Research and analysis

Listeriosis in England and Wales: summary for 2023

Updated 26 September 2024

Applies to England and Wales

Main points for 2023

This report summarises the number, demographics and clinical outcomes of confirmed cases of listeriosis in England and Wales in 2023. The main points are:

• in 2023, 177 cases of listeriosis were reported in England and Wales
• incidence rates of listeriosis were highest in people aged 80 years and over
• overall, the crude incidence of listeriosis was comparable in men and women, but for the age groups 20 to 29 and 30 to 39 reported cases among women were higher, and for those aged 70 and older rates were higher among men
• pregnancy-associated infections accounted for around a sixth of all reported cases and 23.1% of pregnancy associated cases (where known) resulted in still birth or miscarriage
• among non-pregnancy-associated cases of listeriosis, death was reported for 32 cases (21.6%), of whom 11 (34.4%, or 7.4% of all non-pregnancy-associated cases) were known to have listeriosis recorded as a cause of death on the death certificate
• incidence of listeriosis varied geographically, with the lowest incidence in Wales (0.13 per 100,000 population) and the highest in the South West (0.38 cases per 100,000 population)
• there were 7 listeriosis outbreaks investigated in England and Wales in 2023, including national outbreaks associated with smoked fish, semi-soft cheese and beef products

Background

Listeriosis is a foodborne disease caused by the bacterium Listeria monocytogenes (L. monocytogenes). These bacteria are widely distributed in the environment and infection usually occurs after consumption of contaminated raw, chilled, or ready-to-eat foods, which can cause sporadic infections but can also result in outbreaks of disease. People with listeriosis have been reported to develop symptoms between one and 70 days after consuming food contaminated with L. monocytogenes (1).

Compared to other foodborne pathogens, infections in humans are relatively rare, with 2,738 confirmed cases of listeriosis (rate per 100,000 population of 0.62) reported across 24 EU member states and the United Kingdom in 2022 (2), and a median of 167 cases in England and Wales each year (2014 to 2023). However, listeriosis can cause severe clinical outcomes in vulnerable groups, including those aged over 60 years, people who are pregnant and their unborn or newborn babies, and people with impaired immunity. In these groups, listeriosis can present as infection of the bloodstream or brain (3). Due to the severity of infection and high case fatality rate, listeriosis is an important public health concern.

National surveillance of listeriosis in England and Wales has been undertaken since 1992. This report summarises the number, demographics and clinical outcomes of confirmed cases of listeriosis in England and Wales reported in 2023.

Methods

National surveillance of listeriosis in England and Wales is coordinated by the Gastrointestinal Infections and Food Safety (One Health) Division (GIFSOH) at UKHSA, with support from Public Health Wales and involves the collation of routinely collected epidemiological, clinical and microbiological data for cases of listeriosis.

New cases are reported to the national surveillance system by local clinical laboratories, health protection teams and/or by referral of L. monocytogenes isolates to the Gastrointestinal Bacteria Reference Unit for whole genome sequencing (WGS).

Demographic and food history data is collected via a standardised questionnaire (trawling questionnaire) administered to the case or a proxy via health protection teams or the local authority. Clinical data is collected from clinical microbiologists at the frontline diagnostic laboratories via a second separate questionnaire (clinical questionnaire).

The analysis of WGS derived data, combined with clinical and epidemiological data from standardised food and clinical questionnaires, is used to detect clusters of genomically and epidemiologically linked cases, inform outbreak investigations and implement appropriate control measures.

It is likely that the emergence of SARS-CoV-2 (COVID-19) and subsequent non-pharmaceutical interventions (NPIs) implemented to control COVID-19 affected gastrointestinal disease surveillance in a number of ways. These included, but were not limited to, changes which may have impacted ascertainment (for example healthcare seeking behaviour and access to health care) as well as changes which likely impacted incidence (for example closures of hospitality and food businesses, increased awareness of general hygiene, changes or disruption to food supply chains, changes in food consumption or storage habits) which will have also varied over time. It should be noted however that the NPIs implemented during this period would have been less likely to affect reporting of listeriosis cases in a similar manner to other cases of gastrointestinal infections, due to the greater severity of illness (and thus higher rate of hospitalisation) of cases of listeriosis. The low numbers of annually reported cases of listeriosis makes meaningful interpretation difficult however, and it is therefore unclear whether the lower number of cases reported in 2020, and the number of cases reported in 2021, was due to in part to effects of the COVID-19 pandemic or interannual variation which may have occurred even if COVID-19 had not emerged.

As any potential impact of the COVID-19 pandemic upon the 2020 and 2021 case numbers remains unclear, throughout this report data from these years has been excluded when calculating the 5-year median used to compare 2023 data against, with the time period of 2016 to 2019 and 2022 used instead.

Case definitions

For reporting, a confirmed case of listeriosis is defined as (4) a person with a clinically compatible illness (see Clinical criteria below) and (one of the following):

• isolation of L. monocytogenes (or detection of nucleic acid) from a normally sterile site
• isolation of L. monocytogenes (or detection of nucleic acid) from a normally non-sterile site from a foetus, stillborn, newborn, placenta, foetal tissue or from the mother following birth

Clinical criteria

In adults, invasive disease caused by L. monocytogenes (listeriosis) manifests most commonly as meningoencephalitis and/or septicaemia.

Invasive listeriosis principally affects high risk groups including:

• people aged over 60 years
• people with malignancies (especially of the blood)
• kidney disease
• liver disease
• diabetes
• alcohol dependency
• patients on immunocompromising treatment
• pregnant women and their unborn or newborn infants

Symptoms of sepsis include fever, confusion and collapse. Infection during pregnancy may result in foetal loss through miscarriage or stillbirth, neonatal meningitis or bacteraemia. Neonatal infection can range from mild illness to neonatal death, usually within 10 days of delivery.

Cases are further sub-classified as pregnancy-associated or non-pregnancy-associated:

Pregnancy-associated

L. monocytogenes infection in a pregnant patient and neonatal patients aged 28 days or less.

A mother-baby pair is considered a single case.

Non-pregnancy-associated

L. monocytogenes infection in a patient aged over 28 days.

Counts and rates

Data presented relates to confirmed cases reported to the national enhanced surveillance system for L. monocytogenes infection during the period 2014 to 2023.

Population and deaths data is sourced from the Office for National Statistics (ONS). Mid-year 2023 population estimates for England and Wales were used to provide denominators for the calculation of incidence rates. All rates are calculated as per 100,000 population.

National surveillance data for listeriosis

Annual cases of listeriosis

In 2023, 177 cases of listeriosis were reported to national surveillance in England and Wales (Figure 1), representing a 12.7% increase in reported cases compared to the previous 5-year median (n=157, 2016 to 2019 and 2022). The crude incidence of listeriosis in 2023 was 0.29 cases per 100,000 population (95% CI: 0.25 to 0.34), compared to a crude incidence of 0.28 cases per 100,000 in 2022 (95% CI: 0.24 to 0.33).

Figure 1. Annual cases and crude incidence rate of listeriosis reported in England and Wales, 2014 to 2023

Age and sex distribution of cases

Incidence of listeriosis varied by age group and sex, with age-specific incidence rates highest in people aged 80 years and over (Figure 2).

Overall, the crude incidence of listeriosis was comparable in males and females (Table 1, Incidence rate ratio (IRR): 1.05, 95% CI: 0.77 to 1.43), but higher in males compared to females in 3 age groups, 40 to 49 years (IRR: 3.13, 95% CI: 0.56 to 31.75) 70 to 79 years (IRR: 3.03, 95% CI: 1.42 to 7.01) and 80 years and older (IRR: 1.43, 95% CI: 0.73 to 2.79, Figure 2). Incidence was higher in females compared to males in the 20 to 29 (IRR: 0.11, 95% CI: 0.003 to 0.79) and 30 to 39 age groups (IRR: 0.14, 95% CI: 0.16 to 0.61), as pregnancy-associated listeriosis occurs most commonly in these age groups.

Of the 36 cases in the 10 to 19, 20 to 29, 30 to 39 and 40 to 49 age groups, 26 were female, of which 22 cases were associated with pregnancy (range: aged 22 to 44).

Table 1. Sex-specific incidence rates of listeriosis reported in England and Wales, 2023

Gender	Reported cases	Incidence rate	95% CI
Male	89	0.30	0.24 to 0.37
Female	88	0.28	0.23 to 0.35

Figure 2. Age-specific incidence of listeriosis in England and Wales, stratified by sex, 2023 [note 1]

Note 1: For a sub-set of pregnancy-associated listeriosis cases only the clinical isolate for the baby was received by the national reference laboratory. As such, only the age-sex specific data for these babies (rather than their mothers) was available to UKHSA and therefore included in this figure.

Pregnancy associated cases

In 2023, over a sixth of cases were pregnancy associated (n=29, 16.4%), which was comparable to previous years (11.3% to 19.5% between 2016 to 2019 and 2022.

Amongst pregnancy-associated cases, where known (26 of 29), 76.9% of pregnancies resulted in live births and 23.1% resulted in still birth or miscarriage.

Of pregnancy-associated cases of listeriosis in England and Wales in 2023, 69.0% resulted in live births, 6.9% resulted in still births, 13.8% in miscarriage and in 10.3% the outcome was unknown (Figure 3).

Figure 3: Pregnancy-associated cases of listeriosis in England and Wales in 2023

Mortality rate

In 2023, there were 32 deaths among 148 non-pregnancy cases reported to national surveillance (21.6%), which was lower compared to the previous 5 year median calculated from the years of 2016 to 2019 and 2022 (29.4%, range: 23.1% to 44.9%).

Of the 32 deaths, 11 were known to have a clinical manifestation of invasive L. monocytogenes infection recorded as a cause of death (please note this data may be incomplete). This represented a case fatality rate of 7.4%, compared to 12.6% in 2022.

Regional distribution of cases

The incidence rate of listeriosis varied geographically (Table 2; Figure 4). In England, the South West had the highest incidence rate of 0.38 cases per 100,000 population whilst Wales had the lowest incidence rate of 0.13 cases per 100,000 population.

Table 2: Regional incidence of listeriosis reported in England and Wales, 2023

Region	Cases	Incidence rate	95% Confidence Interval
East Midlands	13	0.26	0.14 to 0.45
East of England	16	0.25	0.14 to 0.40
London	29	0.32	0.22 to 0.47
North East	6	0.22	0.08 to 0.48
North West	21	0.28	0.17 to 0.42
South East	29	0.31	0.20 to 0.44
South West	22	0.38	0.24 to 0.57
West Midlands	22	0.36	0.23 to 0.55
Yorkshire and The Humber	15	0.27	0.15 to 0.44
Wales	4	0.13	0.04 to 0.33

Figure 4. Incidence rates of reported listeriosis cases in England and Wales, by region, 2023

Seasonality

October was the peak month for listeriosis reporting in 2023, with no known outbreak activity influencing the peak. In 2022 case numbers peaked in July and November, with no known outbreaks influencing the peaks.

Figure 5. Seasonal trend of reported listeriosis cases in England and Wales (2021 to 2023)

Incidents of listeriosis

There were 7 outbreaks (comprising 2 or more linked cases) of listeriosis investigated in England in 2023 (Table 3), 2 of which also included cases in Wales. Investigations identified the source of the outbreaks for 4 of the 7 known outbreaks and included smoked fish, corned beef, cooked beef tongue products and semi-soft cheese, all considered to be high-risk foods for listeriosis in vulnerable groups.

Table 3. Outbreaks of listeriosis in England investigated in 2023 [note 2]

Outbreak	Clinical cases	Time-frame	Region	Source of contamination
1	15	2020 to 2023	England, Wales	Smoked fish
2	6	2019 to 2023	England	Corned beef
3	7	2021 to 2023	England, Wales	Cooked beef tongue products
4	3	2022 to 2023	England	Semi-soft cheese
5	2	2022 to 2023	England	Unknown
6	2	2023	England	Unknown
7	3	2023	England	Unknown

Note 2: Time-frame indicates the years from which clinical cases associated with each incident were reported.

National outbreak of listeriosis in England associated with smoked salmon, 2020 to 2023

In December 2020, an outbreak of listeriosis was detected in England after 3 people were infected with the same strain of L. monocytogenes, one of whom died. All 3 cases had underlying comorbidities or risk factors for listeriosis, including one pregnancy-related case. One of the outbreak cases reported consuming salmon trimmings and smoked salmon slices purchased from a UK supermarket chain.

Following international inquiry through the European surveillance portal (EpiPulse), WGS analysis of food isolates sampled from smoked salmon in an EU country identified the outbreak strain. The samples were traced back to a smoked salmon supplier in the UK that distributed salmon products to the supermarket chain reported by one of the cases.

Two further cases were identified in 2021, 7 more in 2022 and 3 in 2023. All had underlying comorbidities or risk factors for listeriosis, all were non-pregnancy associated and where it was possible to complete a standardised questionnaire with the case or next of kin, all reported consuming smoked salmon products .

National outbreak of listeriosis in England associated with semi-soft cheese, 2022 to 2023

In March 2023, an outbreak of listeriosis was detected in England after 3 people were infected with the same strain of L. monocytogenes, one of whom died. All 3 cases had underlying comorbidities or risk factors for listeriosis and all reported consumption of the same brand of pasteurised rind washed semi-soft cheese. The affected cheese products were recalled and production immediately ceased voluntarily at the implicated producer, with no further cases having occurred following this action.

Conclusions

Listeriosis remains a rarely reported disease in England and Wales, with 0.29 cases per 100,000 population. The outcome of listeriosis in pregnancy remains severe with 23.1% resulting in miscarriage or stillbirth. Mortality amongst non-pregnancy cases was lower compared to the previous 5-year median (2016 to 2019 and 2022), with a case fatality rate of 21.6% compared to a median of 29.4%. Four national outbreaks were traced back to high-risk foods for listeriosis, supported by the linkage of cases to contaminated food through the analysis of strain relatedness using WGS data.

The number of listeriosis cases in 2023 in England and Wales has increased compared to 2022 but remained at a comparable level with the years preceding the COVID-19 pandemic. While the reason for the decrease in case reporting for 2020 and 2021 is uncertain, it is possible that the impact of the COVID-19 pandemic and consequently its effects upon ascertainment and incidence of listeriosis cases contributed to changes in case reporting in these years when compared to the 5 year median calculated from the 5 years prior to the pandemic (2015 to 2019).

As a predominantly foodborne infection, awareness of high-risk foods for vulnerable groups is vital. While the low number of reported cases complicates the interpretation of trends and any comparisons with previous years, it remains important that cases of illness and clusters of disease continue to be monitored and investigated to inform the continued risk assessment of the food chain and implementation of control measures to protect public health.

Data sources

Data sources are:

• National enhanced surveillance system for listeria monocytogenes infection in England and Wales
• Second Generation Surveillance System (SGSS)
• Gastro Data Warehouse (GDW)

Acknowledgements

We are grateful to the microbiologists, health protection and environmental health specialists who have contributed data and reports to the national surveillance system, as well as providing support to outbreak investigations.

We also thank our colleagues in the:

• Gastrointestinal Bacteria Reference Unit (GBRU) for providing the reference laboratory services and laboratory surveillance functions and expertise
• UKHSA Local Public Health Laboratories and Food Water and Environmental Microbiology Services for providing a surveillance function for GI pathogens and testing of food and environmental samples routinely and during outbreak investigation
• UKHSA health protection and field service teams, as well as local authorities for their contributions to incident investigations

References

1.       Swaminathan B and GernerSmidt P. ‘The epidemiology of human listeriosis’ Microbes and Infection 2007: volume 9, issue 10, page 123,643

2.       European Food Safety Authority and European Centre for Disease Prevention and Control. The European Union One Health 2022 Zoonoses Report. European Food Safety Authority Journal 2023: volume 21, issue 12, page 8,442

3.       Allerberger F and Wagner M. ‘Listeriosis: a resurgent foodborne infection’ Clinical Microbiology and Infection 2010: volume 16, issue 1, page 1,623

4.       Commission implementing decision (EU) 2018/945 of 22 June 2018 on the communicable diseases and related special health issues to be covered by epidemiological surveillance as well as relevant case definitions. Official Journal 2018: L170, page 28