The outbreak sickened 19 people from nine states in the US from mid-2015 to early 2016 and 14 cases of listeriosis were reported in Canada.
In the US, all cases were hospitalized and one died. One illness in a pregnant woman resulted in a preterm live birth. One otherwise healthy child developed meningitis.
In Canada, all cases were hospitalized and three died but the contribution Listeria was unclear.
Link to Dole salads
It was linked to packaged salads from the Dole processing facility in Springfield, Ohio and sold under various brand names.
Dole halted production at the plant and withdrew packaged salad products in January, operations restarted in April.
The firm said in May that it was being investigated by the Department of Justice in relation to the issue.
Researchers said the Listeria Initiative questionnaire has been revised to include additional questions about fresh produce to better identify produce vehicles of the pathogen.
In September 2015, PulseNet identified a cluster of Listeria monocytogenes (Listeria) clinical isolates indistinguishable by two-enzyme pulsed-field gel electrophoresis (PFGE) pattern combination and highly related by whole-genome multilocus sequence typing (wgMLST).
Standardized questionnaire did not identify source
A standardized Listeria questionnaire gathered information about foods consumed in the four weeks before illness from seven people identified by November 30, 2015, with isolation dates occurring July 5 – October 30, 2015.
This did not include leafy green vegetables and failed to identify a common source for infections.
During December 2015 and January 2016, eight new or previously interviewed patients or surrogates participated in open-ended interviews or provided shopper card records, and all reported having leafy greens in the month before illness onset.
It was the first reported outbreak of listeriosis associated with leafy greens and the eighth from fresh produce in the US; all since 2008.
“It is unclear whether the appearance of these outbreaks might be attributed to improved outbreak detection, changes in consumer behavior, or changes in production and distribution,” said the researchers.
“Fresh produce processors are advised to review food safety plans and consider incorporating measures to avoid the growth and persistence of Listeria.”
The Public Health Agency of Canada (PHAC) said cases had onset dates from May 7, 2015 to February 23, 2016.
Six Canadian clinical isolates were compared with US clinical isolates and were highly related by wgMLST. Three cases reported consuming packaged salad processed at the Ohio facility.
In January 2016, the Canadian Food Inspection Agency (CFIA) collected 55 packaged salads from stores representing 12 different products processed at the Ohio facility.
CFIA isolated the outbreak strain and issued a food recall warning in January, for all products processed at the Ohio facility and distributed in Canada.
“Notes from the Field: Outbreak of Listeriosis Associated with Consumption of Packaged Salad — US and Canada, 2015–2016”
Authors: Self JL, Conrad A, Stroika S, et al