An outbreak investigation was launched by HPSC on 22/12/2022 after seven Salmonella Typhimurium isolates were identified as forming a cluster by routine Whole Genome Sequencing (WGS) carried out by the National Salmonella, Shigella and Listeria Reference Laboratory (NSSLRL). The isolates, which were of a less commonly occurring subtype of Salmonella Typhimurium, MLST type ST36, were related to two previous Salmonella Typhimurium isolates (sequenced in the NSSLRL in October 2020 and December 2021) and isolates identified in other European countries since 2020. The source of salmonellosis was not identified for these previous cases. An additional thirteen isolates were added to this cluster following further WGS by the NSSLRL in January 2023, while WGS performed by the Gastrointestinal Bacteria Reference Unit (GBRU) of the UK’s Health Security Agency’s (UKHSA) identified an additional seven cases resident in Northern Ireland and England that fell within the same cluster. In total 27 cases, identified between December 2022 and January 2023, who were resident on or who visited the island of Ireland were investigated as part of this outbreak.
Epidemiological data on outbreak cases were collected in each jurisdiction via national computerised surveillance systems, enhanced surveillance forms and trawling questionnaires. Symptom onset dates for cases ranged between the 30th of November 2022 and the 13th of January 2023, peaking on December 7th. Cases were geographically distributed across multiple areas on the island of Ireland. Fourteen cases were male and thirteen were female. Most cases were adults, ranging in age from 25 to 91 years, and two cases were children (aged <18 years). The median age of cases was 57 years. Forty-one percent (41%) of cases (n=11) were reported to have been hospitalised.
A multi-disciplinary Outbreak Control Team (OCT), chaired by HPSC, was convened in January 2023. This group comprised representatives from HPSC, National Health Protection Service, NSSLRL, Departments of Public Health, the HSE Environmental Health Service, The Food Safety Authority of Ireland (FSAI) and the HSC Public Health Agency of Northern Ireland (PHA) and Food Standards Agency in Northern Ireland (FSA-NI). The OCT met four times during January and February 2023. Enhanced surveillance forms and trawling questionnaires were used to determine food and environmental exposure histories for all cases but no common exposures were identified. Additionally, food business supplier lists were examined and no commonalities were identified. The outbreak was declared over on February 16th 2023 as more than two incubation periods had elapsed without the reporting of new cases associated with this cluster but the source of this outbreak remains unknown.
In January 2023, FSAI instigated a food recall of batches of chicken products due to the detection of Salmonella Typhimurium and the Department of Agriculture, Food and the Marine launched an investigation into the incidence of Salmonella Typhimurium in a number of broiler flocks. There is no microbiological evidence of a link between the cases of salmonellosis investigated as part of this outbreak with the poultry product recall and positive poultry flocks.