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Legionellosis is an infectious disease caused by bacteria belonging to the genus Legionella.[1] Over 90% of legionellosis cases are caused by Legionella pneumophila, a ubiquitous aquatic organism that thrives in warm environments (25 to 45 °C with an optimum around 35 °C).[2]

Legionellosis takes two distinct forms:

  • Legionnaires' disease is the more severe form of the infection and produces pneumonia.[3]
  • Pontiac fever is caused by the same bacterium, but produces a milder respiratory illness without pneumonia which resembles acute influenza.[3]

Legionnaires' disease acquired its name in 1977 when an outbreak of pneumonia occurred among people attending a convention of the American Legion in Philadelphia. On January 18, 1977 the causative agent was identified as a previously unknown bacterium, subsequently named Legionella.

An estimated 8,000 to 18,000 people get legionellosis in the United States each year.[4] Some people can be infected with the Legionella bacterium and have only mild symptoms or no illness at all.

Outbreaks of Legionnaires' disease receive significant media attention. However, this disease usually occurs as a single, isolated case not associated with any recognized outbreak. When outbreaks do occur, they are usually recognized in the summer and early autumn, though cases may occur at any time of year. The fatality rate of Legionnaires' disease has ranged from 5 to 30% during various outbreaks.



Patients with Legionnaires' disease usually have fever, chills, and a cough, which may be dry or may produce sputum. Some patients also have muscle aches, headache, tiredness, loss of appetite, loss of coordination (ataxia) and, occasionally diarrhea and vomiting. Laboratory tests may show that patients’ renal functions, liver functions and electrolytes are deranged, including hyponatremia. Chest X-rays often show pneumonia with bi-basal consolidation. It is difficult to distinguish Legionnaires' disease from other types of pneumonia by symptoms or radiologic findings alone; other tests are required for diagnosis.

Persons with Pontiac fever experience fever and muscle aches without pneumonia. They generally recover in 2 to 5 days without treatment.

The time between the patient’s exposure to the bacterium and the onset of illness for Legionnaires' disease is 2 to 10 days; for Pontiac fever, it is shorter, generally a few hours to 2 days.


Intestinal Infections: These may only occur as part of respiratory infections, and where gastrointestinal symptoms have on occasion been described.

Extraintestinal Infections: L. pneumophila is specifically considered as a pathogen of the respiratory tract, where it is a cause of atypical pneumonia, also known as Legionnaires' disease. Other infections have also been reported, including haemodialysis fistulae, pericarditis and wound and skin infections. Bacteraemia is often associated with Legionnaires' disease.

Animal Infections: None specifically recorded.

Infections of Protozoa: Protozoa such as Harmanella vermiformis and related protozoa have been shown to be able to support the growth of L. pneumophila in tap water. Also Acanthamoeba, Naegleria and Tetrahymena can be infected by L. pneumophila. This pathway may be how these organisms survive in the environment.

Diagnosis and treatment

People of any age may get Legionnaires' disease, but the illness most often affects middle-aged and older persons, particularly those who smoke cigarettes or have chronic lung disease. Immunocompromised patients are also at elevated risk. Pontiac fever most commonly occurs in persons who are otherwise healthy.

The most useful diagnostic tests detect the bacteria in sputum, find Legionella antigens in urine samples, or compare antibody levels to Legionella in two blood samples obtained 3 to 6 weeks apart. The urine antigen test is simple, quick, and very reliable; however it will only detect Legionella pneumophila serogroup #1. Also the urine antigen test will not identify the specific subtyping so it cannot be used to match the patient with the environmental source of infection.

Current treatments of choice are the respiratory tract quinolones (levofloxacin, moxifloxacin, gemifloxacin) or newer macrolides (azithromycin, clarithromycin, roxithromycin). The antibiotics used most frequently have been levofloxacin and azithromycin. Macrolides are used in all age groups while tetracyclines are prescribed for children above the age of 12 and quinolones above the age of 18.
Rifampin can be used in combination with a quinolone or macrolide. Tetracyclines and erythromycin led to improved outcome compared to other antibiotics in the original American Legion outbreak. These antibiotics are effective because they have excellent intracellular penetration and Legionella infects cells. The mortality at the original American Legion convention in 1976 was high (34 deaths in 180 infected individuals) because the antibiotics used (including penicillins, cephalosporins, and aminoglycosides) had poor intracellular penetration. Mortality has plunged to less than 5% if therapy is started quickly. Delay in giving the appropriate antibiotic leads to higher mortality.

Pontiac fever requires no specific antibiotic treatment.

How Legionnaires' disease is spread

Legionellosis infection occurs after inhaling water droplets that originated from a water source contaminated with Legionella. It must be inhaled through a fine aerosol of tiny water droplets that are strung with the bacteria. This often comes from poorly ventilated areas such as prisons where a condensating air conditioner can spread it throughout the entire room, infecting anyone not immune to the strand of bacteria. Potential sources of such contaminated water include cooling towers used in industrial cooling water systems as well as in large central air conditioning systems, evaporative coolers, hot water systems, showers, whirlpool spas, architectural fountains, room-air humidifiers, ice making machines, misting equipment and similar disseminators that draw upon a public water supply. The disease may also be spread in a hot tub if the filtering system is defective. [5] Freshwater ponds, creeks, and ornamental fountains are also potential sources of Legionella.[6]

Legionella will grow in water at temperatures from 20 °C to 50 °C (68 °F to 122 °F). However, the bacteria reproduce at the greatest rate in stagnant water at temperatures of 35 °C to 46 °C (95 °F to 115 °F).

Legionella longbeachae, a organism in the Legionella family, is found in soils and compost.[7] Thus, the dust from purchased bags of soil, compost, or potting mix is also a potential source of Legionella.

Outbreaks of Legionnaires' disease

Below is a list of Legionnaires' disease outbreaks, followed by a discussion of many of them:

Suspected Source  Cases   Deaths  Date Location
Cooling Tower at ice rink183July - August 2007Benalmadena, Spain
Institute of Animal Health1+02007Pirbright, Surrey, UK
Water supply 1675July - August 2007Verkhnyaya Pyshma, Sverdlovsk oblast, Russia [8]
Cooling tower 7031st Dec 2006 - 1st Jan 2007Australia, Sydney [9]
Spa Pool 1150August 2006Sunderland, UK
Cooling tower 262July - Sept 2006Paris, France
Cooling tower 302July 2006Amsterdam, Netherlands
Cooling towers1490June 2006Pamplona, Spain
Cooling tower101February 2006Sydney, Australia
Cooling tower200December 2005Torrevieja, Spain
Cooling tower12721October 2005Toronto, Canada
Cooling tower210June 2005New Rochelle, NY, USA
Indoor ornamental fountain171Summer 2005Rapid City, SD, USA
Air scrubber5210May 2005Fredrikstad, Norway
Cooling tower86212004Pas-de-Calais, France
Cooling tower17272002Barrow-in-Furness, UK
Cooling tower2872001Stavanger, Norway
Cooling tower4496July 2001Murcia, Spain
Whirlpool and humidifier200321999Bovenkarspel, Netherlands
Hospital101281985Stafford, UK
Cooling Tower (widely accepted)221341976Philadelphia, PA, USA

Philadelphia, 1976

The first recognized outbreak occurred on July 27, 1976 at the Bellevue Stratford Hotel in Philadelphia, Pennsylvania, where members of the American Legion, a United States military veterans association, had gathered for the American Bicentennial. Within two days of the event’s start, veterans began falling ill with a then-unidentified pneumonia. Numbers differ, but perhaps as many as 221 people were given medical treatment and 34 deaths occurred. At the time, the U.S. was debating the risk of a possible swine flu epidemic, and this incident prompted the passage of a national swine flu vaccination program. That cause was ruled out, and research continued for months, with various theories discussed in scientific and mass media that ranged from toxic chemicals to terrorism (domestic or foreign) aimed at the veterans. The U.S. Centers for Disease Control and Prevention mounted an unprecedented investigation and by September, the focus had shifted from outside causes, such as a disease carrier, to the hotel environment itself. In January 1977, the Legionellosis bacterium was finally identified and isolated, and found to be breeding in the cooling tower of the hotel’s air conditioning system, which then spread it through the entire building. This finding prompted new regulations worldwide for climate control systems.

Some do not believe that the air conditioning was conclusively proven to be the cause of the outbreak. According to Dr. Victor L. Yu, chief of the infectious disease section at the Oakland Veterans Affairs Medical Center in Pittsburgh, researchers still haven’t identified the exact source.[10] Morbidity and Mortality Weekly Report (MMWR), a weekly publication of the US Centers for Disease Control and Prevention (CDC), stated in 1977 that no source was found in this first outbreak of Legionnaires' disease at the Bellevue Stratford Hotel.[11][12]

United Kingdom, 1985

A large outbreak of Legionnaires' disease was associated with Stafford District General Hospital. A total of 68 confirmed cases were treated in hospital and 22 of these patients died. A further 35 patients, 14 of whom were treated at home, were suspected cases of Legionnaires' disease. All these patients had visited the hospital during April 1985. The BBC reported these statistics as 101 infected with 28 deaths resulting.

Netherlands, 1999

In March 1999, an outbreak in the Netherlands occurred during a flower exhibition in Bovenkarspel. 200 people became ill and at least 32 people died. There is a possibility that more people died from it, but these people were buried before the Legionella infection was recognized. The source of the bacteria were probably a whirlpool and a humidifier in the exhibition area.[13]

Spain, 2001

The world’s largest outbreak of Legionnaires' disease happened in July 2001 (patients began appearing at the hospital on July 7), in Murcia, Spain. More than 800 suspected cases were recorded by the time the last case was treated on July 22; 636-696 of these cases were estimated and 449 confirmed (so, at least 16,000 people were exposed to the bacterium) and 6 died (a case-fatality rate of approximately 1%).

A controlled case study matching 85 patients living outside the city of Murcia with two controls each was undertaken to identify the outbreak source; the epidemiologic investigation implicated the cooling towers at a city hospital (Morales Meseguer Hospital). An environmental isolate from these towers with an identical molecular pattern as the clinical isolates was subsequently identified and supported that epidemiologic conclusion.

Norway, 2001

The first known case of the disease in Norway occurred in 2001 when 28 people were infected in the city of Stavanger, and seven died. At first the authorities were puzzled as several of the victims lived in other locations, including one in Germany and another in England. After a massive investigation a fountain in the small lake of Breiavatnet was suspected as the source of the outbreak. But the fountain had not sprayed the bacteria into the air, the source was a cooling tower at the nearby SAS Radisson hotel. Only three of the infected had stayed at the hotel, but the exit vent of the cooling tower was at ground level next to a public bus stop, explaining the other victims.

United Kingdom, 2002

In 2002, Barrow-in-Furness suffered the U.K.’s worst outbreak of Legionnaires' disease. Six women and one man died as a result of the illness, another 172 people also contracted the disease. The cause was found to be a contaminated cooling tower at the town’s Forum 28 arts centre. [1] Barrow Borough Council later became the first public body in the UK to be charged with corporate manslaughter, but were cleared. They were, however, along with architect Gillian Beckingham, fined for breaches of Health and Safety regulations in a trial that ended in 2006.

France, 2004

Researchers found that the Legionnaires' disease bacteria spread through the air up to 6 kilometers from a large contaminated cooling tower at a petrochemical plant in Pas-de-Calais in northern France. That outbreak killed 21 of the 86 people with laboratory-confirmed infection.[14]

Norway, 2005

In May 2005 there was a second—greater—outbreak in Norway, this time originating in the southeastern town of Fredrikstad. As of 8 June 2005, 52 patients were confirmed infected and ten people were dead. The dead were all from Fredrikstad or nearby cities, in age ranging from 68 to early 90's. The source of the outbreak unexpectedly came from an air scrubber (an industrial air purification facility; this particular one operated by Borregaard Industries in Sarpsborg). Such an installation has never before been reported as a source of Legionellosis anywhere in the world. Although the source was finally identified by DNA matching, it was also fairly well identified by analysing risk increases from people living near suspected sources.[15]

New Zealand, 2005

An outbreak of Legionnaires' disease hit the New Zealand city of Christchurch in mid-2005, with 20 reported cases (three fatal) between late April and August. A cooling tower at the city's Ravensdown Fertiliser Plant was implicated in the outbreak. The outbreak led to plans by the city council to create a registry of all air-conditioning cooling towers within the city.[16]

Toronto, 2005

In October 2005 at least 21 people died [2] and over 100 fell ill during an outbreak at the Seven Oaks Home for the Aged in Toronto, Ontario, Canada. Legionnaire’s disease was originally ruled out as being the cause, but post-mortem examinations confirmed that victims had Legionella bacteria in their lungs. The outbreak is still being investigated, and researchers believe this particular outbreak may be related to a new strain of the bacteria.

Australia, 2007

The outbreak is believed to have started during New Year's Eve celebrations at Circular Quay, on Sydney's harbour. Thought to have started from a cooling tower (where 1,400 cfu/ml Legionella was found) from an adjacent office building, four cases were initially confirmed, but there were concerns due to the fact that Circular Quay is one of the most populated areas in Sydney on New Year's Eve. As a result, there could have been potentially many more cases. On Saturday, January 20 the NSW Health Authority reported that three more cases had developed overnight, bringing the total reported cases to 7.

New York, 2007

A Legionnaires' disease was confirmed in six residents of New York City nursing homes, all of whom were hospitalized and recovered. It was believed to have spread through the heating radiators located at the facilities.

Controlling the potential growth of Legionella in cooling towers

Various studies have shown that some 40 to 60% of cooling towers tested contained Legionella.[17]

A recent research study provided evidence that Legionella pneumophila, the causative agent of Legionnaires' disease, can travel at least 6 km from its source by airborne spread. It was previously believed that transmission of the bacterium was restricted to much shorter distances. A team of French scientists reviewed the details of an epidemic of Legionnaires' disease that took place in Pas-de-Calais in northern France in 2003–2004. There were 86 confirmed cases during the outbreak, of whom 18 perished. The source of infection was identified as a cooling tower in a petrochemical plant, and an analysis of those affected in the outbreak revealed that some infected people lived as far as 6–7 km from the plant.[18]

Temperature affects the survival of Legionellae as follows:[19]

  • 70 to 80 °C (158 to 176 °F): Disinfection range
  • At 66 °C (151 °F): Legionellae die within 2 minutes
  • At 60 °C (140 °F): Legionellae die within 32 minutes
  • At 55 °C (131 °F): Legionellae die within 5 to 6 hours
  • Above 50 °C (122 °F): They can survive but do not multiply
  • 35 to 46 °C (95 to 115 °F): Ideal growth range
  • 20 to 50 °C (68 to 122 °F): Legionellae growth range
  • Below 20 °C (68 °F): Legionellae can survive but are dormant

Removing slime is an effective control process, because up to 1,500 times more chemicals are needed to kill bacteria in slime than free floating bacteria[20] [21].

Action levels following microbial monitoring for cooling towers

The European Working Group for Legionella Infections (EWGLI)[22] was established in 1986 within the European Union framework to share knowledge and experience about potential sources of Legionella and their control. This group has published guidelines[23] about the actions to be taken to limit the number of colony forming units (i.e., the aerobic count) of micro-organisms per mL at 30 °C (minimum 48 hours incubation):

Aerobic countLegionellaAction required
10,000 or less1,000 or lessSystem under control.
more than 10,000
up to 100,000
more than 1,000
up to 10,000
Review program operation. The count should be confirmed by immediate re-sampling. If a similar count is found again, a review of the control measures and risk assessment should be carried out to identify any remedial actions.
more than 100,000more than 10,000Implement corrective action. The system should immediately be re-sampled. It should then be ‘shot dosed’ with an appropriate biocide, as a precaution. The risk assessment and control measures should be reviewed to identify remedial actions.

Aerobic count, cfu/ml at 30°C (minimum 48 hours incubation). Colony count determined by pour plate method according to ISO 6222(21) or by spread plate method on yeast extract agar.

Legionella, bacteria cfu/litre. Determined in accordance with ISO 11731(20).

Expect to find Legionella CFUs because almost all natural water sources contain Legionella. Legionella samples need to be run within 24 hours. Samples should be kept cold with ice packs, but not frozen, and should remain upright.

Guidelines for control of Legionella in cooling towers

Many governmental agencies, cooling tower manufacturers and industrial trade organizations have developed design and maintenance guidelines for preventing or controlling the growth of Legionella in cooling towers. Below is a list of sources for such guidelines:

  • SPX (Marley) Cooling Technologies - ASHRAE Guideline 12-2000 - Minimizing the Risk of Legionellosis
  • EWGLI Technical Guidelines for the Control and Prevention of Legionella in Water Systems
  • Centers for Disease Control and Prevention - Procedure for Cleaning Cooling Towers and Related Equipment (pages 239 and 240 of 249)
  • Cooling Technology Institute - Best Practices for Control of Legionella
  • OSHA - How to maintain these systems - Frequency of cleaning
  • OSHA page on Legionellosis - US Occupational Safety and Health Administration
  • Association of Water Technologies - Legionella 2003
  • California Energy Commission - Cooling Water Management Program Guidelines For Wet and Hybrid Cooling Towers at Power Plants
  • SPX (Marley) Cooling Technologies - Cooling Towers Maintenance Procedures
  • Marley Cooling Technologies - Cooling Tower Inspection Tips {especially page 3 of 7}
  • Tower Tech Modular Cooling Towers - Legionella Control
  • GE Infrastructure Water & Process Technologies Betz Dearborn - Chemical Water Treatment Recommendations For Reduction of Risks Associated with Legionella in Open Recirculating Cooling Water Systems
  • Wisconsin Division of Health, Control of Legionella in Cooling Towers: Summary Guidelines, June 1987, Wisconsin Department of Health and Social Sciences.

Regulations & Ordinances

The guidance issued by the UK government's Health and Safety Executive (HSE) now recommends that microbiological monitoring for wet cooling systems, using a dip slide, should be performed weekly. The guidance now also recommends that routine testing for legionella bacteria in wet cooling systems be carried out at least quarterly, and more frequently when a system is being commissioned, or if the bacteria has been identified on a previous occasion. [24]

The City of Garland, TX requires yearly testing for legionella bacteria at cooling towers at apartment buildings. [25]

Malta requires twice yearly testing for legionella bacteria at cooling towers and water fountains. Malta prohibits the installation of new cooling towers and evaporative condensers at health care facilities and schools. [26]

See also


  1. ^ Ryan KJ, Ray CG (editors) (2004). Sherris Medical Microbiology, 4th ed., McGraw Hill. ISBN 0838585299. 
  2. ^ CDC: Etiologic Agent
  3. ^ a b Winn WC Jr (1996). "Legionella", Baron's Medical Microbiology (Baron S et al, eds.), 4th ed., Univ of Texas Medical Branch. ISBN 0-9631172-1-1. 
  4. ^ CDC: Incidence
  5. ^ Silivanch v. Celebrity Cruises, Inc., 171 F.Supp.2d 241 (S.D.N.Y. 2001) (plaintiff successfully sued cruise line and manufacturer of filter after catching disease on cruise)
  6. ^ Winn WC Jr (1996). Legionella. In: Baron's Medical Microbiology (Baron S et al, eds.), 4th ed., Univ of Texas Medical Branch. (via NCBI Bookshelf) ISBN 0-9631172-1-1. 
  7. ^ CDC: Potting Soil in CA, OR, WA
  8. ^ The cause of the pneumonia outbreak in Sverdlovskaya oblast is Legionella bacterium (in Russian)
  9. ^ HC INFO- Recent Outbreaks of Legionnaires' Disease, Sydney,6 Cases, Jan. 2007
  10. ^ Legionnaires' disease First Outbreak
  11. ^ MMWR, January 18 1977, (reprinted January 24 1997 Vol 46(03); 50-56) special issue, Epidemiologic Notes and Reports Follow-up on Respiratory Illness --Philadelphia
  12. ^ History of Outbreak at Bellevue Stratford Hotel
  13. ^ The Westfriese Flora flower exhibition and fair
  14. ^ Norox Pas-de-Calais
  15. ^ Borregaard Sarpsborg plant in Norway
  16. ^ Television New Zealand report
  17. ^ CTI Legionellosis Guideline: Best Practices for Control of Legionella (WTP-148) (06)
  18. ^ Long-range transmission of Legionella
  19. ^ What is Legionnaires' disease?
  20. ^ Studying Slime Paragraph 12
  21. ^ An Introduction to Biofilms
  22. ^ European Working Group for Legionella Infections
  23. ^ EWGLI: Technical Guidelines for the Control and Prevention of Legionella in Water Systems
  24. ^ UK: Health and Safety Executive Microbiological monitoring (weekly dip slide)
  25. ^ The Dallas Morning News, Garland tough on bacteria
  26. ^ MALTA Control of Legionella Regulations, 2006

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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Legionellosis". A list of authors is available in Wikipedia.
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