Incidence and Trends of Infections With Pathogens Transmitted Commonly Through Food and the Effect of Increasing Use of Culture-Independent Diagnostic Tests on Surveillance

Foodborne Diseases Active Surveillance Network, 10 U.S. Sites, 2013-2016

Ellyn P. Marder, MPH; Paul R. Cieslak, MD; Alicia B. Cronquist, MPH; John Dunn, DVM; Sarah Lathrop, PhD; Therese Rabatsky-Ehr, MPH; Patricia Ryan, MD; Kirk Smith, DVM; Melissa Tobin-D'Angelo, MD; Duc J. Vugia, MD; Shelley Zansky, PhD; Kristin G. Holt, DVM; Beverly J. Wolpert, PhD; Michael Lynch, MD; Robert Tauxe, MD; Aimee L. Geissler, PhD

Disclosures

Morbidity and Mortality Weekly Report. 2017;66(15):397-403. 

In This Article

Discussion

The number of CIDT positive–only infections reported to FoodNet has been increasing markedly since 2013, as more clinical laboratories adopt CIDTs. Initially, increases were primarily limited to Campylobacter and STEC; followed by substantial increases in Salmonella and Shigella beginning in 2015.[6] The pattern continued in 2016, with large increases in the number of CIDT positive–only Vibrio and Yersinia infections. When including both confirmed and CIDT positive–only infections, incidence rates in 2016 were higher for each of these six pathogens. The increasing use of CIDTs presents challenges when interpreting the corresponding increases in incidence. For example, the incidence of confirmed Campylobacter infections in 2016 was significantly lower than the 2013–2015 average. However, when including CIDT positive–only infections, a slight but not significant increase occurred. For STEC and Yersinia, the incidence of confirmed infections alone and confirmed or CIDT positive–only infections in 2016 were both significantly higher than the 2013–2015 average; the magnitude of change approximately doubled when analyzing CIDT positive–only infections.

Because of the ease and increasing availability of CIDTs, testing for some pathogens might be increasing as health care provider behaviors and laboratory practices evolve.[2] Among clinical laboratories in the FoodNet catchment, the use of CIDTs to detect Salmonella, for which the only CIDTs available are DNA-based gastrointestinal syndrome panels, increased from 2 per 460 laboratories (<1%) in 2013 to 59 per 421 laboratories (14%) in 2016 (FoodNet, unpublished data). This increased use paralleled significant increases in incidence of Cryptosporidium, STEC, and Yersinia, and slight but not significant increases in incidence of Campylobacter, Salmonella, Shigella, and Vibrio, all of which are also included in these panel tests. The increase in STEC incidence is driven by the increase in STEC non-O157, which is not typically included in routine stool culture testing because it requires specialized methods. Routine stool cultures performed in clinical laboratories typically include methods that identify only Salmonella, Campylobacter, Shigella, and for some laboratories, STEC O157.[4,5] The increased use of the syndrome panel tests might increase identification, and thus, improve incidence estimates of pathogens for which testing was previously limited.

Results are more quickly obtained using CIDTs than traditional culture methods.[3] Because of this, health care providers might be more likely to order a CIDT than traditional culture.[2] Increased testing might identify infections that previously would have remained undiagnosed. However, sensitivity and specificity vary by test type. Evaluations of DNA-based syndrome panel tests have indicated high sensitivity and specificity for most targets.[3] However, among pathogens for which antigen-based CIDTs are often used, such as Campylobacter and Cryptosporidium, sensitivity and specificity have varied more widely, with a large number of false positive results.[7,8] Including CIDT positive infections to calculate incidence, some of which could be false positives, might provide an inaccurate estimate. When interpreting incidence and trends in light of changing diagnostic testing, considering frequency of testing, sensitivity, and specificity of these tests is important. The observed increases in incidence of confirmed or CIDT positive–only infections in 2016 compared with 2013–2015 could be caused by increased testing, varying test sensitivity, an actual increase in infections, or a combination of these reasons.

These changes in testing are also important to consider when monitoring progress toward Healthy People 2020 objectives.†† The current objectives were created before the use of CIDTs and were based on confirmed infections. In the future, just as incidence measures should adjust for these changes, objectives should also be evaluated in light of changing diagnostics.

CIDTs pose additional challenges because they do not yield the bacterial isolates necessary for essential public health surveillance activities, such as monitoring trends in pathogen subtypes, conducting molecular testing, detecting outbreaks and implicating vehicles, and determining antimicrobial susceptibility. Reflex culture performed to yield an isolate places an additional burden on laboratories' budgets, personnel, and time. Specimen submission requirements differ by state and pathogen, and this responsibility often falls to state public health laboratories.[9] As CIDT use increases and more pathogens are affected, state public health laboratories will be challenged to sufficiently increase their testing capacity and will likely have to prioritize specimens on which to perform reflex culture.[10] Clinical laboratories should review state specimen submission requirements and the Association of Public Health Laboratories guidelines§§ for reflex culture and submission of CIDT positive specimens.

The findings in this report are subject to at least two limitations. First, the changing diagnostic landscape with unknown changes in frequency of testing, varying test performance, and decreasing availability of isolates for subtyping make interpreting incidence and trends more difficult. Second, changes in health care–seeking behavior, access to health services, or other population characteristics might have changed since the comparison period, which could affect incidence.

Foodborne illness remains a substantial public health concern in the United States. Previous analyses have indicated that the number of infections far exceeds those diagnosed; CIDTs might be making those infections more visible.[11] Most foodborne infections can be prevented, and substantial progress has been made in the past in decreasing contamination of some foods and reducing illness caused by some pathogens. More prevention measures are needed. Surveillance data can provide information on where to target these measures. However, to accurately interpret FoodNet surveillance data in light of changes in diagnostic testing, more data and analytic tools are needed to adjust for changes in testing practices and differences in test characteristics. FoodNet is collecting more data and developing those tools. With these, FoodNet will continue to track the needed progress toward reducing foodborne illness.

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