Between August and October 2013 six confirmed and one probable case of Legionella longbeachae were reported in the Lothian and Tayside areas.
The major source of human infection with Legionella longbeachae is considered to be commercial growing media, such as soils and composts which are sold through garden centres, and other composted materials such as bark and sawdust. Older people are more at risk.
A new report by Health Protection Scotland has now found that all seven patients had spent time in the garden or outdoors in the two weeks prior to their illness, and six had recently bought compost.
But the investigators found that all the products used by the patients were bought at different times at different premises. The compost was found to originate five different manufacturers at five different sites in England, Scotland, Ireland and Northern Ireland.
Five samples tested by investigators which were linked to patients tested positive for Legionella longbeachae.
The HPS report said: “This would suggest that growing media was the likely source of infection in these cases.”
However, it was not possible to pinpoint the exact source of the Legionella longbeachae to one particular product.
Instead, the report put the cluster of cases down to a combination of factors which may have increased the risk of the infection, including the warm spell of weather.
The report said: “August and September were unusually warm in Scotland in 2013 and storage of bags post-sale in enclosed spaces (such as inside the home, garage, shed, greenhouse or polytunnel) may have led to the bags being very warm during the day and protected from cold nights by being covered.
“In such conditions, the organism may thrive.”
Dr Kevin Pollock, senior epidemiologist at HPS, said Legionella longbeachae was a very rare disease, with only 19 cases since 2008 in Scotland. He said this had to be taken into account when looking at the risks.
“Gardening is a very good hobby for those who are elderly, who have other conditions,” he said.
“Your assessment of risk has to be proportionate. We would always say good hand hygiene is important, and don’t eat drink or smoke when you’re gardening.
“Bags of compost should also be kept well ventilated, ideally stored outdoors and not inside where they can heat up as we think the warm summer last year may have had something to do with the organism incubating in the compost.”