About Legionella

What is Legionella?

Legionella is a bacteria which can cause a pneumonia-like disease called “Legionnaires’ Disease”. Legionnaires’ Disease is fatal in approximately 15-20% of people infected with the bacteria. Infection occurs when the bacteria is inhaled from aerosols of contaminated water. Aerosols can be produced anywhere water is splashed or sprayed but some of the most common sources associated with Legionnaires’ Disease are cooling towers and showers.

Within the Legionella genus there are approximately 50 different species. Several of these can cause infections in people, however worldwide over 90% of infections are caused by a single species: Legionella pneumophila. With in particular “serogroup 1” of pneumophila causing the vast majority (>80%) of all cases, and being implicated in virtually all major outbreaks.

Where is Legionella found?

Legionella bacteria are naturally occurring bacteria that live in soil and water. Normally the “background” level of bacteria is so low that they are unlikely to cause an infection, however once introduced to man-made water systems the bacteria can quickly multiply and present a health risk. Legionella have been found in a wide range of systems including hot and cold water systems (both large systems serving public buildings and small systems in people’s homes), cooling towers and some forms of air-conditioning system, car-washes, on-board ships, aeroplanes and trains, public fountains, food misters, whirlpool spas and even in car screenwashes.
Within a water system Legionella usually grow within biofilm (slime) found on the insides of pipework, showerheads, cooling tower packing, or water tanks. Bacteria found in the planktonic phase (i.e. suspended in the water itself) are usually an indicator of colonisation of the biofilm.

What level of bacteria are safe?

There is no conclusive evidence on an infectious dose of Legionella, and many factors contribute to determining whether a person becomes ill following exposure. However it is theoretically possible for a single organism to cause an infection. Worldwide the guidelines for responding to Legionella in water vary widely, but generally responsible building owners will take action to deal with Legionella, especially Legionella pnuemophila serogroup 1 bacteria regardless of its concentration. Under the right conditions Legionella bacteria can double in number every 90 minutes meaning that a very low level of bacteria could become thousands of times higher in one or two days.

When was Legionella discovered?

Legionnaires’ Disease was first identified following an unexplained outbreak of pneuomonia like disease at a convention of American ex-Servicemen in 1976. The cause was ultimately traced to a previously unknown bacterium, which was named Legionella. Although the discovery of the bacterium was relatively new the bacteria is believed to have been around for many thousands of years and has been implicated in much older deaths.

When does Legionella proliferate?

Legionella grows well at temperatures between about 20 and 50 deg C. Legionella can withstand freezing and come back alive, and can survive short periods at temperatures up to 60 deg C. Growth is enhanced by slow moving or stagnant water which encourage biofilm formation and rich sources of nutrients such as sludge or sediment at the bottom of tanks / calorifiers etc., rubber or plastic components, and wooden packing in cooling towers. Whilst oxidising biocides can be effective against planktonic Legionella bacteria, cells protected inside biofilm or hidden within scale or other deposits on the inside of water systems, or in dead legs with poor circulation may not receive the same level of exposure allowing the bacteria to multiply.

Who are most likely to be infected?

Anyone could become infected with Legionnaires’ Disease, but males over 50 seem to be particularly susceptible. Those with underlying medical conditions, and smokers are at greater risk.

Further reading:
Wikipedia page on Legionella
World Health Organisation Factsheet