A multi-state outbreak of Listeria monocytogenes in North East USA and Canada was traced to the source by using “DNA fingerprinting” on the listeria bacteria isolated from the ill people.

Public Health investigators used the CDC’s PulseNet system to identify people with Listeria infections (listeriosis), which can be life threatening, and put together a pattern for the outbreak. This would have been hard to do otherwise as the outbreak covered nine states over a period of seven months.

Pulsed-Field Gel Electrophoresis (PFGE) and Whole Genome Sequencing (WGS) were used on samples collected from 19 people hospitalised because of listeriosis, one of whom died. The samples were closely related genetically indicating a common source of infection, which is usually from contaminated food.

Interviews with the ill people revealed a common thread of packaged salad that in many cases could be identified as being produced by Dole.

Routine product sampling by Ohio Department of Agriculture had identified Listeria in a Dole packaged salad. WGS showed it to be closely genetically linked to the samples from the hospitalised people. This enabled the source of the outbreak to be traced to the Dole processing facility in Springfield, Ohio. In January 2016, the company voluntarily recalled all salad mixes produced there.

The facility has recently restarted production after investigations by the Food and Drug Administration, Centre for Disease Control and the Canadian Food Inspection Agency.

Source: www.cdc.gov/listeria/outbreaks/bagged-salads-01-16/