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Legionnaires' Disease - Frequently Asked Questions

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Despite only having been identified some 25 years ago, Legionella pneumophila (the causative agent of Legionnaires' Disease) is no new organism. Medical history is littered with evidence that this organism has wreaked havoc long before people knew its identity. Every year about 5-10% of pneumonia cases are caused by this creature, and the mortality rate can soar to as high as 30% if untreated.

The bacteria's evolution enabled it to exploit modern technology as means of propagation and so Legionnaires' Disease is not an unfamiliar disease to most of us. However, for many, there are still unanswered questions; such as, how can technology favour a dangerous organism, and - more importantly - how safe are we from this airborne threat?

What is Legionella?

Legionella is a Gram negative1, rod-shaped bacteria measuring about 0.3-0.9 microns by 1.5-5 microns2. It is an aerobic microbe, meaning that it requires oxygen to live, and has unusual nutritional requirements - high levels of the amino acid cysteine, inorganic iron and low concentrations of salt - as opposed to a large number of other pathogens. It is also unusually tolerant to high levels of chlorine, low pH and high temperatures (they grow nicely at temperatures between 28 and 45 degrees Celsius) - all of which would strike down the normal pathogen.

There are currently at least 34 species of Legionella, and more than 23 serogroups of L. pneumophila. Serogroup 1 is responsible for more than 80% of all Legionella infections; however serogroups 4 - 6 and other species such as L. bozemanii, L. longbeachae and L. maceachernii have been known to cause Legionnaires' Disease as well.

Where can Legionella be found?

Legionella is ubiquitous in natural freshwater sources such as rivers, ponds and hot springs, usually living as parasites of a number of amoebae. They can also be found in the waters of air-conditioning cooling towers, spas, fountains and potable water sources such as taps, faucets and showerheads. L. longbeachae is usually found in soil, and is transmitted in a most fascinating fashion, which will be discussed later.

Who does Legionella infect?

In the environment, Legionella parasitises free-living amoebae such as Naegleria and Hartmanella. It also infects macrophages and a number of other components of the human cell-mediated immune system.

Scientists have also isolated L. pneumophila from calves who had died of pneumonia. In the second case, fatal pneumonia caused by L. pneumophila was determined as the cause of death.

How does Legionella infect us?

The primary mode of transmission is by inhalation of aerosolised water droplets containing Legionella3. Once inside, it is gobbled up by patrolling alveolar macrophages, who extend a single pseudopod from their cell surface and drag Legionella into them4. Under normal circumstances, a 'captured' bacteria is subsequently packaged inside a membrane called a phagosome, which will then fuse with a lysosome5. However, Legionella has found a way to stop this fusion from occurring, which means that this parasitic little bacteria is now safely tucked away in a little parcel in a large white blood cell that restlessly roams about the lungs. Now that it does not need to fear being attacked by other components of the immune system, it can begin to multiply inside the macrophage, using the cell as a mobile home and the stuff inside it as food. Eventually, when the children and great-grandchildren of the original Legionella have exhausted the food supply, they burst open the macrophage, thus killing it, and surge out in search of new homes. This will continue to go on and on until they are stopped, or until they have done immense damage to the human host.

What are the symptoms of infection by Legionella?

The less severe form of Legionella infection is Pontiac Fever, which manifests itself as a self-limiting flu-like illness. The incubation period is remarkably short - in terms of hours to several days - and symptoms will include malaise, muscle aches (myalgia), fever, chills and headache. Patients will, however, typically recover completely within one week.

Legionnaires' Disease is the more severe manifestation of Legionella infection. The incubation period for Legionnaires' Disease is 2 to ten days. Symptoms typically include a dry (non-productive) cough and difficulty in breathing, as well as flu-like symptoms such as fever, headache, chills, myalgia and overwhelming lethargy. The onset of pneumonia is abrupt. Chest X-rays will reveal signs of pneumonia, either in a distinct area ('lobar') or in a 'patchy' pattern throughout the lungs. Sputum is occasionally produced, often containing inflammation, but Gram staining will reveal no bacteria in the sputum. Failure of the pneumonia to respond to traditional pneumonia-combating antibiotics such as expanded-spectrum cephalosporins and aminoglycosides usually confirms the disease. If the patient is not treated by this stage, prognosis is poor.

Occasionally, other complications may arise from infection by Legionella, including:

  • Heart: pericarditis, endocarditis, myocarditis
  • Lungs: pleural empyema
  • Pancreas: pancreatitis
  • Kidneys: pyelonephritis, acute renal failure
  • Gastrointestinal tract: hepatic abscess, diarrhoea, GI tract-related abscesses
  • Other parts of the body: peritonitis, cellulites, skin rashes, encephalitis, myoglobinuria

Lately, infection by Legionella has also been linked to acute purulent arthritis.

How common is Legionella infection?

Infection by Legionella, despite its recent discovery, is fairly common. Most of us have, at some point in our lives, encountered this bacteria and have subsequently built up antibodies against it. 95% of those who come into contact with it will develop Pontiac Fever, which can easily pass for normal influenza. It has also been speculated that infection by Legionella is responsible for suburban neurosis (aka Sick Building Syndrome) - sore throat, slight headache, tiredness - although findings are, at this time, inconclusive. Most of those who develop full-blown Legionnaires' Disease are those who are old, very young or whose immune systems have been compromised by disease or the environment.

Some major outbreaks of Legionnaires' Disease and other Legionella-caused diseases are summarised in the table below:

YearCountryPlaceDiseaseNo. infectedDeaths
2002Tennessee, USA RestaurantPontiac Fever1000
2001 Murcia, SpainDowntown neighbourhoodLegionnaires' Disease7454
2000Melbourne, AustraliaMelbourne Aquarium Legionnaires' Disease1132
1999NetherlandsFlower ShowLegionnaires' Disease1884
1998Melbourne, AustraliaVictoriaLegionnaires' Disease2700
1997SingaporeAir-conditioned office buildings Sick Building Syndrome5600
1985EnglandStafford HospitalLegionnaires' Disease10128

How dangerous is Legionnaires' Disease?

Research has indicated that Legionella is an opportunistic pathogen, which (theoretically) means that under normal circumstances, it does not cause full-blown Legionnaires' Disease, and will only strike when conditions are right. Originally, it was reported that Legionnaires' Disease is usually contracted through inhalation of aerosols of water droplets containing the organism, and that those who get the disease are primarily the old, the very young, and the immunocompromised, with death occurring in 15 - 30% of the cases. However, there have been several intriguing cases throughout the history of this disease. Four supposedly healthy middle-aged men who worked at an automotive plant in Ohio contracted Legionnaires' Disease in 2001, two of whom died. A medical practitioner supposedly caught Legionnaires' Disease from his pneumonic patient in what was possibly the first person-to-person transmission case of the disease. In the USA, three people came down with the disease, and the source of the organism (L. longbeachae) was traced back to commercial potting soil, which all three had been using. This was not the first case where the organism originated from soil - in 1984, a gravedigger contracted the disease, and Legionella was isolated from the graveyard soil. This suggests that Legionella may be found in more habitats and spread in more ways that we have not yet discovered, and that even supposedly healthy individuals may fall victim to Legionnaires' Disease6.

It would be ridiculous to say that Legionnaires' Disease is not important, or that it does not - and never will - pose as significant a threat as, say, Bubonic Plague or tuberculosis. For just as the plague thrived at a time when regulations on sanitation did not exist and hygiene was practically nil, and that the typhoid fever and cholera organisms still glory in contaminated waters, Legionnaires' Disease is an important disease of the modern world, encouraged and spread by modern technology itself. Even if we try to comfort ourselves by saying that Legionnaires' Disease will only plague the aged and the sick, the fact remains that our modern environment is no longer friendly to our immune system. Smog and haze and noxious fumes emitted by vehicles and machines are among the biggest threats to our lungs' primary defence systems, thus making even the most healthy of us susceptible to infection (of course the degree of susceptibility will differ). Even as children are being born everyday, our citizens grow old and become increasingly prone to disease. The water supply in many places such as Melbourne, Australia have already become tainted with Legionella infestation, and conditions are hard to reverse. The emergence of high-rise buildings with centralised air-conditioning systems and the increasing usage of misting machines, spas and air-conditioning (among others) facilitate propagation of the agents of this disease. Even if the people are healthy enough not to develop Legionnaires' Disease, more than 90% of them will get Pontiac Fever or Sick Building Syndrome, thus contributing to ill-productivity in the workplace and, by extrapolation, posing as an economic problem if not a health one.

However, there is no reason to panic unnecessarily. If detected early on, Legionnaires' Disease can be treated successfully by antibiotic therapy, usually erythromycin (which is a common antibiotic and has no nasty side effects) alone, but sometimes used in conjunction of rifampicin - although in more severe cases, organ damage may occur due to the spread of disease. Besides, the chances of you catching it from someone else who has it are very, very small - which you should be extremely thankful for, seeing as it's a respiratory disease and is transmitted by air.

How do I reduce the risk of catching Legionnaires' Disease?

  • Try to avoid installing misting machines in your home or office. They may offer a certain degree of comfort, but they may also offer Legionella. It would be a good idea to stay clear of them at shopping malls as well.

  • Keep your water tanks clean. Legionella are usually found in tanks that are coated with sludge and slime, and will usually live as layers of scum or as parasites of amoebae. Deny them these privileges and you will reduce the risk of these organisms breeding in your water.

  • If you are running a hospital, regularly monitor the water supply and take corrective action before anything happens - not after! In the event of an outbreak or isolation of the organism, remember to follow up on your monitoring.

  • Keep your hot water hot - that means above 70 degrees Celsius. Cold water storage and distribution temperatures should be kept below 20 degrees Celsius.

  • Chlorination of water sources can wipe out Legionella - but only if the concentration is two parts per million (ppm) or greater. Be warned, however, that studies have shown that the organism will grow in the water once again within two weeks of disinfection. Also, some studies have indicated that the organism may survive chlorination with the help of amoebae, some of which can tolerate high levels of chlorine.

  • Ozonation (0.79mg/litre or greater) of a potable water supply may be useful in eradicating Legionella . This would be more practical for large buildings than for households, however.

  • Stay healthy. Avoid smoking - smokers have the highest incidence of coming down with Legionnaires' Disease - and engaging in activities that could threaten your health. If you are immunocompromised, stay away from things that can generate aerosols such as fountains, whirlpool spas and misting devices. See a doctor if you are sick, and if you come down with pneumonia, make sure they screen you for Legionella. Under no circumstances should you play doctor and prescribe yourself antibiotics. Many of them will not work against this organism, and you may be encouraging antibiotics resistance.


  • Allan, T, H Horgan, H Scaife et al. 2001. Outbreak of Legionnaires' Disease Among Automotive Plant Workers - Ohio, 2001. MMWR Weekly, May 11; 50(18):357-9. (Also available on the CDC website).

  • Anonymous. 1987. Minimising the Risk of Legionnaires' Disease. Technical Memorandum (TM13). The Chartered Institution of Building Services Engineers, London.

  • Barbaree, JM, RF Breiman, AP Dufour (Editors). 1993. Legionella: Current Status and Emerging Perspectives. American Society for Microbiology, Washington DC

  • Bemer, P, S Leautez, E Ninin et al. 2002. Legionella Pneumophila Arthritis: Use of Medium Specific for Mycobacteria for Isolation of L. Pneumophila in Culture of Articular Fluid Specimens. Clinical Infectious Diseases 35:e6-e7.

  • Boldur, I, A Cohen, R Tamarin-Landau et al. 1987. Isolation of Legionella Pneumophila from Calves and the Prevalence of Antibodies in Cattle, Sheep, Horses, Antelopes, Buffaloes and Rabbits. Veterinary Microbiology 13:313-20.

  • Dixon, B. 1994. Power Unseen: How Microbes Rule the World. WH Freeman and Company Limited, New York.

  • Duchin, JS, J Koehler, JM Kobayashi et al. 2000. Legionnaires' Disease Associated with Potting Soil - California, Oregon and Washington, May - June 2000. MMWR Weekly, September 1; 49(34):777-8. (Also available on the CDC website).

  • Edelstein, PH, RE Whittaker, RL Kreiling et al. 1982. Efficacy of Ozone in Eradication of Legionella Pneumophila from Hospital Plumbing Fixtures. Applied and Environmental Microbiology Dec; 44(6):1330-4.

  • Fabbi, M, MC Pastoris, E Scanziani et al. 1998. Epidemiological and Environmental Investigations of Legionella Pneumophila Infection in Cattle and Case Report of Fatal Pneumonia in a Calf. Journal of Clinical Microbiology, July 1998; 36(7): 1942-47.

  • Horwitz, MA, SC Silverstein. 1981. Interaction of the Legionnaires' Disease Bacterium (Legionella Pneumophila) with Human Phagocytes. II. Antibody Promotes Binding of L. Pneumophila to Monocytes but does not Inhibit Intracellular Multiplication. Journal of Experimental Medicine 153:398-406.

  • Marra, A and H Shuman. 1992. Genetics of Legionella Pneumophila Virulence. Annual Review of Genetics 26:51-69.

  • Murray, PR, EJ Baron, MA Pfaller, FC Tenover, RH Yolken (Editors). 1995. Manual of Clinical Microbiology, 6th edition. American Society for Microbiology, Washington DC.

  • Frequently Asked Questions: Legionella Control.

  • Legionnaires' Disease: Unusual Cases.

  • Signs and Symptoms of Legionnaires' Disease.

1That is to say, it is counterstained by the Gram staining method, being unable to hold the primary stain.2One micron/micrometre is 0.000001 metres or 0.0001 centimetres.3This of course depends heavily upon the size of the water droplets as well as the number of organisms that can penetrate the lung's natural collection barriers and get to the alveoli. Large droplets are more liable to be trapped whereas smaller droplets have a high chance of getting through; however, the size of the small water droplets must be big enough to contain the bacteria. It has been determined that water droplets of 1-5 microns are most likely to carry Legionella and penetrate the mucosal layer.4This mechanism is called 'coiling phagocytosis'.5A lysosome is essentially a bag containing an arsenal of deadly enzymes that will, upon release, kill the bacteria.6It makes sense. Think of it - about the only thing that stands between you, a healthy person, and Legionella is your lung's mucosal layer. Should that layer be breached - and it would be no mean feat, in this modern age of pollution - Legionella could easily enter your lungs and wreak havoc. It does not help that your antibodies provide little defence in Legionella infections, since the organism is an intracellular parasite, and can only be dealt with by the cell-mediated immune system - in fact, antibodies may, themselves, promote the uptake of the organism by phagocytosis.

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