Legionnaires’ Disease

We often hear about Legionnaires’ disease during summertime and it is typically associated with gardening, compost and potting mix. But what exactly is Legionnaires’ disease, and what can we do to protect ourselves from it? We sat down with Professor Murdoch, Dean of the University of Otago in Christchurch, who recently led a study on the disease in New Zealand.

What is Legionnaires’ disease?

Legionnaires’ disease is a type of pneumonia caused by legionella bacteria. Several Legionella species are known to cause Legionnaires’ disease. Globally, Legionella pneumophila is reported to be the predominant cause of Legionnaires’ disease, but Legionella longbeachae is the most common cause in New Zealand.

You led a study about Legionnaires’ disease that was published in the Lancet Infectious Diseases – what were the main findings from this study?

This was the world’s first nationwide surveillance study of Legionnaires’ disease and revealed that New Zealand has the highest reported incidence of this disease globally. We don’t necessarily believe that we have more Legionnaires’ disease than any other country, and the study findings probably reflect the fact that we are more rigorous about testing and, consequently, are better at identifying cases of Legionnaires’ disease. The study also showed that the incidence was high in some regions that had historically detected few cases because testing had not been widespread.

Spring is a time when people are getting out in the garden – at this time of year, what are the main risks in our home environments?

Legionella longbeachae is the most common cause of Legionnaires’ disease in New Zealand. Unlike other Legionella species, L. longbeachae is found in soil and compost. So, exposure to compostrelated material is a major risk factor for Legionnaires’ disease. As most gardening activities occur in spring and summer, we see most cases of Legionnaires’ disease at this time of year.

How can we reduce the risk of being exposed to Legionella bacteria?

The standard advice when working in the garden is to wear gloves when handling soil, compost, mulch, or potting mix; wash hands well after handling garden soil, compost, mulch, or potting mix, even if gloves have been worn; avoid opening bags of compost in enclosed areas; and dampen potting mixes and other organic material before use to reduce dust.

What are the symptoms of Legionnaires’ disease and how is it treated?

Symptoms of Legionnaires’ disease include a ‘flu-like’ illness with a rapid onset fever, muscle aches, tiredness, headache, loss of appetite, breathlessness and chest pain and a dry cough leading to pneumonia. Nausea, vomiting and diarrhoea sometimes occur and mental confusion and disorientation may develop. Legionnaires’ disease is treated with antibiotics.

Are some people at a higher risk of developing the disease?

Although people of all ages can get Legionnaires’ disease, those over 50 years of age, smokers, gardeners and people with a compromised immune system are at a higher risk.

How have the results from the study informed practice and policy in New Zealand?

The study highlighted Legionnaires’ disease as a nationwide and relatively common problem in the country. It also demonstrated the importance of diagnostic testing. The study itself has influenced guidelines for the management of pneumonia across many district health boards.

About Professor David Murdoch

Professor David Murdoch is Dean of the University of Otago, Christchurch. He is the co-leader of The Infection Group, the co-director of One Health Aotearoa, a Senior Associate in the Department of International Health at Johns Hopkins School of Public Health, and a clinical microbiologist at Canterbury Health Laboratories.

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