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What is Legionnaires’ disease?

Legionnaires’ disease is an uncommon infection of the lungs (pneumonia) caused by Legionella bacteria. The bacteria are commonly found in the environment, particularly water and soil. Infection occurs two to 10 days after a person breathes in the bacteria in contaminated water vapours or dust.

What are the symptoms of Legionnaires’ disease?

Symptoms of Legionnaires’ disease include fever, chills, a cough and shortness of breath. Some people also have muscle aches, headache, tiredness, loss of appetite and diarrhoea. People can become very sick with pneumonia; most people recover, but around 10 per cent of patients die.

How is it caused?

There are many different species of Legionella bacteria but the two that most commonly cause disease in NSW are Legionella pneumophila (found in water) and Legionella longbeachae (found in soil and potting mix).

Legionella pneumophila bacteria can contaminate air conditioning cooling towers, whirlpool spas, shower heads and other bodies of water. Larger outbreaks tend to be associated with contaminated cooling towers. Cooling towers are part of cooling water systems and these are used in air-condition systems in large buildings.

Legionella longbeachae can contaminate soil or potting mix. People may be exposed to the bacteria at home, at work or in public places. Legionnaires’ disease is not normally spread from person to person.

Who is at risk?

Legionnaires’ disease most often affects middle-aged and older people, particularly those who smoke or who have chronic lung disease. People whose immune systems are suppressed by medications or diseases such as cancer, kidney failure, diabetes or AIDS are also at increased risk.

What is the NSW Health system doing about the Legionnaires’ outbreaks?

NSW Health’s public health units actively investigate cases of Legionnaires’ disease. Their investigation includes:

  • interviewing cases or their families about possible exposures
  • checking the diagnosis with the doctors and laboratories
  • actively seeking additional cases that may have not been reported
  • where a cluster is identified, working with councils to inspect and, if necessary, investigate and test suspicious sources, such as dirty cooling towers


Following investigations into the Legionella outbreaks in the Sydney CBD in 2016, NSW Health established the Legionella Taskforce to consider changes to the already strong regulatory approach to preventing Legionnaires’ disease in NSW. The current requirements for controlling the risk of legionella in cooling towers in NSW are described here: Legionella control.

What precautions do people need to take to reduce their chance of contracting Legionnaires’ disease?

There are no specific precautions people should take in relation to the current clusters, however susceptible people who have been in areas where Legionnaires’ disease clusters have been detected should seek medical attention if they develop the following symptoms: fever, chills, cough, headache, aching muscles, lethargy. Legionnaires’ disease most often affects middle-aged and older people, particularly those who smoke or who have underlying health conditions such as chronic lung disease.

While exposure to contaminated water vapour from cooling towers and other sources may lead to a person to contracting the disease, Legionnaires’ disease can also be contracted through exposure to soil and potting mix where Legionella longbeachae may be found. NSW Health recommends reducing exposure to potting mix dust by following manufacturers’ warnings present on potting mix labels, including:

  • wet down the potting mix to reduce the dust
  • wear gloves and a P2 mask when using potting mix
  • wash your hands after handling potting mix or soil, and before eating, drinking or smoking.

What causes an outbreak of Legionnaires’ disease?

Although 10 per cent or more of cooling towers may be contaminated in some cities at some times, most are never found to cause outbreaks of disease. The reason why some cooling towers are associated with outbreaks is unclear, but probably relates to a range of conditions occurring coincidentally. These are likely to include: contamination of the cooling tower; weather conditions such as the level of humidity, sunlight, temperature and wind direction, and susceptible people being in a position where they can breathe in contaminated water vapour.

Most people who breathe in contaminated water vapour will never get sick. The people most likely to become ill are those who smoke, have underlying illnesses and who are elderly.

Internationally, outbreaks of Legionnaires’ disease can vary in size, ranging up to hundreds of identified patients. In 2015 in New York City, 133 patients were reported with Legionnaires’ disease, 16 of whom died. In 2000 in Melbourne, 125 patients with Legionnaires’ disease were reported in association with an outbreak linked to the Melbourne Aquarium.

What is the public health response to Legionnaires’ disease cases?

Laboratories and hospitals are required to confidentially notify cases of Legionnaires’ disease to public health units in NSW. Public health unit staff carefully interview patients or their carers about their illness and possible exposures. This often includes multiple interviews to identify all possible locations where they travelled, worked, stayed or visited during the two to 10 days before onset of illness. Public health epidemiologists then map these locations and compare them closely with the exposures reported by other patients who have recently been diagnosed with Legionnaires’ disease.

As the bacteria that cause Legionnaires’ disease are common in the environment, it is rarely possible to know how an individual patient became infected. However, where multiple patients are infected, public health epidemiologists search for common areas of exposure. Should multiple patients be diagnosed with the disease, then potential sources of infection can be more accurately pinpointed.

What happens when two patients report common exposures?

Two people with the illness visiting the same place, such as a busy shopping centre, may be coincidental and does not necessarily indicate that’s where they were infected, as people may report common exposures by chance. However public health units take a precautionary approach to such reports. Where two cases report a common exposure, public health unit staff work with local council environmental health officers to review possible sources of infection (such as cooling towers) in the area to assess whether they may have been a risk for infection and, if necessary, require them to be cleaned.

In addition, alerts may be issued to laboratories and doctors serving the area where patients have been reported to check whether other patients may have the disease. This is called active surveillance. Where there is no reasonable evidence that the common exposure area reported by the two cases is the location at which they were infected, public health warnings are not usually necessary. There is the risk that such warnings may have quite negative effects on the community if they inappropriately imply that a certain area was the cause of an outbreak, adversely affecting local businesses.

What happens when three patients report common exposures?

Should active surveillance identify more than two cases reporting common exposures, then there is an increased probability that the source of infection may be in that location. In response, public health units continue with the actions identified above and, in the absence of an alternative source, a public health media alert may be issued to raise awareness about the disease to encourage early diagnosis of other cases, and remind building owners of the need to ensure cooling towers are clean and disinfected. Additional steps including specific alerts may be issued directly to cooling tower operators.

When are deaths publicly reported?

It’s tragic when somebody dies from a communicable disease. Around 10 per cent of people with Legionnaires' disease die despite treatment. Deaths tend to be more common in patients with underlying disease. The proportion of people who die with Legionnaires disease is not related to whether or not the person acquired the infection as part of an outbreak or not. The extent and severity of an outbreak is usually measured by the number of patients who have been affected, rather than the number of deaths.

Strict privacy rules apply to the release of information about patients and it’s important that patients know their privacy is respected. Even if their name is not included, the release of other information about a patient (for example, their date of birth, sex, disease, risk factors and treating hospital) could allow other people to identify that person and infer private information about them. When a death occurs in someone with Legionnaires’ disease, there is need for careful consideration of whether the release of that information would help protect public health, or risk breach of privacy.

How can you determine the source of an outbreak?

It is often very difficult to pinpoint the source of an outbreak with accuracy. The main aim of a public health response is to stop the outbreak continuing as soon as possible. To do this cooling towers and other possible sources of infection in a location suspected to be the cause of the outbreak need to be identified and controlled as quickly as possible. Building owners are warned to maintain cooling towers to ensure they are free of contamination, and environmental health officers carry out door to door inspections of cooling towers.

A cooling tower that is the source of an outbreak may not be identified despite careful investigation. This is because cooling towers may be only transiently contaminated by Legionella bacteria floating through the air, and water vapour from that cooling tower may infect people walking by, as well as contaminate other nearby cooling towers. However continuous disinfection and regular cleaning processes may automatically decontaminate cooling towers, even before infected patients are diagnosed. Should the first cooling tower be tested, it may therefore test negative for Legionella (because it’s been automatically disinfected), even though it’s the real source of patients’ infection, while the nearby cooling towers (contaminated by the first cooling tower, but not yet disinfected) may test positive, even if they have not caused any infections in patients.

NSW Health Pathology has developed special tests (such as genomic sequencing) that can match the strains of bacteria found in patients and in cooling towers. However even if a patient’s strain matches a cooling tower strain, it does not prove it was the source of infection, as both the patient and that cooling tower may simply have been contaminated by another cooling tower that may never be identified (e.g. because it had already been disinfected).

Current as at: Monday 8 January 2024
Contact page owner: One Health