At a glance
Legionella bacteria are ubiquitous in the environment where they can complete their life cycle without infecting humans or other animals. They only infect humans opportunistically. Importantly, they can proliferate in water at relatively warm water temperatures, such as those experienced in shower heads and spa pools, and are found in piped water distribution systems and in storage tanks on ships.
There have been many cases of legionellosis reported on ships. The problem is not restricted to passenger ships. Surveys carried out on general cargo ships have shown drinking water and air conditioning systems to be contaminated with Legionella pneumophila. Therefore, proper management of wet environments aboard is essential to prevent such outbreaks.
Why ships are high-risk environments for legionella
Water storage and distribution systems on ships are complex and could provide opportunities for bacterial contamination as ship movement increases the risk of surge and back-siphonage. Loaded water may vary in temperature, and in some tropical regions, the risk of bacterial growth is increased because of higher water temperatures.
Finally, proliferation is encouraged due to long-term storage and stagnation in tanks or pipes. Legionella can proliferate in hot and cold piped water systems leading to potential exposure through aerosolisation arising from showers and other plumbing fixtures. It is inhalation of bacteria, or aspiration following ingestion, that is thought to lead to disease, rather than swallowing. Those at increased risk are adults 50 years or older, current or former smokers, and people with a weakened immune system or chronic disease.
Poor potable water quality has been linked to outbreaks of Legionnaires’ disease on ship. Increased risk of proliferation of Legionella has been associated with drinking-water subjected to periods of stagnation and in systems operating at temperatures ranging from 25 to 50ºC.
In general, the focus to prevent a possible outbreak onboard must be placed on controlling temperature and adding biocides to keep pathogen numbers down to safe levels and at the same time reducing the opportunities for inhalation of water. Therefore, reliance should not be placed on biocides alone, and multiple barriers should be actively maintained including:
- minimizing residence times of stored water;
- maintaining water temperatures either well above or well below levels that are optimal for Legionella growth;
- maintaining biocides with reliable, well designed and properly maintained dosing systems; and
- reducing the opportunity for aerosol generation and inhalation.
Control measures required to reduce the risk of legionellosis on ships include: prevention of contamination, proper disinfection, filtration and storage of source water, avoidance of dead ends in pipes and regular cleaning and disinfection of recreational water environments.