OBJECTIVES:Little is known about foods associated with sporadic listeriosis, a foodborne bacterial infection. We compared self-reported foods consumed by people with sporadic versus outbreak-associated listeriosis, overall and for certain populations. METHODS:We analyzed data on cases with ≥1 reported food exposure reported during 2016-2022 in the United States to the Listeria Initiative. We used counterfactual random forests to identify foods associated with sporadic and outbreak-associated listeriosis in certain populations (eg, pregnant women). RESULTS:Of 3413 listeriosis cases reported during 2016-2022, 83.5% (n = 2849) were sporadic. Many foods showed no differential association between sporadic and outbreak-associated illness, indicating similar associations or lack thereof with both illness types. While a subset of foods were associated with sporadic versus outbreak-associated listeriosis, these associations varied by population. For example, strawberries (odds ratio [OR] = 2.48; 95% CI, 1.11-5.52) were the only foods associated with sporadic illness among nonpregnant females aged 15 to 49 years. Among adults aged ≥65 years, 10 foods, including deli-counter sliced cheese (OR = 1.78; 95% CI, 1.28-2.47), were associated with sporadic listeriosis. CONCLUSIONS:Foods associated with sporadic listeriosis differed from foods associated with outbreak-associated illness, with differences observed across population groups and by pregnancy status. These findings highlight variation in exposure patterns and underscore the need for further attribution studies focused on sporadic illness across demographic groups, to inform population-specific prevention messaging.

We describe 2 genetically unrelated outbreaks of Listeria monocytogenes infections (outbreak A and outbreak B) linked to packaged salads from 2 different firms that were investigated simultaneously in 2021. Combined, the outbreaks caused 30 illnesses, 27 hospitalizations, and 4 deaths over 8 years. Those investigations led to recalls of product from 2 different firms and highlight how L. monocytogenes contamination can persist for long periods and cause illnesses over many years. Outbreak A was investigated 3 times with illnesses occurring over 8 years, whereas illnesses in outbreak B occurred over 5 years. Both outbreaks illustrate the importance of routine and epidemiologically directed sampling by state partners, without which these outbreaks likely would have gone unsolved. The outbreaks were the second and third multistate outbreaks of listeriosis linked to packaged salads, providing further documentation of the potential for L. monocytogenes infections from consumption of contaminated packaged salads.

Foods implicated in listeriosis outbreaks continue to change over time. Historically, listeriosis outbreaks have been primarily linked to consumption of deli meat and dairy products. More recently, they have been linked to vegetable row crops, fruits, and other produce, including imported specialty mushrooms. Specialty mushrooms, including enoki mushrooms, are popular in Japanese, Chinese, and Korean cuisines, and are commonly used in soups, stir-fries, hotpots, and salads. These mushrooms are imported into the U.S. from a variety of East Asian countries and Canada. Recently, food safety authorities around the world have linked listeriosis outbreaks to contaminated enoki mushrooms, leading to a series of highly impactful recalls. This review examines outbreaks and recalls associated with enoki mushrooms, related risks and food safety concerns, ongoing research, regulatory activities focused on this commodity, consumer preparation and handling practices, and prevention strategies. The exchange of epidemiologic and traceback evidence, genomic data, and international data sharing helped investigators find the source of multiple listeriosis outbreaks linked to enoki mushrooms. Since enoki mushrooms were first linked to listeriosis illnesses in 2020, state, federal, and international partners developed a strategy for outbreak prevention, including enhanced surveillance and an improved investigational approach for this commodity.

In 2022, the U.S. Food and Drug Administration (FDA), the Centers for Disease Control and Prevention (CDC), U.S. state and local partners, the Public Health Agency of Canada (PHAC), and the Canadian Food Inspection Agency (CFIA) conducted a binational sample-initiated retrospective outbreak investigation (SIROI) of Listeria monocytogenes illnesses linked to enoki mushrooms. The FDA and CDC investigated the first known L. monocytogenes outbreak linked to enoki mushrooms from 2016 to 2020, making the 2022 outbreak the second time this pathogen-commodity pair was investigated by FDA and CDC. The 2022 outbreak included six ill people, all of whom were hospitalized. Epidemiologic, laboratory, and traceback evidence led to multiple public health actions, including voluntary recalls by firms, public communications about the outbreak, and FDA’s country-wide Import Alert for enoki mushrooms from China. This SIROI illustrates the importance of surveillance sampling, national and international coordination of efforts, and rapid information sharing to identify and stop foodborne outbreaks on a global scale. To reduce the risk of listeriosis illnesses linked to contaminated enoki mushrooms, public health and regulatory agencies in the United States and Canada remain committed to conducting comprehensive surveillance for Listeria in foods and in people, efficiently investigating identified outbreaks, and implementing control measures to potentially minimize the impact of future outbreaks.

Listeria monocytogenes causes listeriosis, a serious infection with a high mortality rate for persons at higher risk for listeriosis. The first Listeria outbreak linked to frozen vegetables occurred in 2016 and resulted in three deaths. Many frozen vegetables are intended to be consumed after cooking. However, data on consumer behavior are sparse. We characterized consumers' perceptions of contamination of prepackaged frozen vegetables, and preparation methods of prepackaged frozen vegetables to help inform prevention strategies. During September 1-24, 2022, Porter Novelli Public Services conducted the FallStyles survey using the Ipsos KnowledgePanel. Data were weighted to be representative of the U.S. population. Point estimates and 95% CIs were calculated, and differences between respondents were determined using Wald chi square tests. Among 3,008 respondents reporting a preparation and consumption method for frozen vegetables, 8.7% (95% CI = 7.4-10.0%) reported ever consuming the product raw. Respondents who reported having children < 18 years old were more likely to report ever consuming frozen vegetables raw compared with respondents who did not (12.5% vs. 7.4%, p < 0.01). The most reported raw preparation method was adding them directly to a blender for smoothie or juice (5.6%; 95% CI = 4.6-6.7%). Among respondents who reported eating frozen vegetables, 59.6% (95% CI = 57.6-61.6%) reported following package instructions. A third (34.1% [95% CI = 32.2-35.9%]) of respondents agreed that frozen vegetables can be contaminated with germs (like Salmonella, E. coli, and Listeria), with a greater proportion of people with cancer disagreeing compared to those without cancer (32.5% vs 23.4%, p = 0.041). These findings show that some consumers may not be cooking frozen vegetables before eating them. Second, consumers might not be reading instructions on packaging. Both findings highlight the critical importance of preventive controls in the production of frozen vegetables prior to reaching the consumer.

In 2016, the U.S. Food and Drug Administration (FDA), the Centers for Disease Control and Prevention (CDC), and state partners investigated nine Listeria monocytogenes infections linked to frozen vegetables. The investigation began with two environmental L. monocytogenes isolates recovered from Manufacturer A, primarily a processor of frozen onions, that were a match by whole genome sequencing (WGS) to eight clinical isolates and historical onion isolates with limited collection details. Epidemiologic information, product distribution, and laboratory evidence linked suspect food items, including products sourced from Manufacturer B, also a manufacturer of frozen vegetable/fruit products, with an additional illness. The environmental isolates were obtained during investigations at Manufacturers A and B. State and federal partners interviewed ill people, analyzed shopper card data, and collected household and retail samples. Nine ill persons between 2013 and 2016 were reported in four states. Of four ill people with information available, frozen vegetable consumption was reported by three, with shopper cards confirming purchases of Manufacturer B brands. Two identified outbreak strains of L. monocytogenes (Outbreak Strain 1 and Outbreak Strain 2) were a match to environmental isolates from Manufacturer A and/or isolates from frozen vegetables recovered from open and unopened product samples sourced from Manufacturer B; the investigation resulted in extensive voluntary recalls. The close genetic relationship between isolates helped investigators determine the source of the outbreak and take steps to protect public health. This is the first known multistate outbreak of listeriosis in the United States linked to frozen vegetables and highlights the significance of sampling and WGS analyses when there is limited epidemiologic information. Additionally, this investigation emphasizes the need for further research regarding food safety risks associated with frozen foods.

Keeping the global food supply safe necessitates international collaborations between countries. Health and regulatory agencies routinely communicate during foodborne illness outbreaks, allowing partners to share investigational evidence. A 2016-2020 outbreak of Listeria monocytogenes infections linked to imported enoki mushrooms required a multinational collaborative investigation among the United States, Canada, Australia, and France. Ultimately, this outbreak included 48 ill people, 36 in the United States and 12 in Canada, and was linked to enoki mushrooms sourced from one manufacturer located in the Republic of Korea. Epidemiologic, laboratory, and traceback evidence led to multiple regulatory actions, including extensive voluntary recalls by three firms in the United States and one firm in Canada. In the United States and Canada, the Korean manufacturer was placed on import alert while other international partners provided information about their respective investigations and advised the public not to eat the recalled enoki mushrooms. The breadth of the geographic distribution of this outbreak emphasizes the global reach of the food industry. This investigation provides a powerful example of the impact of national and international coordination of efforts to respond to foodborne illness outbreaks and protect consumers. It also demonstrates the importance of fast international data sharing and collaboration in identifying and stopping foodborne outbreaks in the global community. Additionally, it is a meaningful example of the importance of food sampling, testing, and integration of sequencing results into surveillance databases.

BACKGROUND:Frozen foods have rarely been linked to Listeria monocytogenes illness. We describe an outbreak investigation prompted by both hospital clustering of illnesses and product testing.METHODS:We identified outbreak-associated listeriosis cases using whole-genome sequencing (WGS), product testing results, and epidemiologic linkage to cases in the same Kansas hospital. We reviewed hospital medical and dietary records, product invoices, and molecular subtyping results. Federal and state officials tested product and environmental samples for L. monocytogenes.RESULTS:Kansas officials were investigating 5 cases of listeriosis at a single hospital when, simultaneously, unrelated sampling for a study in South Carolina identified L. monocytogenes in Company A ice cream products made in Texas. Isolates from 4 patients and Company A products were closely related by WGS, and the 4 patients with known exposures had consumed milkshakes made with Company A ice cream while hospitalized. Further testing identified L. monocytogenes in ice cream produced in a second Company A production facility in Oklahoma; these isolates were closely related by WGS to those from 5 patients in 3 other states. These 10 illnesses, involving 3 deaths, occurred from 2010 through 2015. Company A ultimately recalled all products.CONCLUSIONS:In this US outbreak of listeriosis linked to a widely distributed brand of ice cream, WGS and product sampling helped link cases spanning 5 years to 2 production facilities, indicating longstanding contamination. Comprehensive sanitation controls and environmental and product testing for L. monocytogenes with regulatory oversight should be implemented for ice cream production.

Foodborne outbreak investigations have traditionally included the detection of a cluster of illnesses first, followed by an epidemiologic investigation to identify a food of interest. The increasing use of whole genome sequencing (WGS) subtyping technology for clinical, environmental, and food isolates of foodborne pathogens, and the ability to share and compare the data on public platforms, present new opportunities to identify earlier links between illnesses and their potential sources. We describe a process called sample-initiated retrospective outbreak investigations (SIROIs) used by federal public health and regulatory partners in the United States. SIROIs begin with an evaluation of the genomic similarity between bacterial isolates recovered from food or environmental samples and clusters of clinical isolates while subsequent and parallel epidemiologic and traceback investigations are initiated to corroborate their connection. SIROIs allow for earlier hypothesis generation, followed by targeted collection of information about food exposures and the foods and manufacturer of interest, to confirm a link between the illnesses and their source. This often leads to earlier action that could reduce the breadth and burden of foodborne illness outbreaks. We describe two case studies of recent SIROIs and present the benefits and challenges. Benefits include insight into foodborne illness attribution, international collaboration, and opportunities for enhanced food safety efforts in the food industry. Challenges include resource intensiveness, variability of epidemiologic and traceback data, and an increasingly complex food supply chain. SIROIs are valuable in identifying connections among small numbers of illnesses that may span significant time periods; detecting early signals for larger outbreaks or food safety issues associated with manufacturers; improving our understanding of the scope of contamination of foods; and identifying novel pathogen/commodity pairs.

Listeria monocytogenes can cause severe foodborne illness, including miscarriage during pregnancy or death in newborn infants. When outbreaks of L. monocytogenes illness occur, it may be possible to determine the food source of the outbreak. However, most reported L. monocytogenes illnesses do not occur as part of a recognized outbreak and most of the time the food source of sporadic L. monocytogenes illness in people cannot be determined. In the United States, L. monocytogenes isolates from patients, foods, and environments are routinely sequenced and analyzed by whole genome multilocus sequence typing (wgMLST) for outbreak detection by PulseNet, the national molecular surveillance system for foodborne illnesses. We investigated whether machine learning approaches applied to wgMLST allele call data could assist in attribution analysis of food source of L. monocytogenes isolates. We compiled isolates with a known source from five food categories (dairy, fruit, meat, seafood, and vegetable) using the metadata of L. monocytogenes isolates in PulseNet, deduplicated closely genetically related isolates, and developed random forest models to predict the food sources of isolates. Prediction accuracy of the final model varied across the food categories; it was highest for meat (65%), followed by fruit (45%), vegetable (45%), dairy (44%), and seafood (37%); overall accuracy was 49%, compared with the naive prediction accuracy of 28%. Our results show that random forest can be used to capture genetically complex features of high-resolution wgMLST for attribution of isolates to their sources.

Listeriosis is a serious infection usually caused by eating food contaminated with the bacterium Listeria monocytogenes. An estimated 1,600 persons become ill with listeriosis each year, among whom approximately 260 die. Persons at higher risk for listeriosis include pregnant persons and their newborns, adults aged ≥65 years, and persons with weakened immune systems. Persons with invasive listeriosis usually report symptoms starting 1-4 weeks after eating food contaminated with L. monocytogenes; however, some persons who become infected have reported symptoms starting as late as 70 days after exposure or as early as the same day of exposure (1). On January 29, 2021, PulseNet, the national molecular subtyping surveillance network coordinated by CDC, identified a multistate cluster of three L. monocytogenes infections: two from Maryland and one from Connecticut (2). CDC, the Food and Drug Administration (FDA), and state and local partners began an investigation on February 1, 2021. A total of 13 outbreak-related cases were eventually identified from four states. All patients reported Hispanic ethnicity; 12 patients were hospitalized, and one died. Rapid food testing and record collection by regulatory agencies enabled investigators to identify a brand of queso fresco made with pasteurized milk as the likely source of the outbreak, leading to an initial product recall on February 19, 2021. Fresh, soft Hispanic-style cheeses made with pasteurized milk are a well-documented source of listeriosis outbreaks. These cheeses can be contaminated with L. monocytogenes unless stringent hygienic controls are implemented, and the processing environment is monitored for contamination (3). U.S. public health agencies should establish or improve communications, including new methods of disseminating information that also effectively reach Hispanic populations, to emphasize the risk from eating fresh, soft Hispanic-style cheeses, even those made with pasteurized milk.

Outbreaks of Listeria monocytogenes (L. monocytogenes) infections have historically been associated with contaminated deli meats, but recent outbreaks have been linked to produce. To date, avocados have not been identified as the source of any outbreaks of L. monocytogenes infections in the United States, but avocado samples have yielded strains that were closely related genetically to clinical L. monocytogenes isolates. To determine whether avocados have been a source of listeriosis, we conducted a retrospective review of epidemiological data for clinical isolates that were genetically related to isolates from avocados. Using a national database, we identified clusters containing clinical and at least one avocado isolate. We then selected clusters based upon isolation dates, cluster and composition size, and available food history data. For each cluster, we assessed whether (1) avocado consumption was higher among case-patients in the cluster than among those with sporadic illnesses, and (2) whether the only food isolates within the cluster were from avocados. If both conditions were met, the link was considered "likely," if one condition was met the link was considered "possible," and if neither condition was met evidence was "limited." Five of fifteen clusters met criteria for assessment. Of these, two were classified as having "limited" evidence for a link to avocados, two as "possible," and one as "likely." For the cluster considered "likely", avocado consumption was significantly higher among case-patients in the cluster compared to sporadic illnesses (Odds ratio: 8.5, 95% CI 1.5-86.5). We identified three clusters that were likely or possibly linked to avocados, suggesting avocados could be a source of listeriosis in the United States. Messaging on safe handling might be warranted for groups at higher risk, but further research is first needed to better characterize the ecology of pathogens on avocados and likelihood of internalization of L. monocytogenes.

Our globalised food supply presents immense challenges to ensuring food safety, as shown by outbreaks of foodborne illnesses associated with imported foods.1Gould LH Kline J Monahan C Vierk K Outbreaks of disease associated with food imported into the United States, 1996–2014.Emerg Infect Dis. 2017; 23: 525-528Crossref PubMed Scopus (33) Google Scholar The speed with which such outbreaks are resolved often depends on how rapidly public health scientists communicate and disseminate actionable data. One such data source is whole-genome sequencing, which is the newest method of molecular subtyping and has superior discriminatory power compared with previous methods.2Allard MW Strain E Melka D et al.Practical value of food pathogen traceability through building a whole-genome sequencing network and database.J Clin Microbiol. 2016; 54: 1975-1983Crossref PubMed Scopus (179) Google Scholar Consequently, whole-genome sequencing has been and continues to be adopted by countries across the world as a tool to combat foodborne pathogens.3Thomas J Govender N McCarthy KM et al.Outbreak of listeriosis in South Africa associated with processed meat.N Engl J Med. 2020; 382: 632-643Crossref PubMed Scopus (53) Google Scholar Sequence data can be made publicly available through numerous databases (eg, the European Nucleotide Archive, the National Center for Biotechnology Information [NCBI] Sequence Read Archive, and the DNA Data Bank of Japan Sequence Read Archive). Laboratories are encouraged to share the genomes they have sequenced4Gardy JL Loman NJ Towards a genomics-informed, real-time, global pathogen surveillance system.Nat Rev Genet. 2018; 19: 9-20Crossref PubMed Scopus (214) Google Scholar and, as new genomes are made public, isolates can be clustered into genetically similar groups to facilitate the detection of potential outbreaks and sources of contamination. Here we discuss a recent case of how international collaboration and sharing of whole-genome sequencing data facilitated the identification of a novel food associated with outbreaks caused by Listeria monocytogenes in Australia, Canada, and the USA. We hope that the events we describe will encourage all public health, food safety, and other authorities to contribute their whole-genome sequencing data to public databases in as close to real-time as possible. By the end of 2019, cluster PDS000011550 in the NCBI L monocytogenes Pathogen Detection database contained 36 clinical isolates with a distinct genetic pattern. The associated illnesses, which included four deaths and six pregnancy-associated infections, began in 2016 and a suspected food vehicle could not be identified from available food exposure histories. In February, 2020, the Canadian Food Inspection Agency (CFIA) uploaded whole-genome sequencing data for a 2016 isolate from a food sample from its pathogen archives to the NCBI Pathogen Detection database. This Canadian domestic mushroom matched the outbreak cluster. Subsequently, another Canadian isolate from an imported enoki mushroom sample was uploaded to the NCBI Pathogen Detection database that matched the outbreak cluster. These isolates provided the first evidence of what contaminated food might be causing human illnesses and deaths. In late February to early March, 2020, US state and federal partners collected enoki mushroom product samples, which yielded 12 isolates whose sequences were uploaded into the NCBI Pathogen Detection database and were found to match the outbreak cluster.5US Food and Drug AdministrationOutbreak investigation of Listeria monocytogenes: enoki mushrooms (March 2020). US Food and Drug Administration, College Park, MDJune 9, 2020https://www.fda.gov/food/outbreaks-foodborne-illness/outbreak-investigation-listeria-monocytogenes-enoki-mushrooms-march-2020Date accessed: September 10, 2020Google Scholar In the middle of March, 2020, US public health officials learned via the EU Epidemic Intelligence Information System that French authorities had collected five L monocytogenes isolates from enoki mushrooms originating from South Korea in 2017. Genomic analyses showed these five food isolates were highly related to the outbreak, but those sequence data are not publicly available. In late March, 2020, further testing by the CFIA identified five samples of enoki mushroom product from South Korea that were positive for L monocytogenes; whole-genome sequencing data was a match to the outbreak cluster. Additionally, through the Food and Agriculture Organization and WHO International Food Safety Authorities Network, US federal officials were notified of six clinical isolates collected in Australia between October, 2017, and March, 2020. These isolates were uploaded to NCBI Pathogen Detection database and found to match the outbreak cluster. In April, 2020, as part of the US domestic outbreak investigation, additional imported enoki mushrooms from South Korea were sampled and L monocytogenes isolates were found that matched the outbreak. This outbreak investigation shows how international sharing of whole-genome sequencing data can decrease the time required to resolve foodborne outbreaks and reduce the burden to public health (appendix pp 1–2). None of the insights described here would have been possible without effective surveillance and international cooperation based on shared genomic data. A common cause of illness over several years and continents would not have been recognised, contaminated products would have remained in the food supply chain, and more people would have been at risk from contaminated food. Delays in aggregating and disseminating data internationally can result in the continued exposure of consumers to potentially contaminated products, whereas routine real-time data sharing can halt this exposure and save lives. We hope this case study provides incentive to all nations doing whole-genome sequencing of foodborne pathogens to upload and share their whole-genome sequencing data in real-time. By doing so, we can reduce the time a foodborne pathogen is present within the global food chain. This online publication has been corrected. The corrected version first appeared at thelancet.com/microbe on Oct 16, 2020 This online publication has been corrected. The corrected version first appeared at thelancet.com/microbe on Oct 16, 2020 We declare no competing interests. The findings and conclusions in this Comment are those of the authors and do not necessarily represent the official position of the US Centers for Disease Control and Prevention. Download .pdf (.25 MB) Help with pdf files Supplementary appendix Correction to Lancet Microbe 2020; 1: e233–34Pettengill JB, Markell A, Conrad A, et al. A multinational listeriosis outbreak and the importance of sharing genomic data. Lancet Microbe 2020; 1: e233–34—In this Comment, in paragraphs six, seven, and eight, mentions of "North Korea" should have been "South Korea". These corrections have been made as of Oct 16, 2020. Full-Text PDF Open Access

Unpasteurized milk can contain harmful bacteria such as Listeria monocytogenes. In 2016, the US Food and Drug Administration notified the Centers for Disease Control and Prevention (Atlanta, GA) that L. monocytogenes isolated from unpasteurized chocolate milk from a Pennsylvania dairy was closely related, by whole-genome sequencing, to L. monocytogenes isolates collected from blood specimens of 2 patients (in California and Florida) in 2014. The California and Florida patients consumed unpasteurized milk from the Pennsylvania dairy. Both were >65 yr old and were hospitalized in 2014; the Florida patient died. Isolates from the 2 patients had indistinguishable pulsed-field gel electrophoresis patterns and were closely related by whole-genome multilocus sequence typing analysis (by 2 alleles) to the isolate from unpasteurized chocolate milk produced by the Pennsylvania dairy in 2015. Together, epidemiologic and laboratory information indicated a common origin. This is the first multistate listeriosis outbreak linked to unpasteurized milk in the United States detected using whole-genome multilocus sequence analysis.

We investigated an outbreak of listeriosis detected by whole-genome multilocus sequence typing and associated with packaged leafy green salads. Nineteen cases were identified in the United States during July 5, 2015-January 31, 2016; isolates from case-patients were closely related (median difference 3 alleles, range 0-16 alleles). Of 16 case-patients interviewed, all reported salad consumption. Of 9 case-patients who recalled brand information, all reported brands processed at a common US facility. The Public Health Agency of Canada simultaneously investigated 14 cases of listeriosis associated with this outbreak. Isolates from the processing facility, packaged leafy green salads, and 9 case-patients from Canada were closely related to US clinical isolates (median difference 3 alleles, range 0-16 alleles). This investigation led to a recall of packaged leafy green salads made at the processing facility. Additional research is needed to identify best practices and effective policies to reduce the likelihood of Listeria monocytogenes contamination of fresh produce.

On December 1, 2017, PulseNet, CDC’s molecular subtyping network for foodborne disease surveillance, identified a cluster of three Listeria monocytogenes clinical isolates. Review of reported exposures indicated that all three patients had consumed prepackaged caramel apples purchased from retail establishments in the month preceding illness onset.

Whole apples have not been previously implicated in outbreaks of foodborne bacterial illness. We investigated a nationwide listeriosis outbreak associated with caramel apples. We defined an outbreak-associated case as an infection with one or both of two outbreak strains of Listeria monocytogenes highly related by whole-genome multilocus sequence typing (wgMLST) from 1 October 2014 to 1 February 2015. Single-interviewer open-ended interviews identified the source. Outbreak-associated cases were compared with non-outbreak-associated cases and traceback and environmental investigations were performed. We identified 35 outbreak-associated cases in 12 states; 34 (97%) were hospitalized and seven (20%) died. Outbreak-associated ill persons were more likely to have eaten commercially produced, prepackaged caramel apples (odds ratio 326·7, 95% confidence interval 32·2-3314). Environmental samples from the grower's packing facility and distribution-chain whole apples yielded isolates highly related to outbreak isolates by wgMLST. This outbreak highlights the importance of minimizing produce contamination with L. monocytogenes. Investigators should perform single-interviewer open-ended interviews when a food is not readily identified.

From 2004–2015, CDC received 82 reports of patients with Listeria monocytogenes (Lm) cultured from non-sterile sites. Whereas invasive listeriosis (Lm isolated from typically sterile sites) is nationally notifiable and well described, information about patients with “noninvasive” Lm (isolated from other sites) is limited to a small number of case reports. We summarize the demographic and clinical characteristics of a convenience sample of patients with noninvasive Lm and compare to patients with invasive Lm reported through routine surveillance. The Listeria Initiative (LI) is a national surveillance system that collects demographic, clinical, laboratory, and food exposure information on laboratory-confirmed listeriosis cases in the United States. While not nationally notifiable, public health agencies can also submit similar information to LI for patients with noninvasive Lm. We reviewed all reports to LI from 2004–2015 and used a standardized approach to characterize isolate sources as invasive or noninvasive. We then compared demographic and clinical factors between patients with invasive and noninvasive Lm using Fisher’s exact and Wilcoxon rank-sum tests. The most common sources of noninvasive isolates (n = 82) were skin or soft tissue (n = 35, 43%), urine (n = 19, 23%), and stool (n = 17, 21%). Compared with patients with noninvasive Lm, patients with invasive Lm (n = 4599) were more likely to be older than 65 years (66% vs. 54% P = 0.03), to be associated with an outbreak (17% vs. 7%, P = 0.02), to be hospitalized (92% vs. 54%, P < 0.01), and to die (20% vs. 4%, P < 0.01). This is the first review of patients with noninvasive Lm using US LI surveillance data. Patients with invasive Lm were older and more likely to be associated with an outbreak, hospitalization, or death when compared with patients with noninvasive Lm. The reasons that patients with noninvasive Lm have a less severe clinical course are not well understood; however, the frequency of hospitalizations and deaths in patients with noninvasive isolates indicates their clinical relevance and public health significance. All authors: No reported disclosures.

In September 2015, PulseNet, the national molecular subtyping network for foodborne disease surveillance, identified a cluster of Listeria monocytogenes (Listeria) clinical isolates indistinguishable by two-enzyme pulsed-field gel electrophoresis (PFGE) pattern combination and highly related by whole-genome multilocus sequence typing (wgMLST). A case was defined as isolation of Listeria with the outbreak PFGE pattern and highly related by wgMLST with an isolation date on or after July 5, 2015, the isolate date of the earliest case in this cluster.

Listeria monocytogenes (Lm) causes severe foodborne illness (listeriosis). Previous molecular subtyping methods, such as pulsed-field gel electrophoresis (PFGE), were critical in detecting outbreaks that led to food safety improvements and declining incidence, but PFGE provides limited genetic resolution. A multiagency collaboration began performing real-time, whole-genome sequencing (WGS) on all US Lm isolates from patients, food, and the environment in September 2013, posting sequencing data into a public repository. Compared with the year before the project began, WGS, combined with epidemiologic and product trace-back data, detected more listeriosis clusters and solved more outbreaks (2 outbreaks in pre-WGS year, 5 in WGS year 1, and 9 in year 2). Whole-genome multilocus sequence typing and single nucleotide polymorphism analyses provided equivalent phylogenetic relationships relevant to investigations; results were most useful when interpreted in context of epidemiological data. WGS has transformed listeriosis outbreak surveillance and is being implemented for other foodborne pathogens.