This article describes the identification and investigation of two extended outbreaks of listeriosis in which crabmeat was identified as the vehicle of infection. Comparing contemporary and retrospective typing data of Listeria monocytogenes isolates from clinical cases and from food and food processing environments allowed the detection of cases going back several years. This information, combined with the analysis of routinely collected enhanced surveillance data, helped to direct the investigation and identify the vehicle of infection. Retrospective whole genome sequencing (WGS) analysis of isolates provided robust microbiological evidence of links between cases, foods, and the environments in which they were produced and demonstrated that for some cases and foods, identified by fluorescent amplified fragment length polymorphism, the molecular typing method in routine use at the time, were not part of the outbreak. WGS analysis also showed that the strains causing illness had persisted in two food production environments for many years and in one producer had evolved into two strains over a period of around 8 years. This article demonstrates the value of reviewing L. monocytogenes typing data from clinical cases together with that from foods as a means of identifying potential vehicles and sources of infection in outbreaks of listeriosis. It illustrates the importance of reviewing retrospective L. monocytogenes typing alongside enhanced surveillance data to characterize extended outbreaks and inform control measures. Also, this article highlights the advantages of WGS analysis for strain discrimination and clarification of evolutionary relationships that refine outbreak investigations and improve our understanding of L. monocytogenes in the food chain.
Bibliographical noteFunding Information:
Thanks are extended to the following for their assistance during this investigation: the London Borough of Enfield, Robert Bartlett; Public Health England (PHE), Dr. Vivien Cleary and Dr. Simon Cathcart; and the Food Standards Agency, Dr. Paul Cook and Dr. Joanne Edge. We also thank Dr. Jim McLauchlin of PHE for reviewing this manuscript and Martin Lodge for advice on microbiological standards. R.E., A.A.-O., T.J.D, A.P., N.E., and K.G. are affiliated with the National Institute for Health Research Health Protection Research Unit (NIHR HPRU) in Gastrointestinal Infections at the University of Liverpool in partnership with PHE, in collaboration with University of East Anglia, University of Oxford, and the Quadram Institute. R.E., T.J.D., A.P., and K.G. are based at PHE, and A.A.-O. is based at the University of Oxford. The views expressed are those of the author(s) and not necessarily those of the National Health Service, the NIHR, the Department of Health, or PHE.
© International Association for Food Protection.
- Listeria monocytogenes
- Whole genome sequencing