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Published on 06/01/2023

What is Legionella and Legionnaires’ Disease?

Many risk factors become amplified when living and working at sea, particularly the spread and development of air and waterborne illnesses. In confined spaces, often customary on sea-going vessels, these highly infectious bugs can quickly transmit and compromise an entire crew.

One of the most common and dangerous types of waterborne bacteria on board ships is Legionella, the bacteria responsible for the development of Legionnaires’ Disease. Here we look at what Legionnaires’ is, how it spreads and how this debilitating illness can be prevented with careful consideration and regular testing. 

Legionnaires’ Disease Explained

Legionnaires’ Disease is a severe type of pneumonia caused by a bacteria known as Legionella. It is most commonly caught and transmitted by inhaling the bacteria through water or soil – and symptoms tend to develop between two and ten days following exposure. 

Common symptoms include: 

The most effective treatment for Legionnaires’ Disease is a course of antibiotics, ideally started as soon as infection has been identified and before major symptoms occur. Alongside swift patient care, there are specific protocols crews should follow in the event of a Legionnaires’ outbreak on board, which we’ve covered in more detail below. 

It’s also important to note that Legionella bacteria is responsible for developing Pontiac fever, a milder flu-type illness producing symptoms such as chills, fever, headache and muscle aches. Pontiac fever usually clears on its own with no need for treatment within two to five days – but untreated Legionnaires’ Disease can be fatal. Being able to distinguish between the two and apply preventive measures in a suspect case of Legionnaires’ Disease could be the difference between life and death for those exposed to Legionella. 

Why is the management of Legionella crucial at sea?

Legionnaires’ is a serious illness and can significantly affect crew and passengers in a marine setting, partly because the spread of bacteria-based illness is challenging to control once a water supply has been compromised. Those at greater risk of complications tend to be over 45 years old, smokers or those with compromised immune systems – and with a large portion of crews occupying this age bracket, the risks of Legionella exposure become greater. Maritime vessels of all shapes and sizes can be affected – with numerous potential risks on ferries, cruise ships and offshore vessels. 

In the past 30 years, there have been over 200 cases of Legionnaires’ on board vessels at sea, including one which resulted in the infection of 50 crew members and one fatality. 

According to recent research by Legionella Control, one-third of cruise ships tested positive for Legionella due to improper management of showers, spa pools and hot tubs, swimming pools and fountains. 

In the case of a Legionnaires’ outbreak on board, there is a strict set of rules to be followed to help contain the outbreak and effectively treat those infected. 

Precautionary and preventive actions include:

How can outbreaks of Legionnaires Disease be prevented at sea? 

Most outbreaks of Legionnaires’ Disease at sea come from mismanagement of a ship’s water supply. Thorough and regular testing is crucial to detect contamination within water supply chains on board, with various infections and viruses associated with poorly maintained water, including Salmonella and E-Coli. 

Legionella thrives at temperatures between 20-50 C, so showers, hot and cold water systems, air conditioning units and drinking water networks are all at risk of contamination. Areas to pay particular attention to include: 

*Any part of your water system that regularly runs at a temperature or stores water between 20–50 °C

*Any outlet that creates and disperses water droplets, including showers, sinks & taps, air conditioning units, fire hoses, and washing equipment

*Anywhere that water is stored or re-circulated

*Areas with deposits containing nutrients that allow bacteria to grow, such as rust, sludge, scale, organic matter, and biofilms

This is where potable water testing and sewage effluent management come into focus. Understanding the risks associated with waterborne illness and ensuring a safe water supply is crucial to protect crew and passengers on board. 

Reducing risks, protecting the crew

In addition to protecting crews, ship operators have a duty of care to take reasonable measures to prevent Legionnaires outbreaks. 

“As an employer, or person in control of premises, you must appoint someone competent to help you meet your health and safety duties and take responsibility for controlling any identified risk from exposure to legionella bacteria.” 


Martek Marine is dedicated to providing sound solutions developed with crew safety and complete compliance in mind. Our sophisticated water testing kits make the testing and management of water on board simpler and safer with quick and easy configuration and low maintenance – with no training required to get up and running. 

The LegionellaMAX Test Kit is the most advanced and accurate of its kind, offering rapid detection of Legionella bacteria in water samples or biofilms taken from any source. The test kit includes everything you’ll need to conduct a water safety test anytime, anywhere, producing on-site results within half an hour.