Legionella on board: how to test, trace and prevent outbreaks
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Legionella is a familiar concern that’s relatively easy to manage – on land. But on board a ship, where water can sit in pipes during layovers, fluctuate in temperature, or stagnate in little-used cabins, the risk takes on a different shape. Especially because the people on board — your crew — are in daily, repeated contact with that water.
And when exposure does happen, the impact isn’t minor. Legionella bacteria can cause severe lung infections, including Legionnaires’ disease, which leads to hospitalization in a significant number of cases. That’s why prevention matters. In this article, you’ll find practical insights on how to test for Legionella, where the risks typically arise, and what you can do to prevent problems before they occur.
When does Legionella become a risk on board?
In theory, the answer is simple: Legionella thrives in warm, stagnant water. In practice, it’s the details that determine your risk. Showers that haven’t been used in days, medical taps that are flushed only during inspection rounds or storage tanks that fluctuate in temperature, for example. In these places, bacteria in the water can multiply rapidly.
The risk increases when:
- Water temperatures remain between 25 and 50°C for extended periods.
- Biofilm builds up in aging pipes or dead ends in the distribution system.
- Chlorine levels are too low, especially at distal points in the system.
- Outlets are rarely used — think of guest cabins, gyms, or treatment rooms.
The good news is: regular water sampling with a specific focus on Legionella can make a big difference in prevention. Let’s look into how that works:
How do you test for Legionella on board?
Legionella is not always part of standard water quality testing. You’ll need to request it separately — ideally as part of a risk-based sampling approach. That means choosing sampling locations based on actual exposure risk: frequency of use, water temperature history and stagnation potential. According to the WHO Guide to Ship Sanitation, testing should include:
Sampling from at least one shower head without cleaning or disinfecting it, to simulate real exposure conditions.
Measuring parameters like temperature, pH, free chlorine and conductivity on-site, before transport to the lab.
Accurate labeling: including location, sample point description, IMO number, time of day and water system status.
Additionally, comparing results over time can help signal biofilm formation, chlorine depletion or process gaps, developing over time. Findings that may be overlooked by a one-time minor detection.
Also read: Marine water sampling: requirement or opportunity?
What to do after a positive Legionella result
If Legionella is detected, the next step depends on the level. In some cases, flushing may be sufficient. In others, you’ll need thermal disinfection, shock chlorination or pipe replacement. A key mistake to avoid: taking action without documenting it properly.
Inspection bodies, including port state control, will want to see not just that an issue was identified, but that a clear and adequate response followed. That includes retesting, keeping records, and adjusting your procedures if needed.
Also read: Failed water test on board: what to do next.
How to prevent Legionella in the first place
The risk of a Legionella infection on your ship, depends on the design and maintenance of the systems on board. These steps help keep things in check:
Maintain hot water above 60°C at the tank and ensure delivery above 50°C at outlets.
Keep cold water below 20°C, especially in storage and distribution lines.
Flush little-used outlets (like crew showers or medical taps) weekly, or include them in the sampling plan.
Track pH, temperature, disinfectant residuals and pressure drops regularly.
Use historical data to identify slow-forming risks, and act before issues escalate.
Small addition, big impact
Preventing Legionella isn’t just a matter of compliance. It’s about protecting crew health, preventing reputational damage and avoiding operational delays. With just a few targeted steps added to your water monitoring routine, you can reduce risks and increase your inspection readiness with confidence.
Want to know how marine water specialists handle Legionella sampling?
Learn more about our services here.
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