Campus Health and Safety
May 2012
Legionnella Pneumophila – whether or not to sample for their presence in water supply systems on campus

Following the finding of the bacteria responsible for Legionnaires' disease in the water supply system in the new Central Government Complex Building earlier, questions were raised as to whether or not we should conduct samplings on campus. The uniform consensus of local experts is NOT to do routine potable water sampling for Legionella. This is also the recommendation of US CDC which suggests sampling only as an epidemiological tool to track down the source of confirmed infection cases. This consensus is based on the following factors:

  • Legionella bacteria are ubiquitous and have been found in many domestic potable water systems (as well as other natural and man-made water bodies) world-wide. In the US, it has been reported present in 30–40 % of domestic water samples. In the latest Hong Kong study done earlier this year the percentage positive for Legionella is about 13 percent and none of the residents has Legionellosis.
  • Although the infection risk of Legionella in water depends on the concentration of the bacteria (together with the likelihood of the water being aerosolized and the immune status of the exposed individual), a grab sample for Legionella cannot offer much information because it has been well demonstrated that the Legionella population in water samples varies significantly on a temporal basis. In other words, it might show no or low count in one sample and higher count in another sample taken at the same spot at a different time. This is related to the ecology of Legionella as they tend to live in biofilm and even inside amoebae.
  • Even if Legionella bacteria are found in domestic water system, it is generally not a concern for healthy individuals. It could be a problem for immuno-compromised individuals such as those with chronic diseases, patients undergoing cancer therapy or who received organ transplant.

There are precautions one can implement to minimize the occurrence of Legionella in domestic water supply systems. These include:

  • Do not store hot/warm water below 50°C as it will provide a favorable growth condition for Legionella. At the UST staff quarters, there is no hot water storage system as all units are equipped with point-of-use heaters.
  • Flush tap water frequently to avoid stagnation
  • If water filters are desired, note that only filters with pore size of 0.2 microns or less are capable of filtering out Legionella. These filters must be changed frequently to ensure effectiveness. (Note: In HK hospital wards where there are immuno-compromised patients, the water filters are changed every 2 weeks.) In reality, a 0.2 micron filter is very restrictive and water will come through it at low speed. In view of the above factors, water filter for controlling exposure to Legionella is not recommended for general use.
  • Make sure to drink boiled water only. Some experts commented that although Legionella is a pathogen for the respiratory system and not the gastrointestinal system, drinking (unboiled) water may be a hazard as the drinking action generates microaerosol in the throat and bacteria in the water can be inhaled into the lung during drinking.
  • If one uses humidifiers at home, make sure the unit is cleaned and disinfected with 1:49 dilution of bleach periodically and that only boiled water is used in the humidifier for aerosolization.