How do you get it ?
Legionnaires' disease is caused by inhaling airborne droplets or particles containing viable legionella
The legionella bacteria is to be found widely in natural waters eg lakes, rivers, streams etc.
In nature numbers appear to be too low to cause any problem. The danger arises when we provide
an environment particularly suited to this bacteria.
To appreciate the chain of events leading up to
exposure and possible infection it helps to know something about the bacteria.
The legionella family is a large one. Over forty different species of the bacteria have been identified
and legionellae-like amoebal pathogens are also proposed. Most members of this family are
However something like seven or eight are known to cause the disease and one in
particular predominates in its virulence to humans.
This particular species is referred to as Legionella pneumophila, and accounts for almost 90%
of legionnaires' disease infections. Of this latter species there are some sixteen sero groups, of which sero group 1 is the most
virulent, accounting for some 50% of L. peumophila infections .
Legionella is a water borne 'gram negative' bacteria.
This means that it is a relatively delicate organism with a
thin cell wall structure.
Typically sausage shaped, and some 2-6 microns long, usually with a thin
flagella when found in nature.
It has a high tolerance of different environmental conditions.
At below 15oC it becomes dormant. It can survive higher temperatures better than most bacteria, even up to 80oC for short periods of time.
Free legionella are destroyed easily by UV light and by a range of chemical biocides.
It has one characteristic that favours its ability to withstand adverse environmental conditions.
It has the ability to infect, grow and multiply within larger tougher cells such as amoeba and protozoa.
It also uses algae as a habitat in which to live. Eventually rupturing and releasing many legionella into
This latter behaviour can make it very difficult to eradicate the presence of legionellae once colonisation of a water
system has occurred.
Where nutrient is severely limited the interface between liquid and solid is more likely to be higher in
nutrient then the bulk of the liquid.
This will encourage the formation of biofilms (or slimes) on
otherwise clean surfaces.
In turn the biofilm will act as both nutrient source and protected environment
Thus acting as a source of outbreak for infection at a later date when conditions are
more favourable. This also has implications for the validity of sampling for legionella and for
determining the effectiveness of cleaning and disinfection operations.