Compost may harbor Legionella spp., the bacteria responsible for Legionnaire's disease, a potentially deadly form of pneumonia. A recent study in the United Kingdom found the bacteria in 15 out of 24 commercially-available composts.
Previous research identified the bacteria in compost from Australia, Japan and Switzerland, according to the Centers for Disease Control (CDC). In Australia, several cases of Legionnaire's disease were traced to contaminated compost in 1989.
"Any environment where you have pathogenic bacteria could be a source of infection," said Tara K. Beattie of the University of Strathclyde and co-author of the recent study published in Clinical Microbiology and Infection in a press release. "It should be emphasized though, that although Legionella seem to be common in compost, human infection is very rare, especially if you consider the volume of compost sold and used."
To avoid risks from bacteria-carrying composts, the study authors recommended opening bags of compost in well-ventilated areas. Gardeners should wash their hands thoroughly after handling compost.
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The composition of the compost may play a role in its bacteria-bearing ability. The study authors noted that UK compost used to be made mostly from peat, but now contains more wood residues, such as sawdust. The contaminated Australia compost from 1989 was made from wood scraps.
"It may be that the change in composition of composts in the UK, moving away from peat based products, could be resulting in species such as Legionella longbeachae being present in compost and therefore more cases of infection could occur," Beattie added.
However, 12 out of 18 peat-based composts contained Legionella as well. Hence, just because a compost contains peat doesn't mean it is Legionella free. The bacteria may have been introduced by other ingredients in the compost mixture, suggested the study authors.
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Most healthy people face little threat from Legionella or Legionnaire's disease. A healthy body can usually fight off the bacteria. However, the CDC warns that Legionella infection risk is greater for older individuals, former smokers, chronic lung disease patients and those with weakened immune systems.
Water distribution systems serves as the most common means of Legionella infections. The bacteria thrive in warm water and can be spread by hot tubs, cooling towers, hot water tanks, decorative fountains, and large plumbing systems, according to the CDC.
For example, a person might inhale the bacteria from the mist rising off of a contaminated hot tub. However, the bacteria do not spread from human to human, so infected individuals pose no risk to others.
IMAGE: Compost (normanack, Wikimedia Commons)