What are the common sources of Legionella?

Sources of Legionella

Legionella bacteria primarily originate from water systems in both healthcare and community settings, with heated potable water distribution systems, cooling towers, and any water source capable of generating aerosols serving as the main reservoirs.

Primary Water System Sources

Healthcare Facilities

• Hospital potable water distribution systems, particularly heated water systems, represent the most frequently implicated source of nosocomial Legionella infections 1, 2
• Hot-water tanks and water heaters maintained at temperatures between 25-42°C (77-108°F) serve as amplification sites where Legionella multiplies optimally 2, 3
• Showers and water faucets act as critical aerosolization points, with bacteria colonizing the inner surfaces of faucet assemblies, aerators, spouts, valve seats, and showerheads 1, 2, 3
• Respiratory therapy equipment filled with non-sterile water poses infection risk, necessitating the use of only sterile water for filling and terminal rinsing of nebulization devices 1
• Locally produced distilled water in hospitals may harbor Legionella, emphasizing the need for rigorous water quality control 2

Building Water Systems

• Cooling towers and evaporative condensers are significant environmental sources requiring routine maintenance to prevent bacterial proliferation 1, 2
• Hot tubs, swimming pools, and decorative fountains can aerosolize contaminated water 4
• Carpet-cleaner water tanks have been identified as potential sources in healthcare facilities 1
• Residential plumbing systems, including home hot-water tanks maintained at warm temperatures (30-54°C), can harbor Legionella 3

Environmental Conditions Supporting Growth

Temperature Requirements

• The ideal multiplication range is 25-42°C (77-108°F), commonly found in inadequately heated or cooled water systems 2
• Hot-water tanks maintained at temperatures below 50°C allow bacterial proliferation, while those at 71-77°C do not support Legionella growth 1, 3

Physical Protection Mechanisms

• Scale and sediment accumulation in water heaters, storage tanks, faucets, and showerheads protects organisms from heat and chlorine disinfection 1, 2
• Biofilm formation with algae and other bacteria increases nutrient availability and provides protection against standard disinfection techniques 5
• Free-living aquatic amoebae support intracellular Legionella growth, offering additional protection from adverse conditions 2, 5
• Water stagnation allows bacterial proliferation, particularly in low-flow areas of plumbing systems 2, 6

Transmission Mechanism

Legionella is transmitted exclusively via inhalation of contaminated aerosols, not through person-to-person contact or drinking water 2, 4

• Aerosol-generating systems facilitate transmission from water to air, with inhalation of these contaminated droplets leading to infection 4, 5
• The concentration of Legionella in environmental samples does not reliably predict disease risk, as multiple factors influence transmission including aerosolization efficiency, proximity to source, host susceptibility, and strain virulence 2

Natural Reservoirs

• Legionella are widespread in natural aquatic environments including lakes, rivers, and streams, existing across varying temperatures, pH levels, and nutrient conditions 5
• These natural sources seed anthropogenic water systems including drinking water distribution networks 5, 6

Common Pitfalls

• Do not assume that visible cleanliness of water systems indicates absence of Legionella—bacteria thrive in biofilms and sediment not visible to the naked eye 1, 2
• Incidence increases during summer months, possibly due to increased cooling tower use and changes in water chemistry at higher outdoor temperatures 4
• The presence of Legionella in plumbing systems does not necessarily correlate with disease occurrence, as infection depends on multiple host and environmental factors 3