Legionella Public Health Risk
Legionella bacteria
Different species of Legionella bacteria occur naturally in aquatic environments (Legionella pneumophila) and in soil (Legionella longbeachae). Legionella pneumophila serogroup 1 is a common cause of Legionnaires’ disease associated with cooling water systems and outbreaks. The growth and multiplication of Legionella bacteria is promoted by:
- A wet and warm environment with a temperature range of 25°C to 42°C (the optimum temperature for multiplication being 37°C.)
- Large body of Stagnant water or low water turnover.
- High concentration of other microorganisms, including algae, amoebae, slime and other bacteria
- Presence of nutrients - biofilm, scale, sediment, sludge, rust or other organic matter. Rust is one of the most common forms of nutrients for Legionella.
- Degraded plumbing components, such as rubber fittings.
- pH fluctuations of the cooling water systems.
Cooling tower systems can provide an ideal environment for the growth of Legionella. This can pose a health risk to employees, contractors, customers or members of the public who have been in or near buildings with a cooling tower.
To limit the growth of legionella bacteria in cooling water systems the water temperature should be maintained at least below 25°C; but below 20°C is ideal as the legionella bacteria will be dormant. The water temperature should be monitored and measured at the return line to validate the operating temperature of your cooling water system.
Legionnaires’ disease
Legionnaires’ disease is an infection of the lungs caused by Legionella bacteria. People can become infected with Legionella bacteria by inhaling very fine water droplets/aerosols that contain the bacterium that bypass the upper airway defences. As Legionella does not colonise the upper airways there is no transmission between humans or between animals and humans (i.e. it is a noncommunicable diseases.)
Legionnaires’ disease usually presents as pneumonia that can vary from mild to severe illness and in some instances death. Legionnaires’ disease has a 10% case fatality rate, meaning that one out of ten people infected will die from it.
Legionnaires’ disease has an incubation period of 2 to 10 days, as a result symptoms may not appear until 10 days after initial exposure. Furthermore, new cases may continue to emerge for up to 10 days after the source of infection has been eliminated. Symptoms include cough, shortness of breath, high fever, muscle pains, and headaches. Nausea, vomiting, and diarrhoea may also occur.
The risk of developing Legionnaires’ disease is greater among people who are older (particularly above 50 years of age), smokers, heavy drinkers or people who have medical conditions that weaken the immune system, examples include cancer, lung disease or being a transplant recipient.
Medical treatments exist but currently no vaccine is available for Legionnaires’ disease.
Outbreak investigation
The Public Health Act 2010 requires all cases of Legionnaires’ disease to be notified to NSW Health. Legionnaires’ disease notification data is published on HealthStats NSW. If more than one case reports a common exposure, the NSW Public Health Unit may initiate an outbreak investigation. This can involve the urgent inspections of cooling towers which could result in issuing improvement notices or prohibition orders on non-compliant cooling water systems.
Peaks in Legionella pneumophila cases often occur in autumn. This seasonal trend may be explained by a prolonged period of cooling tower use over summer, followed by intermittent use during autumn, due to ambient temperature fluctuations.
Impacts on businesses
Outbreaks of Legionnaires’ disease associated with cooling water systems can have significant implications for businesses. During an outbreak, the normal operation of a business will be severely disrupted.
The Public Health Regulation 2022 requires reportable test results to be notified to local government authorities at the levels outlined below:-
- Legionella count ≥1,000 cfu/mL
- heterotrophic colony count ≥5,000,000 cfu/mL
After a notifiable result Environmental Health Officers from Council/NSW Health may inspect the premises and require works to be undertaken to remove the public health risk (including the issue of notices if deemed appropriate).
A number of recent outbreaks have occurred in Sydney NSW:-
Corrective action
Corrective action can involve disinfection, cleaning, maintenance, repair/replacement of the tower, which can have significant cost implications for the occupier. Further water sampling analysis in line with AS3666: 2011 will be required, including two consecutive negative legionella samples to confirm the risk of Legionnaires’ disease has been removed.
Occupiers are responsible for ensuring that monthly testing takes place, and that the Local Council is notified if a reportable test result is received. The reportable result levels are as follows:-
• Legionella count ≥1,000 cfu/mL
• HCC ≥5,000,000 cfu/mL.
Even though the above levels are reportable lower bacterial counts require corrective action as outlined in AS/NZS 3666.3:2011, if legionella counts are above 10 cfu/mL or HCC levels are above 100,000 cfu/mL online disinfection/decontamination of the tower is required.
To limit the public health and commercial impacts from outbreaks of Legionnaires’ disease building owners/occupiers who are responsible for the operation of cooling water systems should ensure that effective maintenance programs are in place to reduce the public health risk posed by legionella to as low as reasonably practicable. This is particularly important in heavily urbanised areas, as there is a higher potential to affect a larger population.
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