Legionnaires' disease is caused by a type of bacteria called Legionella Pneumophila.
It was first recognised in 1976 after a large outbreak of respiratory illness among delegates attending a convention in Philadelphia. There had been previous similar outbreaks in the past, but the bacteria had escaped detection because it failed to grow on the conventional culture media used at that time.
The name comes from the first recognised outbreak at the American Legion convention. The American Legion is an organisation of US war veterans.
Legionnaires disease is often described as an atypical pneumonia. The bacteria is found naturally in lakes, rivers and streams and has been found to grow in hot and cold water systems. The major concern is cooling towers used for air conditioning. The infection is acquired by inhaling droplets carrying the bacterium. It is not acquired through drinking water.
Unlike many respiratory tract infections, the disease is NOT transmitted from person to person. It usually comes from a common source and becomes a public health concern when a cluster of cases occur.
Suspicion for Legionella is often aroused when there is an outbreak of pneumonia-like disease in a geographic area. It can be confused with other outbreaks at this time eg influenza, COVID-19, but early diagnostic measures can usually rule out other causes.
The source must be identified as soon as possible.
The disease can occur at any time of the year but is more common during the summer months and Autumn.
Water sources included:
- Hot water systems in public buildings
- Hot tubs
- Spas
- Birthing baths
- Local misting devices
Guidelines from IDSA (Infectious Diseases Society of America) suggest the following individuals should always be considered as possible Legionellosis:
- Patients who have failed outpatient antibiotic therapy
- Patients with severe pneumonia, particularly those requiring intensive care
- Immunocompromised host with pneumonia
- Patients with pneumonia in the setting of a legionellosis outbreak
- Patients with a travel history [Patients that have travelled away from their home within two weeks before the onset of illness.]
- Patients suspected of healthcare-associated pneumonia
The condition may also be considered in the following circumstances.:
- Smokers
- Diabetes
- HIV/Aids
- Cancer
- Steroid use or other immunosuppressive medication
- Renal disease
- Being over 50 years of age
- Excess alcohol consumption.
It is likely that individuals are commonly exposed to low levels of the organism at work or home, but only rarely does it cause serious disease. A person diagnosed with Legionnaires is not a threat to his/her co-workers, but it is important that the case is notified.
Legionnaires disease is a notifiable disease in Australia.
There are public health guidelines in every Australian State for managing water-cooling systems to prevent Legionella colonisation.
More than 730 cases of Legionellosis have been reported in Australia in the past 12 months (to July 21, 2024)
According to the World Health Organisation (WHO), the rate of occurrence is estimated to be 10-15 cases per million population. This is based on information provided by countries with good surveillance (Europe, Australia and the United States). Cases are more likely to be over 50 years and male.
It is important to note there is another organism, Legionella Longbeachiae, which is reported in Australia. This type is associated with potting mix, gardening soils, mulches, composts and soil conditioners. The individual is exposed by aerosol transmission. It is an infrequent cause of pneumonia in Australia.
