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Nelson Marlborough Health said four were in the Tasman district and one in Marlborough, and all patients had recently purchased and used potting mix, compost or both.
Clinical Director of Public Health Dr Stephen Bridgman said Legionnaires' disease was a serious, but preventable illness which could cause pneumonia and, in some cases, it could be fatal.
Those hospitalised had a stay of between five and 21 days.
Dr Bridgman said potting mix was the most common source of the Legionella bacteria but it was also present in compost, inadequately-treated spa pools and hot water cylinders that were not set to a minimum of 60C.
People at greater risk of severe illness from Legionnaires' disease are those aged over 50, those who smoke, have chronic illnesses that suppress immunity, and who do not take the recommended preventative steps when using potting mix or compost.
Dr Bridgman said the actual number of people infected with Legionnaires' was likely to be higher because the tests were usually only done if someone was sick enough to be admitted to hospital.
Less severe cases could resemble flu or even Covid-19, but unlike these viral illnesses Legionnaires' could not be transmitted from person to person, contracted instead by inhaling soil or compost dust.
"Recent cases in our region have all been connected to soil, compost or potting mix while gardening without using adequate protection.
"At this time of year, when people are getting into their gardens more, we urge people to take care when handling these products, and to follow advice to reduce their risk, in particular the use of masks to prevent inhaling the bacteria when handling potting mix or compost," Dr Bridgman said.
There have been 71 cases in the Nelson-Marlborough region since 2010 and 10 cases in total have been confirmed this year to date.
Early symptoms of Legionnaires' disease can appear two to 14 days after exposure and are similar to the flu. Symptoms include: fever and chills, flu-like symptoms and cough, shortness of breath, muscle aches, headaches and stomach pain, vomiting and diarrhoea.
Anyone with these symptoms should seek health advice straight away by contacting their GP or Healthline, and let them know you have been handling potting mix, soil or compost recently.
Early treatment is effective.