Health officials have been unable to find the source of a Salmonella outbreak that affected almost 30 people in Ireland and England.

The outbreak, which saw 11 people hospitalized, has been declared over by authorities.

Symptom onset ranged between Nov. 30, 2022, and Jan. 13, 2023, peaking on Dec. 7. Patients lived in multiple areas on the island of Ireland. Fourteen patients were male and 13 were female. Most were adults, ranging from 25 to 91 years old, and two were under the age of 18. The median age of cases was 57 years old.

The Health Protection Surveillance Centre (HPSC) started an outbreak investigation in late December after seven Salmonella Typhimurium isolates were seen to form a cluster by Whole Genome Sequencing (WGS). 

They were related to two previous Salmonella Typhimurium isolates sequenced in October 2020 and December 2021 and some were identified in other European countries since 2020. However, the source was not found for these past infections.

Another 13 isolates were added to the cluster following further analysis in January 2023. Whole genome sequencing by the United Kingdom Health Security Agency (UKHSA) identified an additional seven patients who lived in Northern Ireland and England.

In total, 27 patients, identified between December 2022 and January 2023, who lived in or visited the island of Ireland were part of the outbreak.

No connection to chicken recalls or broiler flock
An outbreak control team, chaired by HPSC, was set up in January 2023. It also included representatives from departments of public health, the HSE Environmental Health Service, the Food Safety Authority of Ireland (FSAI), the HSC Public Health Agency of Northern Ireland and the Food Standards Agency in Northern Ireland.

The group met four times during January and February. Enhanced surveillance forms and trawling questionnaires were used to determine food and environmental histories for all cases but no common exposures were identified. Food business supplier lists were also examined but no commonalities were found.

In January and February 2023, there were chicken recalls because of Salmonella. Western Brand recalled various raw chicken products with the possible presence of Salmonella Typhimurium.

The Department of Agriculture, Food and the Marine also investigated the findings of Salmonella Typhimurium in several broiler flocks.

However, there is no microbiological evidence of a link between the outbreak of salmonellosis and the product recall or positive poultry flocks.

In late 2022, the FSAI checked the microbiological quality of chilled and frozen coated poultry meat preparations and meat products intended to be eaten cooked, testing for Salmonella.

HPSC data shows there have been 59 salmonellosis cases from the start of 2023 until the end of February, compared with 29 in the same period in 2022.