The most frequent causes of infection were Salmonella and Campylobacter – a finding consistent with previous years, added the agency.
Last year, FoodNet reported 20,107 confirmed cases (defined as culture-confirmed bacterial infections and laboratory-confirmed parasitic infections), 4,531 hospitalizations, and 77 deaths.
Foodborne Diseases Active Surveillance Network (FoodNet) is a collaboration with CDC, 10 state health departments, the US Department of Agriculture’s Food Safety and Inspection Service (USDA-FSIS), and the Food and Drug Administration (FDA).
It conducts surveillance for laboratory-confirmed infections caused by Campylobacter, Cryptosporidium, Cyclospora, Listeria, Salmonella, Shiga toxin–producing E. coli (STEC), Shigella, Vibrio, and Yersinia in 10 sites covering 15% of the population (an estimated 49m people in 2014).
Compared with incidence in 2012–2014, the 2015 rate of confirmed infections (CI) was significantly higher for STEC non-O157 (40% increase; 21%–62%).
CDC said the increase in incidence of STEC non-O157 infections is attributable, in part or in full, to increases in diagnostic testing. The proportion of laboratories testing for STEC non-O157 increased to 74% in 2015, compared with 55% in 2012.
There were no significant changes for other pathogens compared with previous three year averages.
Incidence of Salmonella serotype Typhimurium infections declined and it has dropped to the third most commonly reported serotype.
CIDT method growth
FoodNet received reports of 3,112 positive culture-independent diagnostic tests (CIDTs) without culture-confirmation, a considerable increase since 2012.
CIDTs detect the presence of a specific antigen or genetic sequence of a germ and do not require isolation and identification of living organisms so are quicker and give results sooner than traditional culturing methods. However, they do not give the information needed to characterize organisms that cause infections.
Because they do not provide isolates (the organism that caused the illness), it is not possible to run tests that determine an organism’s DNA fingerprint, strain or subtype or resistance pattern.
The delay to outbreak investigation means contaminated products may remain on shelves and more people may become sick.
The number of positive CIDT reports, by pathogen were Campylobacter (2,021), Shigella (454), Salmonella (361), and STEC (254).
This represent an increase in positive CIDT reports in 2015 of 92% for Campylobacter, 284% for Shigella, 247% for Salmonella, and 120% for STEC, when compared with the 2012–2014 averages; the overall increase in CIDT reports for these four pathogens was 122%, said CDC.
Increased use of CIDT could affect public health officials’ ability to monitor trends and detect outbreaks, according to the report.
“In the short term, clinical laboratories should work with their public health laboratories to make sure a culture is done whenever a CIDT indicates that someone with diarrheal illness has a bacterial infection.
“For a long-term solution, CDC is working with partners to develop advanced testing methods that, without culture, will give health care providers information to diagnose illness and also give the detailed information that public health officials need to detect and investigate outbreaks.”
Among confirmed infections, the vast majority were diagnosed only by culture; the exception is STEC, for which most were identified by a CIDT.
“Although CIDTs are still most commonly being used for Campylobacter and STEC, the highest percentage increase in use compared with the previous 3-year average was observed for Shigella and Salmonella, most likely due to laboratories using the newly available DNA-based syndrome panels,” said the report.
Positive CIDT results are defined as the detection of antigen or nucleic acid sequences of the pathogen, or for STEC, Shiga toxin or the genes that encode a Shiga toxin, in a stool specimen or enrichment broth using a CIDT.
“Infection 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, 2012–2015”
Authors: Huang JY, Henao OL, Griffin PM, et al.