Legionnaires’ disease, or legionellosis, is a form of pneumonia caused by the bacterium Legionella pneumophilia. As the disease presents symptoms common to other forms of pneumonia specific laboratory tests need to be conducted to determine if the disease is a result of infection by Legionella pneumophilia. As testing methods have improved, we have seen a reduction in fatal cases. The fatality rate, however, still remains rather high with 10-15% of cases being fatal.

Legionella pneumophilia can also cause a much less severe disease called Pontiac fever. Infection occurs when a person breathes in an aerosol (small droplets of water) containing Legionella. These aerosols are created by water striking a hard surface, water spraying from a showerhead or outdoor sprinkler system, “spritzer” cooling sprays, flushing toilets, and by air being bubbled through spa pools (jacuzzis).

People at risk for contracting Legionnaires’ disease include:

• Those belonging to an older age group (≥ 40 years)
• Those with a chronic underlying disease
• Those with immunodeficiencies
• Those having had an organ transplant
• Those having a history of heavy smoking

Where Does Legionella come from?

Legionella is found commonly in low numbers in natural water systems such as lakes, rivers, and ponds. In these areas, it presents very little risk as the numbers are too low to overcome a healthy individual’s immune system and cause an infection. Legionella, however, becomes a risk when it is introduced to man-made water systems. In these systems, conditions are often ideal for the bacteria to grow (multiply) to dangerous levels.

To understand why man-made water systems pose such a health risk, we must first understand what is required for bacteria to grow. Namely a food source, temperature, and time.

1. A food source: Bacteria, like all living organisms, need a source of nutrients to enable them to grow. Legionella is able to grow on many different forms of “food” found commonly in artificial water systems. Slime, scale, dirt and rust often present in these systems all serve as food enabling Legionella to grow.
2. Temperature: Bacteria require specific temperatures to enable them to grow. Legionella bacteria are known to grow optimally at temperatures between 20 and 45° Legionella is even known to survive temperatures as high as 50° Celsius.
3. Time: Legionella needs time to grow to dangerous levels, however, they grow in an exponential fashion whereby 1 bacterium becomes 2, then 4, then 8, then 16 and so on. This allows them to grow to dangerously high levels in a relatively short amount of time.

The following man-made water systems are recognised by the European Legionnaires’ Disease Surveillance Network  (PDF, 1.5MB) as potential sources of Legionnaires’ disease:

• Hot and cold water systems (water storage tanks, water heaters, calorifiers)
• Cooling towers and evaporative condensers
• Heated birthing pools
• Industrial product aeration ponds
• Spa pools, natural pools, thermal springs
• Fountains, sprinklers

These systems pose a risk as they can often be difficult to keep in a good, clean condition, thus supplying any Legionella present with an ample food source.

Any stagnant water system also poses a risk as Legionella is more likely to grow in stagnant water. This is why it is so important to ensure that the water reticulation system doesn’t have any “dead ends” in which water would sit stagnantly.