The investigation into a multi-year, multi-state listeriosis outbreak traced to package salads from Dole might have yielded results several months earlier had sick people been asked if they had eaten leafy greens, according to a report published Thursday.

Nineteen people across nine states were sickened in the U.S. from July 2015 through January this year. One person died. Canadian officials reported 14 people in that country were infected with the outbreak strain of Listeria monocytogenes, which was isolated in Dole’s bagged salads sold in both countries. The salads were produced at Dole’s facility in Springfield, OH. Investigators from the U.S. Centers for Disease Control and Prevention began looking for the cause of the listeria infections in the fall of 2015. They used a standardized questionnaire, which has since been changed. “This (questionnaire) did not include leafy green vegetables and failed to identify a common source for the infections,” according to field notes published by the CDC Thursday. “During December 2015 and January 2016, eight new or previously interviewed patients or their surrogates participated in open-ended interviews or provided shopper card records, and all reported consuming leafy greens in the month before illness onset.” On Jan. 27 Dole voluntarily recalled all salad mixes produced in the Springfield, OH, processing facility. The recall included 22 varieties of packaged salads sold under various brand names. “After the market withdrawal and recall, CDC fielded (more than) 450 inquiries about listeriosis from concerned consumers and clinicians, and the CDC outbreak website received (more than) 787,000 page views, more views than after any other foodborne illness outbreak to date.” The outbreak marked the first time Listeria monocytogenes has been associated with leafy greens, according to the CDC’s field notes. It was the eighth listeriosis outbreak linked to fresh produce, all of which have occurred 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,” the CDC’s report states. “Fresh produce processors are advised to review food safety plans and consider incorporating measures to avoid the growth and persistence of Listeria. The Listeria Initiative questionnaire has been revised to include additional questions about fresh produce to better identify produce vehicles of Listeria.” (To sign up for a free subscription to Food Safety News, click here.)