Variable Legionella Response to Building Occupancy Patterns and Precautionary Flushing

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.

Impact of recommissioning flushing on Legionella pneumophila in a large building during the COVID-19 pandemic

“…However, they are consistent with values measured in cold and hot water at taps after varying lengths of stagnation in buildings located in the same water distribution system than the sports complex sampled in the present study (Bédard et al, 2018;Dias et al, 2019). Observations from this study also appear consistent, yet with slightly higher values of total cell counts, than those measured after a 7-week shutdown in Swiss research buildings during the COVID-19 pandemic (Rhoads et al, 2022). Mean values for the proportion of intact cells (Figure 1D) were in agreement with prior observations of first draw samples in chlorinated water containing 15-41% of viable cells after 24 h to 10 days of distal water stagnation (Bédard et al, 2018).…”

“…The high positivity rates of L. pneumophila by culture-based method and qPCR results (Table 1) recovered in first draw and flushed samples indicate a broad and systemic contamination of the building hot and tempered water systems. Whereas some studies carried out during the pandemic reported either no detectable L. pneumophila (Hozalski et al, 2020) or low positivity and concentrations of L. pneumophila (Dowdell et al, 2022, submitted to Environmental Science and Technology), others measured quantifiable concentrations in similar ranges as those observed in this study for samples representative of points of use (10 1 -10 4 MPN/L or CFU/L; Giglio et al, 2020;Rhoads et al, 2022).…”

“…levels (Wang et al, 2012;Hozalski et al, 2020;Richard and Boyer, 2021), it can lead to momentary detrimental effects. One study also demonstrated increases in L. pneumophila levels after flushing, which was associated to building-specific features and deleterious recommissioning flushing practices such as rapid depletion of boiler temperatures with water turnover in the hot distribution system (Rhoads et al, 2022).…”

Legionella pneumophilaoccurrence in reduced-occupancy buildings in 11 cities during the COVID-19 pandemic

“…Building CH-1 also reported significant heat losses in the primary recirculating line and passive recirculation on individual floor loops, leading to suboptimal temperatures for control of L. pneumophila . 52…”

“…Site-specific investigations were conducted at several of the sampling locations, the results of which are or will be reported elsewhere. 52,53 This study presents a cross-sectional analysis of potential L. pneumophila exposure risks in buildings with diverse uses, plumbing configurations, operation, and climate regions.…”