Green waste compost as potential reservoirs of Legionella in the Netherlands

Legionella is a bacterial species able to cause influenza-like illness (Pontiac fever) or severe pneumonia (Legionnaires disease, LD). We assessed Legionella presence and concentration in composting facilities in The Netherlands. A total of 142 samples from 23 green waste composting facilities were screened for Legionella DNA using qPCR. Of 142 samples, Legionella spp. DNA was detected in 97 (68%), and the subspecies L. pneumophila and L. longbeachae in 33 (23%) and one (0.7%) samples, respectively. Legionella was observed in samples from all composting facilities. The concentration of Legionella spp. DNA ranged from 103 to 105 genomic units (GU)/gram. Compost temperature was negatively correlated with the presence (odds ratio 0.67, 95% CI 0.50–0.92 per 10 degrees increase) and concentration (geometric mean ratio 0.90, 95% CI 0.83–0.97 per 10 degrees) of Legionella spp. Average humidity in the week prior to sampling was negatively correlated with the L. pneumophila concentration (geometric mean ratio 0.73, 95% CI 0.56–0.96 per increase in 10% of humidity). This study suggests that composting facilities can be regarded as reservoirs of Legionella in The Netherlands, but additional studies should target if such facilities represent a human health risk.