Legionnaires’ disease (also legionellosis or legion) is a potentially fatal form of pneumonia caused by any species of gram-negative aerobic bacteria belong to the genus Legionella. Over 90% of cases of Legionnaires’ disease are caused by the bacterium Legionella pneumophila. When infected a large percentage of people will die from the disease unless it is diagnosed and treated early and quickly.

A less severe infection, known as Pontiac Fever and resembling acute influenza, is also caused by other Legionella species. This is a non-fatal but acute respiratory disease that is usually self-resolving and, therefore, often goes undiagnosed.

Legionnaires’ disease was first recognised as an infection amongst humans in 1976, with the first recorded case associated with a ship, a year later in 1977. Since then it has continued to be a public health concern on passenger ships.

Symptoms of Legionnaires’ Disease

The symptoms usually include fever, chills, and a cough, which may be dry or may produce sputum, eventually progressing to pneumonia. Other tell-tale signs would include muscle aches, headache, tiredness, loss of appetite, loss of coordination (ataxia) and, sometimes diarrhea and vomiting.

Persons with Pontiac fever experience fever and muscle aches without pneumonia. They generally recover in 2 to 5 days without treatment. The time between the patient’s exposure to the bacterium and the onset of illness for Legionnaires’ disease is 2 to 10 days; for Pontiac fever, it is shorter, generally a few hours to 2 days.

All these symptoms are often misdiagnosed resulting in significant under recording and miscalculation of the number of cases.

Who is at higher risk?

Men more than women, people aged over 50, smokers, heavy drinkers, diabetics, anyone with an underlying disease and/or a weakened immune system.

The demographics of the cruise passenger make this particular group highly vulnerable to Legionnaires’ disease.

Where is Legionella found?

Legionella bacteria are widespread in natural water systems e.g. rivers, streams, ponds and may even be found in soil. However, the conditions are rarely right for people to catch the disease direct from these sources. They are also found in many recirculating and hot and cold water systems and spa pools, as well as anywhere water is stored or from anything that water is stored within e.g. pressure washers.

How is it contracted/transmitted?

Legionnaires’ disease can only be contracted by inhaling small droplets of contaminated water (aerosol) suspended in the air. The water in these aerosolised droplets rapidly evaporates leaving dry particles (droplet nuclei) containing any bacteria in the original droplet. The aerosolised droplets or particles are too small to see with the naked eye but can enter the lung of a person and start to multiply, causing an infection.

On board ship, for example: when showering, when running sink taps, moist air circulated by air conditioning, heating units and humidifiers, using fire hoses, washing down, pressure washers, hot tubs/spa pools and Jacuzzis, decorative fountains.

Drinking water contaminated with legionella bacteria will NOT cause infection and there is no evidence of person to person transmission.

What supports Legionella colonisation on board ship?

Water temperature between 25-45°C (77-113°F): Due to the extended length of pipes it is difficult to maintain high temperature in all parts of the ship’s hot water system and low temperatures in the cold water system.

Design of the water system: Ship water systems may be complex in nature and can be altered during refits; contain plumbing materials that may no longer be approved; may have dead legs/blind ends present; be difficult to control; have limited access for monitoring, maintenance and repairs.

Standing water: Large capacity water tanks and extended water storage time may result in a low chlorine residual in the water. Low cabin occupancy and water system repairs need to be considered. Standing water encourages formation of biofilms.

Build-up of deposits: Scale, corrosion, and sludge may build up in the base of calorifiers.

Cleaning: Cleaning of water system pipes, taps, showers and tank surfaces may be difficult due to limited access. Removal of deposit, and measures to reduce biofilm and nutrients is required.

Materials: Natural rubber and natural fibres should not be used in washers and seals. Only materials approved for contact with drinking water and shown not to encourage microbial growth should be used for construction of water systems.

Water treatment: Disinfectant and contact time may be used methods to reduce the bacteria.

Piping complexity: Piping of recreational water facilities and other equipment is often complicated and in confined spaces, making it difficult to inspect and maintain.

Knowledge: There may be limited expertise available on board.

System alterations: Water systems on board ship are often complex. Alterations and running repairs can result in dead legs/blind ends.

Preventive measures

Water distribution system

Any WSP established on board the ship must include provisions for Legionella control. Legionella spp. colonisation must be included in the risk assessment of the water distribution system. Requirements for control measures, operational monitoring, record keeping and corrective actions for the potable water distribution are described in Part A, Chapter 4. Control measures such as temperature control, regular cleaning and disinfection, flushing, and actions after system repairs are described below.

Construction – materials

All water systems components should be made of appropriate materials. Materials such as natural rubber, hemp, linseed oil based jointing compounds and fibre washers should not be used in water systems. Materials and fittings for use in water systems should have been shown not to support microbial growth and be suitable for use in contact with potable water.

Water systems should be designed and constructed so as to avoid poor water movement and turnover.

Temperature control

Water systems should:

  • avoid water temperatures between 25°C (77°F) and 49°C (120°F) to prevent Legionella colonisation;
  • ideally, maintain cold water below 25°C (77°F);
  • ideally, maintain hot water above 50°C (122°F).

It is recommended that hot water should be produced or stored at 60°C (140°F) and distributed such that a temperature at least 50°C (122°F), and preferably 55°C (131°F), is achieved within one minute at outlets. Care is needed to avoid much higher temperature because of the risk of scalding.

In addition to the monitoring of the water temperature at the tap it is useful to monitor the water temperature within the pipes by use of a contact thermometer. This is particularly important when thermostatic mixer valves are fitted to outlets. Measurement of the temperature of the hot water in the flow and return loops throughout the ship and not just the combined flows and returns to the water heater can rapidly detect areas of poor circulation. When operating efficiently there should only be a few degrees difference in the temperatures in the individual flows and returns.


Stagnation or slow water movement encourages biofilms to form in the water system.

All taps and showers are to be run in cabins for several minutes at least once a week if they are unoccupied and always prior to occupation.

Regular cleaning and disinfection

The purpose of cleaning is to remove scale, salt, sediments, sludge, dirt and debris from the water tanks and distribution system.

Disinfection must be applied in order to reduce the number of microorganisms in the water to levels that cannot cause harm.

A schedule should be established for regular cleaning and disinfection of all water system components:

−     Filling hoses (flushed for at least three minutes with potable water before use and disinfected at least every six months).

  • Water system pumps (every six months).
  • Water tanks (every year).
  • Pipes and taps of the distribution system (every year).
  • Hot water heaters (every year).
  • Shower heads and taps (every six months or depending on the inspection findings).
  • Hot water storage tanks (emptied when not in use).

Preventive measures during repairs and before cleaning

Before repairs to parts of the water system where water has a low flow rate or is static, water should be drained. Following repairs, that part of the system should be disinfected.

If tanks and calorifiers are heavily contaminated with organic materials, then disinfection is necessary before and after cleaning. Where possible, aerosol generation during cleaning should be avoided.

Regular sampling

Regular sampling of the potable water system is recommended at least every 6 months. Table 6 presents the action levels following Legionella sampling in hot and cold water systems.

Hot tubs and spa pools

Requirements and recommendations for the maintenance of hot tubs and spa pool are described in detail in the recreational water chapter of the manual, (Part A, Chapter 5) and include measures to control the proliferation of legionellae:

  • Spa pools are to be treated with a free residual chlorine level of 3-10 mg/L the levels should be monitored at least every 1 hour.
  • A complete draining, cleaning and renewal of the water should be done regularly.
  • Sand filters are to be backwashed daily after use of the pool has finished.
  • The whole system is to be cleaned and disinfected once a week.
  • Air injection lines should be cleaned and disinfected preferably monthly.