Legionnaires’ disease is a potentially fatal lung infection (pneumonia) that is caused by the bacteria legionella. The condition is called legionnaires’ disease because it was first identified after a mass outbreak at a hotel hosting a convention of a veteran organisation known as the American Legion.

Legionella

Legionella bacteria is commonly found (often in low numbers) in sources of water, such as rivers and lakes. The bacteria sometimes find their way into artificial water supply systems, such as air conditioning systems, hot and cold water services, and cooling towers. Given the right conditions, legionella bacteria can rapidly spread, contaminating these water systems.

Legionnaires’ disease is not contagious

Legionnaires’ disease is contracted by breathing in small droplets of contaminated water. However, the condition is not contagious and cannot be spread directly from person to person.

Large buildings such as hotels, hospitals, museums and office blocks, are more vulnerable to legionella contamination because they have larger, more complex water supply systems, in which legionella contamination can quickly spread.

The actual number of cases of Legionnaires’ disease may be much higher than the reported number of cases. This may be due to people with mild symptoms of the condition being misdiagnosed with more common forms of pneumonia and so these cases may not be reported to the health authorities.

Prognosis

In particularly vulnerable people, such as the elderly, or people with a pre-existing health condition, Legionnaire’s disease can be very serious. An estimated 10% of people who contract Legionnaires’ disease will die from complications arising from infection. Prompt treatment using antibiotics is essential in treating Legionnaires’ disease and reducing the risk of death.

Symptoms of Legionnaires’ disease

The symptoms of Legionnaires’ disease can begin anytime from between 2-10 days after exposure to the initial infection. However, 3-6 days is the most common incubation period. Symptoms usually begin with an initial phase lasting 1-2 days, in which you experience mild headaches, and muscle pain.

This is followed by the onset of more severe symptoms including:

• high fever, usually a temperature of 40C (104F), or above,
• more severe muscle pain, and
• chills.

Once the bacteria begin to infect your lungs, you may also experience:

• a persistent cough, which is usually dry at first but as the infection develops you may start coughing up mucus or possibly blood,
• shortness or breath, and
• chest pains.

About 30% of people with Legionnaires’ disease will also experience gastrointestinal symptoms including nausea, vomiting, diarrhoea, and loss of appetite. About half of those with Legionnaires’ disease will also experience changes to their mental state, such as confusion.

The symptoms of a high fever are almost always due to infection. While it is unlikely to be the result of Legionnaire’s disease, if you have a high fever you should contact your doctor as soon as possible

Causes of Legionnaires’ disease

Legionella bacteria can be found in any freshwater environment, such as rivers and lakes. The bacteria are usually only present in the low numbers because the temperature of the water is often too cold for the bacteria to grow and spread.

However, if the bacteria manage to find their way into an artificial water system, given the right circumstances, they can quickly grow and reproduce, leading to a wide spread contamination of the water system.

The two things the bacteria require to grow and reproduce are:

• a water temperature of between 20-45C (68-113F), and
• impurities in the water that the bacteria can use for food, such as rust, sludge, algae, and limescale.

Water systems known to be vulnerable to legionella infection include:

• hot and cold water systems for large buildings, such as hotels and hospitals,
• air conditioning systems that use water for cooling purposes, and
• cooling towers.

However, any artificial water system is potentially vulnerable to infection. Cases of Legionnaires’ disease have been reported as arising from contaminated:

• baths and showers,
• fountains,
• sprinkler systems,
• whirlpool baths,
• spas, and
• humidifiers that were being used in food display cabinets.

Legionnaires’ disease can be contracted by inhaling contaminated water – for example, by directly drinking the water, or by inhaling small droplets of water mist that are present in the air.

Risk factors

Everyone is potentially vulnerable to Legionnaires’ disease, but there are a number of risk factors that make it more likely that certain individuals will experience a more severe form of the infection. These risk factors include:

• being 50 years of age, or over,
• being a smoker, or having a past history of heavy smoking,
• having diabetes,
• having kidney disease, and
• having cancer, particularly lung cancer, or leukemia.

Having a pre-existing lung condition, such as chronic obstructive pulmonary disease (COPD) is also a risk factor for Legionnaire’s disease.

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