Flushing of stagnant premise water systems after the COVID-19 shutdown can reduce infection risk by legionella and mycobacterium spp

Authors: HOZALSKI, RAYMOND - University Of Minnesota; LAPARA, TIMOTHY - University Of Minnesota; ZHAO, XIAOTIAN - University Of Minnesota; KIM, TAEGYU - University Of Minnesota; WAAK, MICHAEL - Norwegian University; Burch, Tucker; MCCARTY, MICHAEL - University Of Minnesota

Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/24/2020
Publication Date: 11/24/2020
Citation: Hozalski, R.M., Lapara, T.M., Zhao, X., Kim, T., Waak, M.B., Burch, T.R., Mccarty, M. 2020. Flushing of stagnant premise water systems after the COVID-19 shutdown can reduce infection risk by legionella and mycobacterium spp. Journal of Environmental Science and Technology. https://doi.org/10.1021/acs.est.0c06357.
DOI: https://doi.org/10.1021/acs.est.0c06357

Interpretive Summary: The COVID-19 pandemic has resulted in abrupt and prolonged closures of buildings nationwide, including numerous facilities related to food and livestock production. Such closures provide a chance for opportunistic pathogens like Legionella spp. (including L. pneumophila) and Mycobacterium avium complex (MAC) to grow and multiply in premise plumbing. Following re-occupancy of previously closed buildings, exposure to these pathogens through regular water use and without appropriate mitigation procedures could result in increased risk relative to non-pandemic conditions, but the magnitude of this risk and efficacy of potential interventions is not well understood. This study quantified Legionella and MAC concentrations in premise plumbing of several buildings in Minneapolis and St. Paul, Minnesota and investigated the effects of water flushing on those concentrations and public health risk. Pathogen concentrations were measured using a combination of culture and DNA-based techniques, and health risk was predicted using quantitative microbial risk assessment (QMRA). Overall, estimated health risks were relatively low compared to acceptable risk thresholds, except for scenarios where a large percentage of Legionella were assumed to have an infectivity comparable to L. pneumophila. Furthermore, flushing hot and cold water lines for 15 minutes tended to reduce predicted risk. This study provides data informing risk and risk mitigation for stagnant water in premise plumbing nationwide, including food and livestock production facilities shut down for prolonged periods of time due to the COVID-19 pandemic.

Technical Abstract: The unprecedented widespread closing of buildings due to the COVID-19 pandemic has allowed water to stagnate in premise plumbing systems, creating conditions that may facilitate the growth of opportunistic pathogens. In this study, we flushed and sampled showers in buildings that had been unoccupied for approximately two months and quantified Legionella pneumophila using a commercial cultivation-based assay. In addition, all bacteria, Legionella spp., L. pneumophila, L. pneumophila serogroup 1, non-tuberculous mycobacteria (NTM), and Mycobacterium avium complex (MAC) were analyzed using quantitative PCR. Despite low or negligible total chlorine in the stagnant pre-flush water samples, L. pneumophila were not detected by either method; Legionella spp., NTM, and MAC, however, were widespread. Estimated risks of clinical illness from exposure to legionella and MAC via showering were generally low, but risk of subclinical infection via Legionella spp. could exceed a 10^-7 daily risk threshold if just a small fraction (=0.1 %) of those legionellae are highly infectious. Flushing cold and hot water lines restored a total chlorine (as chloramine) residual and decreased all bacterial gene targets to building inlet water levels within 30 minutes. Following flushing, the chlorine residual rapidly dissipated and bacterial gene targets rebounded, approaching pre-flush concentrations after 6 to 7 days of stagnation. These results suggest that stagnant water in premise plumbing may contain elevated levels of opportunistic pathogens; flushing, however, can rapidly improve water quality and reduce the health risk but the improvement will be short-lived if building disuse persists.