What is Legionnaires’ disease and how does it spread?

Legionnaires’ disease is a kind of atypical pneumonia. It is usually caused by the inhalation of water particles in a mist form when there are high levels of the bacteria Legionella pneumophila present in that water source. 

Legionnaires’ disease is a risk that needs to be considered in a wide variety of environments, so below we take a closer look at how it spreads and who it can affect, to give you a better idea of how to fight against it.

The first identified outbreak of Legionnaires’ disease

In late July 1976, at the Bellevue-Stratford Hotel in Philadelphia, the American Legion held a three-day convention attended by around 2,000 Legionnaires. 

In the week following the convention, 130 people who had attended had been hospitalised and 29 had died. All had experienced chest pains, fever and lung congestion. 

The CDC launched an investigation, and by January 1977, it had identified the first known outbreak of Legionnaires’ disease. The investigation had found high concentrations of Legionella bacteria present in the hotel’s water tower, which had then been spread around the building via the air conditioning system. 

This first, tragic, identified case of Legionnaires’ is a classic example of an outbreak, showing both how it can spread and how important it is to ensure outbreaks do not occur.

How does it spread?

Legionella pneumophila bacteria is present in many water sources, often in non harmful quantities. However, in places where water stagnates, especially within certain temperature ranges (typically 25-40c) it can proliferate to dangerously high levels. Where that water is then turned into a fine mist, such as around showers, taps, swimming pools, hot tubs or air conditioning units, those who inhale the mist are at risk of developing Legionnaires’ disease. 

Although it’s possible to catch Legionnaires’ from drinking infected water, it’s uncommon. Infection only occurs in those cases where water accidentally enters the lungs, making it an increased risk for those who have swallowing difficulties.

Legionnaires’ disease is not thought to spread between individuals, although it’s possible that it could occur in very specific circumstances. The fact that the dominant means of infection is via water sources means that water hygiene is the primary way of fighting against Legionnaires’ disease.

Those who are at increased risk

While anyone exposed to high levels of Legionella bacteria can develop Legionnaires’ disease, specific groups are at a far higher risk of getting ill. These include:

  • Individuals in older age groups, typically 50 years and older
  • Individuals with chronic illnesses, including kidney issues, liver issues and diabetes 
  • Individuals who are either former or current smokers
  • Individuals with some form of lung illness (such as chronic obtrusive pulmonary disease)
  • Individuals with cancer

This means that operators of environments with a high concentration of individuals in any of these categories need to be incredibly diligent with water hygiene testing. Such environments could include care homes, hospitals, and community centres.

For further advice tailored to what you need to know about Legionnaires’ disease, head to a dedicated resource such as the Water Hygiene Centre website.