Causes of Legionnaires’ disease

Legionnaires’ disease is a pneumonia or lung infection that is commonly caused by Legionella species of bacteria. (1-8)

The disease is mostly caused by the Legionella pneumophillia.

Legionella pneumonia and other microbes

In legionella pneumonia other microbes may co-infect the lung simultaneously. Common such organisms are tuberculosis bacteria, viruses, fungi and other bacteria.

There are around 40 different species of legionella bacteria that have been identified. Other than L. pneumophillia, others cause milder disease and are reported less often.

Where are Legionella species found?

Legionella species of bacteria commonly live in natural sources of water like rivers and lakes etc. These may find their way into artificial cooling or heating or water works of large buildings like office blocks, hospitals, hotels, shopping malls etc.

How do the bacteria cause Legionnaires’ disease?

Disease occurs when the bacteria enters the breathing atmosphere as fine particles or as small contaminated water droplets in air. These are inhaled deep into the lungs where they multiply and cause the infections.

Other causative organisms

Legionella longbeachae is another causative organism present in soil and compost. Inhaling airborne soil particles that carry Legionella may cause the disease.

What conditions do Legionella bacteria grow in?

Legionella bacteria grow particularly well in water temperature of between 20-45ºC (68-113ºF). The bacteria also prefer water that contains impurities like rust, algae, sludge, amoebae, slime, biofilm, lime scale, corrosion products or other organic matter and other bacteria.

Further conducive environment is where there is stagnation or low water turnover and degraded plumbing materials such as rubber fittings that provide nutrients to enhance legionella growth.

Sources of infection with Legionella bacteria

Sources of infections are:

  • air conditioning systems
  • cooling towers
  • hot and cold water systems spas
  • misting machines
  • fountains
  • spray systems
  • fire extinguishers etc.

Other less common Legionella carriers include:

  • evaporative condensers
  • humidifiers or foggers and water misting systems
  • coolant in industrial milling machines
  • high pressure cooling and cleansing processes
  • potable water aerosols, such as shower heads, nebulizers and ventilator systems in hospitals etc.

Air conditioning systems, especially if poorly maintained, are usually a common cause of the disease. The disease is not spread from person-to-person.

A milder form of Legionella infection is Pontiac fever. Nearly half of the Pontiac fever outbreaks are associated with whirlpool spas or hot tubs.

Who is at risk of Legionnaires’ disease?

Those who are particularly at risk of getting Legionnaires’ disease include:

  • Men, as they are two to five times more likely to get the infection than women
  • Those over the age of 50 are at a greater risk especially if they have other pre-existing ailments
  • Smokers
  • Those with cancers, especially those of lung and blood
  • People with weak immune systems, especially in AIDS patients
  • Those with diabetes
  • Those with chronic lung, liver or kidney disease
  • Alcohol abusers
  • Those with heart disease and heart failure
  • Those who have recently had surgery
  • Those on steroids or related drugs that supress the immune system



Further Reading

  • All Legionnaires' Disease Content
  • Legionnaires’ disease – What is Legionnaires’ disease?
  • Symptoms of Legionnaires’ disease
  • Diagnosis and treatment of Legionnaires’ disease
  • Prevention of Legionnaires’ disease

Last Updated: Apr 19, 2019

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.

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