Legionnaires’ disease

Legionnaires’ disease is a bacterial respiratory infection that can cause pneumonia. It is caused by Legionella bacteria which grow in poorly maintained water systems. Infection occurs when microscopic droplets containing the pathogens are inhaled.

At a glance

  • Legionnaires’ disease is a bacterial respiratory disease caused by Legionella bacteria which can cause severe pneumonia.
  • Older people, people who are chronically ill and people with an immune deficiency are most at risk of becoming infected.
  • Most infections come from contaminated water systems. Transmission from person to person almost never occurs.
  • An infection frequently requires hospital treatment with antibiotics. Rehab and aftercare depend on the severity of the infection.
  • People with Legionnaires’ disease normally make a complete recovery but it is fatal in around 5 to 10% of people.

Note: The information in this article cannot and should not replace a medical consultation and must not be used for self-diagnosis or treatment.

Hand in protective glove showing a positive Legionella antigen test.

What is Legionnaires’ disease?

Legionnaires’ disease, also referred to as legionellosis, is a bacterial respiratory infection that may or may not involve pneumonia.

It is caused by Legionella bacteria. These bacteria can reproduce at certain temperatures, for example in water pipes, and enter people’s airways as microscopic water droplets (aerosols).

Older people, people who are chronically ill and people with an immune deficiency are most at risk of becoming infected with Legionnaires’ disease. Smoking also increases the likelihood of becoming infected.

Infections should always be treated with specific anti-Legionella antibiotics. Hospitalization is often necessary.

There is also a mild form of legionellosis known as Pontiac fever. This is similar to a flu-like infection and often does not involve pneumonia. Most cases of Pontiac fever clear up by themselves within a week.

Interesting fact: Legionnaires’ disease is named after a convention held by the ‘American Legion’ of war veterans in 1976. Large numbers of older people fell ill with severe pneumonia; the source of the infection was the contaminated air conditioning plant at the convention hotel.

What are the symptoms of Legionnaires’ disease?

The symptoms of a Legionella infection include:

  • coughing
  • high temperature
  • muscle pain
  • abdominal pain and diarrhea in some cases

The typical signs of pneumonia may occur 2 to 10 days after infection. These include

  • sudden high temperature and shivering
  • severe feeling of discomfort
  • shortage of breath (dyspnea)
  • coughing and sputum
The symptoms of Legionnaires’ disease (legionellosis) with lung inflammation are sudden high temperature (fever) and shivering, severe discomfort, dyspnea, coughing and expectoration.

Confusion may also occur in older people.

How is Legionella passed on?

Legionnaires’ disease is caused by infection with Legionella bacteria. The bacteria are transmitted via aerosols, mists of microscopic water droplets containing the pathogen, and enter the airways when inhaled.

Older or poorly maintained hot water systems provide favorable conditions for the bacteria to build up and form a ‘biofilm’ of slime in the water pipes. Potential sources of infection include contaminated showers, jacuzzis, humidifiers or cooling towers.

Infection is also possible in rare cases where water containing the pathogens accidentally enters the windpipe instead of the esophagus when swallowed. However there is no risk of infection when drinking the water in the normal way as Legionella bacteria are killed by gastric acid in the stomach. Infection from person-to-person is also almost unheard of. So people who have been infected do not pass the infection on to others.

Important: Legionella are widespread environmental pathogens and occur in small amounts in surface and ground water as well as a wide range of other wetland habitats. They do not usually cause any problems for human beings in their natural eco-systems.

How common is Legionnaires’ disease?

Just under 1,800 cases of Legionnaires’ disease (legionellosis) were reported in Germany in 2019.

Just under 1,800 cases of Legionnaires’ disease (legionellosis) were reported in Germany in 2019.

Around 2 to 4% of all cases of pneumonia are caused by Legionella. However not all people with pneumonia are tested for Legionella, so the actual number of cases of Legionnaires’ disease will be higher.

More cases of Legionnaires’ disease (legionellosis) occur in summer and autumn than in winter and spring. Most of these are individual cases, but minor outbreaks are occasionally reported, for example in hotels or on cruise ships, from a single source. 

People most at risk of Legionnaires’ disease include:

Men are two to three times more likely to be infected than women.

How can Legionnaires’ disease be prevented?

The most important thing is to prevent the Legionella bacteria from reproducing in water piping systems. Buildings should be constructed properly and in accordance with building regulations, and water systems should be properly maintained. There are special guidelines on this for large-scale and commercial facilities.

Water pipes in residential buildings should be used regularly. Basins full of stagnant water (reservoirs) as well as dead-end pipes resulting, for instance, from disconnecting a sink or moving a basin, should be avoided.

Legionella mainly occurs at temperatures between 25 and 45 degrees. Their growth is inhibited when temperatures fall below 20 degrees or rise above 55 degrees; they die at temperatures above 60 degrees. This means that the temperatures in the hot water system should not be below 55 degrees at any point. In order to achieve this the temperature controls on the central hot water tank in a residential property must be set to a minimum of 60 degrees.

There is no vaccination available for Legionella. There is only a limited amount of preventive (prophylactic) measures that can be taken on an individual basis.

How is Legionnaires’ disease diagnosed?

Doctors will ask the patient questions and perform physical examinations in order to diagnose Legionnaires’ disease (legionellosis).

There are also a number of laboratory-based diagnostic methods available. In many cases, Legionella can be detected indirectly in rapid urine tests.

It is also possible to determine the presence of the bacteria by taking a sample of sputum and either growing a culture in the laboratory on special growth media or using a PCR test to detect pathogen DNA in the sputum.

An X-ray is also often taken of the lung.

How is Legionnaires’ disease treated?

Legionnaires’ disease is treated with “macrolide” antibiotics such as azithromycin or clarithromycin. Doctors will often start the treatment by venous injection and then switch to tablets after a few days. Treatment in hospital is often necessary. 

In severe cases fluoroquinolone antibiotics such as levofloxacin or moxifloxacin can also be used. However, use of these drugs requires detailed consideration beforehand due to the risk of severe adverse effects.

Treatment should be initiated as quickly as possible and usually lasts 1 to 2 weeks but may take longer. If the treatment is effective, most people make a full recovery from Legionnaires’ disease (legionellosis) with pneumonia.

People with Pontiac fever do not need to be treated with antibiotics.

Reviewed by the German Legionella Reference Laboratory. As at:

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