With oceans covering around 70% of the Earth’s surface, it is no great surprise that there are a large number of seafaring vessels that set sail every year across them. From cruise liners to ferries to merchant's vessels the number of ships worldwide is vast. For merchant ships alone, in 2021 it was estimated that there were around 55,000 trading internationally(1) with each of these making countless trips every year. And no ship or substantial sea vessel can run without the aid of a crew who undertakes all manner of jobs onboard.
Ships can often be at sea for prolonged periods of time and the isolated crews who live and work within close quarters onboard have to share facilities. Amongst other health and safety essentials, maintaining high water quality for these individuals is of vital importance. The water on board a ship will be used to supply the crew with both potable water for drinking and cooking, water for showering and cleaning and non-potable water for cleaning decks or for fire-fighting purposes.
Both the WHO Guide to Ship sanitation and the Maritime Labour Convention, 2006 regulations state that ship operators/masters should protect crew from waterborne health risks by ensuring the water onboard is kept clean and free from anything likely to cause illness(2, 3). Whilst this covers a host of potential waterborne contaminants, Legionella is among those that ships need to control to minimise the risk of infection. Add to this, the additional complexities of testing for and controlling Legionella compared to some more common waterborne pathogens, it is even more important to give this a high priority.
What are the main challenges for water contamination with microbes including Legionella on board a ship?
Many of the challenges surrounding the water supply safety onboard ships are similar to those of complex buildings on the land, such as complicated pipework networks, dead legs and stagnant water tanks. However, the extra complexities of supply systems on ships add additional challenges and increase the risk of contamination of water supplies. From the water source itself to the transport and storage of water, to the risks of stagnancy during dry docking of ships, there are many factors to be considered with regard to water safety, including Legionella risks (3).
Ships primarily obtain water from ports that are located all around the globe. Whilst this water SHOULD meet the quality recommendations set out by the WHO(2) this may not always be a given, particularly at smaller, more remote ports where the water quality and source may not be known.
Even if the source water is of acceptable quality, the mere process of transferring this water onboard a ship can introduce several stages where contamination may subsequently occur. This process, sometimes referred to as “bunkering” of water, involves a number of different elements for transfer including pipes, hydrants and water ships, where contamination is possible. The presence of stagnant water or biofilm for example within any of these may readily introduce pathogens such as Legionella into the system as it makes its way on board(3).
Finally, the storage of water onboard can add further challenges. It can allow many contaminants that are present within the ideal environment in which to proliferate if the opportunity arises. Maintaining residual disinfection levels will help to control planktonic bacteria in the water but will not be effective against bacteria like Legionella which are able to hide in biofilms or dead legs within the system and resist this disinfection.
In addition, if temperatures of the cold and hot water are not maintained below 20°C and above 50°C respectively, then the risk of Legionella growth, in particular, is much more likely(2). With temperatures of tanks and pipes in the vicinity of the engine room, or in tropical climates, being harder to maintain at low temperatures, the risk from this particular pathogen is considerably higher.
The incidence of Legionella prevalence on large passenger-carrying vessels such as cruise ships has been previously well documented (4) and outbreaks are often associated with the leisure facilities provided on these vessels. However, the prevalence of Legionella contamination on Non-Passenger Merchant Vessels (NPMV), has often gone underreported.
A study in 2016 carried out by Public Health England (PHE) found that of 803 water samples collected from 360 vessels docked at UK ports over a period of three years, over half were positive for Legionella with 46% of the vessels giving L. pneumophila sg1 positive samples(1). Many of these positives were obtained from onboard showers and the level of contamination was frequently seen above the highest UK action level (>1x10e3 cfu/L) which is a worrying combination(1). Although small scale in comparison to global vessel numbers, the findings of the study highlight some major concerns regarding the management of microbial risks, particularly Legionella, onboard NPMV’s and emphasises the need for greater control of water systems on these large ships to keep everyone on board safe.
Whilst reported cases of Legionnaires' disease on merchant ships worldwide, are low, incidences may be under-reported due to the geographical location in which seafarers are treated and milder cases may simply be symptomatically treated onboard whilst at sea with no understanding of the cause. Given that a large proportion of the crew on these types of merchant ships are often male, over 40 and smokers (1) (common risk factors of Legionnaires’ disease), lack of reported cases should not be allowed to lead to complacency that infections will not occur.
With greater knowledge and understanding of the risks of water contamination onboard a ship, the emphasis should be on creating and managing a water safety plan to ensure a solid regime of disinfection, cleaning and testing of water. A good understanding of the transfer system alone will help a ship's crew to identify ideas to focus risk assessments on.
Where Legionella is concerned, regular testing of the water should be carried out at various high-risk locations such as showers or storage tanks to ensure that low levels of the bacteria are picked up before it becomes problematic and a risk to the crew.
Traditional lab culture testing for Legionella might be viewed as challenging in a shipping scenario where the ship itself may be hundreds or even thousands of miles from the nearest testing laboratory. Also, the logistics of sending samples back to shore combined with the several weeks' wait for results are less than ideal. Waiting until docked in a port before taking samples runs the risk of crews being exposed to dangerous levels of bacteria for the weeks in between that they are at sea and often the turnaround time in the dock does not allow for subsequent lab results to be returned before setting sail again.
There is an ideal solution to help protect all personnel on merchant ships
There is an alternative option that will not only give an indication of Legionella pneumophila sg1 contamination much faster (in just 25 minutes!) but is also quick and easy to perform by any member of the crew. The Hydrosense testing solutions for both water and biofilm offer the perfect answer for shipmasters to employ whilst at sea to check their Legionella management controls are working and ensure levels of Legionella are not becoming of concern. It also allows for multiple locations to be easily sampled at one time to check the health of the entire system, rather than just one or two outlets.
These big merchant ships are nothing without their crew and so it stands to reason that those responsible for the ship should take every precaution necessary to ensure the health of that crew allowing plain sailing for each voyage they undertake.
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- High prevalence of Legionella in non passenger merchant vessels. Collins et al. Epidemiol. Infect. (2017), 145
- WHO Guide to Ship Sanitation
- Azara A, et al. Prevalence study of Legionella spp. contamination in ferries and cruise ships. BMC Public Health 2006; 6: 100.