Legionella in the homes

I am a physician at the District Health Office. We have noticed an increase in the number of reported cases of Legionnaires’ disease. These have all been non-nosocomial, non-travel associated Legionnaires’ disease, with a number of patients barely
leaving their houses. It was decided to test the domestic and municipal water supplies for the presence of legionella. We also tested the water of premises where there had been no Legionnaires’ disease. Having found legionella, the correct course of action is not entirely clear. We have identified 3 possible scenarios and would be grateful if you could advise.


  1. Legionella found in the domestic water supply of a known Legionnaires’ disease patient.
  2. Legionella found in the domestic water supply of healthy individuals.
  3. Legionella found in the domestic water supply of those with lung disease of immunocompromised patients, in the absence of Legionnaires’ disease.


  1. Ideally, molecular subtyping of the patient isolate and the environmental isolate would support that the source of the Legionella is from the home. However, I know the legionella isolate from the patient often is not available especially if the diagnosis was made by the urinary antigen test. That said, the home could be disinfected because of the occurrence in the patient’s home. Certainly, the patient and the family would feel safer. Disinfection is also suprisingly easy in a residence. Superheat and flush can be used. The flush should be at least 30 minutes – see our website for details of the superheat and flush (Publications on disinfection by YSE Lin, 1998 ).
  2. No action. The reason is clearcut. Legionella can be found in the majority of buildings over 3 stories tall. Since it is a common colonizer of drinking water, all individuals are exposed on a regular basis to Legionella.
  3. No. See our Reply to Question #2. The logical site to perform culturing of drinking water would be a building in which high risk individuals congregate; in such a situation, environmental culturing would be cost-effective. That specific situation is a hospital and many countries now routinely culture their water supply. The results allow the hospital to make a rational decision on how to prevent a preventable infection. Such a policy has been implemented in Pittsburgh, state of Maryland, Spain, France, Denmark, Taiwan, Netherlands, Italy, and Germany. Two other points: Legionella is not contracted via showering. The major mode of transmission of legionella is not aerosolization, but aspiration. So cooling towers are overemphasized as sources of Legionnaires’ disease.