Enumerating Legionella in water: ANSES reviews the existing methods
Legionellosis (or Legionnaires' Disease) is a lung infection caused by bacteria of the Legionella genus. The disease is monitored mainly through data collected when cases are reported to the authorities, which has been mandatory since 1987. In 2010, 1540 cases were notified to the French Institute for Public Health Surveillance (InVS). In 2010, 159 of the cases notified led to death, i.e. a mortality rate of 11.7% for the disease.
The public health Act of 2004 set a goal to reduce legionellosis cases by 50% between 2004 and 2008. More recently, the Second French National Environment & Health Action Plan 2010-2013 (PNSE2) included among its priorities, improving the investigation of legionellosis cases, preventing the onset of legionellosis related to the contamination of hot water systems and continuing research. ANSES was specified as a partner for this PNSE2 activity.
The main known sources of human exposure to Legionella are environmental in origin. They are linked to proliferation of bacteria in damp environments that can occur under certain conditions and especially in domestic hot water circuits and air-conditioning systems using cooling towers. The regulations governing the environmental monitoring of Legionella concern Legionella spp in cooling towers and Legionella pneumophila in hot water systems. This surveillance is currently based on the culture method, as described in the French Standard NF T90 431 "Recherche et dénombrement de Legionella spp. et de Legionella pneumophila par culture sur milieux gélosés" (Screening and enumeration of Legionella spp. and Legionella pneumophila by culture on agar media). Today, however, several other methods for screening for or enumerating Legionella are being developed or, in some cases, already being used, on a relatively large scale.
Identifying the most effective methods to be used
In this context, on 29 July 2009 the Directorate General for Health (DGS) and the Directorate General for Risk Prevention (DGPR) asked the Agency to investigate enumeration methods. The Agency’s mission was to list and describe all existing methods for the specific enumeration of Legionella in water and to determine whether they could be implemented effectively for monitoring hot water systems and cooling towers.
In response to this request, a collective expert appraisal was undertaken and validated by the Agency’s Expert Committee for “Assessment of risks linked to water and biological agents”.
After reviewing the scientific literature and available data concerning the monitoring of legionellosis cases in France, the working group mandated to carry out the expert appraisal first concluded that the enumeration methods to be chosen should, for public health reasons, cover Legionella pneumophila in hot water systems and the cold water circuits of cooling towers. It also defined the criteria to be taken into account for assessing the effectiveness of a method for enumerating Legionella for the purpose of monitoring hot water systems and cooling towers with a view to adapting the regulations accordingly.
As a result of the review and comparative analysis of the available methods for enumerating Legionella in water, two existing methods appear to be sufficiently effective and robust: culture (in compliance with the NF T90 431 standard) and quantitative PCR (in compliance with the NF T 90 471 standard).
The working group also considered the criteria for interpreting the results obtained with these two methods, and proposed appropriate target enumeration values for the various regulatory contexts.
It also suggested topics for further study and research to extend our knowledge in order to improve methods for enumerating bacteria of the genus Legionella in water and interpreting their results.