A Tale of Four Danish Cities: Legionella pneumophila Diversity in Domestic Hot Water and Spatial Variations in Disease Incidence.

Søren A Uldum,Kelsie Cassell,Sharmin Baig,Lars G Schjoldager

doi:10.3390/ijerph19052530

Abstract

Denmark has one of the highest Legionnaires’ disease notification rates within Europe, averaging 4.7 cases per 100,000 population annually (2017 to 2020). The relatively high incidence of disease is not uniform across the country, and approximately 70% of all domestically acquired cases in Denmark are caused by Legionella pneumophila (LP) strains that are considered less virulent. The aim of this study was to investigate if colonization rates, levels of colonization, and/or types of LP present in hot water systems were associated with geographic differences in Legionnaires’ disease incidence. Domestic water systems from four cities in Denmark were analyzed via culture and qPCR. Serogrouping and sequence typing was performed on randomly selected isolates. Single nucleotide polymorphism was used to identify clonal relationship among isolates from the four cities. The results revealed a high LP colonization rate from 68% to 87.5% among systems, composed primarily of non-serogroup 1. LP serogroup 1 reacting with the monoclonal antibody (MAb) 3/1 was not identified in any of the systems tested, while MAb 3/1 negative serogroup 1 strains were isolated from 10 systems (9.6%). We hypothesize that a combination of factors influences the incidence rate of LD in each city, including sequence type and serogroup distribution, colonization rate, concentration of Legionella in Pre-flush and Flush samples, and potentially building characteristics such as water temperature measured at the point of use.

Highlights

Legionella spp. are environmental bacteria often found in the humid settings of both natural and man-made water systems and are aerosolized through sources such as showers, fountains, mist machines, spa pools, and cooling towers [1]
The Legionella pneumophila (LP) sequence types (STs) distribution among clinical samples ranged from 23 different STs isolated from City B cases to only five different STs among City D cases (Table 2)
Six of the mentioned 10 Mab 3/1-P cases reported in City A (Table 2) had a corresponding sequence type (ST1 or ST42) detected during the case investigation in the sampled residential hot water systems, indicating that the clinical STs are present in the domestic hot water systems (DHW) systems of this city, but likely sporadically

Summary

Introduction

Legionella spp. are environmental bacteria often found in the humid settings of both natural and man-made water systems and are aerosolized through sources such as showers, fountains, mist machines, spa pools, and cooling towers [1]. The bacteria can cause a severe pneumonia called Legionnairesdisease (LD). DK has a high incidence of LD when compared to other countries reporting to the European Centre for Disease Prevention and Control (ECDC) [2–5]. This upward trend in incidence over time is present in several other European countries (e.g., Italy, Slovenia, Czech Republic, and Belgium) where the average annual notification has approximately doubled between 2011 and 2018 [2,4]. The incidence of domestically acquired LD varies greatly from province to province in DK, ranging from 1.6 to 9.6 cases per

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