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30 December 2015
Latest recommendations of Scientific Committee on strategy for prevention and control of Legionnaires' Disease  

     The Centre for Health Protection (CHP) of the Department of Health today (December 30) announced its revised risk-based strategy for environmental investigation and sampling for cases of Legionnaires' disease (LD) recommended by the CHP's Scientific Committee on Emerging and Zoonotic Diseases (SCEZD) in view of the local epidemiology as well as prevention and control practices overseas.
     Locally, the number of reported LD cases has been on the rise since 2011, from 17 in 2011 to 28 each in 2012 and 2013, 41 in 2014 and 66 in 2015 as of yesterday (December 29). Among them, the majority are local cases. Yearly breakdown figures are detailed in Table 1 in the attachment.
     Nevertheless, the CHP highlighted that, in the same period, for the legionella urinary antigen tests conducted by both the CHP's Public Health Laboratory Services Branch (PHLSB) and the Hospital Authority, the percentage positive among patients' specimens received, which gives a scientifically more objective and accurate analysis of the disease trend, had remained stable and had not shown significant increase with time. Yearly figures are detailed in Table 2 in the attachment.
     "Among cases with both respiratory specimens and environmental samples collected from related water systems tested positive for Legionella pneumophila serogroup 1 (Lp1) from 2011 to date, molecular typing studies conducted by our PHLSB so far have not detected the same sequence-based type of Lp1 bacteria between the human and environmental samples. The environmental sites with Lp1 detected were probably incidental findings and unlikely to be the source of infection for the cases," a spokesman for the CHP said.
     The SCEZD has reviewed international practices in environmental investigation and sampling for LD cases. International health or relevant authorities generally do not recommend routine environmental investigation and sampling for sporadic community-acquired cases. In general, environmental sampling is only recommended for investigations involving hospitals, residential institutions and disease clusters or outbreaks where a common potential exposure has been identified through epidemiological investigations.
     Based on the review of the local epidemiology, scientific literature and practices in major developed countries, the SCEZD recommended the following strategy for investigation of LD cases in Hong Kong with effect from January 1, 2016.
     The CHP will continue to conduct epidemiological investigations to identify potential sources of infection, high-risk exposures and clusters. In general, environmental investigation and sampling from potential sources will be carried out only if one of the following criteria is met:
1. A single definite or possible nosocomial case associated with high-risk areas of a hospital (e.g. wards with severely immunosuppressed patients such as transplant unit, intensive care unit, etc.), i.e. the patient stayed in the hospital during the entire or part of the incubation period (IP) (two to 10 days before onset) respectively;
2. The patient spent the whole IP as a resident of a residential institution, such as a residential care home for the elderly (RCHE) or a residential care home for persons with disabilities (RCHD), or as an in-patient in low-risk areas of a hospital;
3. Two patients with onset within six months who had common exposure for a portion of the IP to either a residential institution, such as an RCHE/RCHD, or low-risk areas of a hospital;
4. The patient had exposure to a high-risk source, such as aerosol-generating device (e.g. respiratory equipment), during the IP;
5. The patient visited a high-risk venue, such as spa, jacuzzi or whirlpool, during the IP; and
6. A cluster which is defined as two or more confirmed cases with onset within six months and common exposure to the same potential source of infection during the IP (e.g. a cooling tower, living in the same building etc.).
     Regarding environmental investigation, the CHP will liaise with the Electrical and Mechanical Services Department to collect samples from potential sources for testing for legionella. The following risk-based action levels of total legionella count for control measures are recommended for water samples collected from different settings or potential sources (see Table 3 in attachment):
1. Cooling towers associated with two cases within six months: 10 colony-forming units per millilitre (cfu/ml) or above;
2. Water systems in buildings in the community epidemiologically linked to two cases within six months: 10 cfu/ml or above;
3. Water systems in residential institutions (RCHEs or RCHDs) and low-risk areas of hospitals associated with either any case who stayed there for the whole IP or two cases who spent part of the IP there within six months: one cfu/ml or above;
4. Water systems in high-risk areas of hospitals (e.g. wards with severely immunosuppressed patients such as transplant unit, intensive care unit, etc.) associated with any case: 0.1 cfu/ml or above; and
5. Hot tubs (e.g. spa, jacuzzi or whirlpool) and aerosol-generating devices (e.g. respiratory equipment) associated with any case: 0.1 cfu/ml or above.
     Actions in response to the detection of legionella in the above water samples will depend on the nature of the suspected source, the proportion of specimens with positive laboratory results and specific risk assessment of individual scenarios. Specific actions might include disinfection of the water systems concerned, installation of point-of-use water filters, removal of contaminated tap aerators or shower facilities, etc.
     LD is a type of bacterial pneumonia caused by legionella, most commonly Lp1. The SCEZD noted that legionellae are ubiquitous in aqueous environments including man-made water systems such as potable water supplies systems. Overseas studies show that legionellae were found in about 10 per cent to 60 per cent of various water plumbing systems. It is difficult to achieve long-term elimination of legionella in potable water systems and recolonisation of legionella after disinfection is common. Immunosuppressed patients are the most at risk. The SCEZD stressed the importance of immunosuppressed persons taking measures against LD.
     The consensus summary of the recommended strategy has been uploaded to the SCEZD's page ( The public may also visit the CHP's LD page for more information (
Ends/Wednesday, December 30, 2015
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