As summarized by CIDRAP
- Starting in 2018, illnesses were reported but at that time, epidemiologic investigations suggested that institutional food was the likely source, however there wasn't enough evidence to pinpoint a specific food source.
- There were subsequent investigation in 2021, and 2023, in addition to 2018.
- The CDC reopened the investigation in October 2024 after six new infections were reported. In background information on the outbreak, the Food and Drug Administration (FDA) said 20 of the cases were reported in 2024 and 2025 and that the outbreak and the investigations are ongoing.
- In November 2024, the FDA focused on nursing homes where many of the cases occurred. In its investigation, FDA used food invoices from involved nursing homes to show that since 2024, all nursing homes had received frozen supplement shakes from Lyons ReadyCare or Sysco Imperial brand.
- FDA investigators used environmental sampling and whole-genome sequencing to show that the strain of Listeria found in environmental samples were closely related to isolates from sick patients. "Three of the environmental swabs collected from the processing area tested positive for Listeria monocytogenes. Whole Genome Sequencing analysis determined that the Listeria detected in these samples is closely related to the strain of Listeria that is causing illnesses in this outbreak."
- On February 22, Lyons Magnus, a distributor based in Fresno, California, recalled 4-ounce servings of several flavors of Lyons ReadyCare and Sysco Imperial Frozen supplemental shakes.