Two Concurrent Outbreaks of Listeria monocytogenes Infections Linked to Packaged Salads - United States, 2014-2022

Sep 30, 2026

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Two genetically unrelated listeriosis outbreaks (A and B) were investigated simultaneously in late 2021 and linked to packaged salads from two different firms. Together, the outbreaks caused 30 illnesses, 27 hospitalizations, and 4 deaths across 8 years. Findings supported product recalls from both firms and showed that Listeria monocytogenes contamination can persist in production environments for long periods, leading to illnesses over many years. Routine and epidemiologically directed sampling by state partners enabled identification and resolution of events that likely would otherwise remain unsolved.

LanguageEnglish

CategoryHealthcare

FormatPDF

Pages9

Reading Time23 min

AccessFree to download

Synopsis

Outbreak overview and impact

Background

Persistence of L. monocytogenes and investigation challenges

Prior salad-linked listeriosis context

Methods

Case definition

Epidemiologic data

Genomic relatedness approach

SYNOPSIS Two Concurrent Outbreaks of Listeria monocytogenes Infections Linked to Packaged Salads, United States, 2014–2022 Alexandra Palacios,1 Michael Vasser,1 Asma Madad, Stranjae’ Ivory, Lauren Edwards, Danielle Donovan, Tiffany Greenlee, Tim Muruvanda, Joseph Baugher, Kurt Nolte, Annemarie Buchholz, Karen Blickenstaff, Mabel Low, Grace Pederson, Jasmine Huffman, Carrie Crabtree, Colby Brown, Meghan Hamel, Rima Kandar, Erin Szidonya, Hannah Caird, Katie Cibulskas, Brandi Taylor, Jodi Taylor, Yamir Rosa, Laura Gieraltowski, Amanda Conrad 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. Bedrcee 2015, listeriosis outbreaks linked to proitems 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 simultaneously 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 information 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 environment or product, resulting in seemingly sporadic illnesses over long periods (2,3) . Evidence of contamination in California watersheds connected toa major leafy green production region was described in a 2014 study conducted by the US Department of Author affiliations: Centers for Disease Control and Prevention, Atlanta, Georgia, USA (A. Palacios, M. Vasser, M. Low, G. Pederson, J. Huffman, L. Gieraltowski, A. Conrad); US Food and Drug Administration, College Park, Maryland, USA (A. Madad, S. Ivory, T. Greenlee, T. Muruvanda, J. Baugher, K. Nolte, A. Buchholz, K. Blickenstaff); Michigan Department of Agriculture and Rural Development, Lansing, Michigan, USA (L. Edwards); Michigan Department of Health and Human Services, Lansing (D. Donovan): Georgia Department of Agriculture, Atlanta (C. Crabtree, C. Brown); Public Health Agency of Canada, Ottawa, Ontario, Canada (M. Hamel, R. Kandar); National Microbiology Laboratory, Winnipeg, Manitoba, Canada (E. Szidonya); British Columbia Centre for Disease Control, Vancouver, British Columbia, Canada (H. Caird); Ohio Department of Health, Columbus, Ohio, USA (K. Cibulskas, B. Taylor); Ohio Department of Agriculture, Reynoldsburg, Ohio, USA (J. Taylor, Y. Rosa) DOI: [https://doi.org/10.3201/eid3112.250989](https://doi.org/10.3201/eid3112.250989) 1These first authors contributed equally to this articl

FAQs

What linked outbreaks A and B to packaged salads?

Both genetically unrelated outbreaks were linked to packaged salads produced by two different firms, investigated simultaneously in late 2021.

How many illnesses, hospitalizations, and deaths occurred across both outbreaks?

Across 8 years, the combined outbreaks caused 30 illnesses, 27 hospitalizations, and 4 deaths.

Why can listeriosis outbreaks take many years to solve?

The text explains that epidemiologic information is often limited due to low case numbers, long incubation periods, and illness severity, and investigations may rely on product or environmental sampling.

Sep 30, 2026

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