U.S – A years-long investigation into a Listeria monocytogenes outbreak, spanning from 2014 to 2023, has finally reached a resolution, thanks to advanced genomic analysis by the U.S. Centers for Disease Control and Prevention (CDC), the U.S. Food and Drug Administration (FDA), and state health officials.

Employing Whole Genome Sequencing (WGS), authorities identified a common strain of Listeria responsible for illnesses across multiple states, ultimately tracing it back to cheese products.

The investigation commenced in 2017, with subsequent probes in 2021, as CDC initially identified queso fresco and similar cheeses as potential sources of the outbreak.

However, the lack of specific brand information impeded progress. Renewed efforts in January 2024, triggered by fresh reports of illness in December 2023, led to a breakthrough when the outbreak strain was detected in a cheese sample from Rizo-López Foods in Hawaii.

Leveraging CDC’s PulseNet system, which catalogs foodborne pathogen DNA fingerprints via WGS, investigators discerned a genetic linkage between Listeria isolates spanning the outbreak timeline. This genetic similarity strongly indicated a common food source shared by individuals affected during 2014–2023.

Critical discovery in Hawaii                                                

A pivotal moment occurred when the Hawaii State Department of Health, during routine sampling in January 2024, detected the outbreak strain in aged cotija cheese produced by Rizo-López Foods.

Subsequent FDA inspections corroborated the findings, unearthing the outbreak strain on a container within the facility.

Extent of impact and recalls

The outbreak has affected 26 individuals across 11 states, resulting in 23 hospitalizations and two fatalities.

Epidemiological interviews revealed a significant association between illness and consumption of queso fresco, cotija, or similar cheeses, with specific mention of Don Francisco brand products—manufactured by Rizo-López Foods.

In response, Rizo-López Foods initiated a recall of the implicated aged cotija cheese on January 11, 2024. Following FDA’s discovery of the outbreak strain within the facility, a broader recall encompassing all cheese and dairy products produced at the location was issued on February 6, 2024.

In order to improve foodborne illness surveillance and response, the World Health Organization (WHO) recently released a three-part handbook explaining the use of whole genome sequencing (WGS) as a tool.

The first section deals with WGS for outbreak investigations, the second with WGS for surveillance and response to foodborne diseases, and the third with WGS for routine surveillance.

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