The term 'Legionnaires’ disease' was coined in the 1970s after an outbreak of pneumonia occurred among delegates attending a convention of the American Legion in Philadelphia where 234 people became ill and 34 died. Legionellosis is a potentially fatal infectious disease and takes two distinct forms. One is Legionnaires’ disease which is a more severe form of infection and produces high fever and pneumonia; the second is Pontiac fever caused by the same bacteria but producing a milder respiratory illness without pneumonia that resembles acute influenza. Legionellosis cases are caused by Legionella pneumophila which is a thriving aquatic organism that lives and reproduce in temperatures of between 25°C - 45°C with an optimum temperature of 35°C.
The potentially fatal form of pneumonia and its outbreaks has been a public source of concern and media attention. Every year it is reported that there are 300 to 400 cases of Legionnaires’ disease in the UK in contrast to many thousands of pneumonia cases. The fatality rate of the disease ranges from 5% to 30% during various outbreaks and can approach 50% in hospital-acquired infections when treatment to antibiotics is delayed1. Travellers to holiday destinations, especially warm climates, are at risk and these cases account for up to half of the cases reported from some European countries. Inadequate management, lack of training and poor communication are contributing factors of the outbreaks of Legionnaires’ disease and prevention is through control of the organism in water systems5.
In Edinburgh this year, an outbreak of Legionnaires disease started with the first case identified at the end of May. By mid-July there were 101 confirmed and suspected cases2. Confirmed cases of the disease were predominantly males, smokers, aged over 50 and with underlying health problems. Epidemiological evidence suggested that a common outdoor airborne exposure occurred over the south west of Edinburgh which had probably been spread from cooling towers in the north east of the affected area, although it has not been clearly linked with scientific evidence. This outbreak was the largest to date in Lothian, Scotland and has occurred in a densely populated area of the city. Significant community outbreak of Legionnaires’ disease which has occurred in the UK and in Europe was associated with cooling towers. The low mortality of 4.3% in the confirmed cases of the disease may be due to the timing and the quality of care including hands-on communication3.
Another outbreak emerged on 24th July in Stoke-on-Trent with 20 confirmed cases including two deaths by 7th August. These cases were aged between their late 40’s and mid-70’s and according to the Health Protection Agency; a hot tub has been linked to the outbreak6.
The Health and Safety Executive (HSE) has stressed that effective control measures are required after 90% of the outbreaks come as a result of failure risks and the recognition of possible Legionella problems. The HSE stated that a review of Legionnaires’ disease outbreaks in Britain over the last decade identified common failings in Legionella and that outbreaks of the disease over the past ten years confirmed that cooling towers and evaporative condensers are the most common source of significant outbreaks and that problem can build up rapidly if risks are not controlled7.
Referring to the weekly report of the Health Protection Agency (HPA) published on 2nd March 2012, the pattern of the annual number of cases of confirmed and presumptive cases of Legionellosis for the residents of England and Wales during the years 2000-2010 has increased continuously until 2006 but with the exception in 2002 where 185 cases was associated with the outbreak in Barrow-on-Furness and as from 2006, the number of cases of the disease has stabilised. Out of 1058 Legionnaires’ cases with onset of symptoms during 2008-2010, 650 (61.4%) were classified as community acquired cases, indicating the majority of cases were assumed to have acquired the infection through transmission in the community including travel within the UK. Since 2007, the number of nosocomial Legionnaires’ disease has risen and in 2010, 18 (5.1%) of cases were reported to have acquired their infection in hospitals. Mortality for the nosocomial Legionnaires’ disease cases is much higher than in travel- or community-acquired cases and the case fatality rate for nosocomial Legionnaires’ cases were 22.2% (95% CI 11.2%-37.1%). The possible effects may reflect the increased likelihood that hospitalised patients may have underlying conditions that predispose them to infection such as smoking and heart disease3,4.
Confirmed and presumptive cases of Legionellosis for residents of England and Wales. Source: HPA weekly report 2 March 2012
Confirmed Legionnaires' disease cases by year of onset and category of exposure, 2000-2010. Source: HPA weekly report 2 March 2012
The confirmed cases of Legionnaires disease cases by month and years of onset: 2008-2010 and were at its peak between July and September. The meteorological factors such as temperature and humidity were the likely factors influenced the seasonality of the Legionnaires disease. Since 2000, there has been relatively little change in the case fatality rate (CFR) but in 2008-2010, there were 116 deaths out of a total of 1058 Legionnaires’ disease cases (CFR 11%, range 9.4%-12.8%)4.
Confirmed Legionnaires disease cases by month and year of onset: 2008-2010. Source: HPA weekly report 2 March 2012.
Legionnaires' disease case fatality rate by year, 2008-2010. Source: HPA weekly report 2 March 2012.
The case fatality rate (CFR) increased with age: those under the age of 50 years, the CFR were 5.5% (95% C.I. 2.8%-9.3%) and those aged 70 years and over, the CFR was 21.9% (95% C.I. 17.0%-27.5%). In England and Wales, the number of Legionnaires’ cases associated with travelling abroad has increased between 2000 and 2007 but the numbers decreased in 2008-20104.
Legionnaires' disease cases by age and gender with case fatality rate (%) and 95% CI, 2008-2010. Source: HPA weekly report 2 March 2012.
Legionnaires' disease cases associated with travel by year of onset, 2000-2010. Source: HPA weekly report 2 March 2012.
Based on the report of the Health Protection Agency (HPA) published on 2nd March 2012, nosocomial Legionnaires’ disease also called hospital-acquired infection has been increasing since 2007. Since the first outbreak in July 1976, hospital-acquired infection has been reported in many hospitals and is associated with a contamination of hospital’s water distribution system8. Patients who are immunocompromised are particularly vulnerable when infected and have a high mortality rate.
Blatt et al. performed a case-control study and an environmental study to identify the risk factors and the mode of transmission of Legionella infection during an outbreak of a nosocomial Legionnaires’ disease in a military medical centre. Fourteen cases of nosocomial Legionnaires’ disease were identified by active surveillance following the discovery of 2 culture-proven cases among organ transplant patients. Four control patients were matched to each case by age, sex and date of admission. The cases and control were compared with respect to past medical history and hospital exposure variables. Other variables such as water cultures which revealed Legionella pneumophila serogroup 1, hot water tank and 15% of 85 potable water sites were used to test the significance of nosocomial Legionnaires’ disease. The case-control study indicated that the risk factors that are significant to nosocomial Legionnaires’ disease are immunosuppressive therapy (OR=32.7, CI=4.5 to 302.6) nasogastric tube use (OR=18.4, CI=2.6 to 166.2), bedbathing (OR=10.7, CI=2.2 to 59.0) and antibiotic therapy (OR=14.6, CI=2.9 to 84.4). Shower use (OR=0.1, CI= 0 to 0.4) appeared to be a negative risk factor. The study concluded that potable water was significant in transmitting nosocomial Legionnaires’ disease and that the organisms gained access to the hospital via external water supplies. The risk factors gave the evidence that Legionnaires’ disease may act as a superinfection in a nosocomial setting and is likely acquired by aspiration8.
Several methods can be used to combat nosocomial Legionnaires’ disease in hospitals which include chlorination, heat treatment and the use of filters but re-contamination can easily occur after eradication. The control measures that could be directed at the water distribution system in hospitals is the copper and silver ionisation disinfection methods which is a systematic and long-term eradication of Legionella in water. However, the use of copper and silver ionisation is expensive and can risk water discoloration and a possibility of developing resistance in environmental bacteria9.