In a new article published in the U.S. Centers for Disease Control and Prevention’s (CDC’s) journal Emerging Infectious Diseases, a retrospective analysis of two genetically unrelated listeriosis outbreaks linked to packaged salads underscore the long-term persistence of Listeria monocytogenes contamination in leafy green production and processing environments.

The article’s lead authors are Alexandra Palacios, M.P.H. and Michael Vasser, M.P.H., infectious disease epidemiologists in the Division of Foodborne, Waterborne, and Environmental Diseases, National Center for Emerging and Zoonotic Infectious Diseases (DFWED-NCEZID) at CDC.

Outbreak Background and Context

Investigated simultaneously in late 2021, the outbreaks—referred to as Outbreak A and Outbreak B—resulted in 30 illnesses, 27 hospitalizations, and four deaths across the U.S. and Canada over an eight-year period.

Historically, U.S. listeriosis outbreaks associated with produce were linked to melons, sprouts, and celery. However, since 2015, packaged salads have emerged as a recurring source of contamination. Outbreak A spanned 2014–2022, involving 20 U.S. cases and two Canadian cases, while Outbreak B occurred during 2016–2021, with 10 U.S. cases.

Both outbreaks were solved thanks to the CDC-coordinated Listeria Initiative, as well as routine surveillance sampling and epidemiologically directed sampling conducted by state and local public health partners, illustrating the critical role of proactive surveillance. The Listeria Initiative is a national surveillance system that collects clinical, demographic, and food exposure data for all listeriosis cases.

Additionally, traceback activities were conducted by the U.S. Food and Drug Administration (FDA) as part of the outbreak investigations. 

Outbreak A: Persistent Contamination Across Facilities

Outbreak A was investigated three times before it was resolved. Initial clusters detected in 2019 and 2020 lacked sufficient evidence to identify a source.

In October 2021, routine sampling by the Georgia Department of Agriculture identified L. monocytogenes in an unnamed brand’s packaged garden salad produced at the firm’s North Carolina facility, prompting a voluntary recall. The outbreak investigation was reopened by CDC when additional related cases were identified through CDC’s PulseNet. Subsequent sampling by Michigan public health authorities detected a highly related strain of L. monocytogenes in iceberg lettuce produced by the same firm’s Arizona facility.

Because of limited exposure information, FDA did not conduct formal traceback analysis. Products associated with any one of the firm’s facilities could not account for the geographic distribution of patients in the outbreak. 

The firm conducted a root-cause analysis to determine how two bags of packaged salads, produced seven weeks apart in separate facilities, could contain the same strain of L. monocytogenes. Analysis was complicated by the fact that iceberg lettuce in the salads was sourced from different regions. 

Eventually, the root-cause analysis revealed the outbreak strain on a harvest rig used across multiple facilities, demonstrating how contamination can persist outside of processing environments.

The firm permanently decommissioned the rig, enhanced sanitation protocols, and implemented whole genome sequencing (WGS) testing as part of its pathogen surveillance protocol.

Outbreak B: Environmental Links to Salinas Valley

Outbreak B involved a cluster of clinical isolates of a rare L. monocytogenes sequence type identified through PulseNet. Investigators reviewed National Center for Biotechnology Information (NCBI) data and found that the clinical isolates were previously detected in Salinas Valley, California watersheds as part of a study by the U.S. Department of Agriculture (USDA).

CDC case-case analysis and patient interviews, as well as routine retail samples collected by Michigan public health authorities, linked illnesses to an unnamed brand’s packaged romaine and butter lettuce salad mix. FDA initiated a traceback investigation, which, due to limited exposure information, only included the positive sample collected by Michigan authorities.

The analysis found that the salad mix was produced with produce from three growers and their four respective fields. Of the four fields identified in the traceback investigation, one was located in the same county where the strain was identified in California watersheds. This finding underscores the value of widespread microbiologic sampling of watersheds nationally.

FDA traceback determined that comingling of the products grown in separate fields occurred at the implicated firm’s Illinois production facility.

Despite extensive environmental swabbing by FDA at the Illinois facility, no L. monocytogenes was detected. Based on the Listeria-positive product sample, however, the firm initiated a voluntary recall and corrective actions.

The contamination source was never definitively identified.

Key Learnings

Based on the retrospective analyses of these two unrelated, simultaneous outbreaks, the authors highlighted that:

• Routine sampling solved both outbreaks. Without state-led sampling programs, the two investigations likely would have remained unresolved.
• Environmental persistence is a major risk factor.L. monocytogenes can survive for years in natural environments, harvest equipment, and processing facilities.
• Industry collaboration is crucial. The internal investigation and data-sharing by the firm involved in Outbreak A accelerated regulatory actions and informed preventive measures.
• WGS is a critical tool. Sequencing data for isolates enables firms to track contamination trends and compare strains against national databases.

Industry Implications

Between 2015 and 2024, eight U.S. listeriosis outbreaks were linked to packaged salads, and FDA classified recalls for approximately 240 packaged salad products due to potential L. monocytogenes contamination. Research suggests that both romaine and iceberg lettuce can potentially internalize L. monocytogenes, and packaged salads may pose higher risk due to multiple contact points during processing.

Therefore, the authors stress that the producers of packaged salads should consider:

• Integrating WGS into microbiological monitoring programs
• Conducting surveillance of incoming raw materials
• Performing root-cause analyses beyond the facility, including harvest sites.