Don't worry, the odds of catching legionnaires' disease in Melbourne are pretty slim

The current suspected outbreak of legionnaires’ disease in Melbourne is a serious public health concern. Victorian health authorities say the circulation of the disease may be linked to cooling towers in Melbourne’s CBD.

So how does this happen? And with five people recovering in hospital, should the public be worried?

Cooling towers

Cooling towers recirculate water against a flow of air. This cools down the water that is then used in air conditioning and other appliances. As it goes through the appliances, the water warms up, and warm wet air is then expelled into the environment.

To maximise the cooling effect, such towers are designed to have a large surface area for heat transfer.

Legionella bacteria are relatively common in a water environment. However, they are not abundant, which means they rarely reach large numbers. But once they enter a cooling tower things change.

The combination of recirculating warm water (27 to 35℃), large surface areas for heat exchange and a continuous flow of nutrients from the cooling (and warming) process, creates perfect conditions for the bacteria to grow in slimes called biofilms.

Add the constant release of warm wet air containing the bacteria and it is obvious why they can be a problem.

Cooling towers that are well designed, maintained and disinfected rarely cause a problem. Almost all documented outbreaks are associated with problems with design, maintenance or disinfection – often all three.

When does it happen?

Although outbreaks of disease from cooling towers can happen at any time, they tend to occur in Autumn. This is due to a combination of factors.

At the end of summer, a cooling tower has had plenty of time to incubate legionella if they are allowed. The bacteria inhabit slimes that can coat the large surface area within the system fed by the nutrients from the incoming air supply.

In autumn, as the weather cools down, towers are used less frequently. In Australia long weekends, such as ANZAC weekend and Easter, mean cooling towers are switched off for a few days. The temperatures drop and nutrient supply ceases in the system. This causes the legionella-rich slimes to detach from the surfaces.

When the tower is turned back on, following the long weekend, the immediate effect is to dislodge the slimes and then release them into the environment around the cooling tower.

Autumn assists the travel of the contaminated air as it is cooler, more humid and there are lower UV indexes. This means the fine water particles, called aerosol, can travel for long distances some several kilometres from the source.

Historically in Australia a number of outbreaks of legionnaire’s disease have occurred around Autumn long weekends.

Who is at risk?

In Australia we average just more than 300 cases of this disease per year. Around half of these come from water systems. So the chances of becoming infected are roughly one in 100,000. Cooling towers are identified sources of less than half of these infections, so less than one in 200,000.

Only 5% of people who are exposed to contaminated water will show any signs of disease. Those who do show signs may have anything from a cold or flu-like illness through to very serious and life threatening pneumonia.

The 5% of infected people have common traits. Around two-thirds are males. Heavy smoking and high alcohol consumption are probably the highest risk factor. Being over 50 is another, as our immune systems start going downhill from our thirties onwards.

Another important factor is people with depressed immune systems. This includes transplant recipients and cancer patients who have artificially low immunity. It also includes those with existing lung conditions or other chronic disabilities such as diabetes and kidney problems.

To be infected from a cooling tower the susceptible person must inhale the contaminated aerosol. This means they must be within the vicinity of the source. The disease will show itself within two to 21 days after exposure - most often around five to six days afterwards.

What now?

At this stage in Melbourne there are undoubtedly a lot of concerned cooling tower operators making sure they are not to blame. They will be being assisted by the health department. The problem will most likely be over within 30 days.

The number of new cases, if they are going to increase, will most likely do so in the next five to seven days – a single period of incubation – and then subside.

If you have been in the Melbourne CBD in the last two weeks, it is unlikely you have been exposed and need to worry. If you are one of the high-risk individuals stay calm, but if symptoms of cold or flu appear suddenly with a dry cough and fever over the next couple of weeks seek medical advice immediately.

Your GP should already be on alert for signs of legionnaire’s disease and can probably give you the all clear within an hour.

It is unlikely that you are at any increased risk of catching the disease by going to the CBD now that the cases have been identified and action is being taken. And given the odds quoted above you might have a better chance of winning the lottery.

Richard Bentham has received funding from Australian Research Council, Victorian Health Department, Flinders University Internal Grants schemes. He is a Fellow of the Royal Society for Public Health.