Risk-Based Critical Concentrations of Legionella pneumophila for Indoor Residential Water Uses

Risk-based critical concentrations of enteric pathogens for recreational water criteria and recommended minimum sample volumes for routine water monitoring

Concerns are rising about the contamination of recreational waters from human and animal waste, along with associated risks to public health. However, existing guidelines for managing pathogens in these environments have not yet fully integrated risk-based pathogen-specific criteria, which, along with recent advancements in indicators and markers, are essential to improve the protection of public health. This study aimed to establish risk-based critical concentration benchmarks for significant enteric pathogens, i.e., norovirus, rotavirus, adenovirus, Cryptosporidium spp., Giardia lamblia, Campylobacter jejuni, Salmonella spp., and Escherichia coli O157:H7. Applying a 0.036 risk benchmark to both marine and freshwater environments, the study identified the lowest critical concentrations for children, who are the most susceptible group. Norovirus, C. jejuni, and Cryptosporidium presented lowest median critical concentrations for virus, bacteria, and protozoa, respectively: 0.74 GC, 1.73 CFU, and 0.39 viable oocysts per 100 mL in freshwater for children. These values were then used to determine minimum sample volumes corresponding to different recovery rates for culture method, digital polymerase chain reaction and quantitative PCR methods. The results indicate that for children, norovirus required the largest sample volumes of freshwater and marine water (52.08 to 178.57 L, based on the 5th percentile with a 10 % recovery rate), reflecting its low critical concentration and high potential for causing illness. In contrast, adenovirus and rotavirus required significantly smaller volumes (approximately 0.24 to 1.33 L). C. jejuni and Cryptosporidium, which required the highest sampling volumes for bacteria and protozoa, needed 1.72 to 11.09 L and 4.17 to 25.51 L, respectively. Additionally, the presented risk-based framework could provide a model for establishing pathogen thresholds, potentially guiding the creation of extensive risk-based criteria for various pathogens in recreational waters, thus aiding public health authorities in decision-making, strengthening pathogen monitoring, and improving water quality testing accuracy for enhanced health protection.

Assessment of monitoring approaches to control Legionella pneumophila within a complex cooling tower system

Precise and rapid methods are needed to improve monitoring approaches of L. pneumophila (Lp) in cooling towers (CTs) to allow timely operational adjustments and prevent outbreaks. The performance of liquid culture (ASTM D8429-21) and an online qPCR device were first compared to conventional filter plate culture (ISO 11731-2017), qPCR and semi-automated qPCR at three spiked concentrations of Lp (serogroup 1) validated by flow cytometry (total/viable cell count). The most accurate was qPCR, followed by liquid culture, online and semi-automated qPCR, and lastly, by a significant margin, filter plate culture. An industrial CT system was monitored using liquid and direct plate culture by the facility, qPCR and online qPCR. Direct plate and liquid culture results agreed at regulatory sampling point, supporting the use of the faster liquid culture for monitoring culturable Lp. During initial operation, qPCR and online qPCR results were within one log of culture at the primary pump before deviating after first cleaning. Other points revealed high spatial variability of Lp. The secondary pumps and chiller had the most positivity and highest concentrations by both qPCR and liquid culture compared to the basin and infeed tank. Altogether, this suggests that results from monthly compliance sampling at a single location with plate culture are not representative of Lp risks in this CT due to the high temporal and spatial variability. The primary pump, rather than the CT basin, should be designated for sampling, as it is representative of the health risk. An annual multi point survey of the system should be conducted to identify and target Lp hot spots. Generally, a combination of liquid culture for compliance and frequent qPCR for process control provides a more agile and robust monitoring scheme than plate culture alone, enabling early treatment adjustments, due to lower limit of detection (LOD) and turnover time.

Bioaerosols emission from source facilities in a wastewater treatment plant: Critical exposure time and sensitivity analysis

Overexposure of sewage workers to bioaerosol released from wastewater treatment plants (WWTPs) can cause serious infections, but practical method for controlling their health risk is lacking. In this study, reverse quantitative microbial risk assessment was used to estimate the daily critical exposure time (CET) of sewage workers exposing to Staphylococcus aureus bioaerosol emitted by three emission sources facilities in a WWTP based on either U.S. EPA or WHO benchmark, and sensitivity analysis was conducted to analyze the influence of various parameters on the outcomes of CET. The results showed that the CET of females was always 1.12-1.29 times that of males. In addition, the CET after wearing face masks was 28.28-52.37 times as long as before. The working time can be determined based on the CET results of male workers wearing face masks exposed to the inverted-umbrella aeration tank (14.73-550.98 min for U.S. EPA benchmark and 55.07-1972.24 min for WHO benchmark). In each scenario, the variable parameter exposure concentration (ec) always showed the most influence on the CET results. After wearing the face masks, the removal fraction by employing face masks also had a significant effect on the results, only second to ec. Therefore, the wearing of face mask is the most convenient and effective measure to prolong the CET. Furthermore, practical methods to reducing bioaerosol concentration in WWTPs exposure are also necessary to extend CET and safeguard worker health. This study enriches the application range of reverse quantitative microbial risk assessment framework and provides theoretical support for stakeholders to establish reasonable working time threshold guidelines, and practical method and novel perspective to protect the on-site health risks of sewage workers exposing to various facilities.

Quantitative microbial risk assessment (QMRA) tool for modelling pathogen infection risk to wastewater treatment plant workers

Wastewater treatment plants (WWTPs) provide vital services to the public by removing contaminants from wastewater prior to environmental discharge or reuse for beneficial purposes. WWTP workers occupationally exposed to wastewater can be at risk of respiratory or gastrointestinal diseases. The study objectives were to: (1) quantify pathogens and pathogen indicators in wastewater aerosols near different WWTP processes/unit operations, (2) develop a QMRA model for multi-pathogen and multi-exposure pathway risks, and (3) create a web-based application to perform and communicate risk calculations for wastewater workers. Case studies for seven different WWTP job tasks were performed investigating infection risk across nine different enteric and respiratory pathogens. It was observed that the ingestion risk among job tasks was highest for "walking the WWTP," which involved exposure from splashing, bioaerosols, and hand-to-mouth contact from touching contaminated surfaces. There was also a notable difference in exposure risk during peak (5:00am-9:00am) and non-peak hours (9:00am- 5:00am), with risks during the peak flow hours of the early morning assumed to be 5 times greater than non-peak hours. N95 respirator usage reduced median respiratory risks by 77 %. The developed tool performs multiple QMRA calculations to estimate WWTP workers' infection risks from accidental ingestion or inhalation of wastewater from multiple pathogens and exposure scenarios, which can inform risk management strategies to protect occupational health. However, more data are needed to reduce uncertainty in model estimates, including comparative data for pathogen concentrations in wastewater during peak and non-peak hours. QMRA tools will increase accessibility of risk models for utilization in decision-making.

Innovative microbial water quality management in water distribution systems using in-pipe hydropowered UV disinfection: envisioning futuristic water-energy systems

Hydropower UV disinfection has not been explored as a possible alternative for off-grid disinfection. Hydropowered UV LED technology was developed using off-the-shelf UV-C LEDs and pico - and femto-scale hydro turbine generators and evaluated across point-of-use relevant flow rates. Commercially available UV LED flow through reactors were subjected to microorganism challenge testing with 3 power schemes: wall-plug, hydropower, and hydropower-charged battery. UV LEDs powered by hydropower-charged battery demonstrated similar disinfection as wall-plug powered UV LEDs, achieving 0.5-1.8 MS2 log10 reduction at flow rates 0.5-2.3 L min-1, corresponding to reduction equivalent doses (RED) up to 16 or 30 mJ/cm2 for 254 and 285 nm, respectively. With hydropowered UV LEDs alone, MS2 log10 reduction decreased to <0.3 log10 reduction due to an underperforming and grossly inefficient turbine, with RED of 8 or 18 mJ/cm2 for 254 and 285 nm, respectively. Assessment of existing markets of UV disinfection systems and pico-hydro turbines demonstrated that hydropowered UV systems are already theoretically feasible for scales at point-of-entry (POE) and above. Economic feasibility will improve if turbines and/or UV system efficiencies improve. Prototype hydropower UV LED systems ranged from $145 to 220 depending on the UV LED reactor, and the battery system added $81. This study demonstrates the practicality of sustainable, renewable energy POU UV disinfection technology that can benefit decentralised, off-grid, rural and remote communities. The system may also scale up to provide renewable energy disinfection at larger scales, such as buildings and water distribution systems, for protecting human health in highly populated areas.

Applications of Quantitative Microbial Risk Assessment to Respiratory Pathogens and Implications for Uptake in Policy: A State-of-the-Science Review

BACKGROUND

Respiratory tract infections are major contributors to the global disease burden. Quantitative microbial risk assessment (QMRA) holds potential as a rapidly deployable framework to understand respiratory pathogen transmission and inform policy on infection control.

OBJECTIVES

The goal of this paper was to evaluate, motivate, and inform further development of the use of QMRA as a rapid tool to understand the transmission of respiratory pathogens and improve the evidence base for infection control policies.

METHODS

We conducted a literature review to identify peer-reviewed studies of complete QMRA frameworks on aerosol inhalation or contact transmission of respiratory pathogens. From each of the identified studies, we extracted and summarized information on the applied exposure model approaches, dose-response models, and parameter values, including risk characterization. Finally, we reviewed linkages between model outcomes and policy.

RESULTS

We identified 93 studies conducted in 16 different countries with complete QMRA frameworks for diverse respiratory pathogens, including SARS-CoV-2, Legionella spp., Staphylococcus aureus, influenza, and Bacillus anthracis. Six distinct exposure models were identified across diverse and complex transmission pathways. In 57 studies, exposure model frameworks were informed by their ability to model the efficacy of potential interventions. Among interventions, masking, ventilation, social distancing, and other environmental source controls were commonly assessed. Pathogen concentration, aerosol concentration, and partitioning coefficient were influential exposure parameters as identified by sensitivity analysis. Most (84%, n = 78 ) studies presented policy-relevant content including a) determining disease burden to call for policy intervention, b) determining risk-based threshold values for regulations, c) informing intervention and control strategies, and d) making recommendations and suggestions for QMRA application in policy.

CONCLUSIONS

We identified needs to further the development of QMRA frameworks for respiratory pathogens that prioritize appropriate aerosol exposure modeling approaches, consider trade-offs between model validity and complexity, and incorporate research that strengthens confidence in QMRA results. https://doi.org/10.1289/EHP12695.

Opportunistic Pathogens in Drinking Water Distribution Systems-A Review

In contrast to "frank" pathogens, like Salmonella entrocolitica, Shigella dysenteriae, and Vibrio cholerae, that always have a probability of disease, "opportunistic" pathogens are organisms that cause an infectious disease in a host with a weakened immune system and rarely in a healthy host. Historically, drinking water treatment has focused on control of frank pathogens, particularly those from human or animal sources (like Giardia lamblia, Cryptosporidium parvum, or Hepatitis A virus), but in recent years outbreaks from drinking water have increasingly been due to opportunistic pathogens. Characteristics of opportunistic pathogens that make them problematic for water treatment include: (1) they are normally present in aquatic environments, (2) they grow in biofilms that protect the bacteria from disinfectants, and (3) under appropriate conditions in drinking water systems (e.g., warm water, stagnation, low disinfectant levels, etc.), these bacteria can amplify to levels that can pose a public health risk. The three most common opportunistic pathogens in drinking water systems are Legionella pneumophila, Mycobacterium avium, and Pseudomonas aeruginosa. This report focuses on these organisms to provide information on their public health risk, occurrence in drinking water systems, susceptibility to various disinfectants, and other operational practices (like flushing and cleaning of pipes and storage tanks). In addition, information is provided on a group of nine other opportunistic pathogens that are less commonly found in drinking water systems, including Aeromonas hydrophila, Klebsiella pneumoniae, Serratia marcescens, Burkholderia pseudomallei, Acinetobacter baumannii, Stenotrophomonas maltophilia, Arcobacter butzleri, and several free-living amoebae including Naegleria fowleri and species of Acanthamoeba. The public health risk for these microbes in drinking water is still unclear, but in most cases, efforts to manage Legionella, mycobacteria, and Pseudomonas risks will also be effective for these other opportunistic pathogens. The approach to managing opportunistic pathogens in drinking water supplies focuses on controlling the growth of these organisms. Many of these microbes are normal inhabitants in biofilms in water, so the attention is less on eliminating these organisms from entering the system and more on managing their occurrence and concentrations in the pipe network. With anticipated warming trends associated with climate change, the factors that drive the growth of opportunistic pathogens in drinking water systems will likely increase. It is important, therefore, to evaluate treatment barriers and management activities for control of opportunistic pathogen risks. Controls for primary treatment, particularly for turbidity management and disinfection, should be reviewed to ensure adequacy for opportunistic pathogen control. However, the major focus for the utility's opportunistic pathogen risk reduction plan is the management of biological activity and biofilms in the distribution system. Factors that influence the growth of microbes (primarily in biofilms) in the distribution system include, temperature, disinfectant type and concentration, nutrient levels (measured as AOC or BDOC), stagnation, flushing of pipes and cleaning of storage tank sediments, and corrosion control. Pressure management and distribution system integrity are also important to the microbial quality of water but are related more to the intrusion of contaminants into the distribution system rather than directly related to microbial growth. Summarizing the identified risk from drinking water, the availability and quality of disinfection data for treatment, and guidelines or standards for control showed that adequate information is best available for management of L. pneumophila. For L. pneumophila, the risk for this organism has been clearly established from drinking water, cases have increased worldwide, and it is one of the most identified causes of drinking water outbreaks. Water management best practices (e.g., maintenance of a disinfectant residual throughout the distribution system, flushing and cleaning of sediments in pipelines and storage tanks, among others) have been shown to be effective for control of L. pneumophila in water supplies. In addition, there are well documented management guidelines available for the control of the organism in drinking water distribution systems. By comparison, management of risks for Mycobacteria from water are less clear than for L. pneumophila. Treatment of M. avium is difficult due to its resistance to disinfection, the tendency to form clumps, and attachment to surfaces in biofilms. Additionally, there are no guidelines for management of M. avium in drinking water, and one risk assessment study suggested a low risk of infection. The role of tap water in the transmission of the other opportunistic pathogens is less clear and, in many cases, actions to manage L. pneumophila (e.g., maintenance of a disinfectant residual, flushing, cleaning of storage tanks, etc.) will also be beneficial in helping to manage these organisms as well.

Quantitative Microbial Risk Assessment Framework Incorporating Water Ages with Legionella pneumophila Growth Rates

Water age in drinking water systems is often used as a proxy for water quality but is rarely used as a direct input in assessing microbial risk. This study directly linked water ages in a premise plumbing system to concentrations of Legionella pneumophila via a growth model. In turn, the L. pneumophila concentrations were used for a quantitative microbial risk assessment to calculate the associated probabilities of infection (Pinf) and clinically severe illness (Pcsi) due to showering. Risk reductions achieved by purging devices, which reduce water age, were also quantified. The median annual Pinf exceeded the commonly used 1 in 10,000 (10-4) risk benchmark in all scenarios, but the median annual Pcsi was always 1-3 orders of magnitude below 10-4. The median annual Pcsi was lower in homes with two occupants (4.7 × 10-7) than with one occupant (7.5 × 10-7) due to more frequent use of water fixtures, which reduced water ages. The median annual Pcsi for homes with one occupant was reduced by 39-43% with scheduled purging 1-2 times per day. Smart purging devices, which purge only after a certain period of nonuse, maintained these lower annual Pcsi values while reducing additional water consumption by 45-62%.

Antimicrobial Peptide Assembly on Zwitterionic Polymer Films to Slow Down Biofilm Formation

Formation of biofilms on equipment used in various fields, such as medicine, domestic sanitation, and marine transportation, can cause serious problems. The use of antibiofouling and bactericidal modifications is a promising strategy for inhibiting bacterial adhesion and biofilm formation. To further enhance the antibiofilm properties of a surface, various combinations of bactericidal modifications alongside antibiofouling modifications have been developed. Optimization of the arrangements of antimicrobial peptides on the antibiofouling surface would allow us to design longer-life antibiofilm surface modifications. In this study, a postmodification was conducted with different design using the antimicrobial peptide KR12 on an antibiofouling copolymer film consisting of 2-methacryloyloxyethyl phosphorylcholine, 3-methacryloxypropyl trimethoxysilane, and 3-(methacryloyloxy) propyl-tris(trimethylsilyloxy) silane. The distance of KR12 from the film was adjusted by combining different lengths of poly(ethylene glycol) (PEG) spacers (molecular weights are 2000 and 5000). The density of KR12 was ranged from 0.06 to 0.22 nm-2. When these modified surfaces were exposed to a nutrient-rich TSB suspension, the bacterial area formed by E. coli covered 5-127% of the original copolymer film. We found that a significant distance between the bactericidal and antibiofouling modifications, along with a higher density of bactericidal modifications, slows down the biofilm formation.

Human adenovirus-associated health risk in the recreational waters of the Yal-ku lagoon in the Mexican Caribbean

The study objective was to evaluate human faecal contamination impacts in the Yal-ku lagoon in the Mexican Caribbean and to estimate adenovirus infection and illness risks associated with recreational exposure during water activities. A total of 20 water samples (10 from each site × two sites) (50 L) were collected monthly over a period of 12 months from two selected sampling sites in the swimming area of the Yal-ku lagoon. The occurrence of faecal-associated viruses was explored, and human adenovirus (HAdV) and pepper mild mottle virus (PMMoV) concentrations were quantified. A quantitative microbial risk assessment (QMRA) model was used to estimate exposure and subsequent adenovirus infection and illness risk for 1 h of swimming or snorkelling. Somatic and F + -specific coliphages occurred in 100% of the samples. Both HAdV and PMMoV were detected at a 60% frequency thereby indicating persistent faecal inputs. PMMoV concentrations (44-370 GC/L) were relatively lower than the concentrations of HAdV (64-1,000 GC/L). Estimated mean adenovirus risks were greater for snorkelling than for swimming by roughly one to two orders of magnitude and estimated mean illness risks for snorkelling were >32/1,000. Human faecal contamination is frequent in the Yal-ku lagoon, which is associated with human gastrointestinal illness.

Mitigating risks and maximizing sustainability of treated wastewater reuse for irrigation

Scarcity of freshwater for agriculture has led to increased utilization of treated wastewater (TWW), establishing it as a significant and reliable source of irrigation water. However, years of research indicate that if not managed adequately, TWW may deleteriously affect soil functioning and plant productivity, and pose a hazard to human and environmental health. This review leverages the experience of researchers, stakeholders, and policymakers from Israel, the United-States, and Europe to present a holistic, multidisciplinary perspective on maximizing the benefits from municipal TWW use for irrigation. We specifically draw on the extensive knowledge gained in Israel, a world leader in agricultural TWW implementation. The first two sections of the work set the foundation for understanding current challenges involved with the use of TWW, detailing known and emerging agronomic and environmental issues (such as salinity and phytotoxicity) and public health risks (such as contaminants of emerging concern and pathogens). The work then presents solutions to address these challenges, including technological and agronomic management-based solutions as well as source control policies. The concluding section presents suggestions for the path forward, emphasizing the importance of improving links between research and policy, and better outreach to the public and agricultural practitioners. We use this platform as a call for action, to form a global harmonized data system that will centralize scientific findings on agronomic, environmental and public health effects of TWW irrigation. Insights from such global collaboration will help to mitigate risks, and facilitate more sustainable use of TWW for food production in the future.

The Detection Limits of Legionella According to the EU Directive 2020/2184. Could That Be Too Permissive?

INTRODUCTION

The problem of detecting legionella after a case of legionellosis from the source of environmental contamination has been known since a long time ago. Legionella is a bacterium present in various natural and artificial habitats and especially in surface fresh waters. It is found in greater concentration in warm waters, at temperatures between 20 °C and 42 °C. The greatest risk factor for humans is represented by the presence of Legionella in water distribution systems in hospitals, medical equipment (e.g. respirators, dialyzers, inhalers, humidifiers, water, massage equipment used in balneotherapy) and turbines used in dental practices, especially for hospitalized individuals. In the EU directive 2020/2184, issued by the European parliament on 16/12/2020, the concentration of Legionella was added to the parameters to be determined in assessing the quality of drinking water intended for human consumption. The objectives were to improve the quality standard of drinking water, reduce the consumption of bottled water and consequently reduce plastic waste. The WHO notes that Legionella causes the greatest burden from a health point of view and it is included among the parameters that require careful monitoring with a limit of less than 1000 CFU/L. The aim of this report was to evaluate the new EU directive 2020/2184 on the light of our laboratory experience.

MATERIALS AND METHODS

A total of 459 samples were processed at our Hygiene of food Laboratory - Department of Medical Sciences and Public Health. All statistical analyses were conducted using the SPSS statistical package (version 23 for Windows. SPSS, Inc. Chicago, Ill).

RESULTS AND DISCUSSION

Of the 67 structures examined where the cases occurred, 35 showed samples with at least one over-threshold value considering the reference value of 100 CFU/L, whereas using the new limit of 1000 CFU/L, only 25 structures resulted as having at least one sample above the threshold. In our experience as a regional reference laboratory for Legionella research, the increase from 100 CFU/L to 1,000 CFU/L could lead to a lower alert level. In fact, in the period between October 2017 and October 2021, the median value of CFU/L in presence of a case was 0 (0-100). Despite the large amount of studies on Legionella only a few relate the withdrawals and the consequent CFU/L with the confirmed cases of legionellosis, as in our analysis. The 75° percentile values of the Legionella concentration equal to 100 CFU/L in all samples associated with cases and clusters leads us to hypothesize that the limit equal to 1000 CFU/L that will be introduced for environmental monitoring as per recent European regulations may not be sufficiently protective for minimizing risk in the population, especially in healthcare facilities where fragile patients are assisted.

Legionellosis on the rise: A scoping review of sporadic, community-acquired incidence in the United States

Over the past two decades, the incidence of legionellosis has been steadily increasing in the United States though there is noclear explanation for the main factors driving the increase. While legionellosis is the leading cause of waterborne outbreaks in the US, most cases are sporadic and acquired in community settings where the environmental source is never identified. This scoping review aimed to summarise the drivers of infections in the USA and determine the magnitude of impact each potential driver may have. A total of 1,738 titles were screened, and 18 articles were identified that met the inclusion criteria. Strong evidence was found for precipitation as a major driver, and both temperature and relative humidity were found to be moderate drivers of incidence. Increased testing and improved diagnostic methods were classified as moderate drivers, and the ageing U.S. population was a minor driver of increasing incidence. Racial and socioeconomic inequities and water and housing infrastructure were found to be potential factors explaining the increasing incidence though they were largely understudied in the context of non-outbreak cases. Understanding the complex relationships between environmental, infrastructure, and population factors driving legionellosis incidence is important to optimise mitigation strategies and public policy.

Legionella in drinking water: the detection method matters

Legionella concentrations in drinking water have been regulated for decades and are evaluated with regard to their concentrations in drinking water plumbing systems (DWPS). The respective action levels differ at the international level. In Germany, the Federal Environment Agency (UBA) specifies the application of ISO 11731 for the detection of legionella in drinking water and gives a binding recommendation for the methods to be used for culturing and evaluation. Effective from 01 March 2019, the UBA recommendation was revised. The utilized culture media in the culture approach were altered, consequently affecting the spectrum of legionella colonies detected in drinking water. Using data from a routine legionella monitoring of a large laboratory, over a period of 6 years and 17,270 individual drinking water samples, allowed us to assess the impact of the alteration on the assessment of DWPS. By comparing the amount of action level exceedances before and after the method change, it could be demonstrated that exceedances are reported significantly more often under the new method. Consequently, the corresponding action level for evaluation of legionella contamination and the resulting risk to human health needs to be revised to avoid the misleading impression of increased health risk.

Validation of quantitative real-time PCR for detection of Legionella pneumophila in hospital water networks

BACKGROUND

Rapid monitoring of Legionella pneumophila (Lp) is essential to reduce the risk of Legionnaires' disease in healthcare facilities. However, culture results take at least eight days, delaying the implementation of corrective measures. Here, we assessed the performance of a qPCR method and determined qPCR action thresholds for the detection of Lp in hospital hot water networks (HWNs).

METHODS

Hot water samples (n=459) were collected from a hospital HWNs. Lp were quantified using iQ-Check® Quanti real-time PCR Quantification kits (Bio-Rad) and the results were compared with those of culture. qPCR thresholds corresponding to the culture action thresholds of 10 and 1,000 CFU/L were determined on a training dataset and validated on an independent dataset.

RESULTS

Lp concentrations measured by culture and qPCR were correlated for both the training dataset (Spearman's correlation coefficient ρ = 0.687, p-value < 0.0001) and the validation dataset (ρ = 0.661, p-value < 0.0001). Lp qPCR positivity thresholds corresponding to culture action thresholds of 10 CFU/L was 91 genome units (GU) per liter (sensitivity, 86.4%; negative predictive value - NPV, 93.3%) and that corresponding to culture action thresholds of 1,000 CFU/L was 1,048 GU/L (sensitivity, 100%; NPV, 100%).

CONCLUSION

Detection of Lp by qPCR could be implemented with confidence in hospitals as a complement to culture in the monitoring strategy to speed up the implementation of corrective measures.

Evaluating the threshold limit value of acceptable exposure concentration for exposure to bioaerosols in a wastewater treatment plant: Reverse-quantitative microbial risk assessment and sensitivity analysis

Agitation operations produce numerous pathogenic bioaerosols in WWTPs¹. QMRA² can determine risks of persons exposed to these bioaerosols. However, QMRA framework cannot help stakeholders in immediately deciding whether a risk is intolerable. Thus, evaluating threshold of acceptable exposure concentration is an urgent issue but is still rarely addressed in WWTPs. This study analyzed TLV³ benchmarks of E. coli and S. aureus bioaerosols emitted from a WWTP by reverse-QMRA. Furthermore, variance of input parameters was clarified by sensitivity analysis. Results showed that, under conservative and optimistic estimates, TLV of technicians was 1.52-2.06 and 1.26-1.68 times as large as those of workers, respectively; wearing mask drive TLV up to 1-2 orders of magnitude; TLV of M⁴ was at most 1.33 and 1.31 times as large as that of RD⁵, respectively. For sensitivity analysis, removal fraction by equipping PPE enlarge TLV for effortlessly obtaining an acceptable assessment result; exposure time was dominant when without PPE excepting the scenario of technicians exposed to E. coli bioaerosol. This study helps establish threshold guidelines for bioaerosols in WWTPs and contribute innovative perspectives for stakeholders.

Conceptual model to inform Legionella-amoebae control, including the roles of extracellular vesicles in engineered water system infections

Extracellular vesicles (EVs or exosomes) are well described for bacterial pathogens associated with our gastrointestinal system, and more recently as a novel mechanism for environmental persistence, dissemination and infection for human enteric viruses. However, the roles played by EVs in the ancient arms race that continues between amoebae and one of their prey, Legionella pneumophila, is poorly understood. At best we know of intracellular vesicles of amoebae containing a mix of bacterial prey species, which also provides an enhanced niche for bacteriophage infection/spread. Free-living amoeba-associated pathogens have recently been recognized to have enhanced resistance to disinfection and environmental stressors, adding to previously understood (but for relatively few species of) bacteria sequestered within amoebal cysts. However, the focus of the current work is to review the likely impacts of large numbers of respiratory-sized EVs containing numerous L. pneumophila cells studied in pure and biofilm systems with mixed prey species. These encapsulated pathogens are orders of magnitude more resistant to disinfection than free cells, and our engineered systems with residual disinfectants could promote evolution of resistance (including AMR), enhanced virulence and EV release. All these are key features for evolution within a dead-end human pathogen post lung infection. Traditional single-hit pathogen infection models used to estimate the probability of infection/disease and critical environmental concentrations via quantitative microbial risk assessments may also need to change. In short, recognizing that EV-packaged cells are highly virulent units for transmission of legionellae, which may also modulate/avoid human host immune responses. Key data gaps are raised and a previous conceptual model expanded upon to clarify where biofilm EVs could play a role promoting risk as well as inform a more wholistic management program to proactively control legionellosis.

A Critical Review on the Factors that Influence Opportunistic Premise Plumbing Pathogens: From Building Entry to Fixtures in Residences

Residential buildings provide unique conditions for opportunistic premise plumbing pathogen (OPPP) exposure via aerosolized water droplets produced by showerheads, faucets, and tubs. The objective of this review was to critically evaluate the existing literature that assessed the impact of potentially enhancing conditions to OPPP occurrence associated with residential plumbing and to point out knowledge gaps. Comprehensive studies on the topic were found to be lacking. Major knowledge gaps identified include the assessment of OPPP growth in the residential plumbing, from building entry to fixtures, and evaluation of the extent of the impact of typical residential plumbing design (e.g., trunk and branch and manifold), components (e.g., valves and fixtures), water heater types and temperature setting of operation, and common pipe materials (copper, PEX, and PVC/CPVC). In addition, impacts of the current plumbing code requirements on OPPP responses have not been assessed by any study and a lack of guidelines for OPPP risk management in residences was identified. Finally, the research required to expand knowledge on OPPP amplification in residences was discussed.

Enteric pathogen reduction targets for onsite non-potable water systems: A critical evaluation

Onsite non-potable water systems (ONWS) collect and treat local source waters for non-potable end uses such as toilet flushing and irrigation. Quantitative microbial risk assessment (QMRA) has been used to set pathogen log10-reduction targets (LRTs) for ONWS to achieve the risk benchmark of 10-4 infections per person per year (ppy) in a series of two efforts completed in 2017 and 2021. In this work, we compare and synthesize the ONWS LRT efforts to inform the selection of pathogen LRTs. For onsite wastewater, greywater, and stormwater, LRTs for human enteric viruses and parasitic protozoa were within 1.5-log10 units between 2017 and 2021 efforts, despite differences in approaches used to characterize pathogens in these waters. For onsite wastewater and greywater, the 2017 effort used an epidemiology-based model to simulate pathogen concentrations contributed exclusively from onsite waste and selected Norovirus as the viral reference pathogen; the 2021 effort used municipal wastewater pathogen data and cultivable adenoviruses as the reference viral pathogen. Across source waters, the greatest differences occurred for viruses in stormwater, given the newly available municipal wastewater characterizations used for modeling sewage contributions in 2021 and the different selection of reference pathogens (Norovirus vs. adenoviruses). The roof runoff LRTs support the need for protozoa treatment, but these remain difficult to characterize due to the pathogen variability in roof runoff across space and time. The comparison highlights adaptability of the risk-based approach, allowing for updated LRTs as site specific or improved information becomes available. Future research efforts should focus on data collection of onsite water sources.

Impact of prolonged hotel closures during the COVID-19 pandemic on Legionella infection risks

In general, it is accepted that water stagnation and lack or poor maintenance in buildings are risk factors for Legionella growth. Then, in theory, the prolonged hotel closures due to the COVID-19 pandemic may have increased the risk of Legionella infections. However, there are very few field studies comparing the level of Legionella colonization in buildings before the pandemic and the new situation created after the lockdown. The objective of this study was to analyze these differences in a group of hotels that experienced prolonged closures in 2020 due to the COVID-19 pandemic. We have studied the Legionella spp. results, analyzed by standard culture, from the domestic water distribution systems of 73 hotels that experienced closures (from 1 to >4 months) during 2020, immediately after the reopening. The results were compared with those obtained in similar samplings of 2019. For the comparative analysis, we divided the hotels in two groups: Group A that have suffered closures for ≤3 months and Group B that remained closed for more than 3 months, both in relation to the opening period of 2019. In the Group B (36 sites), the frequency of positive samples in the hot water system increased from 6.7% in 2019 to 14.0% in 2020 (p < 0.05). In the Group A (37 sites), no significant differences were observed. No statistically significant differences were observed in terms of positive sites (defined as hotels with at least 1 positive sample), Legionella spp. concentrations and prevalence of Legionella pneumophila sg1 between the samplings of the two periods studied. The results suggest that hotels that suffered the longest prolonged closures (> 3 months) could have carried a higher risk of exposure to Legionella in the domestic hot water system. These findings highlight the importance of adequate preopening cleaning and disinfection procedures for hotel water systems, and the convenience of considering the most effective disinfection methods especially for hot water systems and after prolonged closure periods.

Shower water contributes viable nontuberculous mycobacteria to indoor air

Nontuberculous mycobacteria (NTM) are frequently present in municipal drinking water and building plumbing, and some are believed to cause respiratory tract infections through inhalation of NTM-containing aerosols generated during showering. However, the present understanding of NTM transfer from water to air is insufficient to develop NTM risk mitigation strategies. This study aimed to characterize the contribution of shower water to the abundance of viable NTM in indoor air. Shower water and indoor air samples were collected, and 16S rRNA and rpoB genes were sequenced. The sequencing results showed that running the shower impacted the bacterial community structure and NTM species composition in indoor air by transferring certain bacteria from water to air. A mass balance model combined with NTM quantification results revealed that on average 1/132 and 1/254 of NTM cells in water were transferred to air during 1 hour of showering using a rain and massage showerhead, respectively. A large fraction of the bacteria transferred from water to air were membrane-damaged, i.e. they had compromised membranes based on analysis by live/dead staining and flow cytometry. However, the damaged NTM in air were recoverable as shown by growth in a culture medium mimicking the respiratory secretions of people with cystic fibrosis, implying a potential infection risk by NTM introduced to indoor air during shower running. Among the recovered NTM, Mycobacterium mucogenicum was the dominant species as determined by rpoB gene sequencing. Overall, this study lays the groundwork for future pathogen risk management and public health protection in the built environment.

Assessment of the risk of Legionella pneumophila in water distribution systems in hospitals of Tehran city

When a sensitive host inhales aerosols containing these bacteria, Legionella infection occurs. Therefore, monitoring and assessing Legionella in the environment and water distribution systems of such places are critical due to the prone population in hospitals. However, the health risks of Legionella bacteria in the environment are not adequately evaluated. In this study, for hospitalized patients, we performed a quantitative health risk assessment of Legionella in selected hospitals in Tehran city using two scenarios of shower and toilet faucet exposure. This study identified Legionella in 38 cases (38%) out of 100 samples collected from toilet faucets and showers in 8 hospitals. The information gathered was used for quantitative microbial risk assessment (QMRA). The microbial load transmitted by inhalation was calculated using the concentration of Legionella in water. Other exposure parameters (inhalation rate and exposure time) were obtained using information from other studies and the median length of hospital stay (3.6 days). The exponential model was used to estimate the risk of infection (γ = 0.06) due to Legionella pneumophila (L. pneumophila) inhalation for each exposure event. For the mean concentration obtained for Legionella (10³ CFU/L), the risk of infection for toilet faucets and showers was in the range of 0.23-2.3 and 3.5-21.9, respectively, per 10,000 hospitalized patients. The results were compared with the tolerable risk level of infection determined by the US EPA and WHO. The risk values exceeded the WHO values for waterborne pathogens in hospitals in both exposure scenarios. As a result, our QMRA results based on monitoring data showed that despite using treated water (from distribution networks in the urban areas) by hospitals, 38% of the samples were contaminated with Legionella, and faucets and showers can be sources of Legionella transmission. Hence, to protect the health of hospitalized patients, the risk of Legionella infection should be considered.

An application for relating Legionella shower water monitoring results to estimated health outcomes

Exposure models are useful tools for relating environmental monitoring data to expected health outcomes. The objective of this study was to (1) compare two Legionella shower exposure models, and (2) develop a risk calculator tool for relating environmental monitoring data to estimated Legionella infection risks and Legionnaires' Disease (LD) illness risks. Legionella infection risks for a single shower event were compared using two shower Legionella exposure models. These models varied in their description of partitioning of Legionella in aerosols and aerosol deposition in the lung, where Model 1 had larger and fewer aerosol ranges than Model 2. Model 2 described conventional vs. water efficient showers separately, while Model 1 described exposure for an unspecified shower type (did not describe it as conventional or water efficient). A Monte Carlo approach was used to account for variability and uncertainty in these aerosolization and deposition parameters, Legionella concentrations, and the dose-response parameter. Methods for relating infection risks to illness risks accounting for demographic differences were used to inform the risk calculator web application ("app"). Model 2 consistently estimated higher infection risks than Model 1 for the same Legionella concentration in water and estimated deposited doses with less variability. For a 7.8-min shower with a Legionella concentration of 0.1 CFU/mL, the average infection risks estimated using Model 2 were 4.8 × 10^-6 (SD=3.0 × 10^-6) (conventional shower) and 2.3 × 10^-6 (SD=1.7 × 10^-6) (water efficient). Average infection risk estimated by Model 1 was 1.1 × 10^-6 (SD=9.7 × 10^-7). Model 2 was used for app development due to more conservative risk estimates and less variability in estimated dose. While multiple Legionella shower models are available for quantitative microbial risk assessments (QMRAs), they may yield notably different infection risks for the same environmental microbial concentration. Model comparisons will inform decisions regarding their integration with risk assessment tools. The development of risk calculator tools for relating environmental microbiology data to infection risks will increase the impact of exposure models for informing water treatment decisions and achieving risk targets.

Design and operational considerations in response to Legionella occurrence in Las Vegas Valley groundwater

Legionella occurrence monitoring is not required by United States Environmental Protection Agency (USEPA) drinking water regulations, and few occurrence studies exist for Legionella in source water or distribution systems. Legionella occurrence was monitored in Las Vegas Valley (Las Vegas, Nevada, USA) drinking water sources, including non-treated surface water, seasonal groundwater (61 wells, before and after chlorination), finished water (after treatment at water treatment facilities), and chlorinated distribution system water (at 9 reservoirs and 75 sample locations throughout the network). Legionella pneumophila was detected at least once at each of the wells sampled before chlorination, with an overall positivity rate of 38% (343/908). During well start-up (time<2 hours; turbidity>3 NTU), L. pneumophila concentrations averaged 2,792±353 MPN/100 mL, with a median of 105 MPN/100 mL, and range of <1 to 90,490 MPN/100 mL across 61 seasonally operated (typically April-October) groundwater wells. After initial flushing (turbidity<3 NTU), the average concentration decreased by more than two orders of magnitude to 24±3 MPN/100 mL but ranged from <1 to >2,273 MPN/100 mL. This trend indicates that stagnation (up to 391 days) contributed to greater initial concentrations, and flushing alone was incapable of complete L. pneumophila elimination. L. pneumophila concentration was significantly, positively correlated with total aqueous adenosine triphosphate (ATP) (p<0.00001, r=0.41-0.71), turbidity (p<0.00001, r=0.27-0.51), orthophosphate (p=0.35-0.076, r=0.51-0.59), and pump depth (p=0.032, r=0.40). During a full-scale assessment of chlorination (Ct=0.7 to 10.5 mg-min/L; T=26.6-28.1°C), substantial reduction of Legionella spp. (up to 2.5 logs) was observed; although, detectable concentrations were still measured. Extrapolating from a Chick-Watson model (log inactivation=0.28*(Ct); R²=0.87) constructed from the full-scale chlorination results, 3- and 4-log inactivation in Las Vegas Valley groundwater would require 10.8 and 14.3 mg-min/L, respectively; at least 3-log inactivation was required to bring Legionella spp. to below detection at the studied well. Chlorine exposure (Ct=0.1 to 10.9 mg-min/L) at most wells discharging directly to the distribution system was insufficient to fully inactivate Legionella spp. After discussing these findings with the state regulatory agency, direct-to-distribution wells (38 of 61 wells) remained out of operation; the distribution system, wells, and reservoirs were monitored for Legionella and chlorine residual, and additional treatment scenarios were identified for further evaluation. Legionella was either not detected or was well controlled in surface water, finished effluent from the drinking water treatment plant, chlorinated reservoirs, and the chlorinated distribution system. This study emphasizes the importance of utility-driven, non-regulatory research in order to protect public health and also identifies the need for greater occurrence monitoring and guidance for Legionella in groundwater supplies.

The Presence of Opportunistic Premise Plumbing Pathogens in Residential Buildings: A Literature Review

Opportunistic premise plumbing pathogens (OPPP) are microorganisms that are native to the plumbing environment and that present an emerging infectious disease problem. They share characteristics, such as disinfectant resistance, thermal tolerance, and biofilm formation. The colonisation of domestic water systems presents an elevated health risk for immune-compromised individuals who receive healthcare at home. The literature that has identified the previously described OPPPs (Aeromonas spp., Acinetobacter spp., Helicobacter spp., Legionella spp., Methylobacterium spp., Mycobacteria spp., Pseudomonas spp., and Stenotrophomonas spp.) in residential drinking water systems were systematically reviewed. By applying the Preferred reporting items for systematic reviews and meta-analyses guidelines, 214 studies were identified from the Scopus and Web of Science databases, which included 30 clinical case investigations. Tap components and showerheads were the most frequently identified sources of OPPPs. Sixty-four of these studies detected additional clinically relevant pathogens that are not classified as OPPPs in these reservoirs. There was considerable variation in the detection methods, which included traditional culturing and molecular approaches. These identified studies demonstrate that the current drinking water treatment methods are ineffective against many waterborne pathogens. It is critical that, as at-home healthcare services continue to be promoted, we understand the emergent risks that are posed by OPPPs in residential drinking water. Future research is needed in order to provide consistent data on the prevalence of OPPPs in residential water, and on the incidence of waterborne homecare-associated infections. This will enable the identification of the contributing risk factors, and the development of effective controls.

Tenets of a holistic approach to drinking water-associated pathogen research, management, and communication

In recent years, drinking water-associated pathogens that can cause infections in immunocompromised or otherwise susceptible individuals (henceforth referred to as DWPI), sometimes referred to as opportunistic pathogens or opportunistic premise plumbing pathogens, have received considerable attention. DWPI research has largely been conducted by experts focusing on specific microorganisms or within silos of expertise. The resulting mitigation approaches optimized for a single microorganism may have unintended consequences and trade-offs for other DWPI or other interests (e.g., energy costs and conservation). For example, the ecological and epidemiological issues characteristic of Legionella pneumophila diverge from those relevant for Mycobacterium avium and other nontuberculous mycobacteria. Recent advances in understanding DWPI as part of a complex microbial ecosystem inhabiting drinking water systems continues to reveal additional challenges: namely, how can all microorganisms of concern be managed simultaneously? In order to protect public health, we must take a more holistic approach in all aspects of the field, including basic research, monitoring methods, risk-based mitigation techniques, and policy. A holistic approach will (i) target multiple microorganisms simultaneously, (ii) involve experts across several disciplines, and (iii) communicate results across disciplines and more broadly, proactively addressing source water-to-customer system management.

Legionella pneumophila in Municipal Shower Systems in Stavanger, Norway; A Longitudinal Surveillance Study Using Whole Genome Sequencing in Risk Management

Following an incidence of Legionnaires disease (LD) in 2007, where a municipal shower system was the likely source of infection, Stavanger municipality initiated a surveillance program for Legionella as part of establishing internal risk evaluation and prevention routines. More than 250 shower systems were examined for cultivatable Legionella pneumophila. The prevalence and diversity of serogroups (sg) and sequence types (STs) of L. pneumophila were mapped using available typing techniques over a period of more than 10 years (2010–2021). The surveillance showed an overall reduction in the L. pneumophila colonisation rate in municipal systems from 11 to 4.5% following prevention measures during the period, with the highest colonisation rate in complex systems (e.g., larger nursing homes and sports complexes). Further, an approximately even distribution between sg1 and 2–14 was seen. Whole genome sequencing (WGS) revealed that only a limited number of STs were detected, and they were consistent at specific locations over time. This study showed that environmental surveillance data in combination with available typing techniques and WGS can give the municipality a better tool for risk management and an overview of ST distributions that can be a valuable asset in future source investigations.

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

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.

Practitioners’ Perspective on the Prevalent Water Quality Management Practices for Legionella Control in Large Buildings in the United States

Managing building water systems is complicated by the need to maintain hot water temperatures high enough to control the growth of Legionella spp. while minimizing the risk of scalding. This study assessed water quality management practices in large buildings in the United States. Surveys conducted with building water quality managers found that more than 85% of buildings have hot water temperatures that are consistent with scald risk mitigation guidelines (i.e., <122 °F/50 °C). However, nearly two thirds and three quarters of buildings do not comply with the common temperature guidance for opportunistic pathogen control, i.e., water heater setpoint > 140 °F (60 °C) and recirculation loop > 122 °F (50 °C), respectively; median values for both setpoint and recirculation loop temperatures are 10 °F (6 °C) or more below temperatures recommended for opportunistic pathogen control. These observations suggest that many buildings are prone to Legionella spp. risk. The study also found that 27% of buildings do not comply with guidelines for time to equilibrium hot water temperature, over 33% fail to monitor temperature in the recirculation loop, more than 70% fail to replace or disinfect showerheads, more than 40% lack a written management plan, and only a minority conduct any monitoring of residual disinfectant levels or microbiological quality. Given the rise in Legionellosis infections in recent years, coupled with highlighted water quality concerns because of prolonged water stagnation in plumbing, such as in buildings closed due to COVID-19, current management practices, which appear to be focused on scald risk, may need to be broadened to include greater attention to control of opportunistic pathogens. To accomplish this, there is a need for formal training and resources for facility managers.

Quantitative Microbial Risk Assessment Applied to Legionella Contamination on Long-Distance Public Transport

The quantitative microbial risk assessment (QMRA) framework is used for assessing health risk coming from pathogens in the environment. In this paper, we used QMRA to evaluate the infection risk of L. pneumophila attributable to sink usage in a toilet cabin on Italian long-distance public transportation (LDT). LDT has water distribution systems with risk points for Legionella proliferation, as well as premise plumbing for drinking water, but they are not considered for risk assessment. Monitoring data revealed that approximately 55% of water samples (217/398) were positive for L. pneumophila, and the most frequently isolated was L. pneumophila sg1 (64%, 139/217); therefore, such data were fitted to the best probability distribution function to be used as a stochastic variable in the QMRA model. Then, a sink-specific aerosolization ratio was applied to calculate the inhaled dose, also considering inhalation rate and exposure time, which were used as stochastic parameters based on literature data. At L. pneumophila sg1 concentration ≤100 CFU/L, health risk was approximately 1 infection per 1 million exposures, with an increase of up to 5 infections per 10,000 exposures when the concentrations were ≥10,000 CFU/L. Our QMRA results showed a low Legionella infection risk from faucets on LDT; however, it deserves consideration since LDT can be used by people highly susceptible for the development of a severe form of the disease, owing to their immunological status or other predisposing factors. Further investigations could also evaluate Legionella-laden aerosols from toilet flushing.

Estimating acceptable exposure time for bioaerosols emission in a wastewater treatment plant by reverse quantitative microbial risk assessment based on various risk benchmarks

Populations exposed to bioaerosols over time in wastewater treatment plants (WWTPs) will be infected. Then, the reverse quantitative microbial risk assessment (QMRA) provides a quantitative framework for the estimation of acceptable exposure time to protect people from excessive exposure and then manage their health risk. In this study, the acceptable exposure time for staffs and visiting researchers exposed to S. aureus or E. coli bioaerosols emitted from aeration ponds in WWTPs was estimated and analyzed by Monte Carlo simulation-based reverse QMRA (using the 1E-4 pppy suggested by the US EPA or 1E-6 DALYs pppy suggested by the WHO as benchmarks). The 1E-3 and 1E-2 pppy were selected as a series of loose annual infection risk benchmarks to calculate a practical acceptable exposure time. The results showed that for the acceptable exposure time in each specific exposure scenario, the exposure of females was consistently 0.3-0.4 times longer than that of males; the exposure of staffs was 3.6-3.9 times shorter than that of visiting researchers; the exposures of populations in the rotating-disc aeration mode were consistently 6.3-6.6 and 2.8-3.1 times longer than those in the microporous aeration mode for S. aureus and E. coli bioaerosols, respectively. The acceptable exposure time with the use of personal protective equipment (PPE) was 33.4-35.0 times as long as that without PPE. The US EPA benchmark is stricter than the WHO benchmark with regard to the estimation of the acceptable exposure time of S. aureus or E. coli bioaerosols. The 1E-3 pppy is more appropriate and practical than the US EPA benchmark, but the 1E-2 pppy is notably too loose for health risk management. This research can assist managers of WWTPs to formulate a justified exposure time and develop applicable administrative and personal intervention strategies. The results can enrich the knowledge bases of reverse QMRA to elect a series of loose health-based target risk benchmarks for health risk management.

Inactivation and risk control of pathogenic microorganisms in municipal sludge treatment: A review

The rapid global spread of coronavirus disease 2019 (COVID-19) has promoted concern over human pathogens and their significant threats to public health security. The monitoring and control of human pathogens in public sanitation and health facilities are of great importance. Excessive sludge is an inevitable byproduct of sewage that contains human and animal feces in wastewater treatment plants (WWTPs). It is an important sink of different pollutants and pathogens, and the proper treatment and disposal of sludge are important to minimize potential risks to the environment and public health. However, there is a lack of comprehensive analysis of the diversity, exposure risks, assessment methods and inactivation techniques of pathogenic microorganisms in sludge. Based on this consideration, this review summarizes the control performance of pathogenic microorganisms such as enterovirus, Salmonella spp., and Escherichia coli by different sludge treatment technologies, including composting, anaerobic digestion, aerobic digestion, and microwave irradiation, and the mechanisms of pathogenic microorganism inactivation in sludge treatment processes are discussed. Additionally, this study reviews the diversity, detection methods, and exposure risks of pathogenic microorganisms in sludge. This review advances the quantitative assessment of pathogenic microorganism risks involved in sludge reuse and is practically valuable to optimize the treatment and disposal of sludge for pathogenic microorganism control.

Water Age Effects on the Occurrence and Concentration of Legionella Species in the Distribution System, Premise Plumbing, and the Cooling Towers

In this study, droplet digital PCR^TM (ddPCR^TM) was used to characterize total Legionella spp. and five specific Legionella species from source (groundwater) to exposure sites (taps and cooling towers). A total of 42–10 L volume water samples were analyzed during this study: 12 from a reservoir (untreated groundwater and treated water storage tanks), 24 from two buildings (influents and taps), and six from cooling towers, all part of the same water system. The approximate water age (time in the system) for all sample locations are as follows: ~4.5, 3.4, 9.2, 20.8, and 23.2 h (h) for the groundwater to the reservoir influent, reservoir influent to the reservoir effluent, reservoir effluent to building Fa (building names are abbreviated to protect the privacy of site location), building ERC and the cooling towers, respectively. Results demonstrated that gene copies of Legionella spp. (23S rRNA) were significantly higher in the cooling towers and ERC building (p < 0.05) relative to the reservoir and building Fa (closest to reservoir). Legionella spp. (23S rRNA) were found in 100% (42/42) of water samples at concentrations ranging from 2.2 to 4.5 Log10 GC/100 mL. More specifically, L. pneumophila was found in 57% (24/42) of the water samples, followed by L. bozemanii 52% (22/42), L. longbeachae 36% (15/42), L. micdadei 23% (10/42) and L. anisa 21% (9/42) with geometric mean concentrations of 1.7, 1.7, 1.4, 1.6 and 1.7 Log₁₀ GC/100 mL, respectively. Based on this study, it is hypothesized that water age in the distribution system and the premise-plumbing system as well as building management plays a major role in the increase of Legionella spp., (23S rRNA) and the diversity of pathogenic species found as seen in the influent, and at the taps in the ERC building—where the building water quality was most comparable to the industrial cooling towers. Other pathogenic Legionella species besides L.pneumophila are also likely amplifying in the system; thus, it is important to consider other disease relevant species in the whole water supply system—to subsequently control the growth of pathogenic Legionella in the built water environment.

A review of legionnaires' disease and public water systems - Scientific considerations, uncertainties and recommendations

Legionella is an opportunistic premise plumbing pathogen and causative agent of a severe pneumonia called Legionnaires' Disease (LD). Cases of LD have been on the rise in the U.S. and globally. Although Legionella was first identified 45 years ago, it remains an 'emerging pathogen." Legionella is part of the normal ecology of a public water system and is frequently detected in regulatory-compliant drinking water. Drinking water utilities, regulators and public health alike are increasingly required to have a productive understanding of the evolving issues and complex discussions of the contribution of the public water utility to Legionella exposure and LD risk. This review provides a brief overview of scientific considerations important for understanding this complex topic, a review of findings from investigations of public water and LD, including data gaps, and recommendations for professionals interested in investigating public water utilities. Although the current literature is inconclusive in identifying a public water utility as a sole source of an LD outbreak, the evidence is clear that minimizing growth of Legionella in public water utilities through proper maintenance and sustained disinfectant residuals, throughout all sections of the water utility, will lead to a less conducive environment for growth of the bacteria in the system and the buildings they serve.

Risk assessment and disease burden of legionella presence in cooling towers of Iran's central hospitals

Regular monitoring and measurement of Legionella in tower water and preventive measures against contamination are particularly important in hospitals. This study aimed at risk assessment and disease burden because of legionella presence in cooling towers of Iran's central hospitals. Then its correlation with temperature, pH, turbidity, residual chlorine, and EC was investigated by the Pearson test. The health risk and burden of diseases caused by Legionella exposure were determined using QMRA and DALY models. Statistical analysis and modeling were performed in MATLAB₂₀₁₈. Of the total samples, 30-43% was infected with Legionella. The mean concentrations in hospital A and B were 5-102.5 ± 10 and 5-89.7 ± 0.7 CFU/L, respectively. Among environmental factors, turbidity and pH were the most effective factors in increasing and decreasing Legionella concentration, respectively. According to the QMRA model, the risks of Legionella infections and annual mortality in both hospitals were 0.2-0.3, 0-0.19, 2-2.9 × 10^-5, and 0-0.7 × 10^-5, respectively, which was higher than the acceptable risk range for Legionella (10-4-10-7). However, the trend of its change was negatively correlated with time (R_B = - 0.77). According to the results, the concentration of Legionella and the exposure risk in both hospitals were higher than the permissible range, which is necessary to decrease to 0.1 current concentrations.

Legionella: A Promising Supplementary Indicator of Microbial Drinking Water Quality in Municipal Engineered Water Systems

Opportunistic pathogens (OPs) are natural inhabitants and the predominant disease causative biotic agents in municipal engineered water systems (EWSs). In EWSs, OPs occur at high frequencies and concentrations, cause drinking-water-related disease outbreaks, and are a major factor threatening public health. Therefore, the prevalence of OPs in EWSs represents microbial drinking water quality. Closely or routinely monitoring the dynamics of OPs in municipal EWSs is thus critical to ensuring drinking water quality and protecting public health. Monitoring the dynamics of conventional (fecal) indicators (e.g., total coliforms, fecal coliforms, and Escherichia coli) is the customary or even exclusive means of assessing microbial drinking water quality. However, those indicators infer only fecal contamination due to treatment (e.g., disinfection within water utilities) failure and EWS infrastructure issues (e.g., water main breaks and infiltration), whereas OPs are not contaminants in drinking water. In addition, those indicators appear in EWSs at low concentrations (often absent in well-maintained EWSs) and are uncorrelated with OPs. For instance, conventional indicators decay, while OPs regrow with increasing hydraulic residence time. As a result, conventional indicators are poor indicators of OPs (the major aspect of microbial drinking water quality) in EWSs. An additional or supplementary indicator that can well infer the prevalence of OPs in EWSs is highly needed. This systematic review argues that Legionella as a dominant OP-containing genus and natural inhabitant in EWSs is a promising candidate for such a supplementary indicator. Through comprehensively comparing the behavior (i.e., occurrence, growth and regrowth, spatiotemporal variations in concentrations, resistance to disinfectant residuals, and responses to physicochemical water quality parameters) of major OPs (e.g., Legionella especially L. pneumophila, Mycobacterium, and Pseudomonas especially P. aeruginosa), this review proves that Legionella is a promising supplementary indicator for the prevalence of OPs in EWSs while other OPs lack this indication feature. Legionella as a dominant natural inhabitant in EWSs occurs frequently, has a high concentration, and correlates with more microbial and physicochemical water quality parameters than other common OPs. Legionella and OPs in EWSs share multiple key features such as high disinfectant resistance, biofilm formation, proliferation within amoebae, and significant spatiotemporal variations in concentrations. Therefore, the presence and concentration of Legionella well indicate the presence and concentrations of OPs (especially L. pneumophila) and microbial drinking water quality in EWSs. In addition, Legionella concentration indicates the efficacies of disinfectant residuals in EWSs. Furthermore, with the development of modern Legionella quantification methods (especially quantitative polymerase chain reactions), monitoring Legionella in ESWs is becoming easier, more affordable, and less labor-intensive. Those features make Legionella a proper supplementary indicator for microbial drinking water quality (especially the prevalence of OPs) in EWSs. Water authorities may use Legionella and conventional indicators in combination to more comprehensively assess microbial drinking water quality in municipal EWSs. Future work should further explore the indication role of Legionella in EWSs and propose drinking water Legionella concentration limits that indicate serious public health effects and require enhanced treatment (e.g., booster disinfection).

Comparing approaches for modelling indirect contact transmission of infectious diseases

Mathematical models describing indirect contact transmission are an important component of infectious disease mitigation and risk assessment. A model that tracks microorganisms between compartments by coupled ordinary differential equations or a Markov chain is benchmarked against a mechanistic interpretation of the physical transfer of microorganisms from surfaces to fingers and subsequently to a susceptible person's facial mucosal membranes. The primary objective was to compare these models in their estimates of doses and changes in microorganism concentrations on hands and fomites over time. The abilities of the models to capture the impact of episodic events, such as hand hygiene, and of contact patterns were also explored. For both models, greater doses were estimated for the asymmetrical scenarios in which a more contaminated fomite was touched more often. Differing representations of hand hygiene in the Markov model did not notably impact estimated doses but affected pathogen concentration dynamics on hands. When using the Markov model, losses due to hand hygiene should be handled as separate events as opposed to time-averaging expected losses. The discrete event model demonstrated the effect of hand-to-mouth contact timing on the dose. Understanding how model design influences estimated doses is important for advancing models as reliable risk assessment tools.

Legionnaires' disease in dental offices: Quantifying aerosol risks to dental workers and patients

Legionella pneumophila is an opportunistic bacterial respiratory pathogen that is one of the leading causes of drinking water outbreaks in the United States. Dental offices pose a potential risk for inhalation or aspiration of L. pneumophila due to the high surface area to volume ratio of dental unit water lines-a feature that is conducive to biofilm growth. This is coupled with the use of high-pressure water devices (e.g., ultrasonic scalers) that produce fine aerosols within the breathing zone. Prior research confirms that L. pneumophila occurs in dental unit water lines, but the associated human health risks have not been assessed. We aimed to: (1) synthesize the evidence for transmission and management of Legionnaires' disease in dental offices; (2) create a quantitative modeling framework for predicting associated L. pneumophila infection risk; and (3) highlight influential parameters and research gaps requiring further study. We reviewed outbreaks, management guidance, and exposure studies and used these data to parameterize a quantitative microbial risk assessment (QMRA) model for L. pneumophila in dental applications. Probabilities of infection for dental hygienists and patients were assessed on a per-exposure and annual basis. We also assessed the impact of varying ventilation rates and the use of personal protective equipment (PPE). Following an instrument purge (i.e., flush) and with a ventilation rate of 1.2 air changes per hour, the median per-exposure probability of infection for dental hygienists and patients exceeded a 1-in-10,000 infection risk benchmark. Per-exposure risks for workers during a purge and annual risks for workers wearing N95 masks did not exceed the benchmark. Increasing air change rates in the treatment room from 1.2 to 10 would achieve an ∼85% risk reduction, while utilization of N95 respirators would reduce risks by ∼95%. The concentration of L. pneumophila in dental unit water lines was a dominant parameter in the model and driver of risk. Future risk assessment efforts and refinement of microbiological control protocols would benefit from expanded occurrence datasets for L. pneumophila in dental applications.

Comparison of Updated Methods for Legionella Detection in Environmental Water Samples

The difficulty of cultivation of Legionella spp. from water samples remains a strenuous task even for experienced laboratories. The long incubation periods for Legionellae make isolation difficult. In addition, the water samples themselves are often contaminated with accompanying microbial flora, and therefore require complex cultivation methods from diagnostic laboratories. In addition to the recent update of the standard culture method ISO 11731:2017, new strategies such as quantitative PCR (qPCR) are often discussed as alternatives or additions to conventional Legionella culture approaches. In this study, we compared ISO 11731:2017 with qPCR assays targeting Legionella spp., Legionella pneumophila, and Legionella pneumophila serogroup 1. In samples with a high burden of accompanying microbial flora, qPCR shows an excellent negative predictive value for Legionella pneumophila, thus making qPCR an excellent tool for pre-selection of negative samples prior to work-intensive culture methods. This and its low limit of detection make qPCR a diagnostic asset in Legionellosis outbreak investigations, where quick-risk assessments are essential, and are a useful method for monitoring risk sites.

Occurrence of Legionella spp. in Man-Made Water Sources: Isolates Distribution and Phylogenetic Characterization in the Emilia-Romagna Region

Legionella species distribution in the Emilia-Romagna region, involving hospital (H) and community (C) environments, was conducted. Legionella culture, agglutination test, and mip-gene sequencing were applied on 240 isolates. The analysis showed a higher prevalence of non-Legionellapneumophila (n-Lp) species (84.1%) compared with L. pneumophila (Lp) (15.9%), with a higher frequency of n-Lp with respect to Lp species in both environments (77.6% and 96.4%, in H and C, respectively). The Shannon index showed a significant difference in Legionella distribution (p = 0.00017), with a significant abundance of Lp in the H compared with C environment (p = 0.00028). The continuous disinfection treatment in H could contribute to adaptive survival of the Lp species. Phylogenetic analysis revealed a conservative clade distribution between H and C: L. feeleii clade with three subclades in C and the Lp clade with five subclades in H and two in C, respectively. Our findings suggest the importance of Legionella surveillance both in H and C, with a focus on n-Lp species less connected to human disease. The Legionella prevalence and diversity found here indicate that geographical and temporal isolate evolution should be considered during surveillance, particularly in the light of global warming and changes in population risk factors.

Microbiological Health Risk Assessment of Water Conservation Strategies: A Case Study in Amsterdam

The aim of this study was to assess the health risks that may arise from the implementation of greywater reuse and rainwater harvesting for household use, especially for toilet flushing. In addition, the risk of cross connections between these systems and the drinking water system was considered. Quantitative microbial risk assessment (QMRA) is a method that uses mathematical modelling to estimate the risk of infection when exposure to pathogens happens and was used in this study to assess the health risks. The results showed that using rainwater without prior treatment for toilet flushing poses an annual infection risk from L. pneumophila at 0.64 per-person-per-year (pppy) which exceeds the Dutch standard of 10-4 pppy. The use of untreated greywater showed a risk that is below the standard. However, treatment is recommended due to the ability of P. aeruginosa to grow in the reuse system. Moreover, showering and drinking with cross-connected water has a high annual infection risk that exceeds the standard due to contact with Staphylococcus aureus and E. coli O157:H7. Several measures can be implemented to mitigate the risks such as treating the greywater and rainwater with a minimum of 5-log removal, closing the toilet lid while flushing, good design of greywater and rainwater collection systems, and rigorous plumbing installation procedures.

Water Sampling Module for Collecting and Concentrating Legionella pneumophila from Low-to-Medium Contaminated Environment

The detection of water contamination with Legionella pneumophila is of critical importance to manufacturers of water processing equipment and public health entities dealing with water networks and distribution systems. Detection methods based on polymerase chain reaction or biosensor technologies require preconcentration steps to achieve attractive sensitivity levels. Preconcentration must also be included in protocols of automated collection of water samples by systems designed for quasi-continuous monitoring of remotely located water reservoirs for the presence of L. pneumophila. We designed and characterized a water sampling module for filtration and backwashing intended for analysis of low-to-medium contaminated water, typically with L. pneumophila bacteria not exceeding 50 colony-forming units per milliliter. The concentration factors of 10× and 21× were achieved with 0.22 and 0.45 µm filters, respectively, for samples of bacteria prepared in clean saline solutions. However, a 5× concentration factor was achieved with 0.45 µm filters for a heavily contaminated or turbid water typical of some industrial water samples.

Flushing of Stagnant Premise Water Systems after the COVID-19 Shutdown Can Reduce Infection Risk by Legionella and Mycobacterium spp

There is concern about potential exposure to opportunistic pathogens when reopening buildings closed due to the COVID-19 pandemic. In this study, water samples were collected before, during, and after flushing showers in five unoccupied (i.e., for ∼2 months) university buildings with quantification of opportunists via a cultivation-based assay (Legionella pneumophila only) and quantitative PCR. L. pneumophila were not detected by either method; Legionella spp., nontuberculous mycobacteria (NTM), and Mycobacterium avium complex (MAC), however, were widespread. Using quantitative microbial risk assessment (QMRA), the estimated risks of illness from exposure to L. pneumophila and MAC via showering were generally low (i.e., less than a 10^-7 daily risk threshold), with the exception of systemic infection risk from MAC exposure in some buildings. Flushing rapidly restored the total chlorine (as chloramine) residual and decreased bacterial gene targets to building inlet concentrations within 30 min. During the postflush stagnation period, the residual chlorine dissipated within a few days and bacteria rebounded, approaching preflush concentrations after 6-7 days. These results suggest that flushing can quickly improve water quality in unoccupied buildings, but the improvement may only last a few days.

Evaluation of Legionella spp. Colonization in Residential Buildings Having Solar Thermal System for Hot Water Production

Despite an increase of literature data on Legionella spp. presence in private water systems, epidemiological reports assert a continuing high incidence of Legionnaires’ disease infection in Italy. In this study, we report a survey on Legionella spp. colonization in 58 buildings with solar thermal systems for hot water production (TB). In all buildings, Legionella spp. presence was enumerated in hot and cold water samples. Microbiological potability standards of cold water were also evaluated. Legionella spp. was detected in 40% of the buildings. Moreover, we detected correlations between the count of Legionella spp. and the presence of the optimal temperature for the microorganism growth (less than 40 °C). Our results showed that cold water was free from microbiological hazards, but Legionella spp., was detected when the mean cold water temperature was 19.1 ± 2.2 °C. This may considered close to the suboptimal value for the Legionella growth (more then 20 °C). In conclusion, we observed the presence of a Legionnaires’ disease risk and the need of some strategies aimed to reduce it, such as the application of training programs for all the workers involved in water systems maintenance.

Required water temperature in hotel plumbing to control Legionella growth

Legionella spp. occurring in hotel hot water systems, in particular Legionella pneumophila, are causing serious pneumonic infections, and water temperature is a key factor to control their occurrence in plumbing systems. We performed a systematic review and meta-analyses of the available evidence on the association between water temperature and Legionella colonization to identify the water temperature in hotel hot water systems required for control of Legionella. Qualitative synthesis and quantitative analysis were performed on 13 studies that met our inclusion criteria to identify the effect of temperature. The Receiver Operating Characteristic (ROC) curve identified 55 °C as a cutoff point for hotel hot water temperature with an Area Under the Curve (AUC) value of 0.914. The odds ratios (OR) for detecting Legionella at temperatures >55 °C compared to lower temperatures from a meta-analysis of three studies was 0.17 [95% CI: 0.11, 0.25], which indicates a strong negative association between temperature and Legionella colonization. A logistic regression on results from multiple studies using both molecular and culture methods found a temperature of 59 °C associated with an 8% probability of detectable Legionella. Only two studies reported sufficiently detailed data to allow a model of concentration vs. temperature to be fit, and this model was not statistically significant. Additional research or more detailed reporting of existing datasets is required to assess if Legionella growth can be limited below particular concentration targets at different temperatures.

Considerations for large building water quality after extended stagnation

The unprecedented number of building closures related to the coronavirus disease (COVID-19) pandemic is concerning because water stagnation will occur in many buildings that do not have water management plans in place. Stagnant water can have chemical and microbiological contaminants that pose potential health risks to occupants. Health officials, building owners, utilities, and other entities are rapidly developing guidance to address this issue, but the scope, applicability, and details included in the guidance vary widely. To provide a primer of large building water system preventative and remedial strategies, peer-reviewed, government, industry, and nonprofit literature relevant to water stagnation and decontamination practices for plumbing was synthesized. Preventative practices to help avoid the need for recommissioning (e.g., routine flushing) and specific actions, challenges, and limitations associated with recommissioning were identified and characterized. Considerations for worker and occupant safety were also indicated. The intended audience of this work includes organizations developing guidance.

Evaluating a transfer gradient assumption in a fomite-mediated microbial transmission model using an experimental and Bayesian approach

Current microbial exposure models assume that microbial exchange follows a concentration gradient during hand-to-surface contacts. Our objectives were to evaluate this assumption using transfer efficiency experiments and to evaluate a model's ability to explain concentration changes using approximate Bayesian computation (ABC) on these experimental data. Experiments were conducted with two phages (MS2, ΦX174) simultaneously to study bidirectional transfer. Concentrations on the fingertip and surface were quantified before and after fingertip-to-surface contacts. Prior distributions for surface and fingertip swabbing efficiencies and transfer efficiency were used to estimate concentrations on the fingertip and surface post contact. To inform posterior distributions, Euclidean distances were calculated for predicted detectable concentrations (log₁₀ PFU cm⁻²) on the fingertip and surface post contact in comparison with experimental values. To demonstrate the usefulness of posterior distributions in calibrated model applications, posterior transfer efficiencies were used to estimate rotavirus infection risks for a fingertip-to-surface and subsequent fingertip-to-mouth contact. Experimental findings supported the transfer gradient assumption. Through ABC, the model explained concentration changes more consistently when concentrations on the fingertip and surface were similar. Future studies evaluating microbial transfer should consider accounting for differing fingertip-to-surface and surface-to-fingertip transfer efficiencies and extend this work for other microbial types.

Considerations for Large Building Water Quality after Extended Stagnation

The unprecedented number of building closures related to the coronavirus disease (COVID‐19) pandemic is concerning because water stagnation will occur in many buildings that do not have water management plans in place. Stagnant water can have chemical and microbiological contaminants that pose potential health risks to occupants. Health officials, building owners, utilities, and other entities are rapidly developing guidance to address this issue, but the scope, applicability, and details included in the guidance vary widely. To provide a primer of large building water system preventative and remedial strategies, peer‐reviewed, government, industry, and nonprofit literature relevant to water stagnation and decontamination practices for plumbing was synthesized. Preventative practices to help avoid the need for recommissioning (e.g., routine flushing) and specific actions, challenges, and limitations associated with recommissioning were identified and characterized. Considerations for worker and occupant safety were also indicated. The intended audience of this work includes organizations developing guidance.

Effect of disinfectant residuals on infection risks from Legionella pneumophila released by biofilms grown under simulated premise plumbing conditions

The ubiquitous presence of biofilms in premise plumbing and stagnation, which commonly occurs in premise plumbing, can exacerbate the decay of chlorine residual in drinking water. Using biofilms grown in a simulated premise plumbing setup fed directly with freshly treated water at two full-scale water treatment plants, we previously determined the mass transfer coefficients for chlorine decay in premise plumbing. These coefficients coupled with inactivation kinetics of L. pneumophila released from biofilms reported previously were integrated into a Monte Carlo framework to estimate the infection risk of biofilm-derived L. pneumophila from 1 to 48 h of stagnation. The annual infection risk was significantly higher when water stayed stagnant for up to 48 h in pipes covered internally with biofilms, compared to clean pipes without biofilms. The decay of residual chlorine due to biofilms during 48-hour stagnation led to up to 6 times increase in the annual infection risk compared to the case where biofilms was absent. Global sensitivity analysis revealed that the rate of L. pneumophila detachment from biofilms and the decay of chlorine residual during stagnation are the two most important factors influencing the infection risks. Stagnation caused by water use patterns and water-saving devices in the premise plumbing can lead to increased infection risk by biofilm-derived L. pneumophila. Overall, this study's findings suggested that biofilms could induce chlorine decay and consequently increase L. pneumophila infection risk. Thus, reducing stagnation, maintaining residual chlorine, and suppressing biofilm growth could contribute to better management of L. pneumophila infection risk.