Page 14: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Enteric Viruses
Appendix C: QMRA model
Mathematical models have been developed as a means to quantitatively assess the potential microbiological risks associated with a drinking water system, including the potential risks associated with bacterial, protozoan, and viral pathogens. These models have been developed by international organizations (Smeets et al., 2008; Teunis et al., 2009), as well as by groups within Canada (Jaidi et al., 2009). QMRA models have also been used to estimate the potential health risks through other routes of exposure (Mara et al., 2007; Armstrong and Haas, 2008; Diallo et al., 2008). Although some of the assumptions vary between models (i.e., the choice of reference pathogen, or selection of dose-response variables), all are based on the accepted principles of QMRA, that is: hazard identification; exposure assessment; dose-response assessment; and, risk characterization.
A QMRA model was developed by Health Canada as part of the risk assessment process for enteric pathogens in drinking water. This probabilistic model explores the potential disease burden, with associated uncertainty, for user-defined scenarios for a drinking water system. The model includes user inputs for the virological quality of the raw water source and the specific treatment train (defined in terms of filtration, and disinfection approaches). For drinking water systems where data is lacking for the above parameters, the model includes values from published literature and from expert opinion as a starting point for the assessment. For source water quality, the model provides users with the choice of four categories: category A (0.1 rotaviruses/100 L), category B (1 rotavirus/100 L), category C (10 rotaviruses/100 L), and category D (100 rotaviruses/100 L). Water sources that are category A or B for enteric viruses are assumed to have very little human faecal contamination. These may be groundwater sources or protected surface water sources were there is very limited human activity on the watershed. As the level of human impacts increase in a watershed, the source water quality may fall into category C or D. The enteric virus concentrations are expressed in number of rotaviruses per volume of water because the QMRA model uses the key characteristics of rotavirus, with the CT values based on HAV and poliovirus, for the risk calculations. These source water quality estimates were developed only be used within the context of the QMRA model for evaluating the impacts of variations in source water quality on the overall microbiological risks. It should be noted that although a source may be category A for enteric viruses, it may have a different source water quality category for bacterial or protozoan pathogens. For treatment processes, the model uses a range of literature values to more accurately represent the range of effectiveness of treatment methodologies.
The QMRA model uses this exposure information, along with the dose-response model and the DALY calculations for rotavirus, to estimate the potential disease burden (in DALYs/person per year) for the site-specific scenario information. The quality of the outputs from the QMRA model are dependent on the quality of the information that is input into the model. Measurements, as opposed to estimates for exposure levels, will result in a higher quality risk assessment output. Even with high quality exposure data, the QMRA model requires numerous assumptions that introduce uncertainty into the assessment:
- It is assumed that the distribution of viruses in water is random (Poisson). However, it is likely that the viruses are not randomly distributed but rather occur in clusters, either loosely associated with each other or tightly bound to or within particles (Gale, 1996). Such clustering means that most consumers will not be exposed, but a small portion will be exposed to one or more viruses. This model does not account for clustering and will therefore underestimate the probability of exposure and infection.
- Treatment efficiency is modelled based on data in published literature for various treatment processes, which may be an under- or overestimate of the performance at a specific site. Also, treatment efficiency data are derived using laboratory-adapted strains of viruses, which may not respond to treatment processes in exactly the same manner as the viruses present in source water.
- Until methods to identify infectious human enteric virus particles become practical for routine monitoring, it is desirable, from a health protection perspective, to assume that all human enteric virus particles detected in source waters are infectious to humans unless evidence to the contrary exists. As well, the dose-response experiments used single laboratory strains; therefore, it is unknown whether organism age, prior environmental exposures or strain type influences infectivity. Furthermore, the experiments are conducted on healthy humans or laboratory animals and do not consider the immune status or secondary transmission in the exposed population (e.g., vulnerable populations). The current risk assessment model assumes that errors caused by overestimating viability are at least partially counterbalanced by virus recoveries, which have been shown to be reasonably efficient (approximately 50%) during the detection procedure.
- The model assumes a daily consumption of unboiled tap water of 1.0 L/person. In a population, there will be a distribution of tap water consumption that is not represented by this point estimate.
- The model is based on risks associated with source water contamination and does not consider potential contamination within the distribution system.
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