Variable Legionella Response to Building Occupancy Patterns and Precautionary Flushing.

William J Rhoads,Céline Margot,Nadine Graf,Meril Sindelar,Frederik Hammes

doi:10.3390/microorganisms10030555

William J Rhoads, Céline Margot + Show 3 more

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https://doi.org/10.3390/microorganisms10030555

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Abstract

When stay-at-home orders were issued to slow the spread of COVID-19, building occupancy (and water demand) was drastically decreased in many buildings. There was concern that widespread low water demand may cause unprecedented Legionella occurrence and Legionnaires’ disease incidence. In lieu of evidenced-based guidance, many people flushed their water systems as a preventative measure, using highly variable practices. Here, we present field-scale research from a building before, during, and after periods of low occupancy, and controlled stagnation experiments. We document no change, a > 4-log increase, and a > 1.5-log decrease of L. pneumophila during 3- to 7-week periods of low water demand. L. pneumophila increased by > 1-log after precautionary flushing prior to reoccupancy, which was repeated in controlled boiler flushing experiments. These results demonstrate that the impact of low water demand (colloquially called stagnation) is not as straight forward as is generally assumed, and that some flushing practices have potential unintended consequences. In particular, stagnation must be considered in context with other Legionella growth factors like temperature and flow profiles. Boiler flushing practices that dramatically increase the flow rate and rapidly deplete boiler temperature may mobilize Legionella present in biofilms and sediment.

Highlights

Foundational support for the widely accepted belief that building water stagnation facilitates Legionella colonization and growth is far less convincing than assumed of such a central dogma
We show the impact of COVID-19 recommissioning flushing and the controlled follow-up boiler flushing experiments
The floor with the highest overall positivity rate (E-floor, 83% positive, n = 12) had very good passive recirculation exchange with the main recirculating loop, while the floor with the lowest positivity rate (C-floor, 29% positive, n = 21) had the poorest exchange (Figure S7); E-floor had 1.5-log higher median L. pneumophila than C-Floor (Dunn Test with Bonferroni correction, p-value = 0.011). This indicates some level of variation in the factors impacting Legionella growth in this system, and these data serve as a baseline to evaluate the impact of periods of low water demand

Summary

Introduction

Foundational support for the widely accepted belief that building water stagnation facilitates Legionella colonization and growth is far less convincing than assumed of such a central dogma. In a controlled pilot-scale experiment, stagnation contributed to growth when it provided relief from inhibitory conditions (e.g., high temperature dissipating) and resulted in no or limited growth when nutrients were scarce (too infrequent water exchange); flow limited or eliminated growth when it delivered inhibitory conditions (hot water, >48 ◦ C in that study) but contributed to growth when inhibitory conditions were absent (water not hot enough) [3,4]. This variability is born out in many field studies, with suboptimal hot water recirculation temperature [5], inadequate

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