Legionellosis in residents of England and Wales: 2017 to 2023 report
Published 21 November 2024
Applies to England and Wales
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Main points
The main messages of this report are that:
- 604 confirmed cases of Legionellosis were reported to the National Enhanced Legionnaires’ Disease Surveillance Scheme (NELSS) in England and Wales in 2023; all cases were identified as Legionnaires’ disease (LD), no cases of Pontiac fever (PF) or non-pneumophila Legionellosis (NPL) were reported
- 388 cases (64.2%) were reported in individuals aged 60 years and over in 2023 – an increase of 141 from 2022 – and 424 cases (70.2%) were male, an increase of 139 from 2022
- the estimated incidence of Legionnaires’ disease in 2023 was 1.0 (95% confidence interval: 0.9 to 1.1) per 100,000 population in England and 1.4 (95% confidence interval: 1.0 to 1.8) per 100,000 population in Wales
- the crude fatality rate (CFR) of Legionnaires’ disease cases with an onset of symptoms in 2023 was estimated to be 3.1% (95% confidence interval: 2.0% to 5.0%), approximately in line with the last 10 years
- most Legionella exposures occurred in the community, accounting for 379 cases (62.7%); travel abroad was associated with 212 cases (35.1%), while 13 cases (2.2%) were linked to healthcare settings (healthcare-associated infections)
- 486 cases (80.5%) in 2023 had at least one underlying health condition or risk factor; the most frequently reported risk factor was smoking which was present in 486 (80.5%) of Legionnaires’ disease cases
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Confirmed cases of Legionellosis in 2023
In 2023, 604 cases fulfilled the case definition (see the Glossary) for a confirmed case of Legionnaires’ disease in England and Wales. There were 0 cases reported with non-pneumonic legionellosis and 0 cases reported with Pontiac fever (Table 1). Overall, the annual number of legionellosis cases was higher in 2023 than in 2022 and has exceeded levels before the COVID-19 pandemic. This increase in cases may in part reflect changes in testing and diagnosis rather than solely increased disease burden.
Table 1. Number of confirmed cases of Legionellosis by disease type and year of symptoms onset, England and Wales, 2017 to 2023
| Disease type | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | 2023 |
|---|---|---|---|---|---|---|---|
| Legionnaires’ disease | 488 | 553 | 540 | 337 | 352 | 413 | 604 |
| Non-pneumonic legionellosis | 0 | 1 | 1 | 0 | 0 | 0 | 0 |
| Pontiac fever | 0 | 0 | 10 | 1 | 0 | 0 | 0 |
| Total legionellosis | 488 | 554 | 551 | 338 | 352 | 413 | 604 |
The remainder of this report will present descriptive analyses of cases that meet the clinical and microbiological criteria for a confirmed case of Legionnaires’ disease in England and Wales between 2017 and 2023.
Trends in Legionnaires’ disease in England and Wales, 2017 to 2023
Cases in 2023 followed the seasonal pattern, with a peak number of cases experiencing onset of symptoms between June and October 2023 (Figure 1). The seasonal peaks were still present when cases associated with travel abroad were removed.
Figure 1. Number of confirmed cases of Legionnaires’ disease by month and year of symptom onset, England and Wales, 2017 to 2023
Note: black lines represent the year in question, grey lines represent all other years.
Age and sex distribution
With the onset of the COVID-19 pandemic there was a decline in reported cases across all age groups. Since the pandemic, there has been an increase in cases returning to pre-pandemic levels (Figure 2). In 2023, 217 cases were reported in individuals aged 70 years and over, comprising 35.9% of reported cases. This is an increase of 99 cases since 2022 and an increase compared with pre-pandemic levels.
Figure 2. Number of confirmed cases of Legionnaires’ disease by year of symptom onset and age group, England and Wales, 2017 to 2023
Of the 604 cases of Legionnaires’ disease reported in 2023, 424 cases (70.2%) were reported in males and 180 (29.8%) reported in females. The overall male-to-female ratio was 2.4 to 1, corresponding with male sex being a known risk factor for Legionnaires’ disease.
Since 2017, more cases in males have been consistently reported relative to females, particularly in older age categories (Figure 3). Single year increases in male cases have been observed in certain age categories, for example those aged under 50 years in 2019 and those aged 60 to 69 years in 2018, both of which were not observed in females. By comparison, the number of female cases has remained relatively stable since 2017.
Figure 3. Number of confirmed cases of Legionnaires’ disease by age group and sex, England and Wales, 2017 to 2023
Geographical distribution
Despite a decrease in the case rate per 100,000 population during the pandemic years in England (2020 to 2021), the rate of Legionnaires’ disease increased in 2022 and 2023. A similar pattern was observed in Wales (Figure 4). Although the rate of Legionnaires’ disease detection was higher in Wales than in England in 2023, the overlapping error bars indicate that this difference might be due to random chance, not an actual difference.
Figure 4. Rate of confirmed cases of Legionnaires’ disease per 100,000 population, England and Wales, 2017 to 2023
Note: error bars represent 95% confidence intervals.
The number of Legionnaires’ disease cases with onset of symptoms reported by the nine UKHSA regional teams in 2023 ranged from 17 cases in the North East to 100 cases in the South East. The East and West Midlands had the joint highest case rate in 2023, with 1.2 cases per 100,000 population (95% confidence interval: 0.9 to 1.5), corresponding to 58 cases in the East Midlands and 71 cases in the West Midlands. The North East was the UKHSA region with the lowest number of cases (17) and the lowest rate of Legionnaires’ disease cases per 100,000 population (0.6 cases per 100,000 population; 95% confidence interval: 0.4 to 1.0).
Figure 5. Rate of confirmed cases of Legionnaires’ disease per 100,000 population by UKHSA regions in England, 2017 to 2023
Note: error bars represent 95% confidence intervals.
Category of exposure
The distribution of cases between principal categories of exposure remained relatively stable outside of the pandemic period (Figure 6). However, during this period (2020 to 2021) the proportion of cases in the community exposure category increased, while the proportion in the travel abroad exposure category decreased. This likely reflects travel restrictions imposed during the pandemic. In 2022 and 2023, with travel patterns returning to normal, the distribution of cases between principal categories of exposure returned to pre-pandemic levels. Note that cases classified under the community exposure category includes cases who travelled domestically within the UK.
Figure 6. Percentage of confirmed cases of Legionnaires’ disease by principal exposure category and year of onset, England and Wales, 2017 to 2023
Note: the community exposure category includes travel within the UK.
Risk factors
Underlying medical conditions such as immunosuppression, chronic respiratory diseases, and liver or kidney diseases, in addition to tobacco smoking, are well-known risk factors for Legionnaires’ disease .
In England and Wales, most Legionnaires’ disease cases (81.2%) from 2017 to 2023 had at least one underlying risk factor (Figure 7). There has been little change in the overall proportion of cases with one or more underlying medical conditions/risk factors for Legionnaires’ disease, between 2017 and 2023.
The most prevalent single risk factor from 2017 to 2023 was smoking. In 2023, 406 cases (67.2%) reported smoking. The most prevalent underlying health condition was diabetes. In 2023, cases were reported as diabetic (Figure 7).
Figure 7. Percentage of underlying medical conditions and risk factors reported in confirmed cases of Legionnaires’ disease, England and Wales, 2017 to 2023
Mortality
A decrease in the case fatality rate was observed in 2023 (Figure 8) despite an increase in cases. This reduction is likely due to a combination of factors rather than a single cause. This includes increased awareness of respiratory diseases following the COVID-19 pandemic, and improvements in Legionnaires’ disease diagnostic testing potentially contributed to earlier diagnoses and interventions.
The annual number of deaths reported among Legionnaires’ disease cases ranged from 19 in 2023 to 36 in 2018, with a median of 29 deaths between 2017 and 2023. In 2023, 19 deaths were reported giving an overall case fatality rate (CFR) of 3.1% (95% confidence interval: 2.0% to 5.0%).
In 2020 the CFR was notably higher than all other years in the 2017 to 2023 period. This may be because of the disruption to healthcare and diagnostic pathways during the COVID-19 pandemic.
Figure 8. Case fatality rate of Legionnaires’ disease, England and Wales, 2017 to 2023
Note: error bars represent 95% confidence intervals.
Among 2023 cases, 13 of the 19 Legionnaires’ disease-associated deaths were in males with a median age of 69 years. The remaining 6 were female with a median age of 78.5 years. The overall male-to-female mortality ratio was 2.2:1.
The number of deaths and the case fatality rate increase with age across both sexes when combining cases from 2017 to 2023 (Figure 9). Those aged under 50 years had the lowest estimated CFR for both males and females. Although the number of deaths was higher in males compared with females across age groups, the case fatality rates (CFRs) were similar between the sexes.
Figure 9. Number of Legionnaires’ disease deaths and case fatality rate (%) by age and sex, England and Wales, 2017 to 2023
Note: error bars represent 95% confidence intervals.
When stratifying the CFR for cases by category of exposure, the CFR in healthcare-associated cases was significantly higher compared with the CFR for other principal exposures (Table 2). By contrast, the CFR of those with travel abroad as a principal exposure was lower than other categories.
Table 2. Case fatality rates for confirmed cases of Legionnaires’ disease by principal category of exposure with 95% confidence intervals (CI), England and Wales, 2017 to 2023
| Exposure category | Number of cases | Number of deaths | Case fatality rate | Lower 95% CI | Upper 95% CI |
|---|---|---|---|---|---|
| Community [note 1] | 2,131 | 143 | 6.7% | 5.7% | 7.9% |
| Travel abroad | 1,056 | 35 | 3.3% | 2.4% | 4.6% |
| Healthcare-associated | 89 | 23 | 25.8% | 17.4% | 36.4% |
| Total | 3,276 | 201 | 6.1% | 5.3% | 7.0% |
Note 1: community exposure category includes travel within the UK.
Deprivation
The Index of Multiple Deprivation (IMD) and the Welsh Index of Multiple Deprivation (WIMD) are designed to measure multiple forms of deprivation across small geographic areas in England and Wales, respectively (see Glossary for details). Both indices rank areas into quintiles, with IMD quintile 1 representing the 20% of areas with the highest levels of deprivation, and quintile 5 representing the 20% of areas with the lowest levels of deprivation.
The most deprived areas (IMD quintiles 1 and 2) report a higher number of cases in both England and Wales, whereas fewer cases are reported in the least deprived quintiles (IMD 4 and 5) (Figure 10 and Table 3). The greater number of cases in the most deprived quintiles was not explained by either tobacco smoking, diabetes or older age.
In 2023, the 2 most deprived quintiles in England (IMD 1 and 2) had 267 confirmed cases, accounting for nearly half (47.7%) of all cases (Figure 10). While the number of cases decreased across all deprivation quintiles during the pandemic, the most deprived quintiles (IMD 1 & IMD 2) saw reductions in cases before returning to pre-pandemic levels by 2023.
Figure 10. Number of confirmed Legionnaires’ disease cases by Index of Multiple Deprivation (IMD) quintile, England, 2017 to 2023
Table 3. Number of confirmed Legionnaires’ disease cases by Welsh Index of Multiple Deprivation (WIMD) quintile, Wales, 2017 to 2023
| Welsh Index of Multiple Deprivation quintile | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | 2023 |
|---|---|---|---|---|---|---|---|
| 1 – most deprived | 6 | 6 | 9 | 5 | 6 | 7 | 11 |
| 2 | 4 | 4 | 9 | 4 | 8 | 4 | 11 |
| 3 | 11 | 8 | 11 | 4 | 7 | 8 | 4 |
| 4 | 5 | 9 | 7 | 5 | 9 | 3 | 6 |
| 5 – least deprived | 2 | 7 | 5 | 2 | 4 | 3 | 11 |
| Total | 28 | 34 | 41 | 20 | 34 | 25 | 43 |
Microbiology
Urinary antigen tests (UAT) remained the most frequently used diagnostic test among Legionnaires’ disease cases in England and Wales, with 3,166 (64.7%) positive UATs reported between 2017 and 2023 (Figure 11). In 2023, 576 positive urinary antigen tests detected Legionnaires’ disease. By comparison, the proportion of culture positive cases decreased from 12.8% in 2017 to 11.3% in 2023, while the proportion of cases positive by polymerase chain reaction (PCR) was similar in 2017 (20.2%) and 2023 (20.8%).
Figure 11. Number of Legionnaires’ disease Cases by diagnostic test, England and Wales, 2017 to 2023
Note: the number of tests can exceed the number of cases; a single case can have multiple tests.
In 2023, 591 cases were infected by L. pneumophila (97.8% of causative organisms) whereas 13 cases were non-pneumophilia (Table 4). Between 2017 to 2023, only 35 cases of L. longbeachae (1.1% of total cases) were identified. However, 10 of these cases died, L. longbeachae therefore has a case fatality rate of 28.6% much higher than the more common L. pneumophila, which has a case fatality rate of 5.8%.
Table 4. Number of Legionnaires’ disease cases by species test, England and Wales, 2017 to 2023
| Species | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | 2023 |
|---|---|---|---|---|---|---|---|
| L. pneumophila | 485 | 548 | 539 | 327 | 341 | 406 | 591 |
| L. longbeachae | 2 | 4 | 1 | 3 | 9 | 6 | 10 |
| L. bozmanii | 0 | 0 | 0 | 0 | 1 | 0 | 0 |
| L. wadsworthii | 0 | 0 | 0 | 1 | 0 | 0 | 0 |
| Not established | 1 | 1 | 0 | 6 | 1 | 1 | 3 |
| Total | 488 | 553 | 540 | 337 | 352 | 413 | 604 |
In line with previous years, the greatest proportion of L. pneumophila cases were of unknown serogroup (71.7%) (Table 5). This is expected, as UATs are widely used, whereas respiratory samples, essential for accurate serogrouping via PCR and culture, made up only 32.1% of tests in 2023. Although UATs are primarily designed to detect L. pneumophila serogroup 1, some test kits may also produce positive results for other serogroups. Therefore, only cases confirmed through additional methods like PCR testing are classified as serogroup 1.
Although numbers are low, there has been an observed increase in confirmed non-pneumophila Legionella in recent years. This increase has been primarily due to L. longbeachae.
Table 5. Number of confirmed Legionnaires’ disease cases by causative organism, England and Wales, 2017 to 2023
| Causative organism | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | 2023 |
|---|---|---|---|---|---|---|---|
| L. pneumophila serogroup 1 | 150 | 169 | 175 | 113 | 132 | 126 | 164 |
| L. pneumophila serogroups 2 to 14 | 2 | 4 | 3 | 1 | 2 | 8 | 3 |
| L. pneumophila serogroup unknown | 333 | 375 | 361 | 213 | 207 | 272 | 424 |
| Non-pneumophila | 3 | 5 | 1 | 10 | 11 | 7 | 13 |
| Total | 488 | 553 | 540 | 337 | 352 | 413 | 604 |
Culture and PCR testing is undertaken when a case tests positive for Legionella infection locally and a lower respiratory tract (LRT) sample is collected from the individual and sent to the UKHSA reference laboratory in Colindale. Culture and PCR testing enables the detection of L. pneumophila non-serogroup 1 and non-pneumophila Legionella species. Additionally, PCR can identify a strain’s sequence type (ST), aiding in pinpointing the infection source to support effective public health action. This highlights the importance of sending respiratory specimens to reference labs.
Across England and Wales, 236 (39.1%) of the confirmed cases had lower respiratory samples (Table 6). Among these, 187 (79.2%) tested positive by either culture or PCR, with 140 (74.9%) of these cases having a complete sequence type (ST) or a partial sequence-based type (SBT) determined from the tests.
Table 6. Number of confirmed Legionnaires’ disease with a sequence type (ST) or a partial sequence-based type (SBT), England and Wales, 2023
| Geography | Confirmed cases | Respiratory samples | Culture and/or PCR positive cases | Cases with ST or partial SBT |
|---|---|---|---|---|
| England | 561 | 222 | 175 | 130 |
| Wales | 43 | 14 | 12 | 10 |
| Total | 604 | 236 | 187 | 140 |
The most common ST identified in clinical samples from cases with onset of symptoms during 2023 was ST 47 (5.7%), followed by ST 62 (4.3%). ST 47 has been the most common ST identified in the last 7 years (Table 7).
Table 7. Top 10 most prevalent sequence types of L. pneumophila identified in clinical isolates from confirmed cases of Legionnaires’ disease, England and Wales, 2017 to 2023
| Sequence type | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | 2023 | Total |
|---|---|---|---|---|---|---|---|---|
| 47 | 14 | 20 | 16 | 3 | 10 | 9 | 5 | 77 |
| 42 | 6 | 10 | 17 | 2 | 6 | 5 | 2 | 48 |
| 62 | 7 | 7 | 6 | 2 | 3 | 7 | 5 | 37 |
| 1 | 8 | 4 | 7 | 5 | 2 | 4 | 1 | 31 |
| 37 | 6 | 5 | 7 | 1 | 4 | 2 | 4 | 29 |
| 82 | 4 | 6 | 3 | 3 | 6 | 5 | 1 | 28 |
| 74 | 3 | 8 | 3 | 2 | 0 | 5 | 4 | 25 |
| 23 | 4 | 3 | 2 | 6 | 2 | 3 | 1 | 21 |
| 48 | 1 | 6 | 2 | 1 | 4 | 3 | 0 | 17 |
| 46 | 3 | 2 | 2 | 3 | 1 | 1 | 2 | 14 |
| Other | 73 | 90 | 109 | 76 | 83 | 95 | 115 | 641 |
| Total [note 2] | 129 | 161 | 174 | 104 | 121 | 139 | 140 | 968 |
Note 2: totals exceed the number of cases as an individual can be infected by multiple sequence types (STs).
In 2023, 50 unique serotypes were found across 78 cases. Of these cases, 54 were associated with community exposure and accounted for 69.2% of serotyped cases. This is in line with community exposure being the more prevalent exposure category.
In 2023, the joint most prevalent serotypes for community exposure (37, 47, 62, 74) accounted for 29.6% of serotyped community exposures cases. A total of 22 (28.2%) serotyped cases were associated with travel abroad. The most prevalent serotype associated with travelling abroad was 42 and accounted for 9.1% of these exposure cases. Only 2 (2.6%) serotyped cases were associated with healthcare-associated exposure (75, 477).
Cases and clusters
A cluster or outbreak is defined as two or more cases linked by proximity in residence or work and timing (within 6 months) that may warrant further investigation (see the Glossary for full details). The number of cases associated with clusters and outbreaks can vary from year to year depending on the characteristics of the individual situation.
Overall, 121 cases in 2023 were linked to one or more additional cases. This accounts for 20.0% of total cases observed in 2023 and represents an increase of 9.3% since 2022.
The number of individuals linked to clusters and outbreaks identified in 2023 has risen compared with the pandemic period (2020 to 2021) but has not yet returned to pre-pandemic levels (Table 8). The decrease during the pandemic period (2020 to 2021) is explained by fewer travel-associated cases both abroad and within the United Kingdom.
In 2023, the number of clusters and outbreaks was greatest in the community exposure category with 57 (47.1%). Healthcare-associated outbreaks have remained relatively stable between 2017 to 2023.
Table 8. Number of cases involved in outbreaks or clusters of Legionnaires’ disease by category of exposure, England and Wales, 2017 to 2023
| 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | 2023 | |
|---|---|---|---|---|---|---|---|
| Community | 35 | 69 | 47 | 55 | 41 | 34 | 57 |
| Healthcare-associated | 4 | 13 | 9 | 8 | 5 | 9 | 7 |
| Travel UK | 14 | 22 | 14 | 3 | 22 | 9 | 13 |
| Travel abroad | 88 | 53 | 50 | 10 | 5 | 35 | 44 |
| Total [note3] | 141 | 157 | 120 | 76 | 74 | 87 | 121 |
Note 3: totals do not include 1 case where exposure type is unknown.
Travel-associated Legionnaires’ disease
The number of Legionnaires’ disease cases associated with travel abroad decreased drastically with the COVID-19 pandemic due to international travel restrictions but has since returned to pre-pandemic levels. (Figure 12). Similarly, the number of cases associated with travel within the UK decreased slightly in 2020, likely because of pandemic restrictions.
Figure 12. Number of confirmed cases of Legionnaires’ disease associated with travel by year of onset of symptoms, England and Wales, 2017 to 2023
Glossary and case definitions
Glossary
Case fatality rate
Case fatality rate is the proportion of people who died from Legionnaires’ disease among all those who tested positive:
Case fatality rate = total deaths from LD/total confirmed cases multiplied by 100.
Confidence interval
A confidence interval is a measure of the degree of uncertainty in an estimate based on a sample distribution. 95% confidence intervals indicates that if we repeated the study many times, 95% of the confidence intervals would contain the true population value. Wider confidence intervals indicate more uncertainty in the estimate. Overlapping confidence intervals indicate that there may not be a true difference between estimates.
Cluster
Two or more cases that initially appear to be linked by area of residence or work, including healthcare or other type of community setting and which have sufficient proximity in dates of onset of illness (for example, 6 months) to warrant further investigation. This is a working definition, and the decision to follow up cases is made locally. Other considerations include:
- the area of residence should take account of population size and density when investigations are planned
- consideration should be given to convening an incident control team if a cluster is identified
- if, after investigation, no common exposures to a potential source of infection are identified for the cases, other than the links mentioned above, then they should be classified as sporadic community acquired cases
Outbreak
Two or more cases where the onset of illness is closely linked in time and where there is epidemiological evidence of a common source of infection, with or without microbiological evidence.
An incident control team should always be convened to investigate outbreaks.
It should be noted that the definitions for cases, clusters and outbreaks shown here were those that were in effect during 2023.
Deprivation
The Index of Multiple Deprivation (IMD) is a composite measure used to assess levels of deprivation in specific geographic areas in England, based on factors such as income, employment, health, education, housing, crime, and environment. It ranks areas in terms of relative deprivation, providing insight into socio-economic challenges.
The Welsh Index of Multiple Deprivation (WIMD) serves a similar purpose in Wales, but the methods, data sources, and weightings for different domains can vary between IMD (used in England) and WIMD. This makes direct comparisons between the two indices problematic. Differences in how deprivation is defined, measured, and weighted for each region reflect local socio-economic conditions, and as a result, the rankings produced by IMD and WIMD are not directly comparable across national boundaries.
Case definitions
Confirmed case of Legionnaires’ disease
A clinical or radiological diagnosis of pneumonia with laboratory evidence of one or more of the following:
- isolation (culture) of Legionella species from a clinical lower respiratory tract specimen
- detection of Legionella pneumophila antigen in a urine specimen (using a urine antigen test)
- detection of Legionella species nucleic acid (such as via PCR) in a lower respiratory tract specimen (such as sputum, bronchoalveolar lavage (BAL))
Probable case of Legionnaires’ disease (following declaration of an outbreak only)
A case with clinical or radiological diagnosis of pneumonia but no microbiological evidence for confirmation of Legionella infection (above) is considered a probable case where the location and onset dates meet the outbreak-specific case definition for the exposure category (see the Category of exposure section).
Category of exposure
The national surveillance scheme supports the public health investigation of sources of infection for cases of legionellosis; this allows control measures to be implemented to prevent further cases. Potential sources of exposure can broadly be divided into 3 categories: community-acquired, healthcare-associated and travel-associated. Definitions for these categories and sub-categories are as follows.
Community-acquired
A case where the most likely potential source of infection is in the community or where there is no evidence of travel or healthcare-associated exposure during the 2 to 10 days before symptom onset.
Travel-associated
A case who either stayed overnight or was considered to have had significant exposure to the water system during a visit (for example the use of spa and leisure facilities) to holiday or business accommodation during the 2 to 10 days before symptom onset. Accommodation sites include hotels, campsites, ships, rented holiday apartments or other tourist facilities.
Travel associated cases can be subdivided into travel abroad, and travel within the UK.
Healthcare-associated
A case who either stayed overnight or was considered to have had significant exposure to a healthcare-associated premises for some or all the 2 to 10 days prior to onset of symptoms.
Healthcare-associated cases can be sub-categorised based on the type (whether the site has been associated to previous cases) and amount of exposure (time spent at the site) the case was subjected to in the 2 to 10 days prior to onset of symptoms.
Definitions for these sub-categories are:
Type A: A case who stayed overnight in healthcare-associated premises for the entire 2 to 10 days before symptom onset.
Type B: A case who stayed overnight or worked as a regular employee in healthcare-associated premises for any of the 2 to 10 days before symptom onset AND the premises have been associated with any previous cases of Legionnaires’ disease or other legionellosis (if the previous cases occurred less than 2 years previously, this should be investigated as a cluster).
Type C: A case who stayed overnight or worked as a regular employee in healthcare-associated premises for any of the 2 to 10 days before symptom onset AND the premises have not been associated with any previous cases of Legionnaires’ disease or other legionellosis.
Type D: A case who visited healthcare-associated premises during the 2 to 10 days before symptom onset, including as an occasional, transient worker but did not stay overnight and did not work at the premises as a regular employee; and a risk assessment indicates investigation of the case as healthcare-associated due to, for example, frequency of visits, nature of treatment, previous cases associated with the site or documented exposure to a source known to be contaminated.
National Enhanced Legionnaires’ disease surveillance scheme (NELSS)
The National Enhanced Legionnaires’ disease Surveillance Scheme (NELSS) for residents of England and Wales was established in 1980 to collect enhanced surveillance data on all cases of Legionnaires’ disease. The scheme is managed by the Acute Respiratory Infections team (ARI) within the UK Health Security Agency (UKHSA).
The primary objectives of NELSS are to:
- detect clusters and outbreaks of Legionella infection.
- identify sources of infection to aid colleagues to apply control measures and prevent further cases.
- disseminate Legionella surveillance data and intelligence to stakeholders involved in the investigation and management of cases in the course of their duty to protect public health.
Data sources and methodology
Population estimates
Population estimates used to calculate incidence rates are sourced from the Office for National Statistics data where available. Estimates for 2023 were unavailable at the time of publication, therefore 2022 estimates were used. If the population grew between 2022 to 2023, incidence rates reported would likely be overestimates, if the population shrinks the incidence would be an underestimate.
Data limitations
Demographic characteristics
Sex is reported as male or female.
Age and age groups were derived from the date of notification and date of birth. Age groupings were chosen to reflect the increased risk of Legionnaires’ disease in older populations and more severe outcomes. The specific grouping additionally captures more granular insights while additionally providing disclosure control.
Background information
Legionellosis is a spectrum of diseases caused by Legionella bacteria. Illness can range from mild (non-pneumonic Legionellosis or Pontiac fever) through to Legionnaires’ disease, which is a form of atypical pneumonia that can be severe and is potentially fatal. Anyone can get infected by the bacteria but certain population groups – such as those aged 50 years and above, smokers, and individuals with weakened immune systems – are more vulnerable. Legionnaires’ disease is a seasonal disease with activity in England and Wales increasing during the summer months, usually reaching a peak between July and September.
Legionella bacteria typically inhabit natural water systems such as streams, rivers, and lakes. However, Legionella bacteria are also able to survive in artificial water systems, for example cooling towers, evaporative condensers, spa pools and hot and cold-water systems. Such human-made water systems mimic the organism’s natural habitat thereby providing an ideal environment for growth. Some species, such as Legionella longbeachae, can also be found in soil, compost, and potting mixes.
Legionellosis is typically contracted when people inhale small water droplets containing Legionella bacteria. The likelihood of illness depends on the concentrations of Legionella in the water source, the production and dissemination of aerosols, host factors such as age and pre-existing health conditions and the virulence of the strain of Legionella.
In the UK, the principal route of infection is likely through direct exposure to aerosols generated and dispersed from colonised human-made sources; however, in many cases, the source of the infection is not identified. A colonised water system which is not appropriately managed has the potential to be a source of major outbreaks.
Prevention of legionellosis is principally through the implementation of stringent control and management of human-made water systems to prevent these systems from becoming contaminated. For this reason, there is legislation governing the use and management of these systems.
Further information and contact details
Feedback and contact information
For feedback and comments on this report, please contact legionella@ukhsa.gov.uk
Official statistics
Our statistical practice is regulated by the Office for Statistics Regulation (OSR). OSR sets the standards of trustworthiness, quality and value in the Code of Practice for Statistics that all producers of official statistics should adhere to. You are welcome to contact us directly by emailing legionella@ukhsa.gov.uk with any comments about how we meet these standards. Alternatively, you can contact OSR by emailing regulation@statistics.gov.uk or via the OSR website.
UKHSA is committed to ensuring that these statistics comply with the Code of Practice for Statistics. This means users can have confidence in the people who produce UKHSA statistics because our statistics are robust, reliable and accurate. Our statistics are regularly reviewed to ensure they support the needs of society for information.
UKHSA will next be conducting a formal review of these statistics in spring 2025. Following this review, an implementation plan will be developed to continue to improve the trustworthiness, quality, and value of these statistics. Key continuous improvements made will be highlighted within future releases of these statistics for transparency.