Quantitative microbial risk assessment to estimate the health risk from exposure to noroviruses in polluted surface water in South Africa.

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 × 10-8 to 5.19 × 10-1) than drinking water exposures (1.04 × 10-5 to 7.24 × 10-1). The annual probabilities of illness varied depending on the type of recreational water exposure with boating (3.91 × 10-6 to 5.43 × 10-1) and swimming (6.20 × 10-6 to 6.42 × 10-1) being slightly greater than playing next to/in the river (5.30 × 10-7 to 5.48 × 10-1). 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.

Comparison of Risk Predicted by Multiple Norovirus Dose-Response Models and Implications for Quantitative Microbial Risk Assessment.

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 1 F1 hypergeometric dose-response model with α = 0.04, ß = 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.

Inactivation of Escherichia coli O157:H7 during thermophilic anaerobic digestion of manure from dairy cattle.

Inactivation of the pathogenic Escherichia coli serotype O157:H7 and a non-pathogenic E. coli strain isolated from dairy cattle manure was evaluated with batch tests at 50 and 55 degrees C in biosolids from a thermophilic anaerobic digester treating the manure. Using differential-selective plating on sorbitol-MacConkey (SMAC) agar to quantify E. coli, the decline in concentrations of both the sorbitol-negative (putative E. coli O157:H7) and sorbitol-positive (putative non-pathogenic E. coli) organisms followed a model that assumed there was a heat-sensitive fraction and a heat-resistant fraction. Inactivation rates of the heat-sensitive fractions were similar for both colony types at each temperature, suggesting that wild-type E. coli can be used as an indicator of inactivation of serotype O157:H7. The decimal reduction time for the heat-sensitive fractions was in the order of 10min at 55 degrees C and ranged from approximately 1-3h at 50 degrees C. Concentrations of heat-resistant organisms at 55 degrees C were 1.4-1.7log(10)cfu/mL. Confirmatory analyses conducted on 30 randomly selected colonies of heat-resistant sorbitol-negative cells from treatment at 55 degrees C indicated that none were serotype O157:H7, nor were they E. coli. Similar analyses on 10 sorbitol-negative isolates from untreated manure indicated that none were serotype O157:H7, although 16S rRNA gene sequence analysis indicated that eight were E. coli or closely related enteric bacteria. These findings suggest that plating on differential-selective media to quantify E. coli, including serotype O157:H7, in effluent samples from thermophilic anaerobic digestion can lead to false positive results. Therefore, more specific methods should be used to evaluate the extent of thermal inactivation of both pathogenic and non-pathogenic E. coli in manure treatment systems.

Laboratory evaluation of thermophilic-anaerobic digestion to produce Class A biosolids. 2. Inactivation of pathogens and indicator organisms in a continuous-flow reactor followed by batch treatment.

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 degrees C with sludges from three different sources and at 51 and 55 degrees 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 degrees C, with greater than 2-log removals across the digester in all cases. Approximately 2-log removal was observed at 51 degrees C, but all samples of effluent biosolids contained at least one viable Ascaris egg at 51 degrees 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.