Why are Listeria Outbreaks so hard to solve?

Fact: “The majority of outbreaks remain unsolved, meaning that the food and/or location source of the outbreak is never identified.” (Locating the source of large-scale outbreaks of foodborne disease https://doi.org/10.1098/rsif.2018.0624).

The incubation period is long. The time from infection to the onset of symptoms—typically referred to as the incubation period can vary to a significant degree. (Goulet V, King LA, Vaillant V, de Valk H. What is the incubation period for listeriosis? BMC Infect Dis. 2013;13:11. Published 2013 Jan 10. doi:10.1186/1471-2334-13-11).

According to the CDC, symptoms of Listeria infection can develop at any time from the same day of exposure to 70 days after eating contaminated food. According to the FDA, gastroenteritis (or non-invasive illness) has an onset time of a few hours to 3 days, while invasive illness can have an onset varying from 3 days to 3 months. According to one authoritative text:

The incubation period for invasive illness is not well established, but evidence from a few cases related to specific ingestions points to 11 to 70 days, with a mean of 31 days. In one report, two pregnant women whose only common exposure was attendance at a party developed Listeria bacteremia with the same uncommon enzyme type; incubation periods for illness were 19 and 23 days.

I challenge you, what did you eat 3 days ago? What did you eat 70 days ago?

Here are some slides I use (I can’t recall who I borrowed them from) to explain the difficulty of figuring out what poisoned people, and even if discovered, why it takes so long.

So, in this recent Listeria Outbreak that according to the CDC, as of July 19, 2024, a total of 28 people infected with the outbreak strain of Listeria have been reported from 12 states – Georgia (2), Illinois (1), Massachusetts (2), Maryland (6), Minnesota (1), Missouri (2), North Carolina (1), New Jersey (2), New York (7), Pennsylvania (1), Virginia (2) and Wisconsin (1). Sick people’s samples were collected from May 29, 2024, to July 5, 2024. Of 28 people with information available, all have been hospitalized. One person got sick during their pregnancy and remained pregnant after recovering. Two deaths have been reported, 1 in Illinois and 1 in New Jersey.

Public health investigators used the PulseNet system to identify illnesses that may have been part of this outbreak. CDC PulseNet manages a national database of DNA fingerprints of bacteria that cause foodborne illnesses. DNA fingerprinting is performed on bacteria using a method called whole genome sequencing (WGS). WGS showed that bacteria from sick people’s samples are closely related genetically. This means that people in this outbreak likely got sick from the same type of food. This means that these 28 people are likely linked, and according to the CDC, likely also linked to delis that sell meat.

Our public health official are still now in the middle of the epidemiologic investigation above. They are trying to identify what these 28 people ate sometime in April to sometime in July. They are trying hard to find a common denominator of a food item each consumed and/or a common food item that could have cross-contaminated the various delis. And, they are trying to figure this out with people who are likely older, sick and like me, cannot tell you what I ate 70 days ago. They may get lucky, but it is hard.

In 2022 the CDC reported an Outbreak of Listeria Infections Linked to Deli Meats and Cheese

There a total of 16 people infected with the outbreak strain of Listeria were reported from 6 states. Sick people’s samples were collected from April 17, 2021, to September 29, 2022. Sick people ranged in age from 38 to 92 years, with a median age of 74, and 62% were male. Of 15 people with race or ethnicity information available, 13 were White, 1 was African American/Black, 1 was Asian, and no one reported Hispanic ethnicity. Eleven people were of Eastern European background or spoke Russian. Of 14 people with healthcare information available, 13 were hospitalized. One person got sick during their pregnancy, resulting in pregnancy loss. Additionally, one death was reported from Maryland.

Of the 12 people interviewed, 11 reported eating meat or cheese from deli counters. Among seven sick people in New York, five bought sliced deli meat or cheese from at least one location of NetCost Market, a grocery store chain that sells international foods. Sick people from other states purchased deli meats or cheeses from other delis. NetCost Market delis are unlikely to be the only source of illnesses because some sick people in the outbreak did not shop at a NetCost Market. A contaminated food likely introduced the outbreak strain of Listeria into delis in multiple states.

But here the investigators got lucky, as in the current outbreak, WGS showed that bacteria from sick people’s samples were closely related genetically. This suggests that people in this outbreak got sick from the same food.

In 2021, health officials in New York state and New York City found the outbreak strain of Listeria in several environmental and food samples:

• Environmental samples from a NetCost Market deli in Brooklyn.

• Several open packages of mortadella and ham that were sliced at the same NetCost Market deli in Brooklyn.

• Sliced salami that a sick person bought from a NetCost Market deli in Staten Island.

So, at least for those people that purchased product at the Brooklyn or Staten Island NetCost Market delis the outbreak, though tragic, was solved. For the rest, however, exactly how they became sick many never have been found.

It will be interesting if the investigators in the current outbreak get a bit of luck.

Republished with permission from Bill Marler and Marler Clark. Copyright (c) Marler Clark LLP, PS. All rights reserved.