Legionella bacteria are widespread in nature and mainly live in natural water systems, such as rivers and ponds. However, the conditions are rarely favourable for people to catch the disease from natural sources.
Outbreaks of the illness usually occur due to exposure to legionella growing in purpose-built systems where water temperatures often encourages growth. The most common systems linked to legionella infections include cooling towers, evaporative condensers, spa pools, and hot water systems used in all sorts of premises both commercial and residential.
Given the prevalence of this bacterium in the natural environment, it is accepted that in order to cause the disease, the number of Legionella in the water would need to be much higher than that found in most normal natural aquatic habitats.
Therefore the presence or absence of Legionella at a particular site is not sufficient for making an accurate assessment of risk. The number and species type present, should also be taken into account in order to assess the risk of infection. Furthermore the condition of the systems will determine the risk of potential bacterial proliferation including legionella bacteria.
It is also important to highlight that Legionella is a fastidious organism. This means it requires very specific conditions for growth. It is these very specific conditions that should be taken into account when conducting risk assessments, inspections, testing and sampling surveys. If we are able to prevent the specific conditions required for growth the system will be inherently hostile to legionella growth.
Whilst legionella can be found anywhere there is water, the risk of infection may be extremely low, particularly where the conditions favourable for growth are avoided and where no susceptible individuals will come into contact with the water.
Legionella Infection Dose
Like all infectious diseases, Legionellosis will require an infectious dose of legionella bacteria. It is accepted that the infectious dose for most individuals will be relatively high. However, this dose may be much lower where particularly susceptible persons are present. The HSC’s ACOP L8 document considers a domestic water system to be under control if legionella is present but maintained below 100cfu/l.
However, the exact number required for humans to become infected varies depending on the susceptibility of the individual so this guideline limit should be treated with caution. Our aim is to create systems which are completely hostile to legionella growth. Therefore our desired control limit is zero legionella.