TORONTO – Monitoring of related Listeria infections over multiple years has seen more than 80 cases and a dozen deaths, according to the Centers for Disease Control and Prevention.

Michael Vasser, from the CDC, gave details about the reoccurring, emerging and persisting (REP) strain during a recent presentation at the International Association for Food Protection.

An REP strain is a reoccurring, emerging and persisting group of bacteria, closely related by whole genome sequencing, that continues to cause illness over time. The CDC recently reported that almost 3,000 infections from a persistent strain of Salmonella Infantis since 2012 are linked to chicken.

“Since the full transition to whole genome sequencing to detect multi-state outbreaks, at CDC we’ve been able to identify strains that continue to see cases over time. We saw an outbreak of Salmonella Newport return in 2018 after it was originally investigated in 2016 and 2017. The outbreak caused more than 400 illnesses that resulted in a very large ground beef recall,” said Vasser.

“That begs the question, had we understood the strain in 2016-17 could we have prevented such a large outbreak from occurring? We decided to collaborate with other state and federal regulatory partners to create this new concept, which is reoccurring, emerging and persisting strains.”

Vasser said approaches to investigating REPs may be different than traditional outbreak methods and about 20 REP strains are currently being monitored.

“We know that most illnesses reported through PulseNet are not linked to a source. The goal is to use REPs to drive down the incidence of illness and to better understand sporadic illness. REP strains offer a means of tracking and investigating problems that are larger in scope than traditional outbreaks but we know we need to keep pushing on translating tracking into action and prevention.”

Listeria example and potato link
Vasser gave an example of long running related Listeria infections. The PulseNet laboratory network identified a cluster of six clinical isolates in February 2017 from four states. People fell ill between May 2016 and February 2017. The same pattern was also seen in two isolates from 2011. Patients reported consumption of ice cream, so multiple production facilities were inspected and samples collected but the outbreak strain was not found.

By March 2018, there were 23 illnesses from 12 states. Open-ended interviews then suggested various frozen foods such as pizzas, meals and snacks. However, in May 2018, isolates from U.S. potatoes were uploaded to the NCBI database by the National Service of Health for Food Safety and Food Quality (SENASICA) in Mexico. Isolates were collected from fresh, raw potatoes grown in the U.S. and tested at export by Mexico.

The REP code was officially assigned in June 2021 with 66 illnesses from 21 states. Potatoes were not on the original Listeria questionnaire but now a potato specific version is being used and all 13 patients interviewed reported consuming potatoes in some form. For nine people, it was frozen potato products.

Currently, 82 cases have been reported from 23 states, with Texas the most affected. Sick people range from less than 1 to 104 years old. The latest case was in April 2023. A total of 66 have been hospitalized, 18 are pregnancy-associated and 12 have died.

“This REP is challenging given the related raw potato isolates and the enormity of items made with fresh potatoes. It is still unclear if potatoes are actually the source of infections or if contaminated potatoes seeded the strain in facilities or environments where other foods are grown or processed. It could be that both potatoes and others foods are contaminated. Sampling of products will likely be our best bet to determining the source of foods causing illness,” added Vasser. 

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