Listeria monocytogenes, a gram positive rod, is the leading cause of fatal foodborne infections in the United States. With an overall fatality rate approaching 20%, it is easy to understand why, even though the infection rates are behind those of Campylobacter, Salmonella, E. coli O157, Vibrio, and Yersinia. This high fatality rate makes the fact that there are 2-3 outbreaks annually reported to the CDC particularly scary.
Back in 2011, there was a listeriosis outbreak that ended up affecting many people in multiple states. This article discusses the inter-agency investigation that led to the discovery of the source of the infection. They ended up identifying 147 patients in 28 states, and per CDC estimates, there are likely twice that many people who were not identified.
Typically, listeriosis has a 1-3 week incubation period, which per the article makes determining the source hard. I mean really, who remembers what they ate 3 weeks ago? This compounded by the fact that Listeria can be found in unpasteurized dairy, undercooked meat, processed meat, and produce, makes the fact that they figured out where it came from even more amazing.
The presence of Listeria on the rind of produce leads to being transferred to the pulp during cutting. Once in the pulp, it can survive even during refrigeration.
One of the more interesting findings from the paper is that there exists something called the “Listeria Initiative” which is an enhanced surveillance system created by the CDC in 2004 to investigate outbreaks. It has a standardized questionnaire, which was modified during this investigation to include melons after multiple people remembered consuming them recently. This allowed them to narrow it down to cantaloupes grown in Colorado, then to a specific farm. It turns out that this farm had changed their processing equipment, and swabs of that equipment and further down the processing line showed the same strains of Listeria.
The investigation ended up determining that 22% of the patients infected died of listeriosis. The median age of death was 81, which is more evidence that elderly patients are at high risk. This outbreak did not have high pregnancy related or neonatal mortality, and while there is a fair amount of conjecture, no real reason is identified for this difference from earlier outbreaks. They do point out that this outbreak was associated with whole produce, instead of commercially prepared products.
Unfortunately, the article does not describe listeriosis treatments, merely discussing the need for oversight of manufacturing and agricultural practices. While a good read and interesting from an epidemiologic standpoint, it is low yield for prevention or treatment from a medical standpoint. The standard travel warning of “cook your food and wash your produce” isn’t terribly earth shattering.
McCollum JT et al. Multistate outbreak of listeriosis associated with cantaloupe. N Engl J Med. 2013 Sep 5;369(10):944-53. [PMID 24004121]
EBM Gone Wild