Whole Genome Sequencing for the Retrospective Investigation of an Outbreak of Salmonella Typhimurium DT 8

Philip M Ashton; Tansy Peters; Linda Ameh; Ralph McAleer; Stewart Petrie; Satheesh Nair; Ivan Muscat; Elizabeth de Pinna; Tim Dallman

doi:10.1371/currents.outbreaks.2c05a47d292f376afc5a6fcdd8a7a3b6

Background:
Salmonella enterica serovar Typhimurium DT8 is uncommon within the European Union. An increase in this phage type was reported in the summer of 2013 in the States of Jersey.

Methods:
A total of 21 human cases with this phage type were microbiologically confirmed. Salmonella isolates from mayonnaise made using raw eggs were also confirmed as being Salmonella Typhimurium DT8. The epidemiological investigations strongly supported a link between mayonnaise consumption and illness. Whole genome sequencing (WGS) was used to retrospectively investigate this outbreak with a view to assess the similarity between the suspect food and the human isolates and to characterise a known point source outbreak to assist in development of algorithms for outbreak detection.

Results:
Sequence data showed that the outbreak associated isolates, including the food isolates, formed a tightly clustered monophyletic group, with a maximum pairwise distance of 3 single nucleotide polymorphisms.

Conclusions:
WGS data is useful in confirming the causative agent of outbreaks where food and clinical isolates are available. This dataset, comprising a known outbreak, will be useful in the development of automatic algorithms for outbreak detection.

In England, several recent campylobacter outbreaks have been associated with poultry liver consumption. Following a lunch event in a hotel in Surrey in November 2013 where chicken liver parfait was served, guests reported having gastrointestinal symptoms. A retrospective cohort study showed 46 of 138 guests became unwell, with a median incubation period of two days and for 11 cases campylobacter infection was laboratory confirmed. Food item analysis identified an association between illness and consumption of roast turkey (aOR=3.02 p=0.041) or jus (aOR=3.55 p=0.045), but not with chicken liver parfait (OR=0.39 p=0.405). The environmental risk assessment did not identify non-compliance with standard food practice guidelines. This study presents a point-source outbreak of campylobacter with a high attack rate and epidemiological analysis results show that the jus or roast turkey was the likely source of infection although this could not be confirmed by the environmental assessment. Consuming the chicken liver dish was not a risk factor for developing symptoms as was initially hypothesised. Prior knowledge on the association between poultry liver food items and campylobacter outbreaks should not overly influence an outbreak investigation to ensure the true aetiology is identified and on-going public health risk is minimised.