Outbreaks of Shiga toxin–producing Escherichia coli (STEC) O157 are
most often associated with beef and leafy greens. Before 2024, **only one
multistate STEC O157 outbreak — in 2011 — had ever been linked to tree
nuts**.
**In 2024, investigators identified organic walnuts as the source of a
multistate STEC O157 outbreak in Washington and California — the first
documented foodborne outbreak in the United States linked to walnuts.**
Why a first matters
Outbreak investigation works by hypothesis. Investigators interview cases
about what they ate, and the questions they ask are shaped by what has caused
outbreaks before. A food that has never been implicated is a food nobody
thinks to ask about — and in a questionnaire built around beef and salad,
walnuts are the thing that does not come up.
Which means the difficulty in this investigation was not the laboratory work.
It was arriving at the idea.
The report's implication is correspondingly short and consequential:
**walnuts should be considered as a possible vehicle for STEC infections in
future outbreaks.**
One line added to the list of foods to ask about, which is how the next
outbreak gets found faster.
What the pattern says about produce
A tree nut is not an obvious vehicle for an organism associated with cattle.
That it turns up in one at all points at the same route contamination takes
into leafy greens: what happens to a crop in the field and in washing, not
what the crop is.
*Source: Barrall AL, Stewart L, Higa J, et al. Notes from the Field: Outbreak
of Escherichia coli O157:H7 Infections Linked to Organic Walnuts — Washington
and California, 2024. MMWR, Centers for Disease Control and Prevention.*
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