Legionnaires' disease (LD) is an infectious disease caused by a type of bacteria called Legionella. The disease was named after an outbreak occurring in a Legion Convention in USA in 1976. In Hong Kong , LD has been made a statutorily notifiable disease since March 1994.

Mode of transmission

Legionella bacteria are found in various environmental settings and grow well in warm water (25 ^oC to 40 ^oC). They can be found in aqueous environment such as water tanks, cooling towers, whirlpool and spas, water fountains and apparatus that support breathing.

People may get infected when they breath in contaminated droplets (aerosols) and mist generated by artificial water systems. They may also get the infection when handling garden soils, compost and potting mixes.

The disease is not transmitted by person-to-person contact, eating or drinking.

Susceptible groups

While anyone may develop LD, the following groups of people are at higher risk:

• Men (3 times more susceptible than women)
• Elder persons (usually aged 65 years or above)
• Smokers
• Alcoholics
• Persons with weakened immunity, especially those with chronic illnesses (such as cancer, diabetes mellitus, chronic lung disease or kidney diseases) and those receiving corticosteroids or drugs that suppress body immunity

The following situation may also increase the risk of infection:

• Disruptions of water supply resulting in "brown" water at the water tap
• Living in areas with old water distribution or plumbing systems; or living near cooling towers or fountains
• Using electric water heater, whirlpool, spas or hot water spring spas
• Recent stay in hotels or ships

Incubation period

The incubation period is 2 to 10 days.

Clinical features

LD mostly presents with fever (39 ^oC to 40.5 ^oC), dry cough, shortness of breath, tiredness, headache, muscle pain, abdominal pain and diarrhoea. In severe cases, neurological symptoms (e.g. confusion) and respiratory failure may appear and some may cause death.

Some patients infected with Legionella bacteria may have a milder, non-pneumonic form of disease called Pontiac fever, which is a self-limiting febrile illness of short duration.

Management

LD can be treated with antibiotics.

Prevention

There is currently no vaccine available for LD.

Design, operate and maintain man-made water systems properly to prevent LD. For more information about the good practices in handling man-made water systems, please visit the website of the Electrical and Mechanical Services Department for the Code of Practice issued by the Prevention of LD Committee < http://www.emsd.gov.hk/emsd/eng/pps/oa_ld_pub_cp.shtml >.

Members of the public should also take heed of the following advice to reduce the risk of infection:

• Avoid smoking and excessive drinking, and maintain good health
• Maintain domestic water filters properly and change them regularly
• Cleanse water tanks of buildings regularly
• Seek and follow doctors' advice regarding the use and maintenance of home respiratory devices
• Take the following precautions when handling garden soils, compost and potting mixes:
• Water gardens and compost gently using low pressure
• Open composted potting mixes slowly and make sure the opening is directed away from the face
• Wet the soil to reduce dust when potting plants
• Avoid working in poorly ventilated places such as enclosed greenhouses
• Wash hands thoroughly afterwards