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Cruising towards Legionnaires' Disease?

Posted by Ruth Thomas on Feb 15, 2022 11:51:45 AM


Risk of Legionella in the Marine Industry – Cruise ship edition

Over the last decade the cruise industry has experienced a massive burst of growth, be it on a luxury “floating hotel”, themed vessel or even a smaller river cruise, as the popularity of cruising in all regions of the world has attracted different age groups, interests and budgets.

From 2009 to 2019 alone, the number of passengers on cruise ships increased from 17.8 million to 30 million per annum with over 270 ships in operation by the end of this period (1). These figures are set to rise further as the world emerges from the temporary shutdown during the Covid-19 pandemic, with a total worldwide ocean cruise capacity in 2021 reaching 323 ships (2). With millions of water tourists ready to set sail aboard these ships once again, there is more need than ever to ensure a safe passage for those looking to enjoy a holiday on the high seas.

Whilst Legionnaires’ disease is often associated with “land-based” locations such as large cooling towers, hotels and hospitals, the risk is no less present on these cruise ship vessels - onboard water systems are responsible for sourcing everything from sinks and showers to swimming and spa pools and even decorative fountains (3).

Add to this, the number of people living on these ships, usually for 1-2 weeks at a time, using these water systems can increase the risk of a potential Legionella outbreak. This is not something that should be overlooked by cruise ship owners.

Cruise ships are considered to be high-risk environments where Legionella spp. may be found for a number of reasons including:

  • the uncertainty of the source water quality
  • the inconsistency of onboard water treatment
  • the complex nature of the storage and distribution systems on board.
Water is often stored within these systems long-term, and stagnation issues, combined with warm water temperatures (from engine room heat or ships cruising through more tropical environments) can quickly lead to the proliferation of any Legionella spp. that reside in the water or are allowed to form in biofilms hidden within the system (4).

Legionella contamination is particularly dangerous when the water has the potential to become aerosolised. Showers on board the ships are one potential source of infection because of this, especially if left unused for long periods of time. This can happen when cruises are running under capacity and not all cabins are in regular use.

A number of leisure activities offered on these cruise ships, which form a major attraction for many passengers enjoying this type of holiday, also pose a significant risk of aerosolising Legionella and causing potential infections (3). For example, spa pools and hot tubs create ideal environments for Legionella spp. to grow and have been associated with several cases of Legionnaires’ disease onboard ships (5).

Legionella has even been implicated with ship potable (drinking) water systems (6) and although less likely to cause a Legionella outbreak due to aerosolisation risk being lower, it still presents a potential danger indicating the importance of monitoring and maintaining ALL water systems.

Due to the nature of travel-associated Legionnaires’ disease, it can prove difficult to pinpoint the source of a Legionella infection. Passengers often combine staying onboard the cruise ship together with overnight stays in hotels at the ports (where cruises depart and return too) as well as enjoying many excursions off the ship to different locations.

By the time symptoms appear in individuals (up to 10 days after exposure) they have often dispersed from the original source of infection. In particular, if not associated with a high case number their commonality may be missed and thus actual cases of Legionella infection, resulting from a cruise ship stay, may be vastly underreported (6).

With cruise ship shutdowns during the Covid-19 pandemic and subsequent recommissioning of vessels in its aftermath, together with the older plumbing systems, the potential ageing populations of passengers and the plethora of perfect breeding grounds via leisure facilities, cruise ships must recognise the importance of testing for Legionella has never been more important. Recommended testing periods are ship dependent and vary from monthly to annually (4).

Despite greater awareness of the risks, there is still a lack of knowledge when it comes to testing for Legionella and the challenges it poses. There is also an implied complication of the testing process.

In the WHO “Guide to Ship Sanitation” (3rd edition) where testing requirements for Legionella are discussed it quotes

the tests are relatively specialized and need to be undertaken by properly equipped laboratories using experienced staff; they are therefore not generally performed by crews or during voyages(4).

We challenge this and it is for this very reason, that at Hydrosense, we developed a rapid on-site testing system for the most serious form of Legionella (L. pneumophila sg1). Our test does not require either experienced staff or properly equipped laboratories. It is a test that can be carried out by anyone and will give an indication of the health of the water system, from your showers to your spa pools in just 25 minutes.

This regime of fast testing can quickly identify Legionella risk areas and is the perfect solution for cruise operators - allowing them to act immediately and protect thousands of passengers from the risk of going home from their cruise with Legionnaires’ disease, rather than simply the lasting memories of a fantastic holiday.


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  1. Cruise lines international association 2019 state of the industry report
  2. https://cruisemarketwatch.com/capacity/
  3. https://legionellacontrol.com/legionella/controlling-legionella-ships/
  4. WHO: Guide to ship sanitation (3rd edition)
  5. WHO (2001) Sanitation on ships. Compendium of outbreaks of foodborne and waterborne disease and Legionnaires’ disease associated with Ships 1970-2000
  6. https://wwwnc.cdc.gov/travel/yellowbook/2020/travel-by-air-land-sea/cruise-ship-travel

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