Legionella - how serious is the threat?

We talk to Brian Shields about the dangers posed by the existence of Legionella in potting mix, garden soils, compost and other organic materials used in horticulture.

Nurseries have been advised to take steps to protect their staff and customers from the threat of exposure to Legionella in organic products. How serious is the threat?
Legionella organisms are very widespread in the natural environment. However when conditions change to provide a suitable environment, concentrations of Legionella can increase to levels where they may pose a serious threat, especially to vulnerable individuals.
Any situation where organic material is warm and damp will encourage the production of Legionella spores. Heaps of chipped tree prunings provide a perfect environment. The warmth created by decomposition favours the growth in numbers of the bacteria. It’s important to remember that the threat is not just to nursery personnel, but to anyone who handles organic materials - and that includes domestic compost. A well-known television personality, for example, contracted the disease from his backyard compost heap.
Legionella causes a disease similar to pneumonia and the potential death rate of people who contract the disease is between 13% and 15%. The most common way of transmitting the disease is from hand to mouth, so washing hands carefully after dealing with organic materials is very important. In the Burnley nursery, we encourage students to wear disposable gloves. However it has just in the last week been brought to my attention that some regular users of disposable gloves (eg in the health industry), develop latex sensitivity over time, so this is something we will need to research further. For anyone who handles these materials regularly, leather gloves are a good idea.
Organic material may contain many latent Legionella spores. When the material becomes dry and dusty, the spores are easily inhaled. When handling this material, steps should be taken to dampen it down to reduce dust. This also applies to domestic potting mix - keep it damp. People who are most susceptible to infection are asthmatics, smokers, diabetics and anyone with chronic lung disease or who are HIV positive. These people should wear face masks when handling organic materials. The disease is more likely to affect elderly or middle-aged people and those whose immune systems are weak eg if they have, or are recovering from, some other illness.
Why have these materials only recently been recognised as potentially hazardous? Has anything changed?
The main thing that has changed is that infection with Legionella is now more likely to be recognised. In the past, people who were infected were most likely diagnosed as having some other disease. The early symptoms are very similar to that of flu - fever, chills, cough, aching muscles, headache, tiredness, loss of appetite and diarrhoea. If the infection is caught early enough, it is treatable, so it is very important to mention to your doctor that you have been handling organic materials such as potting mix, compost or soil, if you are presenting with some, or all, of these symptoms.
The other thing that has changed in the horticultural industry is the move in the last 15 - 20 years to soil-less growing media which is largely based on organic materials. Where these materials continue to decompose and generate heat, Legionella is likely to multiply.
What nursery procedures can be employed to reduce the incidence of Legionella?
A high level of nursery hygiene is important in reducing the incidence of all forms of bacteria. Pasteurising potting media used for cuttings and seed propagation and tubing up is common nursery practice to reduce the incidence of bacteria and other pathogens.
Care with storage of organic-based materials is important. The materials should not be stored for extended periods. They should be kept aerated and prevented from getting dusty.
However the most important thing is that individuals take responsibility for their own safety and not adopt a cavalier attitude. This is particularly important for people who fall within the high risk group.