Page 3: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Enteric Viruses
Part I. Overview and Application
2.0 Executive summary
Viruses are extremely small microorganisms that are incapable of replicating outside a host cell. In general, viruses are host specific, which means that viruses that infect animals or plants do not usually infect humans, although a small number of enteric viruses have been detected in both humans and animals. Most viruses also infect only certain types of cells within a host; consequently, the health effects associated with a viral infection vary widely. Viruses that can multiply in the gastrointestinal tract of humans or animals are known as "enteric viruses." There are more than 140 enteric viruses known to infect humans.
Although there are methods capable of detecting and measuring viruses in drinking water, they are not practical for routine monitoring in drinking water because of methodological and interpretation limitations. The microbiological quality of drinking water continues to be verified by relying on the monitoring of indicators, such as Escherichia coli. The presence of E. coli indicates faecal contamination and the potential presence of enteric viruses. However, the absence of E. coli does not necessarily mean that enteric viruses are also absent. Therefore, E. coli monitoring needs to be used as part of a "source-to-tap" multi-barrier approach to protect the quality of drinking water.
Health Canada recently completed its review of the health risks associated with enteric viruses in drinking water. This Guideline Technical Document reviews and assesses identified health risks associated with enteric viruses in drinking water. It evaluates new studies and approaches and takes into consideration the methodological and interpretation limitations in available methods for the detection of viruses in drinking water. From this review, the guideline for enteric viruses in drinking water is established as a treatment goal of a minimum 4-log reduction of enteric viruses.
2.1 Health effects
The human illnesses associated with enteric viruses are diverse. The main health effect associated with enteric viruses is gastrointestinal illness. The incubation time and severity of health effects are dependent on the specific virus responsible for the infection. In addition to gastroenteritis, enteric viruses can cause serious acute illnesses, such as meningitis, poliomyelitus and non-specific febrile illnesses. They have also been implicated in chronic diseases, such as diabetes mellitus and chronic fatigue syndrome.
The seriousness of the health effects from a viral infection will depend on the specific virus, as well as the characteristics of the individual affected (e.g., age, health status). In theory, a single infectious virus particle can cause infection; however, it usually takes more than a single particle. For many enteric viruses, the number of infectious virus particles needed to cause an infection is presumed to be low.
2.2 Exposure
Enteric viruses cannot multiply in the environment, but they can survive longer in water than most intestinal bacteria and are more infectious than most other microorganisms. Enteric viruses are excreted in the faeces of infected individuals, and some enteric viruses can also be excreted in urine.
Enteric viruses have been detected in surface water and groundwater sources. Recent scientific data have also shown the presence of enteric viruses in groundwater that had been considered less vulnerable to faecal contamination.
2.3 Treatment
The multi-barrier approach is the best approach to reduce enteric viruses and other waterborne pathogens in drinking water. For these types of contaminants, the focus should be on characterizing source water risks and ensuring that effective treatment barriers are in place to achieve safe drinking water. Generally, minimum treatment of supplies derived from surface water sources or groundwater under the direct influence of surface waters should include adequate filtration (or technologies providing an equivalent log reduction credit) and disinfection. Recent published information has shown the presence of enteric viruses in some groundwater sources that were considered to be less vulnerable to faecal contamination (i.e., those not under the direct influence of surface waters). As a result, it is recommended to ensure adequate treatment of all groundwaters to remove/inactivate enteric viruses, unless exempted by the responsible authority.
Once the source water quality has been characterized, pathogen removal/inactivation targets and effective treatment barriers can be established in order to achieve safe levels in the finished drinking water. The removal of enteric viruses from raw water is complicated by their small size and relative ease of passage through filtration barriers. However, viruses are effectively inactivated through the application of various disinfection technologies, individually or in combination, at relatively low dosages. In drinking water supplies with a distribution system, a disinfectant residual should be maintained at all times.
2.4 Quantitative microbial risk assessment
Quantitative microbial risk assessment (QMRA) can be used as part of a multi-barrier approach to help provide a better understanding of risk related to a water system. QMRA uses source water quality data, treatment barrier information and pathogen-specific characteristics to estimate the burden of disease associated with exposure to pathogenic microorganisms in a drinking water source. Through this assessment, variations in source water quality and treatment performance can be evaluated for their contribution to the overall risk. Such analysis can be used to assess the adequacy of existing control measures or the requirement for additional treatment barriers or optimization and help establish limits for critical control points.
Specific enteric viruses whose characteristics make them a good representative of all similar pathogenic viruses are considered in QMRA; from these, a reference virus is selected. Ideally, a reference virus will represent a worst-case combination of high occurrence, high concentration and long survival time in source water, low removal and/or inactivation during treatment, and a high pathogenicity for all age groups. If the reference virus is controlled, it is assumed that all other similar viruses of concern are also controlled. Numerous enteric viruses have been considered. As no single virus has all of the characteristics of an ideal reference virus, this risk assessment incorporates key characteristics of rotavirus together with other enteric viruses to better represent all enteric viruses of concern.
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