A multi-year outbreak of Listeria infections traced to peaches, nectarines, and plums has been declared over by the Centers for Disease Control and Prevention.

In its final update, the CDC reported that the seven-state outbreak had sickened 11 people and killed one. One of the patients was pregnant and went into pre-term labor.

All of the patients except one were hospitalized. The CDC says many more people were likely sickened in the outbreak but were not reported because they did not seek medical attention or were not tested explicitly for Listeria.

Listeria infection can be challenging to diagnose, and it can be hard to identify outbreaks because symptoms can take up to 70 days after exposure to develop. Patients are generally identified through testing because of symptoms. They are interviewed about the foods they ate before becoming ill.

Analysis showed that people in this outbreak were 18 times more likely to eat peaches, nectarines, or plums than sick people not in this outbreak. Sick people’s samples were collected from Aug. 22, 2018, to Aug. 16, 2023.

“On Oct. 23, 2023, the FDA collected a sample of HMC Farms peaches for testing and found Listeria. On Nov. 6, 2023, whole genome sequencing showed that the Listeria in the peaches were closely related to bacteria from sick people. This means that people likely got sick from eating those peaches,” according to the CDC’s final outbreak report. 

On November 17, 2023, HMC Farms recalled whole peaches, plums, and nectarines sold in stores between May 1 and November 15 in 2022 and 2023.

This outbreak is an example of how whole genome sequencing can identify outbreak patients separated by long periods. Using the technique, investigators can match new cases with previous ones by comparing samples in a database. Then, finding a match between contaminated food and a patient sample reveals the source of an outbreak.