Tracking Legionella in air generated from a biological treatment plant: a case study of the outbreak of legionellosis in Norway

Abstract

Two outbreaks of legionellosis occurred in the Sarpsborg/Fredrikstad region southeast of Norway in 2005 and 2008 where more than 60 exposed individuals were infected and 10 case patients died. The air scrubber at Borregaard, a wood-based chemical factory, was identified as the outbreak source. High concentration levels of <i>Legionella</i> species, including the etiological agent <i>L. pneumophila</i> SG1 was found in the aeration ponds, which belongs to Borregaard's biological treatment plant. Results showed that these ponds were able to generate <i>Legionella</i>-containing aerosols that were transported by the wind as such aerosols were measured up to 200 meters downwind of the pond. Our studies did not detect L. pneumophila SG1 isolates, only <i>L. pneumophila </i>SG4 during the air sampling measurement campaign. Furthermore, the operational conditions of the air scrubber proved to be harsh for <i>Legionella</i> growth as the outbreak <i>L. pneumophila</i> strains were not able to grow at 45ºC and pH8 (conditions during the outbreaks). These results, together, lead us to suggest that the aeration pond should be regarded as the primary amplifier and disseminator of <i>Legionella</i> and <i>L. pneumophila </i>and thereby most likely being the outbreak source.