Are foodborne illness outbreaks more common now, or are we simply better at detection? Have the foods and sources associated with foodborne illness changed? Two recent Centers for Disease Control & Prevention reports provide insight.1,2 In 2016, the Foodborne Diseases Active Surveillance Network (FoodNet) detected 24,029 infections, 5,212 hospitalizations, and 98 fatalities.1 FoodNet has 10 sites serving 49 million people (15% of the U.S. population). These 2016 numbers changed only modestly from the 3 prior years.
The big two
The most frequent 2016 foodborne pathogens were Campylobacter and Salmonella, at approximately 8,000 illnesses each, detected by traditional cultures or culture-independent diagnostic tests (CIDTs). (See table.) CIDTs are relatively new molecular-based, mostly multiplex assays that test for more than a dozen pathogens in one assay.
Campylobacter-contaminated domestic food in 2016 was mostly raw/undercooked poultry or unpasteurized milk/fruit drinks. Campylobacter can be detected in up to 88% of chicken carcasses at processing plants and approximately 50% of raw chicken at grocery stores. However, Campylobacter from imported food most often came from fresh produce.2
Overall, Salmonella originated from diverse sources (eggs, poultry, meat, unpasteurized milk/juice/cheese, or raw fruits/vegetables/spices/nuts). But, in 2016, U.S. Salmonella outbreaks were from eggs, alfalfa sprouts, poultry, pistachios, and organic shake/meal products.
Most of the remainder of the 2016 foodborne illnesses were caused by Shigella, with nearly 3,000 cases; shigatoxin-producing Escherichia coli (STEC), with nearly 2,000 cases; and Cryptosporidium, also with nearly 2,000 cases. (See table.)
Hemolytic uremic syndrome (HUS)
HUS rates, mostly resulting from E. coli 0157 H7 in meat, did not vary from 2013 to 2016, with a total 62 pediatric HUS cases in FoodNet (0.56 /100,000 population). Slightly over half (56%) occurred in children under 5 years old at 1.18 per 100,000 population.
Does CIDT increase detection rates?
Detection of the “big two” did not change from 2013 to 2016 or over the past 2 decades. That said, Campylobacter detection was actually down 11% if considering only culture-confirmed cases. That is, if we do not count detections made exclusively by CIDT.
This is important because CIDT – now supplanting culture in many laboratories – identifies pathogens not likely detected by standard culture because culture is generally selective and CIDT is more sensitive. CIDT can increase detection rates (solo and multiple pathogens), even if illnesses do not really increase. The CDC suggested that this contributed to increased STEC and Yersinia detection in 2016. Some would not have been detected if only culture had been utilized.
Viable bacterial/viral isolates are not available from CIDT. A replicating pathogen is needed to characterize shifting/emerging pathogen strains (for example, analysis for mutations or new pathogens via sequencing or antimicrobial susceptibility testing).
To compensate, some CIDT-using laboratories perform “reflex cultures.” CIDT positive specimens also are cultured to provide viable isolates. However, this adds cost to an already costly CIDT test.
The role of imported food
Surveillance systems, such as the Foodborne Disease Outbreak Surveillance System, also track imported foodborne illness. Despite an approximately 50% decrease in overall U.S. foodborne outbreaks since 2000, imported food-related outbreaks increased to 195 during 2006-2014 from 54 during 1996-2004, with 10,685 illnesses, 1,017 hospitalizations, and 19 deaths since 2009. Also, imported food-related outbreaks rose from a mean 3 per year pre-2000 to a mean 18 per year during 2009-2014. Most imported food outbreaks (86% of total) had three causes: scombroid toxin (42% of total), Salmonella (33%), and hepatitis A virus (11%).
A foreign origin for approximately 19% of U.S.-consumed food makes it unsurprising that imported foods increasingly cause foodborne outbreaks. Much imported food is “outbreak prone.” Perhaps surprising is that a staggering 97% of fish/shellfish, 50% of fresh fruits, and 20% of fresh vegetables are imported, according to 2016 estimates by the U.S. Department of Agriculture.
Most imported food illnesses were from Salmonella (4,421 from 52 outbreaks), Cyclospora (2,533 from 33 outbreaks), hepatitis A virus (1,150 from 11 outbreaks), and Shigella (625 from 6 outbreaks). While eggs, ice cream, and poultry are notorious origins for Salmonella in domestic food, most imported Salmonella were from produce: fruits (26%), seeded vegetables (20%), sprouts (11%), nuts/seeds (10%), spices (7%), and herbs (2%).
Seafood/fish caused 55% of outbreaks but few illnesses per outbreak (median 3 illnesses/outbreak), so only 11% of total illnesses were caused by seafood/fish. In contrast, fresh produce caused only 33% of outbreaks but 84% of illnesses (median 40 illnesses/outbreak).
Geographic source, outbreak locations
The origin was known in 91% of outbreaks. Latin America and the Caribbean were most common, followed by Asia.3 Main contributing countries were Mexico (42 outbreaks), Indonesia (17) and Canada (11).
Contaminated fish/shellfish originated from all regions except Europe, most commonly from Asia (the majority of fish/shellfish outbreaks were from Indonesia, Vietnam, China, Philippines, Taiwan, and Thailand) with smaller contributions from the Bahamas and Ecuador.
Contaminated produce originated from all regions, mostly (64%) from Mexico and the Americas (Chile, Guatemala, and Honduras). All but one dairy outbreak originated in Latin America/the Caribbean.3 Outbreaks occurred in 31 states, most commonly California (30), Florida (25), and New York (16). Additionally, 43 (22%) were multistate outbreaks.
Outbreaks from domestic foods decreased, but those from imported foods increased. This makes sense given recent increases in outbreak-prone food imports, such as seafood/fish and produce.
To reduce overall foodborne illness outbreaks, governmental agencies need to:
Individuals can help protect themselves by avoiding inadequately washed or incompletely cooked foods or foods of uncertain origin.
Dr. Harrison is professor of pediatrics and pediatric infectious diseases at Children’s Mercy Hospitals and Clinics, Kansas City, Mo. He said he had no relevant financial disclosures. Email him at firstname.lastname@example.org.
3. Technical appendix in Emerg Infect Dis. 2017 Mar;23(3):525-8.