The application of quantitative microbial risk assessments (QMRAs) to understand and mitigate risks associated with norovirus is increasingly common as there is a high frequency of outbreaks worldwide. A key component of QMRA is the dose-response analysis, which is the mathematical characterization of the association between dose and outcome. For Norovirus, multiple dose-response models are available that assume either a disaggregated or an aggregated intake dose. This work reviewed the dose-response models currently used in QMRA, and compared predicted risks from waterborne exposures (recreational and drinking) using all available dose-response models. The results found that the majority of published QMRAs of norovirus use the F hypergeometric dose-response model with α = 0.04, β = 0.055. This dose-response model predicted relatively high risk estimates compared to other dose-response models for doses in the range of 1-1,000 genomic equivalent copies. The difference in predicted risk among dose-response models was largest for small doses, which has implications for drinking water QMRAs where the concentration of norovirus is low. Based on the review, a set of best practices was proposed to encourage the careful consideration and reporting of important assumptions in the selection and use of dose-response models in QMRA of norovirus. Finally, in the absence of one best norovirus dose-response model, multiple models should be used to provide a range of predicted outcomes for probability of infection.

Thermophilic‐anaerobic digestion in a single‐stage, mixed, continuous‐flow reactor is not approved in the United States as a process capable of producing Class A biosolids for land application. This study was designed to evaluate the inactivation of pathogens and indicator organisms in such a reactor followed by batch treatment in a smaller reactor. The combined process was evaluated at 53°C with sludges from three different sources and at 51 and 55°C with sludge from one of the sources. Feed sludge to the continuous‐flow reactor was spiked with the pathogen surrogates Ascaris suum and vaccine‐strain poliovirus. Feed and effluent were analyzed for these organisms and for indigenous Salmonella spp., fecal coliforms, Clostridium perfringens spores, and somatic and male‐specific coliphages. No viable Ascaris eggs were observed in the effluent from the continuous reactor at 53 or 55°C, with greater than 2‐log removals across the digester in all cases. Approximately 2‐log removal was observed at 51°C, but all samples of effluent biosolids contained at least one viable Ascaris egg at 51°C. No viable poliovirus was found in the digester effluent at any of the operating conditions, and viable Salmonella spp. were measured in the digester effluent in only one sample throughout the study. The ability of the continuous reactor to remove fecal coliforms to below the Class A monitoring limit depended on the concentration in the feed sludge. There was no significant removal of Clostridium perfringens across the continuous reactor under any condition, and there also was limited removal of somatic coliphages. The removal of male‐specific coliphages across the continuous reactor appeared to be related to temperature. Overall, at least one of the Class A pathogen criteria or the fecal coliform limit was exceeded in at least one sample in the continuous‐reactor effluent at each temperature. Over the range of temperatures evaluated, the maximum time required to meet the Class A criteria by batch treatment of the continuous‐reactor effluent was 1 hour for Ascaris suum and Salmonella spp. and 2 hours for fecal coliforms.

This study assessed the risks posed by noroviruses (NoVs) in surface water used for drinking, domestic, and recreational purposes in South Africa (SA), using a quantitative microbial risk assessment (QMRA) methodology that took a probabilistic approach coupling an exposure assessment with four dose-response models to account for uncertainty. Water samples from three rivers were found to be contaminated with NoV GI (80-1,900 gc/L) and GII (420-9,760 gc/L) leading to risk estimates that were lower for GI than GII. The volume of water consumed and the probabilities of infection were lower for domestic (2.91 × 10 to 5.19 × 10) than drinking water exposures (1.04 × 10 to 7.24 × 10). The annual probabilities of illness varied depending on the type of recreational water exposure with boating (3.91 × 10 to 5.43 × 10) and swimming (6.20 × 10 to 6.42 × 10) being slightly greater than playing next to/in the river (5.30 × 10 to 5.48 × 10). The QMRA was sensitive to the choice of dose-response model. The risk of NoV infection or illness from contaminated surface water is extremely high in SA, especially for lower socioeconomic individuals, but is similar to reported risks from limited international studies.

Access to microbiologically safe water is not a reality for many people throughout Sub-Saharan Africa where there is widespread occurrence of viruses in water sources. Exposure to this water can lead to adverse health risks including diarrhoeal disease. To a limited extent in Sub-Saharan Africa, the quantification of the human health risk associated with exposure to virally contaminated water has been done through the use of quantitative microbial risk assessment (QMRA). To understand the scope of the information available on this region, two systematic reviews were done to collect previously published literature from Sub-Saharan Africa on (1) prevalence and quantification of viral contamination in water and (2) QMRAs assessing the risk from exposure to water contaminated by viruses. The results of the 2 reviews were then summarised including, for the QMRAs, exposure and dose-response assumptions, input parameters, and risk outcomes. The results found the prevalence of 10 viruses (1-100%) in drinking, ground, irrigation, surface, and waste waters from eight countries with South Africa having the most information on water contamination by viruses. Quantified viral concentration data was reported for ~50% of the papers, for 6 viruses (entero-, human adeno-, noro-, rota-, sapo-and Hepatitis A virus), and ranged from (10-4-10 11 viruses/liter). Additionally, 22 QMRAs were identified for 6 viruses (entero-, human adeno-, noro-, rota-, coxsackie B, and Hepatitis A virus) from 4 countries demonstrating that QMRA has not been used extensively in this region. The majority of these QMRAs concluded that the risk of infection, illness, or Disability Adjusted Life Year (DALY) was exceptionally high and in excess of acceptable risk limits indicating a public health concern. In conclusion, water is contaminated with viruses, risk from exposure to viruses in water was extremely high for these 4 Sub-Saharan Africa countries, and QMRA is not a widely adopted methodology. Finally, some QMRA limitations were observed such as the need for more viral concentration data, collection of site-or region-specific exposure data, application of commonly used doseresponse models, addressing susceptible populations such those with human immunodeficiency virus (HIV) infection in the risk characterisation, and access to free software.

Quantitative microbial risk assessments (QMRAs) of contaminated drinking water usually assume the daily intake volume is consumed once a day. However, individuals could consume water at multiple time points over 1 day, so the objective was to determine if the number of consumption events per day impacted the risk of infection from Campylobacter jejuni during short-term contamination events. A probabilistic hydraulic and risk model was used to evaluate the impact of multiple consumption events as compared to one consumption event on the health risk from the intake of contaminated tap water. The fraction of the population that experiences greater than 10(-4) risk of infection per event at the median dose was 6.8% (5th-95th percentile: 6.5-7.2%) for one consumption event per day, 18.2% (5th-95th: 17.6-18.7%) for three consumption events per day, and 19.8% (5th-95th: 14.0-24.4%) when the number of consumption events varied around 3.49 events/day. While the daily intake volume remained consistent across scenarios, the results suggest that multiple consumption events per day increases the probability of infection during short-term, high level contamination events due to the increased coincidence of a consumption event during the contamination peak. Therefore, it will be important to accurately characterize this parameter in drinking water QMRAs.