Alternative sources of Legionella bacteria

Abstract

Legionella bacteria can cause Legionnaires’ disease (LD) in humans. Symptoms of LD can range from mild disease to severe pneumonia with sometimes fatal outcome. In the Netherlands, the most important infective agent is Legionella pneumophila. L. pneumophila infection is associated with aquatic sources, such as cooling towers and whirlpools. The majority of the LD cases occur sporadically, i.e. not related to a cluster or outbreak, and the source of infection for these cases is only rarely found. Infection may possible result from exposure to Legionella bacteria in sources that are not yet considered in source investigations. Therefore, the aim of this thesis was to explore alternative sources of Legionella bacteria (i.e. that are not yet considered in source investigations) that might be the cause of sporadic Legionella infections. The focus was on environmental sources, i.e., soil and rainwater. We have isolated pathogenic Legionella species from garden soil. In the Netherlands, L. pneumophila sequence type (ST) 47 is an important cause of sporadic LD. However, since 2002, this ST was only found three times in the environment during outbreak investigations. These investigations all concerned whirlpools in private gardens and from one of the gardens L. pneumophila ST47 was isolated. We speculate that this strain was transmitted from soil to the whirlpool in the garden where it caused the outbreak. Garden soil was further investigated and pathogenic Legionella bacteria were isolated from 12% of 177 sampled gardens. Seven gardens contained L. pneumophila. Several different STs were found and three of these STs were also found in patients in the Netherlands (ST84, ST115, ST477), but they are relatively uncommon. Strikingly however, these three clinically relevant STs belonged to the most frequently isolated STs in garden samples. Furthermore, evidence was found that L. pneumophila can persist in garden soil over time. In another study we isolated Legionella bacteria from rainwater puddles on roads (3.9%) and soil next to roads (30%). L. pneumophila was isolated from 2 rainwater samples and 2 soil samples. STs could be assigned to two L. pneumophila strains isolated from soil, ST710 and ST477, and one strain isolated from rainwater, ST1064. These STs were previously associated with LD patients but only rarely. Last, we found a relationship between Legionella presence and soil type. Compared to Legionella negative soils, the Legionella-positive ones contained an enhanced level of clay and silt, in relation to sand. Furthermore, we found that, in an experimental situation, clayey soil was more favorable for L. pneumophilapersistence over time than sandy soil. In conclusion, soil and rainwater puddles are reservoirs of Legionella bacteria and therefore potential alternative sources of LD. If people are exposed to Legionella bacteria from soil and rainwater is unknown. We recommend to target soil in source investigations, in particular in relation to cases that are infected by a soil specific ST. Furthermore, we recommend to investigate WWTPs and cooling towers; these are known sources of Legionella but they are possibly under-recognized as sources of sporadic LD in the Netherlands.