCC StxAD-positive illnesses were confirmed as Stx-positive at PHLs. In 2010, the rate of all CC STEC was 1.9; adding Stx-confirmed NCC illnesses increased the rate to 2.1.  After adding 36% of Stx-confirmed NCC illnesses to the CC STEC O157, the decrease in STEC O157 incidence was no longer statistically significant (CI=27% decrease–2% increase).

Conclusion: StxAD testing increases detection of non-O157 STEC.  However, we show for the first time that, because of StxAD testing without STEC isolation, culture-based surveillance may not accurately measure incidence.  Inclusion of NCC illnesses led to less certainty of progress made in reducing STEC O157 infection in recent years; this could be partly due to increasing recognition of infections due to increasing StxAD testing.  Moreover, concerns of false-positive StxAD results make expanding surveillance to capture NCC illnesses difficult; validation studies are needed.  To maintain useful surveillance and optimal patient care, diarrheal stools should be cultured for E. coli O157 and simultaneously tested for non-O157 STEC by StxAD with reflex culture of positives.

Subject Category: D. Diagnostic microbiology

Rajal K. Mody, MD, MPH1, Stacy Crim, MPH1, Kathryn Wymore, MPH2, Paula Clogher, MPH3, Amanda Palmer, MPH4, John Dunn, DVM, PhD5, Jennifer Huang, MPH1, Olga Henao, PhD, MPH1 and Patricia M. Griffin, MD1, (1)Centers for Disease Control and Prevention, Atlanta, GA, (2)CA Emerging Infections Program, Oakland, CA, (3)Connecticut Emerging Infections Program, New Haven, CT, (4)Maryland Department of Health and Mental Hygiene, Baltimore, MD, (5)Tennessee Department of Health, Nashville, TN


R. K. Mody, None

S. Crim, None

K. Wymore, None

P. Clogher, None

A. Palmer, None

J. Dunn, None

J. Huang, None

O. Henao, None

P. M. Griffin, None

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