Protracted, Intermittent Outbreak of Salmonella Mbandaka Linked to a Restaurant

Michigan, 2008-2019

William D. Nettleton, MD; Bethany Reimink, MPH; Katherine D. Arends, MPH; Douglas Potter, MBA; Justin J. Henderson, MPH; Stephen Dietrich, MS; Mary Franks, MPH


Morbidity and Mortality Weekly Report. 2021;70(33):1109-1113. 

In This Article


Multiple challenges contributed to delayed source identification. Food histories were incomplete in the early cases. Initial questionnaires were inflexible and focused more on food items than on food establishments. Early cases were not initially identified as a cluster given the sporadic incidence and were hypothesized as a rare or regional PFGE pattern. Further, index of suspicion for a protracted common source early in the outbreak was low given the more typical experience of point source Salmonella outbreaks. Finally, restaurant management was doubtful and required intensive engagement. Both the environmental and clinical testing results were thus essential for continued mitigation efforts.

Salmonellosis outbreaks in the food industry often occur through a point source when undercooked or contaminated food products infect consumers until distribution of the foodborne vehicle ceases.[2,3] In this outbreak, a complex association between the environment and employees of a single restaurant in southwest Michigan demonstrated a protracted and intermittent common source outbreak of Salmonella Mbandaka. A study of 23 restaurant-associated salmonellosis outbreaks found that restaurants with Salmonella-positive environmental samples had a higher proportion of Salmonella-positive food workers and longer outbreak durations than did restaurants with negative environmental samples.[4] The nearly 11-year duration of this outbreak attests to the potential recalcitrance of Salmonella in restaurant environments, the importance of hygienic restaurant policies and practices, and the challenge in source identification when cases occur intermittently and without a clear foodborne vehicle. As WGS is more broadly implemented as a routine subtyping method for Salmonella and other bacterial enteric pathogens, increased discriminatory power might facilitate the identification of more protracted, common-source outbreaks.[5] Whereas initially small numbers of cases might present a challenge to definitively implicating a common source, gathering as much high quality exposure data as possible, including repeated interviewing of patients with cases that are clustered in time using closed-ended questions about exposures of interest, can aid an investigation. In addition, conducting environmental assessments, environmental sampling, and employee testing for Salmonella are best practices that should be considered early in an investigation, particularly when a single foodborne vehicle is not apparent.

Fifteen (42%) of the 36 patients had the outbreak subtype isolated in urine; 12 (33%) patients had urinary symptoms without reporting diarrhea or vomiting. These findings are consistent with the observation that a higher proportion of Salmonella serogroup C1 (including Mbandaka) than of other Salmonella serogroups is isolated from urine.[6,7] Although chronic carriage of Salmonella Typhi after acute infection is widely recognized, asymptomatic carriage of nontyphoidal Salmonella is less well characterized but has been reported in restaurant food and hotel workers as well as in healthy adults and children.[5,8,9]

For most of the time when the reported outbreak investigation was conducted, the restaurant was regulated under a modified version of the 2009 Food and Drug Administration (FDA) Food Code, the latest FDA Food Code that Michigan had adopted. The 2009 FDA Food Code did not include asymptomatic nontyphoidal Salmonella infections among the five specific foodborne pathogens for which exclusion and restriction requirements are delineated. Therefore, the 2017 FDA Food Code was used for guidance because it includes asymptomatic nontyphoidal Salmonella infection as a food worker condition of restriction.[10] Further adoption of the 2017 FDA Food Code will aid public health professionals in disrupting nontyphoidal Salmonella transmission in restaurant settings, particularly as more protracted outbreaks are identified.

Norovirus, Salmonella Typhi, Escherichia coli O157:H7 or Enterohemorrhagic or Shiga toxin-producing E. coli, Shigella spp., and hepatitis A virus.