Legionellosis is a term used to indicate all forms of infection caused by various species of gramnegative aerobic bacteria of the legionella genus. To date more than 40 species of these bacteria have been identified and approximately 90% of cases of Legionnaires’ disease are caused by the most dangerous one: Legionella pneumophila.


The term legionella originates from a tragic event that occurred during a rally of Vietnam war veterans (in jargon the Legionnaires) held in July 1976 at a hotel in Philadelphia (USA) when 221 of the around 2,000 attendees contracted acute pneumonia and 34 of them died. People were even speculating a biological attack by the Russians. Subsequently, it was discovered that the deaths were caused by the action of previously unknown bacteria that had developed in the air-conditioning system and they were fittingly given the name legionella. Following retrospective studies, numerous cases and epidemics of acute pneumonia of which the cause had not been identified were attributed to these bacteria.

Clinical forms of Legionnaires’ disease

From the clinical point of view, legionellosis can occur in two forms: Pontiac fever and Legionnaires’ disease.

Pontiac fever

Pontiac fever breaks out after a variable incubation period ranging from 1 to 2 days and is characterised by high fever, muscle aches, headache and (not always) intestinal disorders. It does not include pneumonia even if in some cases there may be coughing. This form of legionellosis is often mistaken for normal influenza. Antibiotic treatment or hospitalisation may not be required.

Legionnaires’ disease

Legionnaires’ disease breaks out after a variable incubation period ranging from 2 to 10 days (on average 5 or 6 days). It may involve: high fever, muscle aches, diarrhoea, headache, chest pains, generally dry coughing (but also purulent), kidney failure, mental confusion, disorientation and lethargy. It is an infection that cannot clearly be distinguished from other atypical or bacterial forms of pneumonia. Apart from the normal respiratory or systemic support measures, it is treated with antibiotic therapy. The disease, especially if diagnosed late or affecting very weak individuals, may lead to death.

Legionella bacteria
  • Transmission+

    Legionnaires’ disease is transmitted by inhalation of aerosolized water i.e. ultrafine drops. The disease cannot be contracted from drinking contaminated water and it is not transmitted from person to person.

    Individuals most exposed to the disease Legionnaires’ disease can also affect healthy persons, as seen in the case of the Legionnaires in Philadelphia. Nevertheless, predisposing factors are:

    • Chronic diseases
    • Smoking
    • Alcoholism
    • Age
    • Sex

    The graph on the page on the side shows the incidence of cases by age and sex recorded in France in 1998 (source: Dr. Bénédicte Decludt-Janssens, InVS, colloque CSTB/RISE, 16 dècembre 1999).

    Frequency of the disease

    The cases of Legionnaires’ disease in the USA are believed to be at least 11,000 every year. The notified cases in Italy are around 150 per year. Nevertheless, there are valid reasons to believe that the actual cases are at least 10 times more. One of the main reasons why the disease is underestimated is because of the fact (as already mentioned) that Legionnaires’ disease does not have clinical characteristics such that it can clearly be distinguished from other atypical or bacterial forms of pneumonia.

    Utilities at risk

    Based on the above mentioned considerations, the utilities and systems most at risk are:

    • Hospitals, clinics, nursing homes and similar
    • Hotels, barracks, campsites and accommodation structures in general
    • Sports centres and schools
    • Buildings with cooling towers
    • Swimming pools
    • Spas
    • Ornamental fountains and artificial waterfalls
  • Conditions for the development of Legionnaires’ disease+

    Legionella bacteria dwell in rivers, lakes, wells and thermal waters. They can also be present in water mains as they are able to live through the normal purification treatments without suffering too much harm.
    However, the mere presence of these bacteria does not pose a danger to persons. The bacteria become dangerous only when the following conditions exist at the same time:

    1 - Optimal development temperature: varies between 25 and 42°C - Maximum bacterial growth is at about 37°
    2 - Aerobic environment: i.e. an environment where oxygen is present
    3 - Presence of nutritional elements: biofilm, fibrous matter, iron and limestone ions, other microorganisms
    4 - Water atomization with formation of microdrops with variable diameters from 1 to 5 micron
    5 - High contamination level: It is generally thought that this level should exceed 1000 CFU/l

    CFU/l is the unit of measure used to assess water contamination and indicates the amount of microorganisms in one litre of water.As regards the danger threshold, it should be considered that in a very recent circular (September 2002) the French Direction Générale de la Santé fixed the following values:
    1000 CFU/l for areas that receive the public
    100 CFU/l for areas reserved for debilitating treatments or immunodepressed patients.

  • Technological systems and processes at risk+

    The first cases of Legionnaires’ disease were almost exclusively attributed to bacteria originating from cooling towers, evaporative condensers and air treatment units. For several years it was hence thought that air-conditioning systems were mainly, if not solely, responsible for spreading the disease.
    In actual fact, this is not the case: all technological systems and treatments that operate in the conditions indicated in the chart here on the side are at risk. Or more simply said, all technological systems and processes that involve moderate heating of water and its atomization are at risk. In fact, legionella practically always manages to find nutritional substances.
    Below is a list of the systems and the relative “critical” points at greatest risk:

    Cooling towers

    • Open-circuit wet towers
    • Closed-circuit towers
    • Evaporative condensers
    • Air-conditioning systems
    • Wet-pack humidifiers
    • Spray air washers
    • Atomizers
    • Drip separators
    • Filters
    • Silencers

    Sanitary systems

    • Piping
    • Storage tanks
    • Valves and cocks
    • Shower heads
    • Bath spray heads

    Emergency systems

    • Decontamination showers
    • Eye washing stations
    • Firefighting sprinkler systems

    Swimming pools and bathtubs

    • Swimming pools and hydromassage bathtubs
    • Hot baths

    Ornamental fountains
    Oxygen delivery devices
    Machine tool cooling systems

  • Legionella Habitat in the systems+

    Legionella is an enemy and we should know it well.
    Otherwise, we risk fighting it with the wrong and totally inadequate arms.

    Where legionella lives and how it develops
    In technological systems, legionella may be found isolated or in protozoa, such as amoebae, that serve as host. Moreover, whether isolated or in the protozoan host, legionella lives:
    1. freely in water
    2. anchored to biofilms: i.e. aggregates composed of other bacteria, algae, polymers and natural salts.
    It is in these very aggregates that legionella finds the essential medium to live and grow.
    High-level studies on the nature and characteristics of biofilms have been and are still being conducted at Montana State University (MSU) at its dedicated research centre: Center for Biofilm Engineering (CBE). The drawings shown below were taken from publications of this centre. The first represents the exchanges that normally occur between metal surfaces and biofilm; the second, biofilm development when corrosion phenomena occur.

    However, without going into too much detail, it should be considered that biofilms develop where there are (1) the necessary anchoring mediums, (2) nutritional substances, and (3) suitable temperatures. These conditions can, for example, be found in evaporative towers or in pipes that convey hot water at low velocity, i.e. that does not create turbulence such as to hinder biofilm anchorage and growth.

    Where and how legionella can hide itself
    Legionella can not only develop but also hide itself in biofilms. This fact should carefully be considered, as it makes disinfection treatments that act only locally completely unreliable.
    It follows that, for example, in a sanitary hot water system, it is not enough to disinfect (chemical or heat) only the hot water storage hoping that sooner or later the recirculation circuit will carry the bacteria to pass through the hot water storage. It would all be in vain, precisely because the bacteria can find safe shelter in biofilms.

    Contamination measurement
    The presence of biofilms may also lead to significant errors in determining the system contamination levels.
    In fact, during the measurement operations, the biofilms may rupture (as a result of thermal shock, sudden turbulence or mechanical impact) and release large amounts of bacteria which de facto considerably alter the actual level of system contamination. Therefore, the measurements obtained are not always reliable and in case of doubt have to be repeated.

    Actions to combat the formation of biofilms
    So then, it is of the utmost importance to try and combat the formation of biofilms in the fight against legionella. To this end, it may generally be said that:
    - It is advisable to use water containers and pipes with low-adherence surfaces in order to limit the possibility of biofilm anchorage.
    - For the same reason, it is advisable to dimension the pipes for high-velocity flow even if definite values cannot be given.
    - Water should not be left to stagnate and hence hot water storage tanks with large connections, manifolds with too large diameters and dead legs for future utilities should be avoided.

  • Disinfection treatments +

    These are treatments aimed at eliminating or significantly limiting the presence of legionella in the systems.
    On the website legionnellose.com (an example of clarity and scientific precision) it is argued that to date these treatments have had “plus d’échecs que de succés”, meaning more failures than success, and they ascribe it to the following factors:
    Poor knowledge of the problems relating to the presence of biofilms. Incomplete acquisition of the data relating to the specific system characteristics. Little consideration of the phenomena connected with limescale and corrosion. Inadequate knowledge of the required contact time between disinfecting substances and bacteria. These evaluations and considerations in all likelihood hit the mark and we think it is the right premise for the analysis that follows. In fact, very often certain treatments are presented as sure and reliable even when they are not.

    Chlorine is a strong oxidizing agent that has for years been used for drinking water disinfection.
    However, very high doses are required for the antilegionella treatment and this has the following negative effects:
    – Formation of halomethanes (substances partly deemed carcinogenic).
    – Development of severe corrosion phenomena.
    – Instability of the concentration over time.
    – Poor penetration into biofilms.
    – Insufficient action where water stagnates.
    – Alteration of the taste and flavour of the water.

    Chlorine dioxide
    Chlorine dioxide has good antibacterial properties, does not produce halomethanes and remains in the pipes for a relatively long time. In addition, its molecules can penetrate biofilms. It does however have the following drawbacks:
    – It needs to be produced “in situ” with quite complex procedures.
    – It may corrode the pipes even if to a lesser degree than chlorine.
    – It requires fairly high running costs. Positive copper and silver ions. These ions exercise a strong bactericidal action due to the fact that their electrical charge may alter the permeability of the cellular organisms and lead to protein degradation. They can also build up in biofilms and their effect therefore persists (for a few weeks) also after treatment deactivation. These are the main drawbacks:
    – They cannot be used with galvanised surfaces as zinc deactivates the silver ions.
    – Their concentration may not exceed the permitted limits for drinking water.
    – They require high costs.

    Peracetic acid
    Some experiences demonstrate modest effectiveness of this compound in shock treatments.

    Synthetic bactericides
    Placed on the market by companies specialised in water treatment, bactericides may also be active against legionella. Some of these products also exercise an effective action against scale and biofilm. However, the negative effects related to the specificity of the products, their stability over time and the effects on the users are to be verified.

    Ozone can exercise a strong action against legionella and the other bacteria and protozoa present in biofilms.
    Nonetheless, it should be considered that the treatment with ozone:
    – Requires high costs for the production and dosing equipment.
    – Needs careful maintenance.
    – Has a fairly limited effectiveness over time.
    – Degrades some products used for anti-scale and anti-corrosion treatments.
    – May increase the possibility of new infections forming.
    The action of ozone on corrosion is still somewhat controversial. Some authors maintain that it encourages corrosion, others quite the opposite. The latter justify their argument with the fact that ozone may oxidize the nitrogen present in water forming compounds (nitrates and nitrites) that inhibit the corrosion of steels.

    Catalysed oxygenated water
    This is a disinfection technique that associates a catalyst (normally a silver salt) with oxygenated water. Its effectiveness depends on the action of the catalyst. Theoretically, oxygenated water has various advantages among which non-toxic decomposition products. The real advantages and disadvantages are however still little known as practical experience is quite limited.

    The strength of this treatment lies in the possibility of reducing water contamination without adding any chemicals. Two techniques are used:

    • Conventional system with sand filters, which is mainly used with cooling circuits.
    • System with high-capacity microfilters (1 μm and even smaller), which is used both with sanitary hot water circuits and with cooling circuits. Microfilters capable of treating several tens of cubic metres of water per hour are available on the market.

    These are the main drawbacks of filtration:
    – It requires high costs.
    – It needs careful maintenance.
    – Its effectiveness is not constant over time because of progressive filter occlusion.
    – It is exposed to sudden filter rupture.
    – There is a risk of the filters being contaminated with other bacteria.

    Ultraviolet rays (UV)
    UV rays are capable of deactivating the bacteria that pass through the ray emission equipment. It should however be considered that this equipment can exercise only a local action. Moreover, the water turbidity can create shade cones that protect the bacteria. Therefore, other disinfection systems need to be combined with the action of the UV rays.
    There are also limits on the amount of water that can be treated by each piece of equipment. In fact, the fluid flow subjected to the action of the rays must have a small thickness (generally not more than 3 cm) and this considerably reduces the capacity of the equipment used for the treatment.

    Heat treatments
    Like in the case of filtration, the strength of these treatments lies in the fact that they can exercise a complete bactericidal action without adding any chemicals and there is no need (like in the case of UV rays) for integrative systems. Their action is based on the fact that high temperatures cause the death of bacteria in general and legionella in particular. The diagram below indicates the survival times of legionella as the water temperature changes.

    This diagram (taken from a study by J.M. HODGSON and B.J. CASEY) is now internationally accepted as the sure point of reference for legionella heat disinfection and has replaced the old decidedly less reliable and more penalising diagrams. In practice, this diagram assures us that if water is maintained above 50°C, there is no risk of legionella developing, on the contrary, it is eliminated within a few hours. 


    In the next focuses we will examine the limits, performance and real possibility of applicationof heat treatments.