Pine Species Provide a Niche for <i>Legionella Longbeachae</i>

Legionella longbeachae is the commonest cause of Legionnaires’ disease (LD) nationwide in New Zealand (NZ). Most cases occur in spring and summer (October - January) and are associated with the use of commercial potting mix, which usually contains pine bark. L. longbeachae is an environmental organism but its niche has not yet been defined. Bark samples were taken at chest height from trees in three stands of Pinus radiata (Monterey pine) located in the central South Island of NZ. L. longbeachae DNA was detected by qPCR in 28/400 (7%) samples and from 22/50 (44%) different trees. There was a significant difference in the proportion of positive tests by season: summer 0/50 (0%); autumn 0/50 (0%); winter1/50 (2%); spring 22/50 (44%); (p<0.001). Bark samples from non-P. radiata pine species and adjacent mixed species were then tested. More samples from pine species 22/28 (79%) than non-pine species 6/37 (16%) tested positive for L. longbeachae (p<0.001). Pine species appear to be an important ecological niche for L. longbeachae. To our knowledge this is the second human pathogen to have an arboreal niche. The use of bark from P. radiata in commercial potting mix may contribute to the incidence of LD in New Zealand.