Legionellosis (Legionnaires' disease)
Page content: Victorian statutory requirement | Infectious agent | Identification | Incubation period | Public health significance & occurrence | Reservoir | Mode of transmission | Susceptibility & resistance | Control measures | Outbreak measures | Additional sources of information
Victorian statutory requirement
Legionellosis (Group A disease) must be notified immediately by telephone or fax followed by written notification within five days.
School exclusion is not required.
Infectious agent
Legionellae are gram-negative bacilli. There are currently more than 45 known species of Legionellae. 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 over 80% of cases in Victoria.
Identification
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, 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 CXR at the time of presentation.
Other features commonly include hyponatraemia, fever greater than 40°C, renal impairment, diarrhoea and confusion.
Pontiac fever
A non-pneumonic form of the infection has been reported in other countries, presenting as a flu-like illness with fever and malaise lasting two to three days. Although there is said to be a high attack rate (95%), recovery is rapid with no reported deaths.
Method of diagnosis
Various methods of diagnosis for Legionellae 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 five days into the illness and should be repeated if the specimen was taken early in the illness and legionellosis is still strongly suspected.
Serology
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 four fold increase in antibody titre between sera taken in the acute phase and during convalescence three to six weeks later. The two samples should be tested concurrently (in parallel).
Culture
Legionellae are fastidious organisms and will not grow on conventional culture media. Culture for Legionellae 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 to 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 of Human Services.
Incubation period
The incubation period for Legionnaires' disease is two to ten days. For Pontiac fever it is 24 to 48 hours.
Public health significance & occurrence
Sporadic and epidemic forms of Legionnaires' disease occur in Australia. Legionella infections are believed to account for 5-15% of community-acquired pneumonias.
Outbreaks in Australia are generally associated with man-made 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
Legionellae are ubiquitous in the environment. They are often isolated from water and wet areas in the natural environment such as creeks, hot springs, seawater, woodchips, mulch and soil. Potting mix is often colonised with Legionella species, particularly
L. longbeachae.
Legionellae also thrive in man-made water systems if the water temperature is maintained at 20°C-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. Shower-heads, nebulisers, humidifiers, ultrasonic misting systems and fountains have also been implicated.
Evaporative air conditioners like those commonly used for domestic cooling are not associated with Legionella infections.
Mode of transmission
Legionellosis is generally transmitted through inhalation of contaminated aerosols of water or of dust. Microaspiration 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 & resistance
There is a greater risk of more severe legionellosis in persons aged 50 years and over, regular smokers, and the immunosuppressed. More than 70% 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%) when compared to the 7% overall mortality rate in Victoria.
Serological surveys identify Legionella-specific antibody in 10-20% of healthy adults with no history of clinical legionellosis. It is unclear whether this antibody confers protective immunity.
Control measures
Preventive measures
Smoking is an important risk factor for developing symptomatic infection in those exposed to Legionella bacteria, and it is presumed cessation of smoking reduces an individual's risk of infection.
Although total eradication of Legionellae from all artificial systems is not possible, the risk of legionellosis can be minimised through diligent maintenance of aerosol generating equipment and ensuring appropriate placement, 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 inhalation of airborne potting mix
- moisten the contents to avoid creating dust
- wear gloves
- 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 erythromycin to cover the possibility of legionellosis is recommended.
The patient's environmental exposures during their incubation period are established by interview and compared to other cases.
Exposures of particular concern include:
- contact with hospitals and other health care facilities as a nosocomial source presents the greatest risk to others
- exposure to cooling towers
- use of spas
- use of potting mix.
The Department of Human Services routinely investigates workplaces of confirmed cases.
Control of contacts
Although there is no risk of person to person transmission,amongs 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 Legionellae sources, an immediate precautionary disinfection with an oxidizing 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
When two or more cases are linked in time and place an investigation is generally undertaken to identify likely Legionellae sources in the common area. Environmental sources sampled during the Department of Human Services' investigations such as cooling towers and spa baths are generally requested to be disinfected as a precaution while laboratory testing is conducted.
Special settings
Health care 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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