From the archives: A timely reminder of the link between potting mixes, soils and composts and legionnaires’ disease, this this story first appeared in the Listener in summer, 2022. Nicky Pellegrino finds there are ways to mitigate the risks of the potentially fatal respiratory illness.
Spring and early summer is a time for many New Zealanders to get outdoors and do some gardening. As a result, this is when we tend to see the most cases of legionnaires’ disease in this country.
“I always say that when the daffodil season starts then we’re preparing for the legionella season,” says Steve Chambers, a University of Otago respiratory and infectious diseases specialist.
Much of the problem has been traced to commercially produced potting mixes, soils and composts, which house a strain of bacteria known as Legionella longbeachae. When gardeners open the bags and inhale the contaminated dust, an infection can occur, particularly if they are older and have a compromised immune system or lungs.
Digging a home compost pile isn’t without risk, but bagged soil and composted plant material seem to provide a particularly welcoming environment for these organisms to flourish.
“A bag of compost has often been sitting in the sun on pallets covered with plastic and it creates a hothouse environment,” says Chambers.
Once inhaled, the bacteria enter the lungs, where they multiply and may cause a milder illness called Pontiac fever or the more severe legionnaires’.
A 2019 study led by the University of Otago found legionnaires’ disease is more common in this country than previously thought – in fact, we have the highest reported incidence in the world. The researchers arranged for people hospitalised with pneumonia during one calendar year to be tested for the bacteria and three times the usual number of diagnosed cases were found, many of them in patients aged over 65.
Since symptoms include muscle aches, fever, dry cough, breathlessness, headache, loss of appetite and fatigue, the disease may easily be mistaken for a bout of flu – or now Covid-19 – and go undiagnosed.
“I suspect there are people who get a bit unwell with it, then get better by themselves at home, so the burden of disease is even greater than we understand,” says Chambers.
Inhaling the bacteria can cause very serious illness, with some patients needing lengthy hospital stays and intensive care. In most cases, it is treated successfully with antibiotics, but some die and it seems that others may suffer longer-term ill effects – a sort of long legionnaires’ – so it makes sense to take precautions.
Chambers advises wearing a mask and gloves when working with compost, potting mix and soil. Since the bacteria can live on surfaces for a while, he suggests playing safe by washing the gloves before wearing them again, and disposing of the mask.
“The experiments we’ve done with contaminated gloves and masks show the bacteria can persist for hours, days and up to a week,” he says.
Dampening soil and compost before working helps reduce dust. Hand-washing is also called for and Chambers says gardeners should be careful not to touch their face with dirty hands or gloves, as it seems that the organisms can be inhaled from contaminated skin.
Early diagnosis and treatment are important, but this can be challenging because a PCR test using sputum is required – nose or throat swabs don’t cut it – and many patients struggle to cough up enough mucus. At the University of Otago, they are hoping to develop a urine test to make diagnosis quicker and easier.
Researchers there are also working with a manufacturer to see if commercial potting mixes and composts could be treated in some way to make them safer. The theory is the pine bark component, common in New Zealand products, is playing host to these organisms.
Legionnaires’ can also be contracted by inhaling droplets of contaminated water in the air. Spa pools and hot tubs are typically heated to a temperature that is ideal for the bacteria to grow and multiply: 20-45°C. Air-conditioning units in large buildings have also been a source.
Chambers says it is rare to find cases of legionnaires’ in young people. However, gardening is a pursuit very often enjoyed by older people. At age 70, he enjoys it himself.
“It’s a good physical activity and it’s really important that people should enjoy it. So I don’t want to make anyone paranoid, just encourage them to look after themselves as much as possible.”
