In the latest issue of CDC’s Morbidity and Mortality Weekly Report, a report studied the incidence and trends of infections of foodborne pathogens from 2013 – 2016. The report also looked at the effect of the increasing use of culture-independent diagnostic tests (CIDTs) on pathogen surveillance.
Overall, the 2016 incidence of confirmed Campylobacter infections was lower in the United States, but incidences of confirmed Shiga toxin-producing E. coli (STEC), Yersinia, and Cryptosporidium infections were higher. But the report states that culture-independent diagnostics tests (CIDTs) are complicating this data, because testing for pathogens may be occurring more frequently using this method. In 2016, FoodNet, the CDC’s Foodborne Diseases Active Surveillance Network, identified 24,0129 infections, 5,512 hospitalizations, and 98 deaths caused by nine enteric pathogens that are commonly transmitted through food.
But, the report also states that “a large proportion of CIDT positive specimens were not reflex cultured, which is necessary to obtain isolates for distinguishing pathogen subtypes, determining antimicrobial resistance, monitoring trends, and detecting outbreaks.” The authors conclude that increasing use of CIDTs affects the interpretation of public health surveillance data because that test does not yield isolates used to distinguish subtypes, determine antimicrobial resistance, monitor trends, and detect outbreaks.
Thomas Gremillion, the Director of Consumer Federation of America, said in a statement, “This CDC report shows that unsafe food continues to make millions of Americans sick each year. The data underscores the urgent need for policy reforms to address Salmonella and other foodborne pathogens, particularly in meat and poultry. We know how to make food safer, we just need the political will to move forward.”
To get isolates for infections that are identified by CIDTs, federal labs must perform reflex cultures. If they do not, state public health laboratories may have to take up the slack. The authors say that strategies are needed to preserve access to these isolates.
In 2016, the largest number of confirmed or CIDT positive-only infections was for Campylobacter (8,547), followed by Salmonella (8,172), Shigella (2,913), STEC (1,845), Cryptosporidium (1,816), Yersinia (302), Vibrio (252), Listeria (127), and Cyclospora (55). The infections that were CIDT positive without culture confirmation in 2016 was largest for Campylobacter and Yersinia, followed by STEC, Shigella, Vibrio, and Salmonella.
Health care providers may be more likely to order CIDTs because the test is faster and easier to use than traditional culture methods. That could help increase pathogen detection. It’s easy to use CIDTs when trying to diagnose a foodborne illness. Among clinical labs in the FoodNet system, the use of CIDTs to detect Salmonella increased from 2 per 460 labs in 2013 to 59 per 421 labs in 2016.
While this increased testing may identify more infections that would have been undiagnosed in the past, sensitivity and specificity vary by the test type. There is high sensitivity and specificity for most targets, but for pathogens such as Campylobacter and Cryptosporidium, sensitivity and specificity can vary widely, with a large number of false positives.
Since the number of foodborne illnesses in this country is far greater than those that are actually diagnosed, CIDTs are helping uncover those infections. But, the authors say that more data and analytic tools are needed to adjust for this change in the testing process. Foodnet is collecting more data and developing those tools.
