Legionella pneumophila-Epidemiology and Characterization of Clinical Isolates, Slovenia, 2006-2020

Abstract

Legionella pneumophila is the causative agent of severe Legionnaires' disease (LD). Although an increasing number of LD cases have been observed, published data from Slovenia are very limited and data on molecular epidemiology are even scarcer. The present retrospective study (2006-2020) reports the results of the microbiological diagnosis of LD, as well as the epidemiology and characterization of the Legionella clinical isolates. We tested urine samples from 15,540 patients with pneumonia symptoms for L. pneumophila infection by urine antigen test, of which 717 (4.6%) tested positive. Isolation of L. pneumophila was successfully performed from 88 clinical specimens, with 82 (93.2%) being identified as L. pneumophila sg 1 and six (6.8%) as L. pneumophila sg 2-14. Sequence-based typing (SBT) identified 33 different sequence types (STs), the most frequent being ST1 and ST23. Sequence type 1 mainly comprised isolates belonging to the Philadelphia subgroup, and ST23 mostly to Allentown/France. The standard SBT scheme, as well as Dresden phenotyping for L. pneumophila, presented a high diversity among isolates.

Keywords: Legionella pneumophila typing; Legionnaires’ disease; cultivation; diagnosis; epidemiology.

Figure 1

Number of pneumonia patients tested by Legionella pneumophila urine antigen test (UAT), Legionella pneumophila PCR and Legionella pneumophila serology (IFA) per year (Slovenia, 2006–2020). Each patient could be tested with more than one test. UAT, urine antigen test; PCR, polymerase chain reaction; IFA, indirect immunofluorescence assay.

Figure 2

Number of positive male and female patients tested by Legionella pneumophila urine antigen test per year (Slovenia, 2006–2020).

Figure 3

Number of pneumonia patients tested by the Legionella pneumophila urine antigen test (2006–2020). The red and the blue lines represent the number and proportion of positive patients per year, respectively.

Figure 4

Monthly distribution of Legionella pneumophila-positive cases diagnosed by the Legionella urine antigen test (2010–2020).

Figure 5

Legionella pneumophila sequence types (ST) identified among the clinical samples in the period of 2006–2020 (Slovenia). A total of 98 Legionella pneumophila STs are presented.

Figure 6

Minimum spanning tree of 88 Legionella pneumophila clinical isolates, together with 10 profiles obtained by nested SBT (nSBT). The sizes of circles indicate ST sample sizes, while lengths of lines connecting STs indicates extent of variation. STs connected by solid lines indicate one, two, or three allele changes (the shortest thick black line denotes single-locus variants), grey dashed lines represent four changes, and grey dotted lines represent five or more changes. The clonal complexes (CCs) of single-locus variants are circled.