Legionellosis: Wikis


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Classification and external resources
ICD-10 A48.1, A48.2
ICD-9 482.84
DiseasesDB 7366
MedlinePlus 000616
eMedicine med/1273
MeSH D007876

Legionellosis is an infectious disease caused by gram negative, aerobic bacteria belonging to the genus Legionella.[1][2] Over 90% of legionellosis cases are caused by Legionella pneumophila, a ubiquitous aquatic organism that thrives in temperatures between 25 and 45 °C (77 and 113 °F), with an optimum around 35 °C (95 °F).[3]

Legionellosis takes two distinct forms:

  • Legionnaires' disease, also known as "Legion Fever"[4] (archaic), is the more severe form of the infection and produces pneumonia.[5]
  • Pontiac fever is caused by the same bacterium, but produces a milder respiratory illness without pneumonia which resembles acute influenza.[5]

Legionnaires' disease acquired its name in July 1976 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.[3] 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 single, isolated cases not associated with any recognized outbreak. When outbreaks do occur, they are usually 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.


L. pneumophila is specifically considered as a pathogen of the respiratory tract. Other infections have also been reported, including haemodialysis fistulae, pericarditis and wound and skin infections. Bacteraemia is often associated with Legionnaires' disease. Intestinal infections may only occur as part of respiratory infections, and where gastrointestinal symptoms have on occasion been described.

No animal infections have been specifically recorded.

Infections of Protozoa such as Hartmannella 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 suffer from 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 the comparison of Legionella antibody levels to in two blood samples taken 3 to 6 weeks apart. A urine antigen test which is simple, quick, and very reliable will only detect Legionella pneumophila serogroup #1. In addition the urine antigen test will not identify the specific subtypes 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.
Rifampicin can be used in combination with a quinolone or macrolide. Tetracyclines and erythromycin led to improved outcomes 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.

According to the journal "Infection Control and Hospital Epidemiology," hospital-acquired Legionella pneumonia has a fatality rate of 28%, and the principal source of infection in such cases is the drinking-water distribution system.[6]

Pontiac fever requires no specific antibiotic treatment.


Legionellosis infection normally occurs after inhaling an aerosol (suspension of fine particles in air) containing Legionella bacteria. Such particles could originate from any infected water source. When mechanical action breaks the surface of the water, small water droplets are formed, which evaporate very quickly. If these droplets contain bacteria, the bacteria cells remain suspended in the air, invisible to the naked eye but small enough to be inhaled into the lungs.[7] This often occurs in 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.[8] Freshwater ponds, creeks, and ornamental fountains are also potential sources of Legionella.[9] The disease is particularly associated with hotels, cruise ships and hospitals with old, poorly maintained pipework and cooling systems. When the disease is caused by contact with the organism in a hospital, it is called a "nosocomial infection."


Prevention of Legionella growth

Legionella bacteria itself can be inactivated by UV-C-Light. However, Legionella bacteria that grow and reproduce in amoebae or that are sheltered in corrosion particles cannot be killed by UV-light alone. An innovative way is the combination of ultrasonics' and UV-C-light. This uses a two stage process, where ultrasonic cavitation disrupts the amoebae or corrosion particles and leaves the Legionella bacteria exposed for UV radiation. Such combined system are used for example in hot water systems in sensitive areas, such as hospitals, where the inhabitants are more vulnerable than in normal environments.[10]

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

Legionella longbeachae, an organism in the Legionella family, is found in soils and compost.[11] Thus, the dust from purchased bags of soil, compost, or potting mix is also a potential source of Legionella.


Much has been learned about the epidemiology of Legionnaires' disease since the organism was first identified in 1976. National surveillance systems and research studies were established early, and in recent years improved ascertainment and changes in clinical methods of diagnosis have contributed to an upsurge in reported cases in many countries. Environmental studies continue to identify novel sources of infection, leading to regular revisions of guidelines and regulations. Between 1995 and 2005 over 32,000 cases of Legionnaires' disease and more than 600 outbreaks were reported to the European Working Group for Legionella Infections (EWGLI). In the future, there may be an increase in cases as the population becomes more elderly.[2] There is a shortage of data on Legionella in developing countries and it is likely that Legionella-related illness is underdiagnosed worldwide.[12] Improvements in diagnosis and surveillance in developing countries would be expected to reveal far higher levels of morbidity and mortality than are currently recognised. Similarly, improved diagnosis of human illness related to legionella species and serogroups other than legionella pneumophila would improve knowledge about their incidence and spread.[13]

Notable outbreaks

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. They were tachypneic and complained of chest pain. 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.

Subsequent investigations, "revealed that the bacteria thrived in the Bellevue-Stratford hotel's cooling tower. From that water supply the hotel derived its air conditioning and the bacteria were actively pumped into the hotel. Since 1976, air conditioning changed, with federal agencies all over the world requiring more stringent cleaning and hygiene provisions for cooling towers and large scale air conditioning systems."[14]

United Kingdom, 1985

In April 1985 175 patients were admitted to the District or Kingsmead Stafford Hospitals with chest infection or pneumonia. 28 died. The medical diagnosis showed that this was Legionnaires' disease and the immediate epidemiological investigation traced the source of the infection to the air-conditioning cooling tower on the roof of the Stafford District Hospital. A Government Inquiry was set up to investigate how the infection occurred and why it became Britain's largest epidemic of Legionnaires' disease. The infection was linked to one small zone in the hospital, the outpatients department. The initial investigation searched for engineering reasons to explain why this particular zone was the risk area. More detailed and wider epidemiological surveys subsequently showed that staff working in the whole area supplied with fresh air taken adjacent to the tower, had antibodies to the disease. The outpatients department was unique in having a very large transient population of susceptible individuals either receiving treatment or accompanying friends.

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.[15]

Melbourne (Australia), 2000

In April 2000, an outbreak of legionella pnemophillia serogroup 1 occurred in Melbourne, Australia. The outbreak resulted in 125 confirmed cases of legionella, with 95 (76%) hospitalised. It is reported that 4 died from the outbreak. The investigation traced the source of the infection to the cooling tower at the newly opened Aquarium [16]. Since this outbreak legionella infection statistics are required to be reported by the State Government as a notifiable disease [17]. Stingent Regulations were introduced by the State to control legionella in 2001 [18].

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.

United Kingdom, 2002

In 2002, Barrow-in-Furness in the U.K. suffered an 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.[19] 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.

Norway, 2005

In Fredikstad, Norway, 56 people became ill and 10 died from legionnaire's disease caused by bacteria growing in an air scrubber of a nearby factory.

Controlling Legionella growth

Various studies have shown that some 40 to 60% of cooling towers tested contained Legionella.[20]

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 died. 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.[21]

A study of Legionnaires' disease cases in May 2005 in Sarpsborg, Norway concluded that: "The high velocity, large drift, and high humidity in the air scrubber may have contributed to the wide spread of Legionella species, probably for >10 km. ..."[22]

Temperature affects the survival of Legionella as follows:[23]

  • 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 may be an effective control process.[24][25]

Action levels

The European Working Group for Legionella Infections (EWGLI)[26] 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[27] 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 count Legionella Action required
10,000 or less 1,000 or less System 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 100,000 more than 10,000 Implement 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.

Almost all natural water sources contain Legionella and their presence should not be taken as an indication of a problem. The tabled figures assume an aerobic count, cfu/ml at 30 °C (minimum 48 hours incubation) with colony count determined by the pour plate method according to ISO 6222(21) or by spread plate method on yeast extract agar.

Many other 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. However, in the US, there are no regulations requiring testing or maintaining any specified levels in these facilities.

Breeding Ground

The bacteria grow best in warm water, like the kind found in hot tubs, cooling towers, hot water tanks, large plumbing systems, or parts of the air-conditioning systems of large buildings. Indoor ornamental fountains have been confirmed as a cause of Legionnaires' disease outbreaks. In all documented cases submerged lighting as a heat source was attributed to the outbreak. Controlling the growth of Legionella in ornamental fountains is touched on in many of the listed guidelines. However, specific guidelines for ornamental fountains have also been published.[28]

Regulations and 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.[29]

The City of Garland, Texas requires yearly testing for legionella bacteria at cooling towers at apartment buildings.[30]

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.[31]

The Texas Department of State Health Services has provided guidelines for hospitals to detect and prevent the spread of nosocomial infection due to legionella.[32]


  1. ^ Ryan KJ, Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed. ed.). McGraw Hill. ISBN 0838585299.  
  2. ^ a b Swanson M, Heuner K (2008). Legionella: Molecular Microbiology. Caister Academic Pr. ISBN 1-904455-26-3.  
  3. ^ a b "Top 10 things every clinician needs to know about Legionellosis". CDC. 2008-04-22. http://www.cdc.gov/legionella/top10.htm. Retrieved 2008-10-19.  
  4. ^ "Legionnaires' Disease". http://www.scienceclarified.com/Io-Ma/Legionnaires-Disease.html. Retrieved 2008-10-19.  
  5. ^ a b Winn WC Jr (1996). "Legionella". Baron's Medical Microbiology (Baron S et al., eds.) (4th ed. ed.). Univ of Texas Medical Branch. ISBN 0-9631172-1-1. http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed.section.2222.  
  6. ^ Infection Control and Hospital Epidemiology, July 2007, Vol. 28, No. 7, "Role of Environmental Surveillance in Determining the Risk of Hospital-Acquired Legionellosis: A National Surveillance Study With Clinical Correlations" [1]
  7. ^ "HPA - Mode of infection: aerosol formation". UK Health Protection Agency. 2008-04-07. http://www.hpa.org.uk/webw/HPAweb&HPAwebStandard/HPAweb_C/1204186178183?p=1191942128209. Retrieved 2008-10-19.  
  8. ^ 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)
  9. ^ Winn WC Jr (1996). Legionella. In: Baron's Medical Microbiology (Baron S et al., eds.) (4th ed. ed.). Univ of Texas Medical Branch. (via NCBI Bookshelf) ISBN 0-9631172-1-1.  
  10. ^ hielscher. "Ultrasonic Water Disinfection". http://www.hielscher.com/ultrasonics/water_disinfection.htm.  
  11. ^ CDC (September 2000). "Legionnaires' Disease associated with potting soil—California, Oregon, and Washington, May-June 2000". MMWR 49 (34): 777–8. PMID 10987244. http://www.cdc.gov/mmwr/preview/mmwrhtml/mm4934a1.htm.  
  12. ^ World Health Organization 2007. Legionella and the prevention of legionellosis. Geneva, Switzerland: WHO. Available at: http://www.who.int/water_sanitation_health/emerging/legionella.pdf
  13. ^ Fields, B. S.; Benson, R. F.; Besser, R. E. (2002). "Legionella and Legionnaires' Disease: 25 Years of Investigation". Clinical Microbiology Reviews 15 (3): 506–526. doi:10.1128/CMR.15.3.506-526.2002.  
  14. ^ "Legionnaires' Disease History". http://justice.loyola.edu/~klc/BL472/Legionnaire/history.html. Retrieved 2008-10-19.  
  15. ^ The Westfriese Flora flower exhibition and fair
  16. ^ [2]
  17. ^ [3]
  18. ^ [4]
  19. ^ http://www.multiline.com.au/~mg/legionnaires_disease_Barrow-in-Furness.html
  20. ^ CTI Legionellosis Guideline: Best Practices for Control of Legionella (WTP-148) (06)
  21. ^ Long-range transmission of Legionella
  22. ^ An outbreak of legionnaires disease caused by long-distance spread from an industrial air scrubber in Sarpsborg, Norway in May 2005
  23. ^ What is Legionnaires' disease?
  24. ^ Studying Slime Paragraph 12
  25. ^ An Introduction to Biofilms
  26. ^ European Working Group for Legionella Infections
  27. ^ EWGLI: Technical Guidelines for the Control and Prevention of Legionella in Water Systems
  28. ^ Legionella Risk Management - Guidelines for Control of Legionella in Ornamental Features, December 2005.
  29. ^ UK: Health and Safety Executive Microbiological monitoring (weekly dip slide)
  30. ^ The Dallas Morning News, Garland tough on bacteria
  31. ^ MALTA Control of Legionella Regulations, 2006
  32. ^ Report of the Texas Legionnaires' Disease Task Force, Texas Department of State Health Services [5]

29. Article on www.sage.com Article Outbreak of Legionnaires' disease at Stafford District General Hospital http://bse.sagepub.com/cgi/content/abstract/12/2/53

Further reading

Guidelines for Legionella control:

External links

Images of Legionella bacteria


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