Abstract
During the summer of 2012, a major Legionella pneumophila serogroup 1 outbreak occurred in Quebec City, Canada, which caused 182 declared cases of Legionnaire's disease and included 13 fatalities. Legionella pneumophila serogroup 1 isolates from 23 patients as well as from 32 cooling towers located in the vicinity of the outbreak were recovered for analysis. In addition, 6 isolates from the 1996 Quebec City outbreak and 4 isolates from patients unrelated to both outbreaks were added to allow comparison. We characterized the isolates using pulsed-field gel electrophoresis, sequence-based typing, and whole genome sequencing. The comparison of patients-isolated strains to cooling tower isolates allowed the identification of the tower that was the source of the outbreak. Legionella pneumophila strain Quebec 2012 was identified as a ST-62 by sequence-based typing methodology. Two new Legionellaceae plasmids were found only in the epidemic strain. The LVH type IV secretion system was found in the 2012 outbreak isolates but not in the ones from the 1996 outbreak and only in half of the contemporary human isolates. The epidemic strains replicated more efficiently and were more cytotoxic to human macrophages than the environmental strains tested. At least four Icm/Dot effectors in the epidemic strains were absent in the environmental strains suggesting that some effectors could impact the intracellular replication in human macrophages. Sequence-based typing and pulsed-field gel electrophoresis combined with whole genome sequencing allowed the identification and the analysis of the causative strain including its likely environmental source.
Highlights
During the summer of 2012, Quebec City (Quebec, Canada) experienced one of the largest North American outbreaks of Legionnaires’ disease (LD) of the last decades
Characterization of Lp serogroup 1 (Lp1) isolates involved in the 2012 Quebec City outbreak
Lp1 isolates recovered from 23 patients (n = 23 isolates) and from 32 (24%) cooling towers (n = 146 isolates) located in the vicinity of the outbreak discriminated into seven different pulsed- field gel electrophoresis (PFGE) profiles using SfiI (Figure 1)
Summary
During the summer of 2012, Quebec City (Quebec, Canada) experienced one of the largest North American outbreaks of Legionnaires’ disease (LD) of the last decades. Legionella pneumophila (Lp) has been identified as the most common causative agent of LD, and of Pontiac fever, a milder form of the disease with flu-like symptoms [1]. LD is an acute form of pneumonia that can be quite severe, with a casefatality rate ranging between 5–30% and approaching 50% in individuals with compromised health status [2]. LD can occur during outbreaks or as sporadic cases, and is an important cause of nosocomial and community-acquired pneumonia (CAP) accounting for 2 to 15% of the latter [3]. Lp serogroup 1 (Lp1) cause approximately 90% of LD and 95% of community-acquired LD [4,5]
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