Key messages

  • Legionellosis is a Group A disease that must be notified immediately.
  • Legionellosis has two recognised presentations: Legionnaires’ disease and Pontiac fever. Only Legionnaires’ disease has been reported in Australia.
  • Legionellosis patients present as pneumonia cases.
  • 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 requirement for legionellosis

Legionellosis (Group A disease) must be notified immediately by telephone or fax, followed by written notification within 5 days.

This is a Victorian statutory requirement.

Primary school and children’s services centres exclusion for legionellosis

Exclusion is not required.

Infectious agent of legionellosis

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 has 16 identified serogroups. L. pneumophila serogroup 1 has been identified as the cause of more than 80 per cent of cases in Victoria.

Identification of legionellosis

Clinical features

Legionellosis has two recognised presentations: Legionnaires’ disease and Pontiac fever. Only Legionnaires’ disease has been reported in Australia.

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 °C, renal impairment, diarrhoea and confusion.

Pontiac fever

A nonpneumonic form of the infection has been reported in other countries, presenting as a flu-like illness with fever and malaise lasting 2–3 days. Although there is said to be a high attack rate (95 per cent), recovery is rapid, with no reported deaths.


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

Urinary antigen testing

The Legionella urinary antigen test is the most rapid and sensitive test currently available, but will only detect the most common serogroup, L. pneumophila serogroup 1. The 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 strongly 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 significant fourfold increase in antibody titre between sera taken in the acute phase and during convalescence, 3–6 weeks after the first specimen. The two 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. Sputum samples for culture should be attempted for public health reasons even if there are already positive serological or urinary antigen results.

Nucleic acid testing

Detection of Legionella bacteria DNA in clinical specimens using polymerase chain reaction (PCR) techniques is now available in some reference laboratories. The sensitivities and specificities of such tests are variable. Legionella PCR requests should be discussed with the department.

Incubation period of legionellosis

The incubation period for Legionnaires’ disease is 2–10 days, most commonly 5–6 days. For Pontiac fever, the incubation period is most commonly 24–48 hours.

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.

Legionellosis in hospitalised and severely immunosuppressed patients carries a much higher case-fatality rate.

Reservoir for Legionella

Legionella are ubiquitous in the environment. They are often isolated from water and wet areas in the natural environment, such as creeks, hot springs, sea water, woodchips, mulch and soil. Potting mix is often colonised with Legionella species, particularly L. longbeachae.

Legionella also thrive in manufactured water systems if the water temperature is maintained at 20–43 °C, 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. Showerheads, nebulisers, humidifiers, ultrasonic misting systems, car washes and fountains have also been implicated.

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

Mode of 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.

No human-to-human transmission has been recorded.

Susceptibility and resistance to legionellosis

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 much higher case-fatality rate (up to 40 per cent), compared with the 7 per cent overall mortality rate in Victoria.

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 confers protective immunity.

Control measures for legionellosis

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.

To minimise the risk of infection through potting mix, 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.

The same measures are also advisable when handling other gardening material, such as compost.

Only sterile water should be used in the cleaning of nebuliser medication chambers, and 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.

Control of case

Early antibiotic treatment improves survival. Empirical treatment of severe pneumonia with azithromycin to cover the possibility of legionellosis is recommended.

The patient’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.

The department routinely investigates cooling towers in proximity to a confirmed case’s usual residence.

Control of contacts

Although there is no risk of person-to-person transmission, an active search for other people who may have been exposed to the same environmental source is commonly undertaken as part of the investigation of cases.

Control of environment

After sampling of suspected environmental Legionella sources, an immediate precautionary disinfection with an oxidising biocide is undertaken. Disinfection may be impractical and omitted if the source is organic, such as garden potting mix.

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.

Outbreak measures for legionellosis

When two or more cases are linked in time and place, an investigation is generally undertaken to identify likely Legionella sources in the common area. Environmental sources sampled during the department’s investigations, such as cooling towers and spa baths, are generally requested to be disinfected as a precaution while laboratory testing is conducted.

Special settings – healthcare facilities

When a nosocomial source is suspected, immediate testing and disinfection of possible sources is undertaken, and active case finding is conducted throughout the institution.

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