Page 6: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Enteric Viruses
Part II. Science and Technical Considerations
5.0 Sources and exposure
5.1 Sources
The main source of human enteric viruses in water is human faecal matter. Enteric viruses are excreted in large numbers in the faeces of infected persons (both symptomatic and asymptomatic). They are easily disseminated in the environment through faeces and are transmissible to other individuals via the faecal-oral route. Infected individuals can excrete over 1 billion (109) viruses/g of faeces. Some enteric viruses can also be excreted in urine from infected individuals. The presence of these viruses in a human population is variable and reflects current epidemic and endemic conditions (Fields et al., 1996). Sewage plant effluents, sewage lagoon discharges, combined sewer overflows and septic tank leakage can be responsible for contamination of water sources. Enteric virus concentrations have been reported to peak in sewage samples during the autumn/winter, suggesting a possibly higher endemic rate of illness during this time of year or better survival of enteric viruses at cold temperatures. Animals can be a source of enteric viruses; however, the enteric viruses detected in animals generally do not cause illnesses in humans (Cox et al., 2005), although there are some exceptions. As mentioned above, one exception is HEV, which may have a non-human reservoir. To date, HEV has been an issue in developing countries, and therefore most of the information on HEV occurrence in water sources results from research in these countries. There is limited information on HEV presence in water and sewage in developed countries (Clemente-Casares et al., 2003; Kasorndorkbua et al., 2005). It is important to remember that person-to-person and foodborne spread are also important mechanisms for transmission of enteric viruses.
Most of the enteric viruses described above, including noroviruses, rotaviruses, HAV, HEV, enteroviruses, adenoviruses and astroviruses, have been detected in sewage, surface water sources, groundwater sources and drinking water sources around the world, including Canada (Subrahmanyan, 1977; Sattar, 1978; Sekla et al., 1980; Payment et al., 1984, 2000, 2001; Gerba et al., 1985; Raphael et al., 1985a,b; Payment, 1989, 1991, 1993; Bloch et al., 1990; Payment and Franco, 1993; Pina et al., 1998, 2001; AWWA, 1999a; Jothikumar et al., 2000; Scipioni et al., 2000; Van Heerden et al., 2005; Locas et al., 2007). These studies report varying prevalence and concentrations of enteric viruses; however, they cannot be readily compared, given the range of detection methods used (Payment and Pintar, 2006). In general, the level of infectious enteric virions in sewage ranges from 100 to 10 000 infectious units/L (Sano et al., 2004; Sedmak et al., 2005; Geldreich et al., 1990). In contaminated surface water, levels of 1-100 infectious enteric virions/L are common. In less polluted surface water, their numbers are closer to 1-10/100 L (Gerba et al., 1985; Bloch et al., 1990; AWWA, 1999a; Jothikumar et al., 2000; Scipioni et al., 2000; Pina et al., 2001; Dorner et al., 2007). Groundwater sources have been shown to have between 0 and 200 infectious enteric virions/100 L, depending on the level of contamination; however, most contaminated groundwater systems are thought to have very low levels (< 2/100 L) (U.S. EPA, 2006a). These concentrations were generally obtained through targeted studies, since water and wastewater sources are not routinely monitored for enteric viruses. It should also be noted that the concentration of enteric viruses in a source water can have significant temporal and spatial variability depending on whether the pollution source is continuous or the result of a sudden influx of faecal contamination.
Contamination of water sources can occur through various routes, including wastewater plant effluent, disposal of domestic wastewater or sludges on land, septic tank field effluents and infiltration of surface water into groundwater aquifers (Bitton, 1999; Hurst et al., 2001). Migration of enteric viruses into groundwater sources depends on the extent of virus retention in the surrounding soils and their survival rate. For example, research on retention of particles based on subsurface composition has shown that viruses tend to adsorb more strongly to clay than to silt and sand particulate (Goyal and Gerba, 1979). It is important to note that adsorption of viruses in the subsurface does not inactivate viruses, and adsorption is a reversible process, dependent on the ionic characteristics of the percolating water (Bales et al., 1993). Therefore, retained infectious viruses, if desorbed from soil, can potentially still contaminate water sources. Many factors, including rainfall, temperature, hydraulic stresses and soil-specific characteristics, such as pH and soil water content, along with virus-specific attributes, such as isoelectric point, virus size and virus load, can impact subsurface movement of viruses (Schijven and Hassanizadeh, 2000; U.S. EPA, 2003). Under certain conditions, viruses can migrate significant distances. Studies have reported viruses being detected in groundwater samples more than 100 m from known septic sources and in groundwaters from confined aquifers (Gerba and Bitton, 1984; Bales et al., 1993; Borchardt et al., 2007; Locas et al., 2007).
5.2 Survival
As mentioned previously, viruses cannot replicate outside their host's tissues and therefore cannot multiply in the environment; however, they can survive in the environment for extended periods of time. Early experiments investigating the survival of enteric viruses reported survival times ranging from 28 to 188 days (Rhodes et al., 1950; Wellings et al., 1975; Stramer and Cliver, 1984).
Survival depends on numerous factors, including, but not limited to, virus-specific characteristics, the presence of other microorganisms and the characteristics of the water, such as pH, temperature, turbidity and ultraviolet (UV) light levels. Some of these factors have been characterized. Temperature effects on survival rates have been defined for many enteric viruses (Yates et al., 1985; U.S. EPA, 2003). In general, as the temperature increases, the survival time decreases. Exposure to UV light also shortens the survival time of viruses. Other parameters, such as microbial activity, are less well characterized. It has been suggested that bacteria and protozoa can inactivate waterborne viruses, especially in surface waters. Inactivation may be the result of enzymatic activity destroying viral capsid proteins or predation (Herrmann and Cliver, 1973; Pinheiro et al., 2007). In either case, the amount of inactivation is dependent on the microbial ecology and is currently not well understood. Conversely, survival of viruses can be prolonged by factors such as the presence of sediments to which viruses can readily adsorb.
The survival rate varies between types of viruses. Enteric viruses of concern for waterborne transmission are generally non-enveloped viruses. They are generally more resistant to environmental degradation compared with enveloped viruses. Comparisons between enteric viruses also show variability, with adenoviruses potentially surviving longer in water than other enteric viruses, such as HAV and polioviruses (Enriquez et al., 1995).
Enteric virus survival rates also differ from survival rates for protozoa and bacteria. In the environment, enteric viruses have been reported to be more resistant to environmental degradation than bacteria and some protozoa (e.g., Giardia) (Johnson et al., 1997). Survival of viruses through drinking water treatment processes also differs from survival of bacteria and protozoa. For example, enteric viruses have been detected at low levels (i.e., 1-20/1000 L) in treated drinking water free of coliform bacteria in 100-mL samples (Payment, 1989; Gerba and Rose, 1990; Bitton, 1999; Payment et al., 2000; Ehlers et al., 2005). The survival of enteric viruses, and consequently presence in drinking water, can result from the absence of treatment (for many groundwater sources) or from insufficient treatment for the level of virus in the source water (Payment, 1989; Payment and Armon, 1989; Gerba and Rose, 1990; Payment et al., 1997; Bitton, 1999).
5.3 Exposure
Enteric viruses are transmitted via the faecal-oral route. Vehicles for transmission can include water, food (particularly shellfish and salads), aerosols, fomites (inanimate objects, such as door handles that, when contaminated with an infectious virion, facilitate transfer of the pathogen to a host) and person-to-person contact. Poor hygiene is also a contributing factor to the spread of enteric viruses. In addition, the high incidence of rotavirus infections, particularly in young children, has suggested to some investigators that rotavirus may also be spread by the respiratory route (Kapikian and Chanock, 1996; Chin, 2000). There is also some evidence that noroviruses can be spread by contact with vomitus (Marks et al., 2003). For many of the enteric viruses discussed above, outbreaks have occurred both by person-to-person transmission and by common sources, involving contaminated foods, contaminated drinking water supplies or recreational water.
5.4 Waterborne illness
Exposure to enteric viruses through water can result in both an endemic rate of illness in the population and waterborne outbreaks. Endemic rates of enteric illness are difficult to measure or estimate. In Canada, there are roughly 1.3 episodes of enteric illness per capita per year (Majowicz et al., 2004; PHAC, 2007). This estimate includes gastrointestinal illnesses caused by all types of enteric pathogens, not just enteric viruses, and includes all sources of transmission. The numerous routes of transmission and the highly infectious nature of enteric viruses make it difficult to determine what proportion of the endemic enteric illness is specifically related to drinking water sources.
Waterborne outbreaks caused by enteric viruses have been reported in Canada, and these viruses are a common cause of outbreaks worldwide. Some of the viral agents responsible for these outbreaks have only recently been identified (Craun, 1986, 1992; Fields et al., 1996; Payment and Hunter, 2001). The true prevalence of viral-related waterborne outbreaks in Canada and worldwide is unknown. In Canada, between 1974 and 2001, there were 24 reported outbreaks and 1382 confirmed cases of waterborne illness caused by enteric viruses (Schuster et al., 2005). Ten of these outbreaks were attributed to HAV, 12 were attributed to noroviruses and 2 to rotaviruses (O'Neil et al., 1985; Health and Welfare Canada, 1990; Health Canada, 1994; INSPQ, 1994, 1998, 2001; Boettger, 1995; Health Canada, 1996; Beller et al., 1997; Todd and Chatman, 1997; Todd and Chatman, 1998; De Serres et al., 1999; Todd et al., 2001; BC Provincial Health Officer, 2001). There were also 138 outbreaks of unknown aetiology, a portion of which could be the result of enteric viruses, and a single outbreak that involved multiple viral pathogens. Of the 10 reported outbreaks attributed to waterborne HAV, 4 were due to contamination of public drinking water supplies, 2 were the result of contamination of semi-public suppliesFootnote 1 and the remaining 4 were due to contamination of private water supplies. Only 4 of the reported 12 waterborne outbreaks of norovirus infections in Canada occurred in public water supplies, and the remainder were attributed to semi-public supplies. Both rotavirus outbreaks arose from contamination of semi-public drinking water supplies.
In the United States, between 1991 and 2002, 15 outbreaks and 3487 confirmed cases of waterborne viral illness were reported. Of these, 12 outbreaks and 3361 cases were attributed to noroviruses, 1 outbreak and 70 cases were attributed to "small round-structured virus" and 2 outbreaks and 56 cases were attributable to HAV (Craun et al., 2006). During this period, 77 outbreaks resulting in 16 036 cases of unknown aetiology were also reported. It is likely that enteric viruses were responsible for a significant portion of these outbreaks (Craun et al., 2006). Prior to 1991, outbreaks associated with rotavirus contamination had also been reported (Hopkins et al., 1984).
Waterborne outbreaks of noroviruses and HAV occur worldwide (Brugha et al., 1999; De Serres et al., 1999; Brown et al., 2001; Boccia et al., 2002; Anderson et al., 2003; Carrique-Mas et al., 2003). A study on waterborne outbreaks in Finland (1998-2003) found, for the samples analysed for viruses, that the most prominent virus was norovirus (Maunula et al., 2005). Groundwater sources are also frequently reported to be associated with outbreaks of noroviruses and HAV (Häfliger et al., 2000; Maurer and Stürchler, 2000; Parchionikar et al., 2003).
Major waterborne epidemics of HEV have occurred in developing countries (Guthmann et al., 2006), but none has been reported in Canada or the United States (Purcell, 1996; Chin, 2000). Astroviruses and adenoviruses have also been implicated in drinking water outbreaks, although they were not the main cause of the outbreaks (Kukkula et al., 1997; Divizia et al., 2004). However, astrovirus RNA in tap water was correlated with an increased risk of intestinal disease in a study in France (Gofti-Laroche et al., 2003). The development of new detection methods to determine the agent responsible in the numerous outbreaks of unidentified aetiology could potentially link these viruses to outbreaks (Martone et al., 1980; Turner et al., 1987; Hedberg and Osterholm, 1993; Gray et al., 1997; Kukkula et al., 1997, 1999; Lees, 2000).
5.5 Relationship to indicator organisms
Monitoring for enteric viruses still suffers from methodological and interpretation limitations inherent to pathogen detection (Medema et al., 2003; Payment and Pintar, 2006). These limitations include the necessity to concentrate large volumes of water, the need for specialized laboratory equipment and highly trained personnel and the cost of analysis, as well as determining which pathogens to test for, given the multitude of pathogens that may be present, which can vary over time and space. As such, routine monitoring for enteric viruses is currently not practical. Instead, indicator organisms that can be routinely monitored are used to indicate faecal contamination and the potential presence of enteric viruses. Commonly used indicators include bacteria, such as E. coli, enterococci and Clostridium perfringens spores, and viruses of bacteria (i.e., bacteriophages). Total coliforms can also be used, not as indicators of faecal pollution, but to provide general water quality information, especially in groundwater sources (Locas et al., 2007). Non-microbial indicators, such as faecal sterols, caffeine or chloride, have also been used in research studies to indicate faecal contamination (Borchardt et al., 2003; Peeler et al., 2006; Shah et al., 2007; Hussain et al., 2010), however further research on routinely using these indicators is still needed.
There are many published studies investigating the relationship between the various indicator organisms and the presence of enteric viruses in treated drinking water, surface water and groundwater. Determining a relationship between pathogens and faecal indicators has some inherent difficulties, foremost because of methodological differences related to the volumes of water analysed. For indicator organisms, usually 100-mL sample volumes are tested, whereas for pathogens, tens to hundreds of litres of water are concentrated, and then a portion of this volume is analysed. Even with these limitations, faecal indicators have been found to be useful for indicating the potential presence of enteric viruses in various water sources. The most appropriate indicator (or indicators) will depend on whether they are being used to provide information on virus presence in groundwater or surface water sources or as indicators of removal or inactivation of viruses by treatment processes and treated drinking water quality.
5.5.1 Treated drinking water
The indicator organisms routinely monitored in Canada as part of the multi-barrier "source-to-tap" approach for verifying drinking water quality are E. coli and total coliforms. The presence of E. coli in drinking water indicates recent faecal contamination and the potential presence of enteric pathogens, including enteric viruses. Total coliforms, however, are not faecal specific and therefore cannot be used to indicate faecal contamination (or the potential presence of enteric pathogens). Instead, total coliforms are used to indicate general water quality issues. Further information on the role of E. coli and total coliforms in drinking water quality management can be found in the Guideline Technical Documents on E. coli and total coliforms (Health Canada, 2006a,b). As mentioned previously, the survival of bacteria and viruses differs in the environment and through treatment processes. As a result, although the presence of E. coli indicates the potential presence of enteric viruses, the absence of E. coli does not necessarily indicate that all enteric viruses are also absent. However, if a multi-barrier, source-to-tap approach is in place and each barrier in the drinking water system has been controlled to ensure that it is operating adequately based on the quality of the source water, then E. coli and total coliforms can be used as part of the verification process to show that the water has been adequately treated and is therefore of an acceptable microbiological quality.
Other faecal indicators that may be used to verify the adequacy of treatment include enterococci, Clostridium perfringens spores and various bacteriophages (somatic coliphages, F-specific RNA coliphages and phages of Bacteroides). As enterococci and Clostridium perfringens spores are both bacterial indicators, they suffer from the same limitations as E. coli, in that their survival and response to treatment processes differ from those of enteric viruses. The concentration of these indicators in source waters can also be much lower than the concentrations of E. coli and total coliforms, making them less useful than E. coli and total coliforms for routinely verifying treatment processes. Bacteriophages, since they are viruses (of bacteria), have survival rates that are more similar to those of enteric viruses, and they are often used as surrogates for enteric viruses for determining treatment efficiencies. However, their concentrations in source waters are generally insufficient to make them useful for verifying treatment adequacy on a routine basis.
5.5.2 Surface water sources
Several studies have investigated the relationships between indicator organisms and the presence or absence of human enteric viruses in surface water sources. Escherichia coli and Clostridium perfringens have been found to be associated with the presence of enteric viruses in surface waters that are impacted by human faecal pollution (Payment and Franco, 1993; Payment et al., 2000; Ashbolt et al., 2001; Hörman et al., 2004). Bacteriophages have also been found to be associated with the presence of enteric viruses in some studies (Skraber et al., 2004; Ballester et al., 2005; Haramoto et al., 2005), but not in others (Hörman et al., 2004; Choi and Jiang, 2005). Both correlations to coliform bacteria (Haramoto et al., 2005) and lack of correlations to coliform bacteria (Skraber et al., 2004; Ballester et al., 2005; Choi and Jiang, 2005) have also been observed. Based on these studies, it is evident that no one faecal indicator can be used to indicate enteric virus presence in all surface water sources. The most suitable indicator or indicators will depend on the surface water source and its site-specific faecal pollution inputs. Although not used for routine monitoring, targeted studies can also be carried out to determine enteric virus concentrations directly, as opposed to using indicator organisms.
5.5.3 Groundwater sources
Microbial indicators normally used to indicate faecal contamination of a water source, such as E. coli, do not necessarily migrate through the subsurface or have a survival rate in the environment comparable with that of enteric viruses. Several recent studies have investigated the usefulness of E. coli and total coliforms for indicating enteric virus contamination of groundwater sources.
The presence of any of the indicator organisms in a groundwater source is a good indication that the source may be at risk of faecal contamination and potentially adversely affect human health. In a study by Craun et al. (1997) on groundwater consumption, it was found that the presence of coliforms correlated very well with the presence of viral gastroenteritis. However, the absence of indicators may not necessarily indicate the absence of enteric viruses. A study of private wells in the United States found that 8% of the wells tested by PCR were positive for one or more enteric viruses, however, none of the contaminated wells contained indicators of faecal contamination (i.e., E. coli, enterococci, coliphages) and only 25% of the virus impacted wells were positive for total coliforms (Borchardt et al., 2003). Several other studies conducted in the United States have also reported that they found no link between the detection of an indicator organism and the detection of enteric viruses in a groundwater sample (Abbaszadegan et al., 1998, 2003; Borchardt et al., 2004) with approximately 15 % of samples testing positive for enteric viruses in the absence of indicators (Abbaszadegan et al., 2003). However, some studies observed that upon repeat sampling, if a site tested positive for pathogens it usually tested positive, at some point in time, for one of the microbial indicators (Lieberman et al., 2002; Abbaszadegan et al., 2003). A study on Canadian groundwater quality that monitored 23 municipal wells with a history of acceptable bacteriological quality found that wells that underwent repeat monitoring (122 samples collected from 16 wells) tested positive in a small number of samples (7/122 samples). Indicator organisms were detected in 4 of the 16 wells, while enteric viruses were detected in only 1 of the 16 wells. However, the positive site for enteric viruses was not one of the sites positive for indicator organisms (Locas et al., 2008). An additional study investigating several groundwater aquifers in various countries determined that using a combination of a bacterial indicator and a bacteriophage was more useful for assessing groundwater contamination than using only bacterial indicators (Lucena et al., 2006). Based on these studies, ongoing routine monitoring with bacterial indicators in combination with bacteriophages (in some instances), and using data collected from sanitary surveys and vulnerability assessments, can be used to provide a useful assessment of enteric virus presence in groundwater sources.