Legionnaires' disease: water droplets and lung macrophages
Legionnaires' disease is a pneumonia caught by breathing in water droplets that contain Legionella bacteria, which then multiply inside macrophages (white blood cells that normally engulf germs) in the lungs.
Remember
Legionnaires' disease comes from breathing in contaminated water droplets and does not usually spread between people, and the bacteria multiply inside lung macrophages, so treatment needs an antibiotic that reaches the inside of cells.
Legionnaires' disease is caught by inhaling aerosolised water, which means breathing in tiny droplets of water in the air that are contaminated with Legionella pneumophila.
The most likely place of infection is somewhere the water is at a high enough temperature for the bacteria to grow, such as a poorly maintained shower system or an air conditioning unit. The NHS also names spa pools and hot tubs, and taps and showers that are not used often. It is usually caught in places such as hotels, hospitals or offices rather than at home. It does not usually spread between people, and drinking water that contains the bacteria does not usually cause the disease.
Once in the lungs, the bacteria are taken up by alveolar macrophages (white blood cells that normally engulf and destroy germs in the air sacs). Instead of being killed, they multiply inside the cell. The organism lives intracellularly, that is, inside host cells.
This is why treatment needs an antibiotic that reaches the inside of cells, such as clarithromycin, a macrolide. Fluoroquinolones and doxycycline are other antibiotics that reach good levels inside cells.
Symptoms usually begin 2 to 10 days after exposure to the bacteria. The illness frequently begins with headache, muscle pain and fever, and it can progress to cough, diarrhoea and confusion.
Key facts
- Route
- the disease is caught by breathing in aerosolised water (tiny droplets of water in the air) contaminated with Legionella pneumophila.
- Sources
- air conditioning systems, spa pools and hot tubs, and taps and showers that are not used often.
- Between people
- Legionnaires' disease does not usually spread between people.
- Inside cells
- the bacteria are taken up by alveolar macrophages but multiply inside them instead of being killed.
- Treatment
- an antibiotic that reaches the inside of cells is needed, such as clarithromycin, a macrolide.
- Timing
- symptoms usually begin 2 to 10 days after exposure.
Exam traps
- Direct spread between people through respiratory droplets is not the usual route for Legionnaires' disease, so isolating patients is not routinely needed.
- Eating contaminated food is not the usual route: drinking water that contains the bacteria does not usually cause Legionnaires' disease, which depends on inhalation into the lungs.
- Insects do not carry Legionella. A mosquito acting as the vector (the carrier that transmits the organism) belongs to infections such as malaria and dengue, not to Legionnaires' disease.
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