Morbidity and Mortality Weekly Report

Preliminary Incidence and Trends of Infections Caused by Pathogens

Transmitted Commonly Through Food — Foodborne Diseases Active
Surveillance Network, 10 U.S. Sites, 2016–2021
Jennifer P. Collins, MD1; Hazel J. Shah, MPH1; Daniel Lowell Weller, PhD1; Logan C. Ray, MPH1; Kirk Smith, DVM, PhD2; Suzanne McGuire, MPH3;
Rosalie T. Trevejo, DVM, PhD4; Rachel H. Jervis, MPH5; Duc J. Vugia, MD6; Tamara Rissman, MPH7; Katie N. Garman, MPH8; Sarah Lathrop, DVM, PhD9;
Bethany LaClair, MPH10; Michelle M. Boyle, MPH11; Stic Harris, DVM12; Joanna Zablotsky Kufel, PhD13; Robert V. Tauxe, MD1;
Beau B. Bruce, MD, PhD1; Erica Billig Rose, PhD1; Patricia M. Griffin, MD1; Daniel C. Payne, PhD1

To evaluate progress toward prevention of enteric infections laboratory-diagnosed infections in 2021 by 2020 U.S. Census
in the United States, the Foodborne Diseases Active Bureau population estimates for the surveillance area. The
Surveillance Network (FoodNet) conducts active population- percentage change in incidence during 2021 compared with
based surveillance for laboratory-diagnosed infections caused the average annual incidence during 2016–2018 was estimated
by Campylobacter, Cyclospora, Listeria, Salmonella, Shiga toxin- using a new Bayesian, negative binomial model with penalized
producing Escherichia coli (STEC), Shigella, Vibrio, and Yersinia thin plate splines that adjusted for state-specific trends and
at 10 U.S. sites. This report summarizes preliminary 2021 data changes in population over time (4).
and describes changes in annual incidence compared with the Surveillance for physician-diagnosed postdiarrheal hemolytic
average annual incidence for 2016–2018, the reference period uremic syndrome (HUS), a complication of STEC infection,
for the U.S. Department of Health and Human Services’ is conducted through a network of nephrologists and infection
(HHS) Healthy People 2030 goals for some pathogens (1). preventionists and by hospital discharge data review. This
During 2021, the incidence of infections caused by Salmonella report includes HUS cases in children and adolescents aged
decreased, incidence of infections caused by Cyclospora, Yersinia, <18 years for 2020, the most recent year with available data.
and Vibrio increased, and incidence of infections caused This activity was reviewed by CDC and was conducted
by other pathogens did not change. As in 2020, behavioral consistent with applicable federal law and CDC policy.**
modifications and public health interventions implemented During 2021, FoodNet identified 22,019 infections, 5,359
to control the COVID-19 pandemic might have decreased hospitalizations, and 153 deaths (Table 1). Incidence was
transmission of enteric infections (2). Other factors (e.g., highest for Campylobacter (17.8 cases per 100,000 population)
increased use of telemedicine and continued increase in use and Salmonella (14.2). Overall, 8% fewer infections were
of culture-independent diagnostic tests [CIDTs]) might have reported during 2021 than the average during 2016–2018;
altered their detection or reporting (2). Much work remains incidence decreased for Salmonella, increased for Cyclospora,
to achieve HHS Healthy People 2030 goals, particularly for Vibrio, and Yersinia, and was unchanged for Campylobacter,
Salmonella infections, which are frequently attributed to Listeria, Shigella, and STEC. The percentage of infections
poultry products and produce, and Campylobacter infections, resulting in hospitalization and the percentage of outbreak-
which are frequently attributed to chicken products (3). associated infections were stable. Overall, 7% of infections in
FoodNet is a collaboration among CDC, 10 state health 2021 were associated with international travel compared with
departments, the U.S. Department of Agriculture’s Food 13% during 2016–2018 (Figure).
Safety and Inspection Service (USDA-FSIS), and the Food Two thirds (67%) of bacterial infections were diagnosed using
and Drug Administration (FDA). FoodNet’s catchment area CIDT in 2021, compared with approximately one half (49%)
(Connecticut, Georgia, Maryland, Minnesota, New Mexico,
Oregon, Tennessee, and selected counties in California, * Admission to an inpatient unit or an observation stay of >24 hours within
Colorado, and New York) includes approximately 15% of 7 days before or after specimen collection or determined to be related to the
the U.S. population (an estimated 50 million persons in infection if beyond this time frame.
† Attributed to infection when they occurred during hospitalization or within
2020). Bacterial infections were diagnosed by culture or 7 days after specimen collection for nonhospitalized patients.
CIDT; Cyclospora infections were diagnosed by microscopy § Generally defined as two or more cases of similar illness associated with a

or polymerase chain reaction (2). The frequencies of common exposure; some sites also stipulate that illnesses be from more than
one household.
hospitalizations,* deaths,† outbreak-associated infections,§ ¶ International travel before illness began: 30 days for Listeria and Salmonella
and international travel–associated infections¶ were calculated serotypes Typhi and Paratyphi, 14 days for Cyclospora, and 7 days for other
pathogens.
overall and by pathogen; unknown results were classified as ** 45 C.F.R. part 46. 102(I)(2), 21 C.F.R. part 56; 42 U.S.C. Sect. 241(d);
“no.” Incidence was calculated by dividing the number of 5 U.S.C. Sect. 552a; 44 U.S.C. Sect. 3501 et seq.

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 Morbidity and Mortality Weekly Report

during 2016–2018 (Table 2). In 2021, 37% of bacterial infections 72.1%) and Infantis (23.7%; 95% CrI = 2.9% to 48.7%), lower
were diagnosed using only CIDT (i.e., the specimen had a for I 4,[5],12:i:- (−33.4%; 95% CrI = −45.4% to −17.9%),
negative culture result or was not cultured) compared with 26% Typhimurium (−29.2%; 95% CrI = −35.7% to −22.4%),
during 2016−2018. A reflex culture†† was performed for 70% of and Enteritidis (−24.7%; 95% CrI = −33.6% to −15.6%),
infections diagnosed by CIDT in 2021, similar to 2016–2018. and unchanged for Javiana (−23.0%; 95% CrI = −44.0% to
Reflex culture attempts decreased for Campylobacter, Listeria, 12.4%) and Newport (−8.7%; 95% CrI = −28.5% to 19.2%).
STEC, Vibrio, and Yersinia. The percentage of reflex cultures Enteritidis, Newport, Typhimurium, Javiana, and I 4,[5],12:i:-
that yielded a pathogen ranged from 24% for Yersinia to 89% have been among the five most common serotypes since 2010.
for Listeria. Infantis has been among the 10 most common since 2013.
Among 6,110 Salmonella isolates, 5,442 (89%) were During 2021, Oranienburg caused a multistate outbreak linked
serotyped in 2021. The seven most common serotypes were to onions;¶¶ before that, Oranienburg had last been among
Enteritidis (908; 17%), Newport (596; 11%), Typhimurium the 10 most common serotypes in 2009.
(510; 9%), Javiana (406; 7%), I 4,[5],12:i:- (304; 6%), Among 1,203 STEC isolates in 2021, serogroup O157 was
Oranienburg (247; 5%), and Infantis (232; 4%). Compared most common (314; 26%), followed by O26 (179; 15%),
with 2016–2018, incidence§§ was higher for Oranienburg O103 (140; 12%), and O111 (116; 10%). During 2020,
(38.6% increase; 95% credible interval [CrI] = 14.2% to FoodNet identified 49 cases of postdiarrheal HUS in children
†† Culture of a specimen with a positive CIDT result.
and adolescents aged <18 years (0.4 cases per 100,000),
§§ 2021 incidence (per 100,000): Enteritidis (1.8), Newport (1.2), Typhimurium
(1.0), Javiana (0.8), I 4,[5],12:i:- (0.6), Oranienburg (0.5), and Infantis (0.5). ¶¶ https://www.cdc.gov/salmonella/oranienburg-09-21/details.html

TABLE 1. Number of laboratory-diagnosed bacterial and parasitic infections, hospitalizations, deaths, outbreak-associated infections, crude
incidence, and percentage change compared with 2016–2018 average annual incidence, by pathogen — Foodborne Diseases Active Surveillance
Network, 10 U.S. sites,* 2021†
2021
No. (%)
Outbreak-associated % Change in infection incidence
Pathogen No. of Infections§ Hospitalizations¶ Deaths** infections†† Crude incidence§§ (95% CrI¶¶), 2016–2018 to 2021
Total 22,019 5,359 (24) 153 (0.7) 861 (4) — —
Bacteria
Campylobacter 8,974 1,822 (20) 33 (0.4) 51 (0.6) 17.8 −5.5 (−11.4 to 0.9)
Salmonella 7,148 1,974 (28) 52 (0.7) 597 (8) 14.2 −10.0 (−16.9 to −3.2)
STEC*** 2,542 600 (24) 10 (0.4) 79 (3) 5.0 8.8 (−6.8 to 27.0)
Shigella 1,699 532 (31) 8 (0.5) 67 (4) 3.4 −14.8 (−33.8 to 6.0)
Yersinia 683 146 (21) 3 (0.4) 2 (0.3) 1.4 79.0 (49.4 to 116.1)
Vibrio 461 117 (25) 9 (2) 8 (2) 0.9 45.5 (26.9 to 66.3)
Listeria 148 140 (95) 37 (25) 9 (6) 0.3 4.6 (−8.5 to 20.1)
Parasite
Cyclospora 364 28 (8) 1 (0.3) 48 (13) 0.7 443.2 (195.9 to 1,134.2)
Abbreviations: CIDT = culture-independent diagnostic test; CrI = credible interval; STEC = Shiga toxin–producing Escherichia coli.
* Data were obtained from laboratories in Connecticut, Georgia, Maryland, Minnesota, New Mexico, Oregon, Tennessee, and selected counties in California, Colorado,
and New York.
† 2021 data are preliminary.
§ Bacterial infections diagnosed by culture or CIDT. Cyclospora infections diagnosed by microcopy or polymerase chain reaction.
¶ Admission to an inpatient unit or an observation stay of >24 hours within 7 days before or after specimen collection or determined to be related to the infection
if beyond this time frame. Absolute change in percentage of infections resulting in hospitalization during 2021 compared with annual average for 2016–2018:
Campylobacter (0.3), Salmonella (0.3), STEC (1), Shigella (8), Yersinia (−4), Vibrio (−5), Listeria (−2), Cyclospora (2), and overall (0.6). Unknown hospitalization status
(10% of infections during 2021 and 4% during 2016–2018) was classified as not hospitalized.
** Attributed to infection when deaths occurred during hospitalization or within 7 days after specimen collection for nonhospitalized patients. Absolute change in
percentage of infections resulting in death during 2021 compared with annual average for 2016–2018: Campylobacter (<0.1), Salmonella (0.3), STEC (<0.1), Shigella
(0.4), Yersinia (−0.7), Vibrio (−0.2), Listeria (6), Cyclospora (0.1), and overall (0.2). Unknown death status (8% of infections during 2021 and 3% during 2016–2018)
was not classified as a death.
†† Generally defined as two or more cases of similar illness associated with a common exposure; some sites also stipulate that illnesses be from more than one
household. Absolute change in percentage of outbreak-associated infections during 2021 compared with annual average for 2016–2018: Campylobacter (0.2)
Salmonella (1), STEC (−1), Shigella (−1), Yersinia (0.2), Vibrio (−2), Listeria (1), Cyclospora (−10), and overall (<0.1). Unknown outbreak-association status (0.02% of
infections during 2021 and 0% during 2016–2018) was classified as not outbreak-associated.
§§ Cases per 100,000 population. Domestic incidences (cases with no or unknown travel) by pathogen during 2021: Campylobacter (17.0), Salmonella (13.1), STEC
(4.6), Shigella (3.0), Yersinia (1.3), Vibrio (0.8), Listeria (0.3), and Cyclospora (0.6).
¶¶ Percentage change reported as increase or decrease. Some increases are likely due to increasing use of CIDTs by clinical laboratories.
*** Compared with the annual average for 2016–2018, the incidence of STEC O157 infections (0.6 per 100,000) changed by −21.7% (95% CrI = −32.4% to −11.5%),
and the incidence of non-O157 STEC infections (1.8) changed by −11.6% (95% CrI = −26.2% to 7.0%).

US Department of Health and Human Services/Centers for Disease Control and Prevention MMWR / October 7, 2022 / Vol. 71 / No. 40 1261
 Morbidity and Mortality Weekly Report

including 21 (43%) in children aged <5 years (0.7 per 100,000). Discussion
The overall incidence of HUS was similar to that during The 8% decrease in enteric infections reported to FoodNet
2016–2018 (−7.6% change; 95% CrI = −21.1% to 8.4%). The during 2021 compared with the annual average during 2016–
2020 incidence of STEC O157 infections decreased 16.8% 2018 suggests ongoing effects of the COVID-19 pandemic.
(95% CrI = −25.0% to −9.3%) compared with the average Previously published FoodNet data (2) and other studies
during 2016–2018. Overall, 37 (76%) HUS cases had evidence using data from 2020 (5–7) support the occurrence of two
of STEC infection; 18 of 23 (78%) HUS cases with culture- pandemic-related phenomena: decreased transmission and
confirmed STEC infection were serogroup O157. incidence of enteric infections (i.e., due to pandemic control
measures) and underascertainment of infections related to
changes in health care–seeking behaviors (e.g., increased use

FIGURE. Number of laboratory-diagnosed bacterial and parasitic infections and percentage of persons with international travel,* by month —
Foodborne Diseases Active Surveillance Network,10 U.S. sites,† 2016–2018 and 2021§
3,500 100
Average 2016–2018
45
3,000 2021
Average % with international travel 2016–2018 40

% With international travel

2,500 % With international travel 2021 35
No. of cases

30
2,000
25
1,500
20

1,000 15

10
500
5

0 0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Month
* History of international travel before illness began: 30 days for Listeria and Salmonella serotypes Typhi and Paratyphi, 14 days for Cyclospora, and 7 days for other
pathogens. Unknown international travel (25% of infections during 2021 and 17% during 2016–2018) was classified as no travel.
† Data were obtained from laboratories in Connecticut, Georgia, Maryland, Minnesota, New Mexico, Oregon, Tennessee, and selected counties in California, Colorado,
and New York.
§ 2021 data are preliminary.

TABLE 2. Percentage of bacterial infections diagnosed by a culture-independent diagnostic test, only by a culture-independent diagnostic
test, with a reflex culture, and percentage of reflex cultures that yielded a pathogen — Foodborne Diseases Active Surveillance Network, 10
U.S. sites,* 2016–2018 and 2021†
Infections diagnosed Infections diagnosed Infections with Reflex culture yielded
by CIDT, %§ only by CIDT, %¶ a reflex culture, %** a pathogen, %††
Pathogen 2016–2018 2021 2016–2018 2021 2016–2018 2021 2016–2018 2021
Overall 49 67 26 37 71 70 65 64
Campylobacter 53 70 36 46 60 56 55 62
Listeria 4 13 0 2 100 95 88 89
Salmonella 30 49 9 15 79 85 88 83
Shigella 49 76 29 44 69 83 58 51
STEC 100 100 43 53 88 80 65 59
Vibrio 45 61 31 46 83 73 38 33
Yersinia 69 85 46 71 69 68 48 24
Abbreviations: CIDT = culture-independent diagnostic test; STEC = Shiga toxin–producing Escherichia coli.
* Data were obtained from laboratories in Connecticut, Georgia, Maryland, Minnesota, New Mexico, Oregon, Tennessee, and selected counties in California, Colorado,
and New York.
† 2021 data are preliminary.
§ Includes specimens that had a culture performed, regardless of the result, and those not cultured. Denominator is total infections.
¶ Includes specimens that had a negative culture result and those not cultured. Denominator is total infections.
** Specimens with a positive CIDT result that had a culture performed, regardless of the result. Denominator is infections diagnosed by CIDT.
†† Denominator is specimens with a reflex culture.

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 Morbidity and Mortality Weekly Report

of telemedicine). The relatively low percentage of infections

Summary
associated with international travel during 2021 (7%) and
What is already known about this topic?
2020 (5%) (2) support occurrence of the former. Lifting of
pandemic control measures might have contributed to the stable During 2020, the number of infections reported to the
Foodborne Diseases Active Surveillance Network (FoodNet)
or increased incidence for some pathogens during 2021. The decreased compared with the average reported during 2016–
stable percentage of hospitalizations during 2021 suggests that 2018. Pandemic-related measures likely decreased occurrence of
underascertainment was similar to baseline levels. However, the some infections and limited ascertainment of others.
stable incidence of HUS coincident with a decrease in incidence What is added by this report?
of STEC O157 infections during 2020 suggests that these During 2021, the number of infections reported to FoodNet
infections and perhaps others were underascertained; the severity decreased 8% compared with the 2016–2018 average, likely
of HUS makes it a more reliable measure (i.e., less affected by related to the pandemic. Most infections were caused by
changes in health care delivery or health care–seeking behaviors). Campylobacter or Salmonella; the five most common Salmonella
A better understanding of how pandemic control measures serotypes remained predominant. Use of culture-independent
diagnostic tests increased.
influenced enteric infections might help identify interventions
to sustainably decrease their incidence. What are the implications for public health practice?
Increasing use of CIDTs complicates the interpretation Comprehensive efforts are needed to improve food safety.
Substantial progress is needed to achieve national goals,
of surveillance trends, with factors such as test platform and
particularly for Salmonella and Campylobacter. Reflex cultures
pathogen affecting the accuracy of results. Molecular tests have remain essential for surveillance of enteric infections.
high sensitivity for many pathogens (8) but might not indicate
viable organisms. Variable specificity of CIDTs for FoodNet
The findings in this report are subject to at least three
pathogens can result in false-positive results, most notably
limitations. First, infections resulting from all modes of
for Vibrio (9). Reflex cultures remain essential for public
transmission (i.e., not exclusively foodborne) are included.
health functions, including determining antibiotic resistance,
Second, changes in incidence might not reflect sustained
detecting outbreaks, and determining serotypes.
trends, particularly in the context of the COVID-19 pandemic.
Comprehensive efforts are needed to address the root causes
Finally, the percentage of cases with hospitalization, death,
of foodborne illness, and substantial progress is needed to
and international travel might be underestimated because
achieve HHS Healthy People 2030 goals, particularly for
unknown results were classified as “no”; preliminary 2021 data
Salmonella and Campylobacter (1). The most recent report
have a higher percentage of unknown results than do finalized
from the Interagency Food Safety Analytics Collaboration
2016–2018 data.
attributed 23% of foodborne Salmonella illnesses to chicken
FoodNet’s 2021 data demonstrate ongoing effects of
and turkey and 42% to produce items (3). The predominance
the COVID-19 pandemic on reported cases of infections
of five Salmonella serotypes for >10 years emphasizes the need
transmitted commonly through food. As CIDT use continues
for more robust measures to identify and address Salmonella
to increase, reflex cultures remain essential for public health
contamination in food by serotype. In October 2021, USDA-
functions. Identifying novel strategies and implementing
FSIS announced plans for stronger efforts to reduce Salmonella
known strategies to address the root causes of illness are needed
infections associated with poultry products, including before
to sustainably decrease infections and achieve HHS Healthy
harvest and in slaughter and processing facilities, and began
People 2030 goals.
working with a national advisory committee*** (10). Targeted
efforts are also needed to address Salmonella contamination Acknowledgments
of produce and Campylobacter infections from chicken
Workgroup members, Foodborne Diseases Active Surveillance
products (3). Improving agricultural water safety, as FDA Network (FoodNet), Emerging Infections Program, CDC; Robert
has proposed,††† might decrease infections with pathogens Breazu, Carey Devine, Staci Dixon, Stacey Riddick, Division of
transmitted commonly by produce, including Salmonella, Foodborne, Waterborne, and Environmental Diseases, National
STEC O157, and Listeria. Center for Emerging and Zoonotic Infectious Diseases, CDC.
Corresponding author: Jennifer P. Collins, jcollins3@cdc.gov, 404-718-6903.
*** The National Advisory Committee on Microbiological Criteria for Foods.
††† https://www.fda.gov/food/food-safety-modernization-act-fsma/
fsma-proposed-rule-agricultural-water

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 Morbidity and Mortality Weekly Report

1Division of Foodborne, Waterborne, and Environmental Diseases, National 4. Weller DL, Ray LC, Payne DC, et al. An enhanced method for calculating
Center for Emerging and Zoonotic Infectious Diseases, CDC; 2Minnesota trends in infections caused by pathogens transmitted commonly through
Department of Health; 3New York State Department of Health; 4Oregon Health food. medRxiv. [Preprint posted online September 17, 2022]. https://
Authority; 5Colorado Department of Public Health and Environment; 6California doi.org.10.1101/2022.09.14.22279742
Department of Public Health; 7Connecticut Emerging Infections Program; 5. Armistead I, Tran A, White AE, Wilson E, Scallan Walter EJ. Trends in
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New Mexico; 10Georgia Department of Public Health; 11Maryland Department COVID-19 pandemic, 2020. Foodborne Pathog Dis 2022;19:290–2.
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Administration, Silver Spring, Maryland; 13Food Safety and Inspection Service, 6. Kraay ANM, Han P, Kambhampati AK, Wikswo ME, Mirza SA,
U.S. Department of Agriculture, Washington, D.C. Lopman BA. Impact of nonpharmaceutical interventions for severe acute
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