T-Safe In Focus

What is Legionella? Transmission, Symptoms, and Prevention Explained

Legionella is a genus of bacteria, and within this genus, there are currently over 60 identified species and over 70 serogroups. However, the most well-known and clinically relevant pathogenic species is Legionella pneumophila, which is responsible for the vast majority of Legionnaires' disease cases.


The bacteria was first discovered in 1976, during an outbreak of a mysterious illness among attendees of an American Legion Convention in Philadelphia. The bacteria were isolated, and subsequently, the species Legionella pneumophila was identified as the causative bacteria of the illness. The disease was named Legionnaires’ disease in reference to the affected group. Since then, other species of Legionella have been identified as causing similar respiratory illnesses, collectively known as Legionellosis.


  • Legionnaires’ Disease is a severe form of pneumonia with symptoms including high fever, chills, cough, muscle aches, and shortness of breath. In severe cases, it can lead to complications such as respiratory failure, kidney failure, septic shock, and potentially being fatal.
  • The bacteria are primarily transmitted through Inhalation of contaminated water droplets, often in the form of aerosols generated by water systems like air conditioning, hot tubs, or showers. However, other methods of contagion are, Intubation and Aspiration. It is not known to be transmitted from person to person.

Infection Statistics

The incidence of Legionnaires’ disease can vary from year to year and across different regions. Surveillance systems track cases of the disease, and outbreaks may occur in specific settings where there is exposure to contaminated water sources.

In Europe and the UK, as in other parts of the world, cases of Legionnaires’ disease are reported to health authorities, and data is collected to monitor trends and respond to outbreaks.

According to the European Centre for Disease Prevention and Control (ECDC), there were 8,499 confirmed cases of Legionnaires’ disease reported in the European Union and European Economic Area (EU/EEA) countries in 2019. The incidence rate was 1.6 cases per 100,000 population.

It is important to note that specific statistics can change over time, and the latest and most accurate information can be obtained from health authorities or organisations responsible for disease surveillance in the respective regions. Public health measures, strict water system maintenance, and adherence to government guidelines can help prevent Legionella infections.

Control of Legionella

Controlling Legionella bacteria presents several challenges due to their ability to thrive in various water systems and environments.

Some of the key challenges include:

Complexity of Water Systems

Legionella can colonize and proliferate in a wide range of water systems, including cooling towers, hot water tanks, showers, and decorative fountains. The complexity of these systems, along with factors such as temperature fluctuations and stagnation, can make it difficult to effectively control bacterial growth.

Biofilm Formation

Legionella bacteria have the ability to form biofilms, which are slimy layers that adhere to surfaces within water systems. Biofilms provide a protective environment for bacteria, making them more resistant to disinfection measures and allowing them to persist over time.

Temperature Requirements

Legionella bacteria thrive in warm water environments, with optimal growth occurring at temperatures between 20°C and 45°C. Maintaining water temperatures outside of this range can help control bacterial growth, but this may not always be feasible or practical in all water systems.


Stagnant water and low flow rates can create favourable conditions for Legionella growth by allowing bacteria to concentrate and multiply. Regular flushing of water systems and ensuring adequate flow rates can help reduce the risk of bacterial colonisation.

Monitoring and Testing

Detecting and monitoring Legionella contamination in water systems require specialised testing methods, and results may not always be immediately available. This can make it challenging to identify and respond to potential risks in a timely manner.


Compliance with regulations and guidelines for Legionella control varies across jurisdictions and industries. Ensuring that water systems meet regulatory requirements and implementing appropriate control measures can be resource-intensive and may require ongoing maintenance and monitoring.

Addressing these challenges typically requires a multifaceted approach that includes regular planned preventive maintenance and inspection of water systems, implementation of water treatment and control measures, monitoring for Legionella contamination, and training for personnel responsible for managing water systems.

Collaboration between public health authorities, water management professionals, and stakeholders in various industries is also essential for effectively controlling Legionella and reducing the risk of associated illnesses.

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