Sources of Legionella Pneumonia
Legionella pneumonia is acquired primarily from contaminated water systems that aerosolize the bacteria, with the most important sources being hospital potable water systems (heated water distribution systems, showers, faucets), cooling towers, evaporative condensers, and respiratory therapy equipment filled with non-sterile water. 1
Primary Environmental Reservoirs
Man-Made Water Systems
Legionella species are commonly found in various natural and man-made aquatic environments and may enter hospital water systems in low or undetectable numbers. 1 The key sources include:
- Hospital potable water distribution systems - particularly heated water systems, which provide optimal growth conditions 1
- Cooling towers and evaporative condensers - significant sources requiring routine maintenance 1
- Showers and water faucets - common aerosolization points in both healthcare and residential settings 1
- Hot-water tanks and water heaters - serve as amplification sites 1
- Respiratory therapy equipment - when filled or rinsed with non-sterile water 1
- Locally produced distilled water in hospitals 1
Community Sources
For community-acquired disease, the evidence identifies:
- Residential potable water systems - assigned "definite rank" as a source of sporadic community-acquired disease 2
- Large building water systems - contribute substantially to sporadic cases 2
- Car air-conditioner water leakage - definite source for sporadic cases 2
- Solar-heated potable water - probable source 2
Optimal Growth Conditions
Legionella amplification in water systems is enhanced by specific environmental factors: 1
- Temperature range of 25-42°C (77-108°F) - ideal for bacterial multiplication 1
- Water stagnation - allows bacterial proliferation 1
- Scale and sediment accumulation - protects organisms from disinfection 1
- Presence of free-living aquatic amoebae - support intracellular growth of Legionella 1
Transmission Mechanism
The bacteria are transmitted via inhalation of contaminated aerosols, not through person-to-person contact or drinking water. 3, 4 Water sources that create respirable aerosol droplets pose the greatest risk, including:
- Showers and faucets with aerosolizing fixtures 1, 3
- Cooling towers that release aerosols into the air 1, 3
- Hot tubs and swimming pools 3
- Fountains 3
Critical Clinical Pitfall
The concentration of Legionella organisms in environmental samples does not reliably predict disease risk. 1 Multiple factors influence actual transmission risk beyond bacterial presence, including the degree of aerosolization into respirable droplets, proximity of the aerosol to the host, host susceptibility, and strain virulence. 1 This explains why some water systems with high bacterial counts may not cause disease, while others with lower counts do.
Nosocomial vs. Community Acquisition
In hospitals, the heated potable water distribution system is the most frequently implicated source during outbreak investigations. 1 Hospital-acquired cases carry a 40% mortality rate compared to 20% for community-acquired cases, reflecting the severity of underlying disease in hospitalized patients. 1
For community-acquired disease, residential potable water and large building water systems appear to contribute to the majority of sporadic cases, though the exact source cannot be definitively identified for most patients. 2 Cooling towers remain a potentially significant source, but definitive linkage to individual sporadic cases is difficult to establish. 2
Seasonal Variation
Disease incidence is higher in summer months, possibly due to increased cooling tower use for air conditioning and changes in water chemistry at higher outdoor temperatures. 3