Legionella / Legionnaires' disease

The work of ANSES

Legionella is a bacterium that may cause a respiratory infection known as legionellosis or Legionnaires' disease. The main known sources of human exposure to this bacterial agent are environmental, specifically through water heating systems and wet cooling towers. As part of the objective of reducing cases of Legionnaires' diseaseby 50%, ANSES has carried out several studies on the health risks related to proliferation of Legionella in water within cooling towers in nuclear power stations, and on detection and enumeration methods for this bacterium in water.

Legionnaires' diseaseis a respiratory disorder caused by bacteria called Legionella. Monitoring of this disease is carried out primarily on the basis of data from reported cases. Reporting has been mandatory since 1987. In 2011, 1170 cases of Legionella infection were reported in France (InVS data published on 22 May 2012). The disease was fatal in 10.8 % of reported cases, corresponding to 114 deaths in 2011.

The main known sources of human exposure to this bacterial agent are environmental, specifically through water heating systems and wet cooling towers. This bacterial genus can multiply in water up to 45°C, a lethal temperature that prevents multiplication of most other bacteria, and can survive in water up to 66°C.

The second French National Environment and Health Action Plan for 2009-2013 (PNSE2) identified as a priority improvement the investigation of cases of Legionella infection, prevention of cases related to contamination of water heating systems, and continued research efforts. ANSES has been designated as a partner in this action item of the PNSE2. Within this framework, the Agency has issued two reports on the subject.

Assessment of the health risks related to proliferation of Legionella in water in the cooling towers of nuclear power stations

The national electricity producer EDF operates 19 nuclear power plants, including 11 with cooling towers that fall under Basic Nuclear Installation (BNI) regulations. These 11 sites have a total of 32 cooling towers, with 28 nuclear power plant cooling towers supplied with river water, and subject to the recommendations of the Directorate General of the Institute for Radiological Protection and Nuclear Safety concerning the risk of Legionella proliferation in cooling towers that fall under BNI regulations.

As per these recommendations, EDF was required to implement by 1 August 2012 suitable prevention methods to maintain Legionella concentrations below a bacterial density threshold of 5.10CFU/L (Colony-forming units per litre) in water in the cooling circuits of all nuclear power plant cooling towers, with the exception of those at the Chinon site, for which the required Legionella concentration was 5.105 CFU/L.)

Discharge of liquid and gas effluents and withdrawing water from these circuits are subject to authorisation granted at the ministerial level, as indicated in Decree No. 95-540 of 4 May 1995 concerning BNIs

In 2004, as part of the risk prevention objective concerning Legionella, the Agency received a formal request from the Ministries of Health and the Environment and the Nuclear Safety Authority for an opinion on intervention levels and on the prevention, monitoring as well as the actions strategy, implemented by EDF. The Agency was also asked to assess exposure of the population around nuclear power stations, and to evaluate the health and environmental risks related to water and gas discharge from these installations with respect to Legionella. This expert appraisal was intended to assist the health authorities in confirming or rejecting currently used thresholds for Legionella in the water of nuclear plant cooling towers.

These studies were conducted in two phases and resulted in the publication of two reports in 2006 and 2007:

  • the first specifically concerned the relevance of action thresholds recommended at the time of the expert appraisal;
  • the second focused on the relevance of treatments recommended by EDF to reduce concentrations of Legionella spp. and their impact on health and the environment.

Detection and enumeration methods for Legionella in water

Regulations have been implemented for environmental monitoring of Legionella. These regulations concern Legionella spp. in cooling towers and specifically the Legionella pneumophila species in water heating systems. Since 1 August 2012, monitoring has been based on culture methods (as per Standard NF T90-431 "Detection and enumeration of Legionella spp. and of Legionella pneumophila by culture in agar media". However, there are several detection and enumeration methods for Legionella that are under development or that are currently in use to greater or lesser extents.

In this context, the Agency received a formal request in 2009 from the Directorate General for Health (DGS) and the Directorate General for Risk Prevention (DGPR) concerning enumeration methods for these bacteria. The aim was to catalogue and describe all available methods specifically for enumeration of Legionella in water and to study the relevance of their use for testing of water from heating systems and cooling towers.

Results of these studies were published in July 2011. Firstly, ANSES found that the enumeration methods to adopt, from a health point of view, should concern Legionella pneumophila in water heating systems and in water from the cooling circuits of cooling towers. The Agency also defined criteria to take into account when evaluating the relevance of an enumeration method for Legionella intended to test water heating systems and cooling towers with a view to adapting the regulations.

After cataloguing and comparative analysis of the available methods for enumeration of Legionella in water, two currently available methods were considered to be sufficiently relevant and robust: the culture method (as per Standard NF T90 431) and quantitative PCR (as per Standard NF T90 471).

The criteria used to interpret the results obtained using these two methods were also assessed, and led to the proposal of target count values suitable for the various regulatory contexts.

Other possible studies and research avenues were also proposed to gain further knowledge with a view to future optimisation of enumeration methods for bacteria of this genus.