How does Legionella transmit?

Legionella Transmission Mechanisms

Legionella primarily transmits through inhalation of aerosols containing the bacteria from water sources contaminated with Legionella. 1 This is the most established and well-documented mode of transmission for this pathogen.

Primary Transmission Routes

Aerosol Inhalation

• Legionella enters a person's respiratory tract when they breathe in mist or vapor containing the bacteria 1
• Common sources of contaminated aerosols include:
  • Cooling towers
  • Showers and faucets
  • Respiratory therapy equipment
  • Room-air humidifiers 1
  • Hot tubs/swimming pools
  • Decorative fountains 2

Secondary Transmission Route

• Aspiration of contaminated potable water has been proposed as an alternative mode of transmission in certain patients 1
• Aspiration of pharyngeal colonizers (bacteria present in the throat) may also lead to infection 1

Important Characteristics of Transmission

• No person-to-person transmission: Unlike many respiratory pathogens, Legionella is not transmitted from person to person 1
• Seasonal variation: Higher incidence occurs in summertime, possibly due to:
  • Increased use of cooling towers for air conditioning
  • Changes in water chemistry during warmer temperatures 2
• Water quality factors: Several water quality variables influence Legionella growth and transmission:
  • Total organic carbon levels
  • Heterotrophic bacterial counts
  • Disinfectant type and concentration 3, 4

Risk Factors for Infection

The risk of acquiring legionellosis after exposure depends on:

1. Type and intensity of exposure to contaminated aerosols 1
2. Host factors that increase susceptibility:
   • Severe immunosuppression
   • Chronic illnesses (hematologic malignancy, end-stage renal disease)
   • Advanced age
   • Diabetes mellitus
   • Chronic lung disease
   • Smoking 1

Relative Risk of Different Exposure Sources

Research assessing the relative potential for Legionella inhalation exposure from common water uses shows significant variability between exposure pathways, with doses differing by over five orders of magnitude 5. From highest to lowest exposure risk:

1. Ultrasonic and cool mist humidifiers
2. Showers and faucets
3. Toilets and therapy pools 5

Clinical Implications and Prevention

• Understanding transmission routes is crucial for implementing effective prevention strategies
• Water system maintenance and disinfection are key preventive measures
• Healthcare facilities require special attention due to higher risk populations
• Monitoring water quality parameters (disinfectant levels, heterotrophic bacteria) may help reduce legionellosis risk 4

Common Pitfalls in Understanding Transmission

• Assuming all water sources pose equal risk (they don't - aerosol-generating sources pose higher risk)
• Overlooking the role of water quality in transmission potential
• Focusing solely on shower exposure when other sources may pose greater risk 2
• Failing to recognize that conventional disinfection methods may not eliminate Legionella completely, as the bacteria can survive inside amoebae 4