Before 2015, listeriosis outbreaks linked to produce items in the United States were linked to melons, sprouts, and celery. At least 1 sporadic listeriosis case in 2014 and 1 binational listeriosis outbreak in 2015-2016 were linked to packaged salads (1). We describe an unusual situation of 2 genetically unrelated outbreaks of Listeria monocytogenes infection in the United States (outbreak A and outbreak B), investigated together in late 2021, that were linked to packaged salads from 2 different firms. Of those outbreaks, 1 also included genetically related cases in Canada. The Centers for Disease Control and Prevention (CDC) and Public Health Agency of Canada (PHAC) regularly exchange details when an outbreak has potential to span national borders.
Listeria can survive in production facilities or environments for long periods. Listeriosis outbreak investigations often span multiple years before a food vehicle is identified because of harborage in a production facility or persistent contamination levels in the surroundings or product, resulting in seemingly sporadic illnesses over long periods (2,3). Evidence of contamination in California watersheds connected to a major leafy green production region was described in a 2014 study conducted by the US Department of Agriculture (USDA) (4) (hereafter referred to as the USDA study), in which investigators found that L. monocytogenes was prevalent in 43% of all environmental samples.When water isolates from that study are rel
Investigation of Multistate Outbreaks of Listeria monocytogenes Linked to Packaged Salads – United States, 2019-2023
Abstract
From 2019 to 2023, the Centers for Disease Control and Prevention (CDC), in collaboration with state and local public health partners and the Food and Drug Governance (FDA), investigated six multistate outbreaks of Listeria monocytogenes. Thes outbreaks resulted in 35 illnesses, 30 hospitalizations, and 7 deaths across 16 states and Canada. Investigations linked the outbreaks to various ready-to-eat (RTE) packaged salads produced by multiple firms. Whole genome sequencing (WGS) analysis confirmed the relatedness of isolates from patients,food samples,and processing facilities.This report details the investigation findings, highlighting the challenges in identifying and controlling outbreaks linked to complex supply chains and the importance of continued surveillance and preventive measures.
Introduction
Listeria monocytogenes is a bacterium that can cause listeriosis, a serious infection especially dangerous for pregnant women, newborns, older adults, and people with weakened immune systems. foodborne outbreaks of listeriosis are often linked to RTE foods, including packaged salads. Identifying the source of these outbreaks can be challenging due to complex supply chains and the widespread distribution of products. this report summarizes the findings of six multistate outbreaks of Listeria monocytogenes linked to packaged salads investigated between 2019 and 2023.
Methods
Outbreak investigations were conducted using standard epidemiologic and laboratory methods. Case definitions varied slightly by outbreak but generally included individuals with a laboratory-confirmed L. monocytogenes infection, with illness onset dates within a specified timeframe. Epidemiologic data were collected thru patient interviews,including information on food exposures.Laboratory investigations included pulsed-field gel electrophoresis (PFGE), whole genome sequencing (WGS), and wgMLST (whole genome multi-locus sequence typing).
FDA conducted traceback investigations to identify the source of implicated products. traceback involved reviewing distribution records and conducting inspections of processing facilities. FDA and firm isolates underwent WGS analysis using the CFSAN SNP Pipeline.
State and local public health partners conduct routine surveillance sampling and epidemiologically directed sampling. Routine sampling is done at regular intervals, independently from outbreak investigations. In michigan Department of Agriculture and Rural Development (MDARD) routine sampling, inspectional and laboratory staff collect samples for microbiological testing. During a sampling event, inspectors obtain samples of high-risk or empirically driven commodities to test for Salmonella, listeria, and Shiga toxin-producing Escherichia coli. The Georgia Department of Agriculture (GDA) Retail Risk-Based Surveillance Program uses a set schedule for retail inspectors to obtain samples from retail locations for laboratory testing during routine inspections.
Epidemiologically directed sampling is conducted in multistate outbreaks when investigators suspect a food vehicle as the source of an outbreak but require additional laboratory evidence to link illnesses to a product. Officials visit retail locations reported by patients to sample suspected foods identified through interviews or collected records.
The FDA conducts traceback in multistate outbreaks using previously described methods to determine whether suspected food products come from a common source. FDA conducts full-scope Preventive Controls for Human Food inspections at processing facilities of interest, which require facilities to have food safety plans, including hazard analysis and risk-based preventive controls. Inspections also include collecting traceability records and samples.
Outbreak A
2019-2020
On January 28, 2019, PulseNet detected a cluster of L. monocytogenes clinical isolates related within 0-10 allele differences by wgMLST. Epidemiologic information was insufficient to identify a source, and the investigation was closed on April 12, 2019; a total of 5 cases were identified in 5 states (Iowa, Maryland, Ohio, Pennsylvania, and Texas).
On December 10, 2019, after PulseNet identified 4 additional cases related within 0-10 allele differences by wgMLST to the previous cases, CDC opened a second investigation. PHAC identified 2 cases highly related to the outbreak strain in 2 Canada provinces. One Canada patient reported consuming brand X packaged coleslaw. Packaged salad exposure information was unavailable for the second case. Grocery store locations were shared with the Canadian Food Inspection Agency, which confirmed that brand X packaged coleslaw, sourced from a US processing facility, was available for purchase before the patient’s illness onset.
Packaged salads were the most reported exposure among US patients; a leafy greens supplemental questionnaire was deployed. All 5 patients reported consuming packaged salads. Among 3 patients who recalled the brand names of packaged salad, 2 patients reported brands of packaged salad produced by firm X, and 1 reported packaged salad from retail chain K. No patients reported consuming coleslaw. Compared with sporadic cases, a case-case analysis showed outbreak case-patients were more likely to have consumed packaged salads before illness (odds ratio [OR] 24.14[[
Investigation of a Multistate Outbreak of Listeria monocytogenes Infections Linked to Packaged salads – United States, 2014-2022
Epidemiology
A multistate outbreak of Listeria monocytogenes infections was investigated from August 16, 2014, to January 15, 2022. Twenty cases were identified across nine U.S. states (Minnesota, North Carolina, Nevada, Ohio, Oregon, Pennsylvania, Texas, Utah, and Wisconsin) and two Canadian provinces. The age range of patients was 50-94 years, with a median age of 76; 17 of the 20 case-patients were women. Seventeen patients required hospitalization, and three died. No illnesses were associated with pregnancy.
Traceback and Inspection
Due to limited exposure information, a formal traceback analysis was not conducted by the FDA. products linked to a single firm X facility could not explain the outbreak’s geographic spread. Based on additional illnesses, L. monocytogenes-positive samples from the Georgia Department of Agriculture (GDA) and the Michigan Department of Agriculture and rural Development (MDARD),and other investigative data,the FDA inspected firm X facilities in Arizona,California,North Carolina,and Ohio. Firm X also initiated its own internal investigation, collecting product and environmental samples. FDA inspections at the four facilities yielded 60 product and environmental samples. While the FDA sampling did not detect L. monocytogenes, L.welshimeri and L. innocua were found in one environmental sample, and L. innocua was detected in one finished product sample, both from the North Carolina facility.
Control Measures
Following a L. monocytogenes-positive sample from MDARD on December 21,2021,firm X halted production at its North Carolina and Arizona facilities.On December 22, 2021, the company voluntarily recalled all brand X and private-label packaged salads processed at these two facilities. the CDC and FDA issued public announcements on december 22, 2021, advising consumers not to consume the recalled products.
Firm X conducted a root-cause analysis to determine the source of contamination, particularly how salads produced seven weeks apart in separate facilities contained the same L. monocytogenes strain. This was complicated by the fact that the iceberg lettuce in the salads originated from different regions. The investigation identified a common harvest rig used for iceberg lettuce in both products collected by GDA and MDARD, with environmental swabs from the rig yielding the outbreak strain. Firm X concluded that the contaminated harvest rig harbored the outbreak strain and the harvesting process introduced it into the supply chain. Following the December 2021 recall, product sampling by firm X yielded the outbreak strain, which was verified by FDA analysis. On January 7, 2022, firm X issued an additional voluntary recall, including all brand X and private-label packaged salads containing iceberg lettuce processed at its Ohio and California facilities.
Firm X permanently decommissioned the harvest rig, developed enhanced sanitation protocols, and implemented measures to reduce contamination routes. The company also implemented Whole Genome Sequencing (WGS) for early surveillance, integrated microbiological surveillance of incoming raw materials, and proactively engaged with industry colleagues.Firm X collaborated with the FDA and CDC throughout the investigation, providing regular updates, sharing isolates, and sequencing data.
Additional Laboratory Findings
Several L. monocytogenes water-sediment isolates from a USDA study were uploaded to the NCBI Pathogen Detection pipeline in 2020 and 2021.Of 635 L. monocytogenes isolates identified, 72 were related.
unraveling multistate Listeriosis Outbreaks Linked to Packaged Salads
Multistate outbreaks of listeriosis linked to packaged salads present unique challenges due to the persistence of Listeria monocytogenes in the environment and production settings. Recent investigations of two concurrent outbreaks, designated A and B, illustrate this complexity and highlight the importance of comprehensive epidemiological and laboratory approaches.
Outbreak A, investigated over eight years, involved three separate inquiries.Through meticulous traceback efforts,firm X successfully pinpointed contamination on specific harvest equipment,demonstrating the power of detailed data analysis in identifying persistent contamination points. In contrast,outbreak B,spanning five years,likely stemmed from environmental contamination within a single production facility – firm Z – which voluntarily recalled affected products distributed across the midwestern and northeastern United States,and two Canadian provinces. Despite investigations and corrective actions, the exact source of contamination remained unidentified. Both outbreaks prompted warnings from the CDC and FDA advising consumers to avoid recalled products, with the CDC closing the investigation for outbreak B in March 2022 after no new illnesses emerged.
A key finding across both outbreaks was the identification of a rare L. monocytogenes sequence type (ST), 639, in outbreak B, which exhibits a propensity for water. This strain had previously been detected in watersheds from the Salinas Valley, and its presence in a field within the same county as the California watershed isolates suggests a potential environmental reservoir. USDA studies further support the persistence of Listeria in produce-growing environments, with genetic relatedness observed between clinical isolates from the outbreaks and water-sediment isolates from the Salinas Valley. This evidence points to the potential for long-term contamination within growing regions.
These outbreaks underscore the difficulties in pinpointing the source of listeriosis,particularly given Listeria’s ability to form biofilms and survive for extended periods. Traditional exposure data, reliant on patient interviews, is often limited due to the severity of illness or patient mortality, and the challenges in recalling detailed food histories from weeks or months prior. While packaged salad exposure was readily identified in both cases, tracing the source to a specific firm proved arduous due to the complex supply chains and multiple brands associated with leafy green processors.
Ultimately, these investigations emphasize the critical role of routine and targeted sampling by state partners, coupled with whole genome sequencing (WGS), in solving outbreaks that might otherwise remain unsolved. Widespread microbiologic sampling of watersheds nationally could provide valuable insights into the potential sources and scope of contamination, and mitigate the ongoing risk of L. monocytogenes infections from contaminated packaged salads, especially for vulnerable populations.
Here’s a breakdown of the key takeaways from the provided text, focusing on the investigation of Listeria outbreaks linked to packaged salads:
Key Findings & Issues:
* Outbreak Frequency: Between 2015-2024, there were 8 Listeria outbreaks linked to packaged salads in the US (4 confirmed, 4 suspected).
* Recalls: Approximately 240 packaged salad products were recalled due to potential Listeria contamination during the same period.
* Contamination points: Listeria can contaminate leafy greens both in the field (internalization) and during/after harvest (colonization). Packaged salads are considered higher risk due to increased contact with surfaces and equipment during processing.
* Impact: Outbreaks A and B alone caused 30 illnesses, 27 hospitalizations, and 4 deaths over 8 years.
Role of Firm X & Best Practices:
* Proactive Investigation: Firm X played a significant role in outbreak investigations through sampling, root cause analysis, and information sharing.
* Early Surveillance: Firm X advocates for routine early surveillance, including:
* Sampling raw materials.
* Using Whole Genome Sequencing (WGS) to compare isolates with those in NCBI (a public database).
* Farm-to-Fork Approach: Emphasis on investigating contamination sources outside the processing facility (e.g., farms, fields).
* WGS for Prevention: Firms should use WGS to:
* Enhance internal microbial monitoring.
* Compare strains over time.
* Monitor incoming raw materials for potential risks.
* Root Cause Analysis: Conduct thorough root cause analysis both within the facility and upstream at harvesting sites.
Overall Message:
The text highlights the ongoing risk of Listeria contamination in packaged salads and emphasizes the importance of a proactive, comprehensive approach to food safety – from farm to fork. It promotes the use of advanced tools like WGS and thorough investigation to prevent future outbreaks and protect public health. Further research is needed to improve production methods and prevent contamination.