Summary of Possible Multistate Enteric (Intestinal) Disease Outbreaks in 2023

Key Findings

1. During 2023, CDC with state and regulatory public health partners investigated 181 possible multistate outbreaks, of which 84 were determined to be outbreaks. Of these, outbreak investigators solved 50 outbreaks (60%). A suspected source was identified for 22 outbreaks and a confirmed source was identified for 28 outbreaks.
2. The 84 multistate outbreaks investigated, both solved and unsolved, resulted in 3,153 illnesses, 942 hospitalizations, and 10 deaths.
3. Of the 50 solved outbreaks, 32 were linked to contaminated food and caused 1,219 illnesses, 421 hospitalizations, and 9 deaths. The remaining 18 solved multistate outbreaks were linked to animal contact and caused 1,220 illnesses, and 290 hospitalizations.
4. Fruits were identified as the source of the most solved foodborne outbreaks (5) during 2023. Vegetable row crops (4) and chicken (4) were also identified as common sources of foodborne outbreaks in 2023.
5. Backyard poultry was identified as the source of both the most solved outbreaks (14) and the most outbreak-associated illnesses (1,127) of any animal contact category in 2023.

Possible multistate outbreaks

This analysis includes investigations of 181 possible multistate enteric (intestinal) outbreaks caused by Salmonella, Shiga Toxin-Producing Escherichia coli (STEC), Listeria monocytogenes, or Campylobacter during 2023. Some possible outbreaks were excluded from this report because they were determined to be single state outbreaks or linked to international travel. After evaluation, 84 (46%) of these were determined to be multistate outbreaks, and investigators solved 50 (60%).

Multistate Outbreaks

Consistent with findings of previous summaries, Salmonella caused the highest number of multistate outbreaks (54) and illnesses (2,848), making up 64% of the multistate outbreaks investigated and most of (90%) of the illnesses. More multistate outbreaks were investigated by CDC in 2023 than any year since 2016. The number of multistate outbreaks investigated in 2023 increased from 2022 for all pathogens except Listeria, which caused the same number of outbreaks in 2022 and 2023.

Figure 1: Number of Multistate Outbreaks Investigated by CDC, by Year and Pathogen, 2016–2023*

*Investigation of possible multistate campylobacteriosis outbreaks by Outbreak Response and Prevention Branch began in 2022.

The 84 investigated multistate outbreaks resulted in 3,153 illnesses, 942 hospitalizations, and 10 deaths. These numbers substantially underrepresent the true number of illnesses caused by these outbreaks because many people do not seek medical care or do not get tested to see what could be causing their illness. Without testing, they cannot be identified as being part of a multistate outbreak.

Solved Multistate Outbreaks

Among the 84 investigated multistate outbreaks, 50 (60%) were solved, meaning a confirmed or suspected source of the outbreak was identified, including 28 (56%) outbreaks with confirmed sources and 22 (44%) with suspected sources. Among the 50 solved outbreaks, 32 (64%) were linked to contaminated foods, and 18 (36%) were linked to animal contact. The percent of solved multistate outbreaks remained about the same compared to 2022.

Figure 2: Solved Multistate Foodborne and Animal Contact Outbreaks, 2016–2023

Figure 3: Illnesses in Solved Multistate Foodborne and Animal Contact Outbreaks, 2016–2023

Solved multistate outbreaks caused at least 2,439 illnesses. Solved multistate outbreaks linked to contaminated food caused 1,219 illnesses, 421 hospitalizations, and 9 deaths, while multistate outbreaks linked to animal contact caused 1,220 illnesses and 290 hospitalizations.

Highlights

Listeria monocytogenes and Enoki Mushrooms

Why was this outbreak notable?

The first known outbreak of Listeria monocytogenes linked to enoki mushrooms occurred in 2020 and was from enoki mushrooms imported from Korea. Illnesses were identified in the United States, Canada, and Australia. As a result of this outbreak, the U.S. Food and Drug Administration (FDA) implemented import controls to prevent enoki mushrooms contaminated with Listeria monocytogenes from entering the food supply. In 2023, a second outbreak of Listeria monocytogenes linked to enoki mushrooms was investigated after new illnesses were found to be genetically linked to enoki mushrooms from China. This outbreak included five illnesses in the United States, and Canadian partners also identified an illness and related enoki mushroom isolates in Canada. Contamination levels of the enoki mushrooms were very high, and multiple strains of Listeria monocytogenes were identified in mushroom samples.

This outbreak underscores the continuing risk of listeriosis caused by the consumption of enoki mushrooms, highlights international collaboration, and demonstrates the global impact of enoki mushrooms as a source of listeriosis.

What happened as a result of this outbreak?

CDC suggests that people at high risk for listeriosis only eat enoki mushrooms that are thoroughly cooked and prepare and handle raw enoki mushrooms separately from other foods. CDC and FDA are working on assessing how people in the United States typically prepare and consume enoki mushrooms. As a result of this outbreak, FDA placed China on a country-wide import alert for enoki mushrooms and multiple firms promptly recalled enoki mushrooms in the United States and Canada.

Salmonella and Dog Food

Why was this outbreak notable?

In October 2023, a state surveillance program isolated Salmonella Kiambu from a bag of dried dog food purchased at a retail store. This isolate was genetically related to isolates obtained from ill people earlier in the year, most of whom were children 1 year of age or younger. Most patients had the same brand of dried dog food in their homes before illness onset.

This outbreak demonstrates the human health risk associated with contacting contaminated dried dog food, which was last confirmed as the vehicle of a United States multistate salmonellosis outbreak in 2012. Patients could have acquired Salmonella from direct contact with the food, though for infant patients, it is likely that transmission occurred indirectly via caregivers who handled the food, by contact with contaminated feeding equipment, or through interaction with dogs that ate the food. It is important to educate caregivers about the risk of Salmonella transmission between children, pets, and pet food of any type and about the steps to prevent illness, including handwashing after feeding pets and regular cleaning of pet feeding equipment. Additionally, surveillance for enteric pathogens in pet food remains crucial to detect these outbreaks. These outbreaks can last a long time because of the long shelf life of dried dog food.

What happened as a result of this outbreak?

In collaboration with the FDA and CDC, the manufacturer voluntarily recalled the affected brand of dried food as well as all other brands of dried dog and cat food produced at the same facility. Investigating agencies notified the public of this outbreak and disseminated information about how to prevent illnesses associated with pets and pet food.

Salmonella and Onions

Why was this outbreak notable?

In 2023, investigators linked a multistate outbreak of Salmonella Thompson infections to the consumption of fresh diced onions. A small number of ill people in this outbreak lived in five long-term care facilities (LTCFs). All five LTCFs they lived in had diced yellow onions supplied by the same processor. FDA and state officials conducted inspections at the onion processor and grower and the outbreak strain was identified in the onion growing environment.

What happened as a result of this outbreak?

As a result of this outbreak investigation, fresh diced onion products were voluntarily recalled, removing potentially contaminated products from the market. CDC and FDA issued public notices describing the outbreak and recall. FDA continues to implement a strategy to prevent salmonellosis outbreaks associated with bulb onions. CDC launched a representative survey to help estimate how often the general United States population eats different types of onions. The comparison of onion consumption in the general population to onion consumption of ill people in an outbreak can reveal increased exposure, which allows investigators to identify onions as a possible source of illness in outbreak investigations. CDC is also working to assess foodborne outbreaks associated with healthcare facilities and evaluate how food safety is incorporated into food procurement decisions at LTCFs to identify prevention opportunities. This outbreak, in addition to two onion-associated outbreaks in 2020 and 2022, highlights the risks of foodborne illness from eating raw onions, the susceptibility of LTCF residents to foodborne illness, the challenges of preventing contamination on farms, and the ongoing emergence of onions as a source of Salmonella outbreaks.

Salmonella and Backyard Poultry and Ground Turkey

Why was this outbreak notable?

Each year, CDC and state partners investigate outbreaks of Salmonella linked to backyard poultry (such as chickens, ducks, and turkeys), with most illnesses occurring between April and August. In 2023, there were 13 individual outbreaks of 6 different Salmonella serotypes linked only to backyard poultry, resulting in 1,072 illnesses. Additionally, CDC, the U.S. Department of Agriculture's Food Safety and Inspection Service (USDA-FSIS), and state partners investigated a separate outbreak of Salmonella Hadar that began in February 2023. Some patients reported contact with backyard poultry, and a smaller number reported consuming ground turkey only. The outbreak strain was isolated from one patient's backyard poultry (chickens), and the outbreak strain was also isolated from ground turkey collected for routine testing from processing establishments by USDA-FSIS and from retail stores by FDA's National Antimicrobial Resistance Monitoring System.

Since its emergence in 2020, this strain of Salmonella Hadar has caused more than 2,000 illnesses. Previous outbreaks of Salmonella Hadar were linked to either contact with backyard poultry or consumption of ground turkey. This was the first Salmonella Hadar outbreak linked to both backyard poultry and consumption of ground turkey. These findings indicate that this strain of Salmonella Hadar persists in both the backyard poultry and ground turkey industries.

What happened as a result of this outbreak?

CDC and USDA-FSIS investigators shared outbreak information with the National Turkey Federation and with food processing establishments involved in the outbreak. Additionally, CDC met with the backyard poultry hatchery that was identified in traceback as the single common supplier to patients who reported purchasing backyard poultry before illness onset. CDC continues to collaborate with governmental agencies, members of backyard poultry and food poultry industries, and academic partners to understand how transmission of this strain continues to persist in both industries.

Listeria monocytogenes and Caramel Apples

Why was this outbreak notable?

In 2023, caramel apples were a suspected source of an outbreak of Listeria monocytogenes. Since the first outbreak of listeriosis linked to caramel apples in 2014, caramel apples have been identified as the suspected source of listeriosis illnesses in two additional outbreaks. After the 2014 outbreak, caramel apples were added to the standard questionnaire used specifically to interview people sick with Listeria. Given that caramel apples are not commonly eaten year-round, ill people typically remember if they consumed them when asked directly during interviews. This allowed investigators to rapidly identify caramel apples as the likely source of two outbreaks in 2017 and 2023. These outbreaks both occurred in the fall and in the same geographic area. Efforts by state and local health departments and FDA identified the same producer in both the 2017 and 2023 outbreaks. Sampling and inspection were conducted at the firm, but the outbreak strain was not identified, and the products ill people consumed were no longer on the market due to their short shelf life. These outbreaks highlight the potential for caramel apples as a source of listeriosis, and the challenges related to investigating potential sources that, like caramel apples, have a relatively short shelf-life. For example, outbreaks may not be identified until after contaminated product is no longer on the market, thus limiting the potential for confirming the food source.

What happened as a result of this outbreak?

As a result of these outbreaks, FDA conducted research on caramel apples as a source of illnesses in foodborne outbreak investigations to better understand how caramel apples become contaminated. This research provides insight to producers on how to make safer products for consumers.

Multistate Outbreaks Linked to Food

Figure 4: Solved Multistate Foodborne Outbreaks, by Year and Pathogen, 2016–2023

Of the 50 total solved multistate outbreaks, 32 were linked to contaminated foods. A suspected source was identified in 56% of foodborne outbreaks and a confirmed source was identified for 44%. These solved multistate foodborne outbreaks were associated with 1,219 illnesses, 421 hospitalizations, and nine deaths. Ill people in these outbreaks ranged in age from less than 1 to 100 years, with a median age of 47 years; 57% were female. Among all illnesses related to solved foodborne outbreaks, 11% occurred in children under 5 and 30% occurred in adults 65 years and older, which are age groups at higher risk for severe illness. When compared with Salmonella, STEC, and Campylobacter infections, a higher proportion of solved multistate foodborne outbreaks of Listeria infections were in adults aged 65 and over (61% compared to 30%, 11%, and 33%, respectively).

Figure 5: Percent of ill people in solved foodborne outbreaks by reported race and ethnicity (n=949)

Of the 50 total solved foodborne outbreaks, race and ethnicity data were reported for 949 (78%) ill people. Of those, 716 (75%) were White race (non-Hispanic), 107 (11%) were Hispanic or Latino ethnicity, 70 (7%) were Black or African American race (non-Hispanic), 32 (3%) were Asian race (non-Hispanic), 10 (1%) were American Indian or Alaska Native race (non-Hispanic), and 2 (<1%) were Native Hawaiian or Pacific Islander race (non-Hispanic). Twelve (1%) people reported multiple races (non-Hispanic).

Figure 6: Solved Multistate Foodborne Outbreaks by Pathogen, 2023

Salmonella caused most of the solved multistate foodborne outbreaks with 16 outbreaks (50%), followed by Listeria with seven outbreaks (22%), STEC with seven outbreaks (22%), and Campylobacter with two outbreaks (6%).

Figure 7: Illnesses in Solved Multistate Foodborne Outbreaks, 2023

Foodborne Salmonella outbreaks caused more illnesses per outbreak (median: 32 illnesses, range: 9–407) compared to outbreaks caused by STEC (median: 12 illnesses, range: 5–37), Campylobacter (median: 12 illnesses, range: 11–13), and Listeria (median: 5 illnesses, range: 2–19).

Figure 8: Hospitalizations in Solved Foodborne Outbreaks, 2023

As in previous years, illnesses associated with solved multistate foodborne Listeria outbreaks were more severe.

Figure 9: Percent of Ill People Hospitalized in Solved Foodborne Outbreaks by Pathogen, 2023

Overall, 98% of people in Listeria outbreaks were hospitalized, and 4% died, compared with 35% of people hospitalized and <1% of people who died in multistate Salmonella outbreaks, 52% of people hospitalized and 0% died in STEC outbreaks, and 38% of people hospitalized and 0% died in Campylobacter outbreaks. Pregnant women are considered a high-risk group for listeriosis. In total, three pregnant women were part of three separate outbreaks of listeriosis. There were no fetal losses reported from these pregnancies.

CDC's Outbreak Response and Prevention Branch (ORPB) assigned food categories for specific foods linked to outbreaks based on the Interagency Food Safety Analytics Collaboration (IFSAC) categorization scheme. Fruits were identified as the source of the most solved foodborne outbreaks (5) during 2023. Fruits identified in outbreaks included melons (2), stone fruit (1), avocado (1), and caramel apples (1).

Figure 10: Solved Multistate Foodborne Outbreaks by Food Category, 2023

Figure 11: Number of Illnesses Associated with Solved Multistate Foodborne Outbreaks, by Food Category, 2023

Fruits accounted for the most illnesses out of any food category, with 439 illnesses, 177 hospitalizations, and seven deaths. One outbreak linked to cantaloupe accounted for 407 illnesses, 158 hospitalizations, and six deaths. The second most common sources of outbreaks were chicken and vegetable row crops (4 each). Chicken-related outbreaks accounted for 177 illnesses, 36 hospitalizations, and no deaths. Vegetable row crops accounted for 57 illnesses, 40 hospitalizations, and one death. Outbreaks that were categorized in the "Other" category included raw dough, fresh pico de gallo, coleslaw, and Caesar salad. The contaminated ingredient in the raw dough, pico de gallo, coleslaw, and Caesar salad was not identified.

Outbreaks Linked to Animal Contact

Of the 84 multistate outbreaks investigated in 2023, 18 were linked to contact with animals or an animal's food, and all 18 outbreaks (100%) were solved. These outbreaks were associated with 1,220 illnesses and 290 hospitalizations. During 2023, Salmonella caused 17 outbreaks and STEC caused one.

Figure 12: Percent of ill people in solved zoonotic outbreaks by reported ethnicity, 2023 (n=834)

Ill people in multistate outbreaks linked to animal contact ranged in age from less than 1 to 94 years, with a median age of 31 years; 23% of illnesses occurred in children younger than 5, and 14% occurred in adults aged 65 and older. The proportion of ill people younger than 5 was higher for solved multistate animal contact outbreaks (23%) than for solved multistate foodborne outbreaks (11%). Over half (56%) of ill people were female. Of the 834 (68%) ill people with available information, 653 (78%) were White race (non-Hispanic), 99 (12%) were Hispanic or Latino ethnicity, 41 (5%) were Black or African American race (non-Hispanic), 11 (1%) were American Indian or Alaska Native race (non-Hispanic), and 10 (1%) were Asian race (non-Hispanic). Twenty (2%) people reported multiple races (non-Hispanic). No ill people were Native Hawaiian or Pacific Islander race (non-Hispanic).

Figure 13: Total Number of Illnesses from Multistate Animal Contact Outbreaks linked to Backyard Poultry, 2016–2023

Among all multistate animal contact outbreaks, 32% of ill people were hospitalized and none died. During 2023, there were 14 solved multistate outbreaks, 1,127 illnesses and 256 hospitalizations linked to contact with backyard poultry.

Figure 14: Number of Outbreaks Associated with Solved Multistate Animal Contact Outbreaks by Animal Contact Category, 2023

Similar to previous years, animal contact outbreaks linked to contact with backyard poultry outnumbered those linked to all other animal sources combined. Backyard poultry, such as chickens, ducks, geese, and turkeys can carry Salmonella even if they appear clean and show no signs of illness.

Figure 15: Number of Illnesses Associated with Solved Multistate Animal Contact Outbreaks by Animal Contact Category, 2023

Small turtles (<4 inches in shell length) caused the second highest number of multistate outbreaks linked to animal contact, despite the federal ban on their sale as pets. The two multistate outbreaks linked to small turtles in 2023 caused 80 illnesses, 32 hospitalizations, and no deaths. Children under the age of 5 years accounted for 36% of the illnesses associated with these small turtle outbreaks. Although any turtle can carry Salmonella germs that can spread and make you sick, turtles with shells less than 4 inches long are a known source of illness. The other two solved multistate outbreaks linked to animal contact were caused by ruminants (e.g., cattle, goats, and sheep) and canines (dog food).

What's being done

When investigators find the food or animal source of a multistate outbreak, agencies can take actions, such as issuing outbreak notices or requesting that companies recall adulterated products. Warning the public quickly about contaminated foods or animal contact outbreaks can prevent illnesses and save lives. Not every outbreak with a confirmed source results in product actions or outbreak notices. Outbreaks may end by the time the source is confirmed, or investigators may determine that the contaminated food is no longer available for sale.

In 2023, CDC issued outbreak notices for 11 investigations. The 11 investigations included 8 linked to contaminated foods (7 with a confirmed source, 1 with a suspected source) and 3 linked to animal contact or pet food. Among the 7 investigations with a confirmed food source, 5 resulted in food recalls, and 1 resulted in the food item not being sold temporarily. Among the 3 animal contact or pet food investigations, 1 resulted in a pet food recall. In total, the outbreak notices received more than 2 million page views, including a large Salmonella outbreak linked to fresh diced onions which received over 1 million views.

CDC uses social media and news media to disseminate outbreak advice to the public. In 2023, outbreak posts on CDC's Instagram, X (previously Twitter), and Facebook received more than 9 million impressions in total. A Facebook post about a Listeria outbreak with a suspected source of leafy greens received over 101,000 total engagements.

In 2023, CDC created 4 videos to go on social media sites. Two of the videos were related to foodborne outbreaks. Both contained prevention tips for Listeria outbreaks, one linked to deli meats and cheeses and the second linked to enoki mushrooms, and received nearly 300,000 views on Instagram. The other two videos, titled "CDC Coops," were created to prevent outbreaks linked to backyard poultry (Episode 1 and Episode 2). The videos received 41,000 views on YouTube.

For more information on how CDC decides to communicate during foodborne outbreaks, please see the Issuing Foodborne Outbreak Notices web page.

Methods

The data sources and methods used for the analyses in this report have been previously described in detail in the previous "Summary of Possible Multistate Enteric (Intestinal) Disease Outbreaks from 2017–2020" and in the MMWR report of 2016 data. Briefly, CDC maintains an internal tracking database in the System for Enteric Disease Response, Investigation, and Coordination (SEDRIC) of investigations coordinated by CDC of possible multistate outbreaks caused by Salmonella, Shiga Toxin–Producing Escherichia coli (referred to as STEC for simplicity throughout this report), Listeria monocytogenes (referred to as Listeria throughout this report), and Campylobacter. Investigation of possible multistate outbreaks of Campylobacter began in 2022. These analyses were based on data available in SEDRIC on July 30, 2024, and may differ from previously published reports. An investigation of a possible multistate outbreak was included in this report if it began on or after January 1, 2023, and ended on or before March 31, 2024; or if it began before 2023 but ended before March 31, 2023. For outbreaks linked to specific foods, outbreak vehicles were provisionally assigned food categories based on the fourth-level IFSAC categorization scheme. These data may not reflect all of ORPB's outbreak investigation activities as some enteric pathogen strains are categorized as reoccurring, emerging, or persisting (REP) strains. REP strains can reoccur and periodically cause acute outbreaks. They can also emerge and increase in frequency or persist and cause illnesses over periods of months or years, despite investigation and prevention efforts. Outbreak illnesses represent a small proportion of all enteric illness. People who are sick with an enteric illness may not seek medical care or be tested for enteric pathogens. Similarly, race and ethnicity data presented in this report represent those who had access to care, sought care, and had testing performed for enteric pathogens. Race and ethnicity data was missing for 25% of solved outbreak illnesses.

The number of multistate outbreaks reported here will differ from the number of outbreaks reported in CDC's Foodborne Disease Outbreak Surveillance System (FDOSS), which includes both single state and multistate outbreaks and a larger group of pathogens. In addition, the investigation year in this report, which is based on when CDC investigated the possible outbreak, may differ from the outbreak year in FDOSS, which is based on when outbreak-associated illnesses occurred. ORPB provisionally assigned IFSAC food categories to outbreaks in this report. Final IFSAC food category assignments for both single state and multistate outbreaks are made during the annual FDOSS outbreak closeout process.

Acknowledgements

This work would not be possible without the collaboration and critical work of many partners, including state, local, tribal, and territorial health and agriculture departments; the PulseNet database team, CDC; the Outbreak Response and Prevention Branch (ORPB), CDC; Food and Drug Administration Coordinated Outbreak Response and Evaluation Network, Washington, DC; and the U.S. Department of Agriculture's Food Safety and Inspection Service Office of Public Health Science, Applied Epidemiology Staff, Washington, DC. We would like to also acknowledge Lexi Palacios, Grace Vahey, Allie Busbee, Samantha Jefferson, Laura Gieraltowski, Kate Varela, Misha Robyn, Katie Werner, and Sarah Collier (ORPB) for their efforts during the writing of this report.