Research and analysis

HPR volume 17 issue 9: news (28 July and 1 August 2023)

Updated 28 December 2023

Ongoing investigation into listeriosis associated with consumption of smoked fish products in England, Scotland and Wales

The UK Health Security Agency (UKHSA), Public Health Scotland (PHS), Public Health Wales (PHW) – together with the Food Standards Agency (FSA), Food Standards Scotland (FSS) and local authorities – are continuing to investigate an ongoing outbreak of listeriosis across England, Scotland and Wales.

Listeriosis is predominately a foodborne bacterial infection caused by Listeria monocytogenes. This bacterium is widely distributed in the environment and infection commonly occurs after consumption of contaminated ready-to-eat foods, causing sporadic cases and outbreaks. Most cases are asymptomatic or experience self-limiting, mild gastroenteritis. However, the infection may cause bacteraemia, meningoencephalitis or severe sepsis in the elderly, immunocompromised or those with underlying chronic conditions (1). Vertical and peripartum transmission during pregnancy can lead to miscarriage, neonatal meningoencephalitis and sepsis.

Since 2015, UKHSA (previously Public Health England) has routinely utilised whole genome sequencing (WGS) for confirmation of Listeria species and strain typing. A multi-agency investigation was initiated in July 2021, informed by the detection of L. monocytogenes CC217 t10:12 in a sample of smoked salmon and 4 cases. Multiple meetings have been held with key stakeholders, the aim being to coordinate the epidemiological, microbiological and food chain investigations.

UKHSA identified 19 cases of listeriosis between November 2020 and June 2023, of which 2 were reported in the last 12 weeks and none in the last 4 weeks. They were all infected by the same strain of L. monocytogenes (CC217 t10:12), indicating these cases were most likely to have been exposed to a common source of infection. The confirmed cases were resident in England, Scotland or Wales. All 19 cases were hospitalised and 4 cases, to date, are deceased. One case was pregnant at the time of their infection; 16 cases were over 65 years of age. All cases or their next of kin were asked to complete a questionnaire detailing the food items the case had consumed in the 30 days prior to their onset of symptoms of listeriosis. Epidemiological analysis of this information has indicated that the most commonly consumed food among cases is smoked fish (predominantly smoked salmon) (2).

This investigation led NHS England to update guidance on foods that may be high risk for listeriosis so as to include reference to smoked fish in 2022 (3, 4). One of the retailers identified during the investigation updated the labels on the packaging of their smoked salmon products from February 2023 to highlight that consumption of this product may pose a risk of listeriosis for vulnerable groups.

Despite these measures, cases continue to be reported, therefore the investigation remains ongoing to identify the source of contamination, to enable the implementation of control measures and to prevent further cases.

References

1. Scobie A, Kanagarajah S, Harris RJ, Byrne L, Amar C, Grant K, and others (2019). ‘Mortality risk factors for listeriosis – a 10 year review of non-pregnancy associated cases in England 2006-2015’. Journal of Infection: volume 78 issue 3, pages 208 to 214.

2. UKHSA (2023). ‘Smoked fish and risk of listeriosis in high risk groups, including pregnancy and the immunocompromised’

3. NHS England. Listeriosis.

4. NHS England. Foods to avoid in pregnancy.

Infection reports in this issue of HPR

Laboratory confirmed cases of measles, rubella and mumps in England: January to March 2023