Legionella and Legionnaires’ disease: Section 2

Legionnaires’ disease

Legionnaires’ disease is a potentially fatal form of pneumonia caused by the bacterium Legionella pneumophila. Legionella species can also cause less serious illnesses that are not permanently debilitating. The group of infections caused by species of Legionella is known as legionellosis.

Legionella bacteria occur naturally in the environment. They are commonly found in lakes, rivers, creeks and soil. People usually contract Legionnaires’ disease by breathing in Legionella bacteria in very fine droplets of water called aerosols. Artificial water systems, including showers, spa pools, fountains, car washes and cooling towers, may provide environments that allow Legionella bacteria to multiply in large numbers. Legionella can then be spread by aerosols.

The main risk factors for an outbreak of the disease are:

• the presence of Legionella pneumophila bacteria
• conditions suitable for multiplication of the organism, including a suitable temperature (20–50 °C), and a source of nutrients such as sludge, scale, rust, algae or other organic matter
• a means of creating and spreading breathable droplets, such as the aerosols generated by a cooling tower, shower or spa
• exposure of susceptible people to these aerosols.

Incubation period

Legionnaires’ disease has an incubation period of 2-10 days. This means that symptoms do not appear until 2–10 days after a person has been exposed to Legionella bacteria. It also means that cases may continue to emerge for up to 10 days after the source of infection has been successfully eliminated.

Cooling tower systems and Legionnaires’ disease

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 general public who have been in or near buildings with a cooling tower.

In the past, owners of cooling tower systems usually learned of cases of Legionnaires’ disease when public health officers from the department investigated possible sources of infection associated with their location.

Who is at risk?

Most people exposed to Legionella bacteria do not become infected. The risk of disease increases with age, especially among smokers. People with chronic medical conditions that weaken the body’s immune system – including people with cancer, lung disease or diabetes, and transplant recipients – may be at increased risk of Legionnaires’ disease.

Impacts on health

Many people with Legionnaires’ disease are admitted to hospital for long periods and spend some of this time in intensive care. For a minority of sufferers, the disease is fatal. A small percentage may suffer some permanent disablement.

Since the Legionella Risk Management Strategy was introduced in Victoria in 2001, the number of cases of Legionnaires’ disease has decreased. Since 2000, it is suspected that testing for Legionnaires’ disease in patients with pneumonia-like symptoms has significantly increased as a result of increased awareness of Legionnaires’ disease and the introduction of the urinary antigen test (in 1999).

The Public Health and Wellbeing Regulations 2019 require that all cases of legionellosis are notified to the department. The department publishes these data on the internet.

Figure 1 shows the incidence of legionellosis in Victoria over the past decade. Legionella pneumophila cases are commonly associated with cooling tower systems.

Figure 1: Notified cases of legionellosis, Victoria, 2012–2020

* Not further specified

The above graph shows the number of cases of Legionellosis notified to the Department of Health each year from 2012 until 2020 and the rate of legionellosis per 100,000 population. The number of cases each year has increased from 68 in 2012 to 130 in 2020. The rate per 100,000 population has increased from 1.2 in 2012 to 2.1 in 2020.

Impacts on businesses

Outbreaks of Legionnaires’ disease associated with a cooling tower system can have devastating effects on a business.

Owners and occupiers of land may face prosecution for not complying with the Public Health and Wellbeing Act 2008, the Public Health and Wellbeing Regulations 2019, and occupational health and safety legislation. Legal action for damages suffered by individuals or companies as a result of an outbreak of Legionnaires’ disease is also likely.

During an outbreak, the normal operation of a business is likely to be severely disrupted. In some cases, the business may have to suspend all operations until the source of the outbreak is located and treated. Negative media attention is likely, and the business may suffer significant loss of trade and customer goodwill for a long time after the outbreak has been contained.