Legionnaires’ disease is a type of pneumonia caused by Legionella bacteria. These microbes naturally inhabit fresh water, but they can also thrive in man-made systems such as hot tubs or cooling towers where they can form stubborn biofilms that are challenging to remove. If these water systems are not properly maintained, the microbes can multiply and disperse through airborne water droplets.

Individuals contract the disease by inhaling small water droplets containing the bacteria. While most healthy people don’t develop Legionnaires’ disease after exposure, those with lung conditions, smokers, older adults (people over 40 years of age for the Melbourne outbreak), or people with weakened immune systems are at risk. Symptoms typically appear 2 – 10 days after exposure. Treatment involves antibiotics, and the disease is not usually transmitted from person to person.

To contain outbreaks, rapid identification of the infection source is crucial. Implementing thorough disinfection procedures can prevent further spread. Keep in mind that Legionella is a slow-growing microbe, so confirming the source may take up to 10 days. That is why health authorities in Victoria have undertaken the mammoth task of disinfecting 41 cooling towers in the area of the suspected outbreak while awaiting the laboratory results to confirm the source of the current outbreak. 

Legionella is an environmental bacterial infection that causes pneumonia (a lung infection). It tends to be more severe in older people, smokers, and those with chronic lung and kidney disease. Unfortunately, it can be fatal, especially in these groups.
 
The likely source of this outbreak is a cooling tower, which allows the Legionella bacterium to be aerosolised for quite a distance (think a kilometre or more). 

Legionella likes living in warm water (25-45 degrees C) such as can be found in cooling towers. This is why cooling towers require mandatory periodic testing for the presence of Legionella.
 
Two positives about Legionella are that it can’t be transmitted between people, and that commonly used antibiotics are effective against it. 
 
The public health investigation will be trying to identify the culprit cooling tower, which will likely involve a combination of epidemiological studies of cases and uninfected controls, as well as testing of cooling towers in the suspect areas.

Legionella pneumophila is commonly found in the environment and can cause Legionnaires’ disease - a severe pneumonia, and Pontiac fever - a milder flu-like illness.

In many cities, Legionnaires’ disease cases peak during autumn and summer, often following periods of high rainfall and humidity. The disease is more frequently diagnosed in men over 50, particularly smokers or those with underlying health conditions.

Globally, the incidence of Legionnaires’ disease has increased significantly, with notable rises in the USA and Hong Kong. This increase may be due to a growing number of susceptible individuals, better diagnostic methods, and improved reporting.

Our research focused on how Legionella pneumophila spreads in urban environments and explored the relationship between weather conditions and Legionnaires’ disease outbreaks. Interestingly, while increases in humidity and rainfall are linked to a rise in Legionnaires’ disease cases, the major outbreak clones of L. pneumophila did not show a direct connection to weather changes in our study.

These findings highlight the importance of high-resolution bacterial typing (genome sequencing) and considering weather data when investigating Legionnaires’ disease outbreaks, especially when clinical samples are limited. This approach can help improve strategies for managing and preventing Legionnaires’ disease in urban areas.

There are different species of bacteria called Legionella, which are commonly found in the environment and some cause illness in people. Infection with Legionella pneumophila is called Legionnaires’ disease.
 
Air conditioner cooling towers have been implicated in outbreaks previously, as have aerosol-producing devices such as spa baths/spa pools, water misters and decorative fountains.
 
Transmission occurs through the inhalation of contaminated aerosols. Risk of infection is increased by smoking, chronic lung or heart disease, diabetes, some forms of cancer and immunosuppression. Person to person transmission does not occur.
 
To minimise the risk of growth of Legionella in cooling towers, they should be properly maintained and monitored.
 
People of any age may be infected, though the disease is more common in older people and people with weakened immune system.

Infection is treated with antibiotics.