Legionellosis is associated with two clinically and epidemiologically distinct illnesses:
- Legionnaires' disease (characterised by fever, myalgia, cough, and pneumonia);
- Pontiac fever (a milder illness without pneumonia).
Legionnaires' disease is an uncommon and potentially fatal form of pneumonia caused by Legionella bacteria. Legionella bacteria are distributed widely in both natural and artificial water supplies. In most cases, disease is caused by the inhalation of water containing Legionella. Sources include showers, air conditioning cooling towers, humidifiers, whirlpool spas and fountains.
Legionella are fastidious gram-negative bacilli. They grow best in warm water (25-45ºC) which is stagnant and are associated with bio-film and amoebae. However, Legionella have been found to survive in temperatures ranging from 6oC to 60oC.
The majority of cases of legionellosis are caused by Legionella pneumophila. There are 16 recognised serogroups of L. pneumophila, of which serogroup 1 (Sg1) is the most common. In addition there are over 50 other species of Legionella that have been shown to cause human disease. There is evidence of previous exposure to Legionella species in a high proportion of the population, as determined in seroprevalence studies in blood-donor blood. However, evidence of previous exposure to L. pneumophila Sg1 is more uncommon.
The incidence of legionellosis in Scotland is low, and there are usually between 20 and 40 cases per year, the majority of which are contracted overseas. Older age and male gender are both associated with increased risk, as is smoking and underlying respiratory disease.
Legionellosis surveillance in Scotland
Health Protection Scotland has undertaken enhanced surveillance of Legionella infections in conjunction with the Scottish Haemophilus, Legionella, Meningococcus and Pneumococcus Reference Laboratory (SHLMPRL). The purpose of this enhanced surveillance is to characterise the Legionella species and identify likely sources of infection.
In April 2009, the former Scottish Legionella Reference laboratory merged with the Scottish Meningococcal Pneumococcal Reference laboratory to become the Scottish Haemophilus, Legionella, Meningococcus and Pneumococcus Reference Laboratory (SHLMPRL).
The majority of cases of legionellosis are detected in hospitalised patients. Patient samples may be tested locally, but are not confirmed as positive until tested and confirmed at the reference laboratory. Once cases are confirmed, the NHS board where the patient is being treated is requested to complete an enhanced surveillance report form. This collects demographic data, information on clinical presentation and risk factors for each patient, and detailed information on travel away from home during the incubation period of the illness (2-10 days prior to onset of symptoms). All cases suspected to be travel-related are immediately reported to ELDSNet.
HPS publishes two-yearly summaries of legionellosis in Scotland. Readers are referred to previous reports for more detailed information about past years.1 This report provides an update on cases of legionellosis in Scotland to the end of December 2010.
Legionnaires' disease surveillance in Europe
Only the severe form of legionellosis, Legionnaires' disease, is monitored at a European level. This monitoring is administered by the European Centre for Disease Control (ECDC), through the European Legionnaires' Disease Surveillance Network (ELDSNet).2,3
- Legionnaires' disease is monitored in two ways:
- annual submission of datasets of all Legionnaires' disease cases in member states;4,5
immediate reporting of travel-related cases of Legionnaires' disease as they are diagnosed by member states.6 This aims to improve knowledge and information on the epidemiological and microbiological (both clinical and environmental) aspects of Legionnaires' disease, to locate sources of infection and to prevent further cases of infection.
ECDC provides case definitions for Legionnaires' disease, which all member states use. In local outbreaks these definitions may be modified to be more inclusive, allowing more rapid identification of an infection source. For the purposes of routine surveillance and reporting to ECDC, HPS and SHLMPRL use the following definitions (taken from ECDC).7
A confirmed case of Legionnaires' disease must have clinically defined pneumonia and at least one of the following three laboratory criteria:
- isolation of Legionella species from respiratory secretions or any normally sterile site;
- detection of Legionella pneumophila antigen in urine;
- Legionella pneumophila serogroup 1 specific antibody response.
Probable cases of Legionnaires' disease must have clinically defined pneumonia and at least one of the following laboratory criteria:
- detection of Legionella pneumophila antigen in respiratory secretions or lung tissue;
- detection of Legionella species nucleic acid in a clinical specimen;
- specific antibody response to Legionella pneumophila non-serogroup 1 or other Legionella species;
- Single high titre of specific antibody for Legionella pneumophila serogroup 1, other Legionella pneumophila serogroups or other Legionella species.
Cases are linked epidemiologically if at least one of the following criteria are met:
- environmental exposure;
- exposure to the same common source.
The case definition detailed above is the latest version available on the ELDSNet web pages, taken from the Commission Decision of 28 April 2008, when ECDC case definitions were reviewed and revised.7 This differs from the case definitions in the current Health Protection Guidance,8 which incorporates the old ECDC case definitions. This update in definitions reflects changes to laboratory methods for detection of Legionella bacteria. HPS and SHLMPRL use the revised ECDC definitions for normal surveillance activities.
Characteristics of cases
The number of SHLMPRL-confirmed cases of legionellosis diagnosed in Scotland between 1995 and 2010 is shown in Figure 1. In 2009 there were 25 cases, of which 23 were confirmed and two were probable cases, and in 2010 there were 16 cases, of which 14 were confirmed and two were probable. In 2010 there were an additional two cases of legionellosis in Scottish residents that were diagnosed and treated overseas - these are not included in the data in this report.
The annual incidence rate of legionellosis in Scotland was 4.8 cases per million population in 2009 and 3.4 in 2010. This incidence remains relatively low when compared to the rest of Europe, which had an average incidence of 11.8 per million population for the period 2007-084 and 11.2 per million population in 2009.5
Cases of legionellosis were diagnosed in all quarters in 2009 and 2010, with the highest numbers of cases diagnosed in quarter 3 (July to September) in both years, see Figure 2. This is in line with patterns at a European level where the largest number of cases is diagnosed in August or September.4,5
Table 1 shows the breakdown by sex of cases in Scotland from 1995 to 2010. In 2009, 68% of cases were male and in 2010 50% of cases were male. The general pattern of higher incidence amongst males is typical for Legionnaires' disease, with on average twice as many male cases in Scotland than female cases. In Europe in 2009, 72.6% of cases were male and 27.4% were female.5
Legionellosis is more common in older age groups. Figure 3 shows the age band distribution of cases in the years 1995-2010. In 2009 all cases were older than 40 years and in 2010 more than 90% of cases were in those older than 40 years. During 2009 and 2010, the most common age range of cases was 60-69 years (16 cases, 39%). Figure 4 compares numbers of cases in each age band for 2001-02, 2003-04, 2005-06, 2007-08 and 2009-10.
Smoking and lung disease are known risk factors for legionellosis. For the cases in 2009 and 2010, 54% were smokers, 27% did not smoke and for 19% of cases smoking status was not known or not recorded. This compares to 44% smokers in 2007-08 (35% not smokers, 21% smoking status not known or not recorded).
Immunocompromised individuals have increased risk of infection with a variety of pathogens. For the cases in 2009 and 2010, 20% were immunocompromised, 75% were not immunocompromised and for 5% this was not known or not recorded. This compares to 24% immunocompromised in 2007-08 (63% not immunocompromised and 13% not known or not recorded).
All of the reported cases in 2009 and 2010 were Legionnaires' disease, with no cases of Pontiac fever diagnosed. In 2009, 24 cases presented with pneumonia (96%) and in 2010, 14 cases presented with pneumonia (87%). Other clinical symptoms for these cases included fever, shortness of breath, cough, headache, muscle pain, lethargy, confusion, nausea, vomiting, diarrhoea and collapse.
There were three recorded deaths among cases of legionellosis reported in 2009 and none in 2010 (Table 2). Mortality data captured through this surveillance scheme does not infer Legionella infection as the cause of death recorded on the death certificate. For 2009 and 2010 the mean case fatality rate was 7.3%. This is lower than the rate seen in Scotland for 2007-08 (10.3%) but similar to the rate seen previously in 2004-06 (7.5%). However, the small number of deaths makes case fatality rates subject to large annual variation and are therefore difficult to interpret.
Suspected source of infection
Of the 41 cases reported in 2009 and 2010 the majority (66%) were travel-related; 27% were community-acquired; 2% were hospital-acquired; and 5% were of unknown origin. This distribution is shown in Figure 5, which shows an increase in the number of community-acquired cases from the previous reporting period 2007-08 (75% travel-related, 15% community-acquired; 1% hospital-acquired, 9% unknown origin).
The countries associated with travel-acquired cases are listed in Table 3. For 2009 and 2010 travel to Spain, Greece and travel within the UK (including Scotland) accounted for the most travel-related cases.
In 2009, 15 travel-related cases were reported to EWGLI (European Working Group for Legionella Infections now replaced by ELDSNet). Five have been attributed to clusters – that is where travellers from different places are suspected to have contracted legionellosis from the same source accommodation in a two-year period. In 2010, of the 12 cases reported to EWGLI, eight cases were linked with six different clusters, with two cases attributed to each of two of these clusters.
Of the 11 community-acquired cases in 2009 and 2010 only two cases had strongly suspected specific sources based on environmental testing. One case with case L. pneumoniae Sg1 subtype Philadelphia had the same strain isolated from their home hot water tap. One case with Legionella longbeachae had the same strain isolated from branded, bagged potting compost that they had been using. These cases and related cases where a source was suspected but typing could not be fully confirmed are discussed in the laboratory section of this report. Cooling towers were not suspected in any of the community acquired cases. Due to the widespread and ubiquitous environmental distribution of Legionella bacteria, a specific source for most sporadic cases of community-acquired disease is never identified.
HPS was not informed of any Scottish outbreaks of epidemiologically linked cases of Legionnaires' disease or Pontiac fever in 2009 and 2010.
Only one case of hospital-acquired legionellosis was reported in 2009 and 2010. For this case, water sampling of the ward identified L. pneumoniae Sg1 strain Olda and this was confirmed in the patient. This is discussed in more detail below.
Cases of legionellosis described in travellers to Scotland
In 2009-10 there was one Scottish case of legionellosis associated with travel within Scotland (this case is included in the data for this report). We are not aware of any cases of legionellosis in visitors to Scotland in 2009 and 2010, whose suspected source of infection was Scottish accommodation.
SHLMPRL testing for legionellosis in 2009 and 2010
A total of 2,941 specimens for the diagnosis of legionellosis were submitted to SHLMPRL in 2009, a decrease of 23% on 2008. In 2010, 2,700 specimens were submitted, a decrease of 8% on 2009 (Table 4). This drop in specimen numbers is largely due to a decrease in the numbers of sera being submitted and an increase in the number of sending laboratories performing urinary antigen assays on site. However, SHLMPRL would encourage all laboratories to send any positive samples for confirmation before reporting to local public health departments.
Characteristics of laboratory confirmed cases of Legionella infection
In 2009 and 2010, 32 (44%) of all positive samples were confirmed by urinary antigen test, 15 (21%) by PCR, 12 (17%) by seroconversion, six (8%) by single high antibody titre and seven (10%) by culture isolation (Table 5). A total of seven cultured isolates were obtained from human illness in 2009 and 2010. Of these, five were confirmed as L. pneumophila Sg1 and two as L. longbeachae Sg1. The breakdown of L. pneumophila Sg1 isolates by monoclonal subtype was Philadelphia (two), Benidorm (two) and Allentown/France (one).
A total of 73 environmental cultures were received in 2009 and 39 in 2010. This compares with 168, 252, 90, 183 and 207 in years 2008, 2007, 2006, 2005 and 2004 respectively. The species, serotypes and subtypes are shown in Table 6. L. pneumophila Sg1 was the most common serotype confirmed with 48 of 112 (43%). Of these, the most common monoclonal subtype was Philadelphia (17; 35%). Of the other L. pneumophila serogroups Sg4 (5%), Sg6 (13%) and Sg14 (5%) were the most frequently found in the environment. L. anisa and L. bozemanii accounted for the second most commonly isolated Legionella species at 22%.
All patient and related environmental isolates of L. pneumophila are routinely genotyped using Sequence Based Typing (SBT). One patient isolate and an isolate from the water supply in the patient's home were typed and both found to be identical (L. pneumophila Sg1 Philadelphia subtype 37). Two further unrelated urinary antigen positive cases (which were L. pneumophila Sg1 but no further typing could be done) had L. pneumophila Sg1 Philadelphia subtype 37 isolated from their home water supply.
Each year the lab participates in external quality assurance schemes for SBT, detection of Legionella DNA in respiratory samples by PCR, detection Legionella urinary antigen and isolation of Legionella species from environmental water samples. The lab continues to perform satisfactorily in all four schemes.
New developments in testing
Currently, a 16sRNA PCR ELISA is used as the screening PCR for the identification of L. pneumophila and Legionella species in respiratory secretions. Any positives by PCR are further genotyped using a nested SBT typing method that allows the classification of a sequence type from DNA extracted from a patient sample. This enables genotyping in culture negative cases that are positive by urinary antigen and/or serology. There has been an increase in PCR positives (21%) compared to culture alone (10%) (Table 5).
Nested SBT was utilised in a case linked to an outbreak in Wales in 2010. One patient was urinary antigen positive, PCR positive but culture negative. A nested SBT was performed on DNA from sputum and L. pneumophila Sg1 subtype 62 was identified. On further environmental testing of an associated hotel, an isolate from the water was found to be a L. pneumophila Sg1 subtype 62.
Similarly a urinary antigen positive, culture negative nosocomial case was identified as L. pneumophila Sg1 subtype 1 by nested SBT that was indistinguishable to an isolate of L. pneumophila Sg1 Olda subtype 1 isolated from the water supply in the patient's hospital room.
SHLMPRL would encourage all sending laboratories to forward all respiratory samples from urinary antigen and serology positive cases for further typing.
L. longbeachae in potting compost in 2008, 2009 and 2010
SHLMPRL routinely types all Legionella species using macrophage infectivity potentiator (mip) speciation. The resultant sequence has allowed all Legionella to be identified to species level which complements and enhances serotyping results. In 2008 and 2009 there was an increase in cases of L. longbeachae infection in Scotland which was reported in Eurosurveillance and received considerable media attention.9
In the years 2009-10, three unrelated community-acquired cases of Legionnaires' disease were identified who were all keen gardeners and had been working with different brands of bagged compost prior to onset of disease. Two of these cases were culture positive for L. longbeachae Sg1. In two of these cases, L. longbeachae Sg1 was isolated from the associated compost. When the strains were compared by Amplified Length Polymorphism (AFLP), in one case the patient isolate was linked to the isolate in the compost they had been working with prior to becoming ill. As compost is commonly used, but L. longbeachae infection seemingly rare, further work is required to ascertain the prevalence and predictors of L. longbeachae in compost and the conditions which facilitate transmission and generate an aerosol of the bacteria.
Most cases of legionellosis are diagnosed by urinary antigen test that is L. pneumophila specific and does not detect infection with L. longbeachae. We would encourage clinicians in cases of community-acquired pneumonia with a history of gardening and compost exposure, to send serum and respiratory samples to SHLMPRL for analysis.
European Legionnaires' disease surveillance
European surveillance of travel-acquired Legionnaires' Disease was co-ordinated by EWGLI (European Working Group for Legionella Infections), and administered by HPA, Colindale, London on behalf of ECDC up to March 2010. In April 2010 ECDC assumed direct control of the management of this surveillance and this was renamed ELDSNet (European Legionnaires' Disease Surveillance Network). More details of this scheme can be found on their website.2
Guidance on the management of Legionella outbreaks and clusters
Scottish guidance on Legionella outbreak management was produced by the Health Protection Network in March 2009 and is available on the HPN website.8 This evidence-based guidance is aimed at professionals of the wider health protection community in Scotland, considering issues around initial management response; epidemiological investigations; environmental investigations; sampling; risk assessment; communication; reporting and control. Following outbreaks of legionellosis in Scotland 2011, this guidance will undergo review in 2012.
The authors would like to thank the microbiologists, consultants in public health medicine, clinicians, nurses and other staff who assisted in the submission of samples to SHLMPRL and in the completion of surveillance forms.
- HPS web pages on Legionella http://www.hps.scot.nhs.uk/resp/legionella.aspx
- ELDSNet web pages on the ECDC website http://ecdc.europa.eu/en/activities/surveillance/ELDSNet/Pages/index.aspx
- ECDC web pages on legionellosis http://ecdc.europa.eu/en/healthtopics/legionnaires_disease/Pages/index.aspx
- Joseph CA, Ricketts KD, on behalf of the European Working Group for Legionella Infections. Legionnaires' disease in Europe 2007–2008. Euro Surveill. 2010;15(8):pii=19493. Available online: http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=19493
- ECDC Surveillance Report : Legionnaires' Disease in Europe 2009, September 2011. ISBN 978-92-9193-312-9. See ECDC web page http://ecdc.europa.eu/en/healthtopics/legionnaires_disease/Pages/index.aspx
- Joseph CA, Ricketts KD, Yadav R, Patel S, on behalf of the European Working Group for Legionella Infections. Travel-associated Legionnaires' disease in Europe in 2009. Euro Surveill. 2010;15(41):pii=19683. Available online: http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=19683
- ECDC case definitions http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2008:159:0046:01:EN:HTML
- HPN website for Scottish Legionella guidance http://www.hps.scot.nhs.uk/about/publications.aspx
- Pravinkumar SJ, Edwards G, Lindsay D, Redmond S, Stirling J, House R, Kerr J, Anderson E, Breen D, Blatchford O, McDonald E, Brown A. A cluster of Legionnaires' disease caused by Legionella longbeachae linked to potting compost in Scotland, 2008-2009. Euro Surveill. 2010;15(8):pii=19496. Available online: http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=19496