Multistate Outbreak of Listeriosis Associated With Packaged Leafy Green Salads

United States and Canada, 2015-2016

Julie L. Self; Amanda Conrad; Steven Stroika; Alikeh Jackson; Laura Whitlock; Kelly A. Jackson; Jennifer Beal; Allison Wellman; Marianne K. Fatica; Sally Bidol; Paula Pennell Huth; Meghan Hamel; Kristyn Franklin; Lorelee Tschetter; Christine Kopko; Penelope Kirsch; Matthew E. Wise; Colin Basler


Emerging Infectious Diseases. 2019;25(8):1461-1468. 

In This Article

Abstract and Introduction


We investigated an outbreak of listeriosis detected by whole-genome multilocus sequence typing and associated with packaged leafy green salads. Nineteen cases were identified in the United States during July 5, 2015–January 31, 2016; isolates from case-patients were closely related (median difference 3 alleles, range 0–16 alleles). Of 16 case-patients interviewed, all reported salad consumption. Of 9 case-patients who recalled brand information, all reported brands processed at a common US facility. The Public Health Agency of Canada simultaneously investigated 14 cases of listeriosis associated with this outbreak. Isolates from the processing facility, packaged leafy green salads, and 9 case-patients from Canada were closely related to US clinical isolates (median difference 3 alleles, range 0–16 alleles). This investigation led to a recall of packaged leafy green salads made at the processing facility. Additional research is needed to identify best practices and effective policies to reduce the likelihood of Listeria monocytogenes contamination of fresh produce.


Invasive Listeria monocytogenes infections (listeriosis) are the third leading cause of death from foodborne illness in the United States and cause an estimated 1,500 infections, 1,400 hospitalizations, and 250 deaths each year.[1] Although incidence of listeriosis is lower than for many foodborne illnesses, it often results in severe illnesses, including sepsis and meningitis, and is associated with a high case-fatality rate and fetal loss in pregnant women.[2–4] Populations at highest risk for invasive listeriosis include elderly persons, immunocompromised persons, and pregnant women and their newborns.[5] Listeriosis can have a long incubation period (median 11 days, range 0–70 days) between exposure and symptom onset.[5,6]

L. monocytogenes was first recognized as a foodborne pathogen after an outbreak in Canada during 1981 that was linked to cabbage in coleslaw.[7] Outbreaks of listeriosis in the United States have historically been associated with ready-to-eat delicatessen meats and dairy products, but have more recently been associated with fresh produce, including sprouts, celery, cantaloupe, stone fruit, and caramel apples.[5,8,9]

In October 2015, PulseNet USA (, the national molecular subtyping network for foodborne disease surveillance, identified a cluster of 8 clinical L. monocytogenes isolates from 6 states that were closely related genetically to one another by whole-genome multilocus sequence typing (wgMLST) (median difference 5 alleles, range 0–12 alleles); the Centers for Disease Control and Prevention (CDC; Atlanta, GA, USA) initiated a cluster investigation. During a weekly foodborne cluster investigation call on January 13, 2016, CDC informed the Public Health Agency of Canada (PHAC) of the cluster investigation, which had increased to include 13 cases of listeriosis. PHAC informed CDC of a cluster of 6 cases of listeriosis in Canada that had a pulsed-field gel electrophoresis (PFGE) pattern combination indistinguishable from that of the US cases. The United States and Canada conducted collaborative investigations to determine the source of the outbreak.