Department of Health

Key messages

  • Legionellosis is an ‘urgent’ notifiable condition that must be notified immediately to the Department of Health by medical practitioners and pathology services.
  • Legionellosis presents as a febrile illness or pneumonia, which may be severe.
  • Outbreaks in Australia are generally associated with manufactured water systems, including water cooling towers and spa baths.
  • Smokers and immunocompromised people are at increased risk of contracting legionellosis.

Notification and school exclusion requirements for legionellosis

Legionellosis is an ‘urgent’ notifiable condition and must be notified by medical practitioners and pathology services immediately by telephone upon initial diagnosis (presumptive or confirmed). Pathology services must follow up with written notification within 5 days. This is a Victorian statutory requirement.

Exclusion is not required for children at primary school and children’s services centres.

Legionellosis species

Legionella species are gram-negative bacilli. There are currently more than 45 known species of Legionella. Those that are known to cause disease in Australia include L. pneumophila, L. longbeachae, L. micdadei and L. bozemanii.

L. pneumophila serogroup 1 causes most cases in Victoria.

Identification and incubation period

Clinical features

Legionellosis has 2 recognised presentations - Legionnaires’ disease and Pontiac fever.

Legionnaires disease

  • Legionnaires’ disease - This is the pneumonic form of the illness. There is often a severe flu-like prodrome, with anorexia, nonproductive cough, malaise, myalgia and fever. Upper respiratory tract symptoms such as runny nose and sore throat are rare.
  • Patients may present with any form of pneumonia. As a group, they are more likely than other community-acquired pneumonias to fulfill criteria for severe disease. There are nearly always radiographic changes on chest X-ray at the time of presentation.

Other features commonly include hyponatraemia, fever greater than 40 degrees celsius, renal impairment, diarrhoea and confusion.

Pontiac fever

  • A non-pneumonic form of the infection has also been reported, presenting as a flu-like illness with fever and malaise lasting 2–3 days.
  • Recovery is usually rapid.

Incubation period

The incubation period for Legionnaires’ disease is 2–10 days, most commonly 5–6 days.

For Pontiac fever, the incubation period is usually 24–48 hours.


Methods of diagnosis for Legionella infection include urinary antigen testing, serology, culture and nucleic acid testing.

Urinary antigen testing

  • Legionella urinary antigen testing is rapid and sensitive, but most tests in use only detect the most common serogroup, L. pneumophila serogroup 1.
  • The urinary antigen test may not become positive for up to 5 days into the illness, and should be repeated if the specimen was taken early in the illness and legionellosis is still suspected.


  • Positive Legionella antibody results (both IgG and IgM) are common in healthy adult populations.
  • The presence of antibodies is not necessarily indicative of recent infection, especially in acute-phase sera.
  • Diagnosis is made by the observation of a fourfold increase in antibody titre between sera taken in the acute phase and during convalescence, 3–6 weeks after the first specimen. The 2 samples should be tested concurrently (in parallel).


  • Legionellae are fastidious organisms and will not grow on conventional culture media.
  • Culture for Legionella must be specifically requested if the illness is suspected.
  • Culture is the gold standard and the only method by which human specimens can be compared with environmental samples. Culture should be attempted for public health reasons even if the diagnosis has already been made by other means.

Nucleic acid testing

  • Detection of Legionella bacteria DNA in respiratory specimens, sterile site fluid, or fresh biopsy specimens using polymerase chain reaction (PCR) techniques is available in some laboratories.
  • The sensitivities and specificities of such tests vary.

Public health significance and occurrence of legionellosis

Sporadic and epidemic forms of Legionnaires’ disease occur in Australia. Legionella infections are believed to account for 5–15 per cent of community-acquired pneumonias.

Outbreaks in Australia are generally associated with manufactured water systems, including water cooling towers and spa baths.

Home and institutional warm water systems are potential sources of Legionella infection, but are only rarely implicated in Australia. Legionella outbreaks due to contaminated warm water systems are regularly reported from other countries.

Reservoirs for Legionella

Legionella are prevalent in the environment. They are often isolated from water and wet areas in the environment, such as:

  • creeks
  • hot springs
  • sea water
  • woodchips
  • mulch
  • soil
  • potting mix (often colonised with Legionella species, particularly L. longbeachae.

Legionella also thrive in manufactured water systems if the water temperature is maintained at 20–43 degrees celsius, which favours the proliferation of the bacteria.

These may include cooling water towers associated with air-conditioning and industrial processes, spa baths and household warm water systems for bathing. The following reservoirs have also been implicated:

  • showerheads
  • nebulisers
  • humidifiers
  • ultrasonic misting systems
  • car washes
  • fountains.

Evaporative air-conditioners, such as those commonly used for domestic cooling, are not associated with Legionella infections.

Transmission of Legionella

Legionella are generally acquired through inhalation of contaminated aerosols of water or of dust. Micro-aspiration of contaminated water may be an important mode of transmission in certain subgroups, such as intubated patients and those receiving nasogastric feeding.

In general, Legionella does not spread from human-to-human transmission.

Susceptibility, prevention and control measures

There is a greater risk of more severe legionellosis in people aged 50 years and over, regular smokers and the immunosuppressed. More than 70 per cent of infections in Victoria occur in patients over 50 years of age. The disease is extremely rare in children.

Nosocomial infections and infections in severely immunosuppressed patients have a high case-fatality rate (up to 40 per cent).

Serological surveys identify Legionella-specific antibody in 10–20 per cent of healthy adults with no history of clinical legionellosis. It is unclear whether this antibody provides protective immunity.

Preventive measures

Smoking is an important risk factor for developing symptomatic infection in those exposed to Legionella bacteria, and it is presumed that cessation of smoking reduces an individual’s risk of infection.

Although total eradication of Legionella from all artificial systems is not possible, the risk of legionellosis can be minimised through diligent maintenance of aerosol-generating equipment, as well as ensuring appropriate placement and design, and compliance with legislation requirements by owners.

Only sterile water should be used in the preparation of aerosol solutions for use in nebulisers or humidifiers. Flushing and instillation of drinking water through nasogastric tubes in intubated or immunosuppressed patients should also only be performed with sterile water.

To minimise the risk of infection through potting mix, compost or other gardening materials gardeners should be advised to:

  • open the bag with care to avoid inhaling airborne potting mix
  • moisten the contents to avoid creating dust
  • wear gloves and an appropriate mask
  • wash hands after handling potting mix, even if gloves have been worn.

Control of cases

Early antibiotic treatment improves survival. Australian treatment guidelines for of severe pneumonia include empirical cover for legionellosis.

A case's environmental exposures during their incubation period are established by interview and compared with other cases.

Exposures of particular concern include:

  • contact with hospitals and other healthcare facilities, as a nosocomial source presents the greatest risk to others
  • exposure to cooling towers
  • use of spas
  • use of potting mix.

Although there is no risk of person-to-person transmission, active case finding may be undertaken in co-exposed individuals in certain high-risk settings.

Control of environment

Environmental investigation is routinely undertaken for cases of legionellosis with non-longbeachae species with Victorian exposures.

The purpose of environmental investigation and disinfection is to ensure potential sources of legionella infection are controlled to minimise the ongoing risk to others.

All cooling towers in Victoria are required by law to be registered, and to undergo regular maintenance and water testing. Records of treatment may be sought, and further disinfection may be required, depending on the circumstances of the case and in accordance with regulations.

Department outbreak measures for legionellosis

Legionella outbreaks and clusters (2 or more cases linked in time and place), are assessed and investigated by the department on a case-by-case basis. In special settings, such as healthcare facilities, when a nosocomial source is suspected, immediate testing and disinfection of possible sources may be undertaken, and active case finding conducted.

Reviewed 08 October 2023


Contact us

Do not email patient notifications.

Communicable Disease Section Department of Health GPO Box 4057, Melbourne, VIC 3000

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