Research and analysis

Listeriosis in England and Wales: summary for 2021

Updated 5 February 2024

Applies to England and Wales

Main points for 2021

This report summarises the number, demographics and clinical outcomes of confirmed cases of listeriosis in England and Wales in 2021:

• in 2021, 160 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 lower in men than women, but for the age groups 70 to 79 and 80 years and over reported cases among men were higher compared to women
• pregnancy associated infections accounted for around a fifth of all reported cases and 20.7% of pregnancy associated cases (where known) resulted in still birth or miscarriage
• among non-pregnancy associated cases of listeriosis, death was reported for 22 cases (17.5%), of whom 12 (54.5%, or 9.5% 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 (0.16 per 100,000 population) and the highest in London (0.39 cases per 100,000 population)
• there were 3 listeriosis outbreaks investigated in England and Wales in 2021, including a national outbreak associated with smoked fish

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 1 to 70 days after consuming food contaminated with L. monocytogenes (1).

Compared to other foodborne pathogens, infections in humans are relatively rare, with 2,183 confirmed cases of listeriosis (rate per 100,000 population of 0.49) reported across 27 EU member states in 2021 (2), and a median of 160 cases in England and Wales each year (2012 to 2021). 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 2021.

Methods

National surveillance of listeriosis in England and Wales is coordinated by the Gastrointestinal Infections and Food Safety (One Health) Division (GIFSOH) at the UK Health Security Agency (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 are collected via a standardised questionnaire (trawling questionnaire) administered to the case or a proxy via Health Protection Teams or the Local Authority. Clinical data are 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 gastroinrestinal 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 gastroninestinal 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 case numbers remains unclear, throughout this report 2020 data has been excluded when calculating the 5-year median used to compare 2021 data against, with the time period of 2015 to 2019 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:

• isolation of L. monocytogenes (or detection of nucleic acid) from a normally sterile site

or:

• 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.

Sub-classification of confirmed listeriosis cases

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 relate to confirmed cases reported to the national enhanced surveillance system for L. monocytogenes infection during the period 2012 to 2021.

Population and deaths data were sourced from the Office for National Statistics (ONS). Mid-year 2021 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 2021, 160 cases of listeriosis were reported to national surveillance in England and Wales (Figure 1), representing a 1.9% increase in reported cases compared to the previous 5-year median (n=157, 2015 to 2019). The crude incidence of listeriosis in 2021 was 0.27 cases per 100,000 population (95% CI: 0.23 to 0.31), compared to a crude incidence of 0.24 cases per 100,000 in 2019 (95% CI: 0.21 to 0.29).

Figure 1. Annual cases and crude incidence rate of listeriosis reported in England and Wales, 2012 to 2021

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 lower in males than females (Table 1, Incidence rate ratio (IRR): 0.87, 95% CI: 0.63 to 1.21), but higher in males aged 70 to 79 compared to females aged 70 to 79 (Figure 2, IRR: 1.98, 95% CI: 0.96 to 4.26). Incidence was also higher in males compared to females in the 80 and over age group.

Of the 35 cases in the 10 to 19, 20 to 29 and 30 to 39 age groups, 28 were female, of which 24 cases were associated with pregnancy (range: aged 19 to 39).

Table 1. Sex-specific incidence rates of listeriosis reported in England and Wales, 2021

Gender	Reported cases	Incidence rate	95% CI
Male	73	0.25	0.20 to 0.31
Female	87	0.29	0.23 to 0.35

Figure 2. Age-specific incidence of listeriosis in England and Wales, stratified by sex, 2021*

*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 2021, over a fifth of cases were pregnancy associated (n=34, 21.3%), which was higher than previous years (10.2% to 19.5% between 2012 and 2019).

Amongst pregnancy associated cases, where known (23 of 29), 79.3% of pregnancies resulted in live births and 20.7% resulted in still birth or miscarriage.

Of pregnancy associated cases of listeriosis in England and Wales in 2021 67.6% resulted in live births, 8.8% resulted in still births, 8.8% in miscarriage and in 14.7% the outcome was unknown (Figure 3).

Figure 3. Pregnancy associated cases of listeriosis in England and Wales in 2021

Mortality rate

In 2021, there were 22 deaths among 126 non-pregnancy cases reported to national surveillance (17.5%), compared to the previous 5 year median calculated from the years of 2015 to 2019 (39.2%, range: 28.5% to 45.4%).

Of the 22 deaths, 12 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 9.5%, compared to 12.8% in 2019.

Regional distribution of cases

The incidence rate of listeriosis varied geographically (Table 2; Figure 4). In England, London had the highest incidence rate of 0.39 cases per 100,000 population whilst Wales had the lowest incidence rate of 0.16 cases per 100,000 population.

Table 2. Regional incidence of listeriosis reported in England and Wales, 2021

Country	Region	Cases	Incidence rate	95% Confidence interval
England	East Midlands	10	0.20	0.10 to 0.38
	East of England	13	0.20	0.11 to 0.35
	London	34	0.39	0.27 to 0.54
	North East	10	0.38	0.18 to 0.69
	North West	21	0.28	0.18 to 0.43
	South East	20	0.22	0.13 to 0.33
	South West	18	0.32	0.19 to 0.50
	West Midlands	12	0.20	0.10 to 0.35
	Yorkshire and The Humber	17	0.31	0.18 to 0.50
Wales	Wales	5	0.16	0.05 to 0.38

Figure 4. Incidence rates of reported listeriosis cases in England and Wales, by region, 2021

Contains Ordnance Survey data © Crown copyright and database right 2023.

Seasonality

September was the peak month for listeriosis reporting in 2021, with no outbreak activity influencing the peak. In 2019 case numbers peaked in May, with one outbreak influencing the peak (Figure 5).

Figure 5. Seasonal trend of reported listeriosis cases in England and Wales (2019 to 2021)

Incidents of listeriosis

There were 3 outbreaks (comprising 2 or more linked cases) of listeriosis investigated in England in 2021 (Table 3). The source of the outbreaks were cooked beef tongue products, corned beef and smoked fish, all considered to be high-risk foods for listeriosis in vulnerable groups.

Table 3. Outbreaks of listeriosis in England investigated in 2021

Outbreak	Clinical cases	Time-frame	Region	Source of contamination
1	5	2020 to 2021	England	Smoked fish
2	4	2019 to 2021	England	Corned beef
3	3	2021	England	Cooked beef tongue products

‘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 2021

Between November and 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 (EPIS, now known as EpiPulse), whole genome sequencing (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. These 2 cases also had underlying comorbidities or risk factors for listeriosis, both were non-pregnancy associated and reported consuming smoked salmon products.

Conclusions

Listeriosis remains a rarely reported disease in England and Wales, with 0.27 cases per 100,000 population. The outcome of listeriosis in pregnancy remains severe with over a fifth resulting in miscarriage or stillbirth. Mortality amongst non-pregnancy cases was lower compared to the years preceding the COVID-19 pandemic (2015 to 2019), with a case fatality rate of 17.5% compared to a median of 39.2%. Three 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 2021 in England and Wales has returned to comparable levels with the years preceding the COVID-19 pandemic. While the reason for the decrease in case reporting for 2020 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 the 21% decrease in case reporting in 2020 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

• National Enhanced Surveillance System for Listeria monocytogenes infection in England and Wales
• Second Generation Surveillance System (SGSS)
• Gasto 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, GernerSmidt P. The epidemiology of human listeriosis. Microbes and Infection 2007: volume 9, issue 10, page 1,236 to 1,243

2. European Food Safety Authority and European Centre for Disease Prevention and Control. The European Union One Health 2021 Zoonoses Report. EFSA Journal 2022: volume 20, issue 12:7666

3. Allerberger F, Wagner M. Listeriosis: a resurgent foodborne infection. Clinical Microbiology and Infection 2010: volume 16, issue 1, page 16 to 23

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