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Legionella spp. is a key contributor to the United States waterborne disease burden. Despite potentially widespread exposure, human disease is relatively uncommon, except under circumstances where pathogen concentrations are high, host immunity is low, or exposures to small-diameter aerosols occurs. Water quality guidance values for Legionella are available for building managers but are generally not based on technical criteria. To address this gap, a quantitative microbial risk assessment (QMRA) was conducted using target risk values in order to calculate corresponding critical concentrations on a per-fixture and aggregate (multiple fixture exposure) basis. Showers were the driving indoor exposure risk compared to sinks and toilets. Based on aggregate fixture exposures, critical concentrations depended on the dose response model (infection vs. clinical severity infection, CSI), risk target used (infection risk vs. disability adjusted life years on a per-exposure or annual basis), and fixture type (conventional vs. water efficient or “green”). Median critical concentrations based on exposure to a combination of toilet, faucet, and shower aerosols ranged from ~10-2 to ~100 CFU per L and ~101 to ~103 CFU per L for infection and CSI dose response models, respectively. As infection model results for critical L. pneumophila concentrations were often below a feasible detection limit for culture-based assays, the use of CSI model results for non-healthcare water systems with a 10-6 DALY pppy target (the more conservative target) would result in an estimate of ~20 CFU per L (arithmetic mean of samples across multiple fixtures and/or over time). Single sample critical concentrations with a per-exposure-corrected DALY target at each fixture would be 1050 CFU per L (faucets), 4.3 × 105 CFU per L (toilets), and 25.2 CFU per L (showers). The absence of detectable L. pneumophila may be appropriate for healthcare or susceptible population settings.
Purpose of review: The global importance of Legionnaires' disease (LD) and Pontiac fever (PF) has grown in recent years. While sporadic cases of LD and PF do not always provide contextual information for evaluating causes and drivers of Legionella risks, analysis of outbreaks provides an opportunity to assess these factors. Recent findings: A review was performed and provides a summary of LD and PF outbreaks between 2006 and 2017. Of the 136 outbreaks, 115 were LD outbreaks, 4 were PF outbreaks, and 17 were mixed outbreaks of LD and PF. Cooling towers were implicated or suspected in the a large portion of LD or PF outbreaks (30% total outbreaks, 50% confirmed outbreak-associated cases, and 60% outbreak-associated deaths) over this period of time, while building water systems and pools/spas were also important contributors. Potable water/building water system outbreaks seldom identify specific building water system or fixture deficiencies. The outbreak data summarized here provides information for prioritizing and targeting risk analysis and mitigation strategies.