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Legionella pneumophila is an ubiquitous bacterium, which means that it has the ability to grow in its natural habitat (hot water) and adapt to other environments with similar characteristics including the human body, which has an ideal temperature for its growth. If inhaled, Legionella pneumophila can make its way into the lungs to replicate until causing a potentially life-threatening infection: Legionnaires’ disease.

water and legionella
water and legionella

Habitat and development

Legionella pneumophila bacteria are naturally present in water and moist soils. They colonize water distribution networks (hot sanitary water), cooling circuits (wet cooling towers, collective air conditioning) which conditions promote their proliferation. temperature between 25 and 45 degrees Celsius, oxygen, corrosion (metal ions: iron, zinc, aluminum), tartar (calcium), biofilm (organic layer containing microorganisms and nutrients needed by legionella).

All water networks with these characteristics therefore pose a risk.

As human contamination occurs by inhaling contaminated water, the area of health risk will depend on the projection range of microdroplets and aerosols: a few meters to a few tens of meters for showers, spas, misting systems, public fountains… to several kilometers in cooling towers.


Climate impact

In cold countries (or tempered), legionella (Legionella pneumophila) therefore grows mainly in the heated water networks, with cold water networks rarely exceeding 20°C. In regions with warmer climates, cold water temperature regularly exceeds 25°C, becoming a conducive environment for proliferation.

Legionnaires’ disease: a potentially fatal disease

The pathogenic bacterium Legionella pneumophila causes a serious form of pneumonia, legionellosis (or Legionnaire’s disease), which is fatal in more than 10% of cases. Mortality rate can reach 50% in populations at risk (age, respiratory diseases, smoking, immunodeficiency …).

Although discovered 40 years ago, Legionnaires’ disease still affects about 9,000 people in Europe and tens of thousands of people in the United States each year, causing several thousand deaths worldwide. These figures, however, remain greatly underestimated, as diagnostic infrastructures are limited in many regions (Africa, Asia Pacific, etc.)


Increase in Legionnaires' disease cases

Despite stricter regulations that have passed over the last years (more frequent mandatory tests, higher number of facilities concerned…), the incidence of Legionnaires’ disease keeps growing in Europe: 4,921 cases in 2011, 9,238 cases in 2017. The reasons for this increase are uncertain today: the effect of warming water on bacteria proliferation, greater monitoring of pneumonia symptoms (thus compensating the underestimation of previous cases).

Furthermore, Legionnaires’ disease is a severe pneumopathy, which requires a substantial level of care for infected patients (intensive care, specific infrastructures). Many countries do not have adequate healthcare infrastructures. The cost incurred by legionnaires’ disease care in the United States, according to a study published in 2016, is more than $42,000 per case resulting in estimated annual costs of nearly $340 million for the Medicare program.

Regulatory framework for legionella monitoring in water systems

In order to fight this microbiological risk, regulations based on Legionella pneumophila enumeration have become stricter over the years and require public buildings as well as condominiums and companies that own air cooling towers (ACTs) to implement preventive management methods, regular plant testing, and corrective methods.

If tested positive (Legionella pneumophila concentration greater than 1,000 Colony Forming Units per liter of water – CFU / L), even with no infection reported cases, costs can exceed € 1M (operating loss due to facility closure, disinfection, technical unemployment…) in addition to fines by the public authorities.


What types of tests exist?

Regulatory tests are required for facilities owners / operators at regular intervals to monitor legionella concentration in their networks. Testing frequency depends on the installation type. The only regulatory approved method in most Western countries is the ISO 11731 (or NF T90-431) culture method in a certified laboratory.

Self-monitoring tests are mandatory between the intervals in ACTs and are strongly recommended in public buildings which must provide evidence of proper monitoring of water systems (civil and criminal liability is incurred in the event of human contamination, as well as potential heavy compensation to survivors and family members of the deceased). The method chosen for these self-monitoring tests is left to the discretion of the facility owner.

Methods to prevent legionella risk

Legionella and water system temperature

In order to prevent legionella proliferation in sanitary hot water systems, these must be permanently heated above 50°C, while the average consumption temperature is 40°C (according to a study by the French Environment & Energy Management Agency). Many facilities, including health facilities and residential buildings, try to limit the risk by heating their networks to 55-60 degrees Celsius, and by performing regular thermal shocks (at 70 degrees Celsius for more than 10 minutes). This preventive method results in a considerable energy bill, considering that an average of 15°C (55°C-40°C, or 27% of the heating temperature) is attributable to legionella control.

Legionella and biocides

ACTs or cooling towers, are also subject to preventive treatments. These consist in routinely and sometimes massively injecting biocides into the networks (mostly chlorine combined with softening and anti-corrosion products, or hydrogen peroxide combined with silver ions). In addition to the significant costs of such treatments (from tens to several hundred thousand euros per year depending on the cooling tower power and the network size), the chemical pollution caused by the discharges can have dramatic consequences for biodiversity. Biocide residues also destroy microorganisms in the environment, impacting the entire food chain and local ecosystems.

Some Legionella pneumophila concentration tests are not always reliable

 The ISO 11731 (or AFNOR NF T90-431) culture method is the most effective and the only approved for mandatory regulatory tests. Bacteria culture is the only method to exclusively detect Legionella pneumophila (There are many species of legionella, Legionella spp stands for for “all legionella species”), all serogroups (there are 16 serogroups of this pathogenic bacterium, their prevalence in causing Legionnaires’ disease varies from one geographical area to another), living (thus capable of replication, a condition of their pathogenicity), and of to count them (CFU/L: Colony-Forming Units by Water Liter).

The culture analysis, although the most efficient, can only be carried out in specific laboratories by qualified technicians (this test is very operator-dependent). This method also requires more than 10 days to provide results, enabling the legionella concentration to replicate from an undetectable threshold to a >100,000 CFU/L threshold during this period of time.

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