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Legionella : Legionnaires Disease A Risk Management Approach

John A. Herbert

Your building systems can harbour a particularly dangerous, potentially fatal bacteria named Legionella Pneumophila, more commonly known as Legionella.

legionella, Kelcroft, legionella risk prevention

Legionella is an aqueous organism, it lives in the nature, given the opportunity it can infest and breed in your water systems, including cooling towers, air conditioners, humidifiers, hot water systems and other water systems.

Given the right environmental conditions Legionella will rapidly multiply and reach a point where it could infect you, your staff or the general public.

Legionella Control and Risk Management is a methodology to assess your potential risk.




What is Legionnaires' disease?

How do you get Legionnaires' disease?

Where do the Legionella bacteria come from?

How do Legionella outbreaks occur?

Video briefing



Legionnaires' Disease (退 伍軍人病) is a form of pneumonia that may have serious consequences for some people, especially people in the older age groups and those otherwise susceptible to disease.

Legionellosis is a term used to describe any illness caused by the Legionella bacterium.

The bacterium responsible for legionnaires disease was initially identified in 1977, by the CDC in Atlanta, following a large outbreak at a Bellevue Stanford hotel, Philadelphia, USA in July 1976.

The disease was named from the group of people primarily affected in this outbreak. They were retired American service personnel who were attending a legion convention at the stricken hotel.

Since this first outbreak, sporadic cases and major outbreaks have been regularly reported across the globe, many of them linked to hotels, healthcare, and holiday accommodation. 

What is Legionnaires' disease?

Legionnaires disease is a severe form of pneumonia. The disease has no particular clinical features that clearly distinguishes it from other types of pneumonia, therefore only laboratory tests can confirm the diagnosis.

It normally takes between 2-10 days to develop symptoms (typically five to six days but very rarely some cases may take two to three weeks to develop symptoms).

Patients usually start with a dry cough, fever, headache and sometimes diarrhoea and then pneumonia. The literature states that people over the age of 50 are more at risk than younger people and males more than females. Although discovered nearly thirty years old, little is known, and the indications are that the age pre-disposition is not an important factor as once thought, even Neonates have contracted Legionnaires disease.

Effective antibiotic treatment is available if the diagnosis is made early in the illness. Death occurs in about 5-15% of people who get the disease, depending on their age and individual health status. Smokers are more at risk than non-smokers. If patients in hospitals contracted Legionnaires disease the mortality risk increases up to 40%. 

How do you get Legionnaires' disease?

People are infected when they breathe air that contains tiny droplets of water, known as aerosols, inside of which are the Legionella bacteria.

If the bacteria is inhaled deep into the lungs infection could follow.

Legionnaires disease cannot be caught from water you drink that enters your stomach in the normal way – the bacterium has to get into the lungs through breathing it in.

Spread from person to person has not been documented.

The infectious dose is not known, although outbreaks traced back to a source many kilometres away indicate that the infectious dose maybe only be one bacteria. 

Where do the Legionella bacteria come from?

Legionella is common, found naturally in environmental water sources such as rivers, groundwater, lakes and reservoirs, usually in low numbers.

When the bacteria enter domestic water systems used in our buildings, they sometimes cause a risk to humans if people get exposed to them through air conditioning or air cooling systems, or through contaminated water systems used for baths or showers, etc.

How do outbreaks occur?

Experience indicates that outbreaks are associated with cooling towers, or domestic water distribution systems.

If the bacteria is in the water in quantities that can cause infection, someone taking a shower would inhale the bacteria trapped inside the tiny aerosols that are created when the shower water hits the hard surfaces of the shower unit or bath. They may also be affected by other water systems that cause aerosols, for example whirlpool spas and fountains.

In contrast, large explosive outbreaks in the community are mostly associated with cooling towers. Cooling towers are devices used to cool buildings.

They are also called “wet air conditioning systems” because the process of cooling air involves extensive contact between water and air, thereby creating aerosols.

When the Legionella bacteria are present in these systems they can cause legionnaires’ disease. Air conditioning units that use water to cool the air can also pose a risk.

However, many air conditioning systems are “dry” and these pose no risk for legionnaires’ disease.

When an outbreak of legionnaires disease occurs, the source may be found through two types of investigation.

One collects information on the activities and whereabouts of the patients with legionnaires' disease to look for links between cases such as staying at or visiting the same places before they became ill.

The other involves looking for the Legionella bacteria in the suspected water sources and in clinical specimens from the patients. If the bacteria are found in both, specialised laboratory methods are used to see if they are of the same type.

Sources of Legionella Infection

The media tends to only report the explosive Legionella outbreaks which tends to be misleading when you consider the range of equipment and devices that have been associated Legionella infection.

Here is a table complied by Director John Herbert, citing devices and equipment, with references, that have caused Legionella Infection Legionnaires Disease (3 pages PDF format).



Click on this link to learn more about how Legionella causes infection and once inside the lung how Legionella grows: Legionella infection and multiplication video


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Further Information

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