The source of Melbourne’s deadly legionnaires’ outbreak has been found, with authorities confident the peak has passed.

A cooling tower at Laverton North in Melbourne’s west has been identified as the source after dozens of cooling towers in the suburb and neighbouring Derrimut were disinfected, Victorian health authorities said.

“I am hopeful we’ve contained the source of the outbreak,” Victoria’s chief health officer Clare Looker said on Monday.

“Further work is being undertaken now that we have this positive result to quantify the amount of legionella in the sample.

“Recent weather patterns could explain the wider-than-usual dispersal of cases.”

The Department of Health is investigating an outbreak of Legionnaires’ disease involving a significant increase in notified cases across metropolitan Melbourne & surrounding areas since Friday afternoon. To find out about symptoms & more information visit: https://t.co/3t2Xi7IGK1 pic.twitter.com/c2O0NjPsC8

— Victorian Department of Health (@VicGovDH) July 29, 2024

The findings come after the deaths of a man in his 60s and a woman in her 90s.

As of Monday, 77 confirmed cases remain in hospital.

Deakin University chair in epidemiology Catherine Bennett told The New Daily many more people were likely infected, but would present with little to no symptoms.

There is little risk of transmission within the wider community.

“[Legionnaires’] can’t spread person to person from an environmental contamination source,” she said.

“The incubation period can be anywhere from two to 12 days … that exposure period [may have] passed, and we’re unlikely to see lots of cases going ahead if they have, in effect, shut down the source.”

What is legionnaires’ disease?

Legionnaires’ disease is a severe form of lung inflammation caused by legionella bacteria.

The many species of the bacteria are commonly found in damp environments such as water and soil, and so can contaminate air-conditioning cooling towers, humidifiers, shower heads and potting mix.

People can get infected with legionnaires’ disease by breathing in contaminated dust or water vapour.

Bennett said strict cleaning standards applied to cooling towers. While they may not always be entirely effective, they did reduce the number of outbreaks of the disease.

Symptoms were usually “quite mild”, and the disease could present as a simple dry cough.

Other possible symptoms include, but are not limited to, chest pains, nausea, diarrhoea, chills and fatigue.

Some infected people don’t feel any symptoms.

But smokers, people with chronic illnesses or weakened immune systems, and people aged over 50 should are more likely to develop severe pneumonia.

“This outbreak is probably larger than we know, but it’s not a risk to those other people,” Bennett said.

“For the majority, [they] will have a mild infection or no signs, and they’ll have recovered by now.

“For people who have had a cough and can’t shake it, or are feeling quite unwell with it, it’s probably worth seeing the doctor.”

What can you do to decrease infection risk?

There are no vaccines against legionnaires’, and risk of infection is likely out of an individual’s control.

For most, it is a case of “wrong place, wrong time, in the wrong weather”.

“It would be unfortunate if you happen to be in the proximity of a tower that had just started to form a biofilm [and] was actually shedding legionella into the … mist,” Bennett said.

“If you have no wind so it doesn’t dissipate, if it drifts down to ground level or into other air intakes, that’s when we have a problem.

“It does just reinforce how cooling tower maintenance and the cleaning regimen is so critical.”

National health advice service Healthdirect recommends careful cleaning of spaces and objects such as air-conditioning units, pools, and showers to help reduce the risk of legionella bacteria growth.

To avoid infection from potting mixes, follow any available instructions for the mix, wet it to reduce dust, wear gloves and a mask, and wash your hands after handling.