CDC, public health and regulatory officials in several states, and the U.S. Food and Drug Administration (FDA) investigated a multistate outbreak of Listeria monocytogenes infections. Epidemiologic data showed that leafy greens were a likely source of the outbreak. However, there was not enough other data to identify a specific type or producer of leafy greens.

As of June 13, 2023, a total of 19 people infected with the outbreak strain of Listeria were reported from 16 states. Sick people’s samples were collected from July 3, 2018, to March 31, 2023.

Public health officials collected many different types of information from sick people, including their age, race, ethnicity, other demographics, and the foods they ate in the month before they got sick. This information provided clues to help investigators identify the source of the outbreak.

Sick people ranged in age from less than 1 to 96 years, with a median age of 72, and 63% were female. Among 19 people with race information available, 17 people were White, 1 was African American/Black, and 1 reported “Other” race. Among 19 people with ethnicity information available, two people were Hispanic. Eighteen people were hospitalized, and no deaths were reported.

State and local public health officials interviewed people about the foods they ate in the month before they got sick. Of 14 people who answered questions about leafy greens, 13 (93%) people ate leafy greens, 13 (93%) ate iceberg lettuce, and 10 (71%) ate romaine lettuce. Twelve (86%) people ate packaged salads. CDC conducted a case-case analysis, comparing foods that sick people in this outbreak reported eating to foods that people sick with Listeria reported eating who were not part of an outbreak. The analysis showed that people in this outbreak were 8 times more likely to eat iceberg lettuce (p=0.035), 5 times more likely to eat romaine lettuce (p=0.038), and 4 times more likely to eat packaged salads (p=0.049) than sick people not in this outbreak. This suggests that leafy greens were a likely source of this outbreak. Three people ate leafy greens at the long-term care facilities they lived in, and one person ate leafy greens at a hospital they worked in. People bought leafy greens and different brands of packaged salads from several stores.

Public health investigators used the PulseNet system to identify illnesses that were 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 were closely related genetically. This suggests that people in this outbreak got sick from the same food.