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Inclusion of Rapid Test Results Leaves Gaps in CDC Foodborne Illness Data

For the first time, the CDC’s yearly report on foodborne illnesses in the U.S. included infections that were identified only with rapid diagnostic tests, which can speed up treatment for patients but lead to important gaps in data, researchers said.

Culture-independent diagnostic tests (CIDTs) are increasingly being used by clinicians to detect enteric infections because they produce results more quickly than traditional culture methods, researchers from the CDC and other public health facilities across the country wrote in MMWR. However, used alone, these tests tell only part of the story, leaving out important information on pathogen subtypes and antimicrobial resistance, and making it difficult to monitor trends or link infections to outbreaks, the researchers said.

Such information can be obtained only if a reflex culture is performed on the CIDT-positive specimen, the researchers said. But according to their report, which compared surveillance data for 15% of the U.S. population from 2013 to 2016, a large proportion of positive CIDT results do not receive these much-needed cultures.

“We need foodborne-illness trend data to monitor progress toward making our food supply safer,” Robert Tauxe, MD, MPH, director of the CDC’s Division of Foodborne, Waterborne, and Environmental Diseases, said in a statement. “It’s important that laboratories continue to do follow-up cultures on CIDT-positive patients so public health officials can get the information needed to protect people from foodborne illness.”

Potential for false-positive results

Including CIDT results in the CDC data for 2016 raised the incidence rates for six of the most reported bacterial foodborne illnesses. However, interpreting these increases is complicated, the researchers said.

As an example, they noted that the incidence of culture-confirmed infections with Campylobacter — the most commonly reported bacterial foodborne illness — was “significantly lower” in 2016 compared with the average over the previous 3 years. However, a “slight but not significant” increase occurred when infections that were diagnosed only through CIDTs were included.

Further, Campylobacter is one of the pathogens for which antigen-based CIDTs are often used. These tests can produce a large number of false-positive results, leading to skewed estimates of incidence, the researchers said.

“When interpreting incidence and trends in light of changing diagnostic testing, considering frequency of testing, sensitivity, and specificity of these tests is important,” they wrote. “The observed increases in incidence of confirmed or CIDT positive–only infections in 2016 compared with 2013 to 2015 could be caused by increased testing, varying test sensitivity, an actual increase in infections, or a combination of these reasons.”

FoodNet data

The researchers looked at preliminary 2016 data from the CDC’s Foodborne Disease Active Surveillance Network, also called FoodNet, which monitors cases of nine foodborne disease at 10 sites in Connecticut, Georgia, Maryland, Minnesota, New Mexico, Oregon, Tennessee and certain counties in California, Colorado and New York, and compared the incidence rates with data from 2013 to 2015.

The sites reported 24,029 foodborne infections in 2016, including 5,512 hospitalizations and 98 deaths. Campylobacter was the leading cause of bacterial foodborne illness with 8,547 reported infections, followed by Salmonella (8,172), Shigella (2,913), Shiga toxin-producing Escherichia coli (1,845), Cryptosporidium (1,816), Yersinia (302), Vibrio (252), Listeria (127) and Cyclospora (55).
Salmonella typhimurium infections, which often are linked to beef and poultry, decreased 18% in 2016 compared with the average incidence for 2013 to 2015. The reduction is possibly due to regulatory action by the USDA to reduce Salmonella contamination in poultry and vaccination of chicken flocks, according to a CDC news release.

The CDC said increases in Yersinia, Cryptosporidium, and Shiga toxin-producing E. coli infections were likely due to the increased use of CIDTs.

“This report provides important information about which foodborne germs are making people sick in the United States,” Tauxe said. “It also points out changes in the ways clinicians are testing for foodborne illness and gaps in information as a result.”

The researchers said previous analyses have indicated that the number of foodborne infections in the U.S. “far exceeds those diagnosed” and that CIDTs might be making them “more visible.” But they said more tools are needed to accurately interpret FootNet surveillance data in light of these changes to testing practices.

“FoodNet is collecting more data and developing these tools,” they wrote. “With these, FoodNet will continue to track the needed progress toward reducing foodborne illness.” – by Gerard Gallagher


Marder EP, et al. MMWR Morb Mortal Wkly Rep. 2017;doi:10.15585/mmwr.mm6615a1


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