Background Information on Legionella and Legionnaires Disease

2.1     Background Information on Legionella and Legionnaires Disease

Legionella is the bacterium which causes Legionnaires disease. Of this bacterium,

Legionella pneumophila is one species and is the one most commonly associated with disease outbreaks. Legionnaire’s disease is identified as a pneumonia type of infection of the lower respiratory tract. The infection is most commonly acquired by the inhalation deeply into the lungs of airborne droplets or particles (<5μ) containing viable Legionella. Exposure to Legionella can also cause a short feverish illness without pneumonia known as Pontiac Fever.  Legionella principally affects those who are susceptible due to age (the very young and the very old), illness or immunosuppression.

Research and investigations indicate that the occurrence of Legionella contamination is greatest in water cooling towers, evaporative condensers, hot and cold-water services, water spray humidifiers and air washers, spa baths and pools where water is agitated and recirculated. The contamination from a cooling water tower will cover a far larger area than any other likely source.

Sediment, scale, and organic materials present in water systems can provide nutrients and give protection for Legionella. Legionella have been shown to colonise certain types of water fittings, pipework and materials used in the construction of water systems. The presence of these materials may provide nutrients for Legionella and make eradication difficult. Other organisms in water systems such as bacteria, amoeba and algae can provide a suitable nutrient and habitat in which Legionella can survive and multiply.

The presence of micro-organisms within water systems can provide ideal conditions for Legionella to grow.

Legionella is most likely to proliferate in water systems that operate at a temperature of between 20⁰C and 50⁰C. Human blood temperature of approximately 37⁰C is the ideal temperature for proliferation. Stagnant water within the above temperature range appears to provide ideal conditions to promote colonisation by Legionella.

Legionella will survive at temperatures below 20⁰C but is considered to be in a dormant state with no growth activity. The bacterium does not survive temperatures maintained consistently at 60⁰C or above.

For water samples collected and returned to the laboratory, Legionella pneumophila is recovered by propagation of the organism on a specially supplemented nutrient growth medium. Such samples are normally then incubated at around 37⁰C. It may take up to 7 days for colonies of Legionella to appear. Legionella can be recognised by visual examination of the colonies followed by a number of laboratory techniques to identify species and serogroup.