What is Legionella?

Legionella is a genus of Gram-negative bacteria that includes many species. They live in freshwater and can colonize artificial systems when conditions are favorable.

Legionella pneumophila is the species most often associated with waterborne cases and outbreaks of legionellosis. Other species can also cause disease, but they are identified less frequently.

A result expressed as Legionella spp. must therefore not be automatically interpreted as a result for L. pneumophila. Always check the exact parameter requested and the unit used by the laboratory.

Where are Legionella naturally found?

Legionella are present in natural freshwater, particularly in lakes, rivers, streams, springs, and damp soil. In these environments, they are generally found in low concentrations.

The risk increases significantly when the bacteria enter an artificial system that offers them better conditions for multiplication: favorable temperature, low water turnover, internal surfaces, corrosion, scale, sludge, biofilms, and the presence of host microorganisms.

What conditions favor the proliferation of Legionella?

The WHO indicates that Legionella can live and multiply in water systems between approximately 20 and 50 °C (68-122 °F), with a particularly favorable zone around 35 °C (95 °F). However, the risk never depends on temperature alone.

• water temperature in a favorable range;
• stagnation or low water turnover;
• dead legs, oversized pipes, or points that are rarely used;
• biofilms on internal walls;
• deposits of scale, sludge, sediment, or corrosion products;
• weakening of disinfectant residuals;
• mixing of hot and cold water in certain areas of the network;
• insufficient maintenance of water heaters, showerheads, and equipment.

Why do biofilms play an important role?

A biofilm is a community of microorganisms attached to a surface and surrounded by a protective matrix. Pipes, seals, showerheads, hoses, tanks, and internal surfaces can harbor these communities.

Legionella can survive and multiply inside certain free-living protozoa, particularly amoebae. This mechanism partially protects them against unfavorable conditions and contributes to their persistence in water networks.

Occasional disinfection can strongly reduce concentrations without lastingly eliminating all niches. Prevention therefore relies on design, temperature, water turnover, maintenance, and monitoring, rather than a single treatment.

How can a person be exposed?

The most common route is the inhalation of contaminated aerosols: very small droplets produced by a shower, spray, mist, nebulization, or equipment that generates fog.

An infection can also occur through aspiration, when contaminated water or ice accidentally enters the lungs, particularly in certain hospitalized patients or people with swallowing disorders.

Which equipment can produce contaminated aerosols?

• showers and handheld showerheads;
• hot tubs, spas, and hydromassage jets;
• cooling towers and evaporative condensers;
• decorative fountains and misting devices;
• humidifiers and certain respiratory equipment;
• equipment using warm or recycled water;
• improperly balanced or insufficiently maintained domestic hot water systems;
• cold water systems where the temperature rises significantly.

Not all of this equipment poses the same level of risk. Factors such as temperature, quantity of aerosols, dispersion distance, frequency of use, quality of maintenance, and the populations exposed must be taken into account.

What are the health effects of Legionella?

Legionnaires' disease

This is a pneumonia that can be serious. The incubation period is generally 2 to 10 days, with longer periods observed in some episodes. Symptoms may include fever, cough, breathing difficulties, fatigue, muscle pain, headaches, digestive issues, or confusion.

Antibiotic treatment is necessary. The severity depends on age, associated illnesses, immune status, and the speed of diagnosis and care.

Pontiac fever

This non-pulmonary form resembles an acute flu-like syndrome. It appears more quickly, generally heals spontaneously within a few days, and is not associated with pneumonia.

Who is at higher risk?

• people over 50 years old;
• current or former smokers;
• people with chronic respiratory disease;
• people with compromised immune systems;
• patients with cancer or those receiving certain treatments;
• people suffering from kidney disease or other chronic illnesses;
• people who have recently undergone surgery, intubation, or ventilation;
• patients at risk of aspiration.

In healthcare facilities, certain points of use are subject to stricter requirements when they are accessible to patients identified as particularly vulnerable.

What is the European framework for Legionella?

Directive (EU) 2020/2184 introduces a risk assessment for internal water distribution systems. It sets a parametric value for Legionella of < 1,000 CFU/L, intended for the application of its provisions regarding internal networks and corrective measures.

This European value must be interpreted within its regulatory context: it does not mean that all water containing less than 1,000 CFU/L is risk-free for all people and uses. Measures must be adapted to priority buildings, equipment, exposed populations, and the results of the risk assessment.

What rules apply in France?

Collective domestic hot water systems

The order of February 1, 2010, applies in particular to collective domestic hot water installations serving high-risk points of use in healthcare facilities, social and medico-social establishments, prisons, hotels, tourist residences, campsites, and other public-access buildings.

The count of Legionella pneumophila must be less than 1,000 CFU/L at all concerned high-risk points of use. In healthcare facilities, it must be below the detection limit at points accessible to patients identified as particularly vulnerable.

Internal cold water installations

The order of December 30, 2022, regarding the risk assessment of internal distribution systems provides, in its annex, an objective below the detection limit for Legionella, a quality limit of 1,000 CFU/L for L. pneumophila, and a quality reference of 1,000 CFU/L for Legionella spp., within the framework of this assessment.

How to perform a Legionella water sample?

Sampling must be carried out according to a strategy defined based on the installation and the objective: regular monitoring, network diagnostics, inspection after repairs, exceedance, case investigation, or validation of corrective measures.

1. precisely identify the point and network involved;
2. document the type of sample: first flush or post-flushing;
3. measure the temperature according to the selected protocol;
4. note the duration since the last use;
5. use the bottle and neutralizer provided by the laboratory;
6. avoid cross-contamination;
7. respect transport conditions and deadlines;
8. ensure complete traceability of the sample.

The choice between a first-flush sample and a post-flushing sample changes the nature of the inquiry. The former can better reflect local contamination at the point of use, while a post-flushing sample can better represent the network upstream.

How is Legionella analyzed in a laboratory?

Culture

The reference method is based on culture. The sample is processed and then plated on suitable media. After incubation, suspicious colonies are confirmed and counted. The result is generally expressed in CFU/L, colony-forming units per liter.

Q-PCR

Quantitative PCR searches for genetic material and can provide a result more quickly. It can detect bacteria that would not form colonies under culture conditions. Its result, often expressed in genome units, is not directly interchangeable with a result in CFU/L.

Always verify the targeted species, the method, the limit of detection, the limit of quantification, the analyzed volume, and the reported unit.

How to interpret a Legionella result?

A result must be interpreted in conjunction with the network plan, temperature, sampling point, protocol, history, and exposed population. An isolated value does not by itself describe the entire installation.

Result below the limit of detection

This means that no colonies were detected above the method’s capabilities in the sample. This does not guarantee an absolute absence throughout the network.

Positive result

It may reveal a local or more extensive colonization. Additional sampling, examination of temperatures, checking for stagnation, and inspection of the installation may be necessary.

Regulatory exceedance

In installations subject to the decree of February 1, 2010, an exceedance requires immediate corrective measures, protection of users, and verification of the effectiveness of the measures through new analyses.

How does temperature help control Legionella?

Thermal management is an essential barrier. The WHO recommends maintaining hot water above 50 °C in the network, with a production outlet of at least 60 °C, and maintaining cold water below 25 °C, ideally below 20 °C, when technically possible.

These goals must be balanced with burn prevention. Mixing devices may be necessary as close as possible to points of use, without creating stagnant lukewarm volumes.

Why must stagnation be limited?

When water remains stationary for a long time, its temperature may approach the range favorable to Legionella, residual disinfectant may decrease, and biofilms may develop.

• eliminate or reduce dead legs;
• correctly size pipes and storage units;
• balance hot water loops;
• identify points rarely used;
• implement documented flushing;
• check buildings after prolonged closure;
• avoid unused equipment remaining filled with water.

Simply running a tap occasionally does not correct poor design, a fouled tank, an unbalanced network, or extensive colonization.

What role does installation maintenance play?

Maintenance reduces deposits, scale, sediment, and microbiological niches. It must concern production, storage, loops, terminal points, and equipment that produces aerosols.

• inspect and clean tanks according to the maintenance plan;
• descale and clean showerheads, hoses, and aerators;
• replace degraded or difficult-to-clean components;
• monitor temperatures and their stability;
• keep a sanitary logbook in affected establishments;
• document work, disinfections, and analysis results;
• check the effectiveness of corrective measures.

Excessively aggressive or poorly controlled maintenance can damage materials, promote corrosion, or create new imbalances. Operations must comply with technical and sanitary requirements.

Which treatments can reduce Legionella?

Strategies can combine thermal management, chemical disinfectants, cleaning, flushing, filtration, and hydraulic correction. No single treatment guarantees lasting control without network monitoring.

The CDC emphasizes that no single measure is sufficient to guarantee Legionella control in a potable water system. A management program must combine several barriers and verify their effectiveness.

Does point-of-use filtration protect against Legionella?

In some healthcare facilities, validated terminal microbiological filters can be used as a temporary or targeted barrier at points of use intended for highly vulnerable patients.

Their effectiveness depends on the membrane, device integrity, installation, duration of use, and replacement. They do not treat biofilms or colonization in the upstream network.

A poorly maintained filter can become a zone of stagnation or microbiological growth. It should never be presented as an autonomous solution for a contaminated network.

How can the risk be reduced in a home?

• run hot and cold water from rarely used points at least once a week;
• flush water points after a prolonged absence before taking a shower;
• regularly clean and descale showerheads and hoses;
• avoid permanently lukewarm settings in hot water storage;
• have the tank maintained and checked for proper operation;
• remove unused equipment that holds water;
• seek advice from a professional in case of a complex network or unstable temperature.

In an individual home, an analysis is not systematically necessary. It may be relevant in the presence of a highly vulnerable person, at-risk equipment, suspicion linked to a case, or a persistent network problem.

What measures are essential in establishments?

Affected establishments must organize structured monitoring: network knowledge, sampling strategy, temperature measurements, analyses, maintenance, traceability, and procedures in case of exceedance.

• keep network plans up to date;
• identify at-risk points of use and vulnerable users;
• monitor production, storage, and loop returns;
• schedule regulatory sampling;
• prepare for recommissioning after closure;
• train technical teams;
• keep results and actions in the sanitary logbook;
• coordinate measures with the ARS (Regional Health Agency) when a case is reported.

What to do in case of exceedance or a suspected case?

1. immediately protect exposed users, especially vulnerable persons;
2. limit or suspend aerosol-producing uses based on the assessment;
3. inform the competent health and technical authorities;
4. examine temperatures, flow rates, loops, storage, and recent events;
5. implement appropriate corrective measures;
6. perform additional sampling if necessary;
7. verify the effectiveness of actions through new analyses;
8. document all decisions and operations.