Epidemiological analysis of Legionnaires’ disease in Scotland: a genomic study

A new paper has been published by the Lancet Microbe analysing the spread and journey of Legionnaires Disease in Scotland. The paper begins as follows:

Legionella is a genus of globally ubiquitous, intracellular, freshwater bacteria that naturally coexists with protozoa. More than 30 Legionella species can invade immune cells and cause opportunistic human respiratory infections. However, over 90% of infections are caused by a single species, Legionella pneumophila, which is typically inhaled from contaminated environmental aerosols. Legionellosis varies from a mild flu (Pontiac fever) to Legionnaires’ disease, an atypical, often severe form of pneumonia with a 10% mortality rate. Most Legionnaires’ disease cases are sporadic and unlinked to other infections, with many acquired during travel. However, outbreaks of Legionnaires’ disease occur when a cluster of infections are linked to a common source such as plumbing, air-conditioning systems, or industrial cooling towers. Additionally, commercial compost or soil can be a reservoir for outbreaks of Legionnaires’ disease caused by Legionella longbeachae. L pneumophila bacteria are challenging to eradicate from environmental reservoirs and strains might persist in these environments, posing a threat to public health.

To identify the source of outbreaks or sporadic infections, clinical and environmental isolates are compared on the basis of knowledge of possible exposures. Legionnaires’ disease cases are typically classified as either travel-associated Legionnaires’ disease (TALD), hospital-associated Legionnaires’ disease (HALD), or community-acquired Legionnaires’ disease (CALD), depending on the patient’s history of potential exposure in the 10 days before disease onset. Although sequence-based typing is considered the current gold standard method for discrimination of L pneumophila, whole-genome sequencing (WGS) methods have a much higher level of resolution to distinguish closely related isolates and to determine the source of infections. Applied retrospectively, WGS has been used to resolve the source of Legionnaires’ disease outbreaks that remained ambiguous with standard molecular methods, and provides enhanced resolution to cases affected by recent genetic recombination events that complicate phylogenetic reconstruction.

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