Page 4: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document - Enteric Protozoa: Giardia and Cryptosporidium

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Partie II. Science et considérations techniques

5.0 Sources and exposure

5.1 Giardia

5.1.1 Sources

Human and other animal faeces, especially cattle faeces, are major sources of Giardia. Giardiasis has been shown to be endemic in humans and in over 40 other species of animals, with prevalence rates ranging from 1% to 100% (Olson et al., 2004; Pond et al., 2004; Thompson, 2004; Thompson and Monis, 2004). Table 3 summarizes the prevalence of Giardia among humans and selected livestock animals and highlights the relatively high levels of giardiasis in cattle. Giardia cysts are excreted in large numbers in the faeces of infected humans and other animals (both symptomatic and asymptomatic). Infected cattle, for example, have been shown to excrete up to one million (10⁶) cysts per gram of faeces (O'Handley et al., 1999; Ralston et al., 2003; O'Handley and Olson, 2006). Cysts are easily disseminated in the environment and are transmissible via the faecal-oral route. Beaver, dog, muskrat and horse faeces are also sources of Giardia, including human-source G.lamblia (Davies and Hibler, 1979; Hewlett et al., 1982; Erlandsen and Bemrick, 1988; Erlandsen et al., 1988; Traub et al., 2004, 2005; Eligio-García et al., 2005). Giardia can also be found in bear, bird, cat and other animal faeces, but it is unclear whether these strains are pathogenic to humans (refer to Section 5.1.3).

Giardia cysts are commonly found in sewage and surface waters and occasionally in drinking water. In a cross-Canada survey of 72 municipalities performed between 1991 and 1995, Wallis et al. (1996) found that 72.6%, 21% and 18.2% of raw sewage, raw water and treated water samples, respectively, contained Giardia cysts. Table 4 highlights a selection of studies that have investigated the occurrence of Giardia in surface waters in Canada. Typically, Giardia concentrations in surface waters ranged from 2 to 200 cysts/100 L. Concentrations as high as 8700 cysts/100 L were reported and were associated with record spring runoff, highlighting the importance of event-based sampling (see Section 7.0; Gammie et al., 2000). The typical range reported above is at the lower end of that described in an international review (Dechesne and Soyeux, 2007). Dechesne and Soyeux (2007) found that Giardia concentrations in source waters across North America and Europe ranged from 0.02 to 100 cysts/L, with the highest levels reported in the Netherlands. Source water quality monitoring data were also gathered for nine European (France, Germany, the Netherlands, Sweden and the United Kingdom) water sources and one Australian source. Overall, Giardia was frequently detected at relatively low concentrations, and levels ranged from 0.01 to 40 cysts/L. An earlier survey by Medema et al. (2003) revealed that concentrations of cysts in raw and treated domestic wastewater (i.e., secondary effluent) typically ranged from 5000 to 50 000 cysts/L and from 50 to 500 cysts/L, respectively.

Treated water in Canada is rarely tested for the presence of Giardia. When testing has been conducted, cysts are typically not present or are present in very low numbers (Payment and Franco, 1993; Ong et al., 1996; Wallis et al., 1996, 1998; EPCOR, 2005; Douglas, 2009), with some exceptions. In 1997, a heavy spring runoff event in Edmonton, Alberta, resulted in the presence of 34 cysts/1000 L in treated water (Gammie et al., 2000). Cysts have also been detected in treated water derived from unfiltered surface water supplies (Payment and Franco, 1993; Wallis et al., 1996).

5.1.2 Survival

Giardia cysts can survive in the environment for extended periods of time. Survival in water can range from weeks to months (or possibly longer), depending on a number of factors, including the characteristics specific to the strain and of the water, such as temperature. The effect of temperature on survival rates of Giardia has been well studied. In general, as the temperature increases, the survival time decreases. For example, Bingham et al. (1979) observed that Giardia cysts can survive up to 77 days in tap water at 8°C, compared with 4 days at 37°C. DeRegnier et al. (1989) reported a similar effect in river and lake water. This temperature effect is, in part, responsible for peak Giardia prevalences reported in winter months (Isaac-Renton et al., 1996; Ong et al., 1996). Exposure to ultraviolet (UV) light will also shorten the survival time of Giardia. A detailed discussion of the effects of UV light on Giardia is provided in Section 7.1.4.

It is commonly assumed that the viability of Giardia cysts found in water is high, but monitoring experience suggests otherwise. Cysts found in surface waters are often dead, as shown by propidium iodide (PI) dye exclusion (Wallis et al., 1995). Observations by LeChevallier et al. (1991b) also suggest that most of the cysts present in water are non-viable; 40 of 46 cysts isolated from drinking water exhibited "non-viable-type" morphologies (i.e., distorted or shrunken cytoplasm). Studies have frequently revealed the presence of empty cysts ("ghosts"), particularly in sewage.

5.1.3 Exposure

Person-to-person transmission is by far the most common route of transmission of Giardia (Pond et al., 2004; Thompson, 2004). Persons become infected via the faecal-oral route, either directly (i.e., contact with faeces from a contaminated person, such as children in daycare facilities) or indirectly (i.e., ingestion of contaminated drinking water, recreational water and, to a lesser extent, food). Animals may also play an important role in the (zoonotic) transmission of Giardia, although it is not clear to what extent. Cattle have been found to harbour human-infective (assemblage A) Giardia, as have dogs and cats. Assemblage A Giardia genotypes have also been detected in wildlife, including beavers and deer.

Although there is some evidence to support the zoonotic transmission of Giardia, most of this evidence is circumstantial or compromised by inadequate controls. Thus, it is not clear how frequently zoonotic transmission occurs or under what circumstances. Overall, these data suggest that, in most cases, animals are not the original source of human-infective Giardia, but may amplify zoonotic genotypes present in other sources (e.g., contaminated water). In cattle, for example, the livestock Giardia genotype (assemblage E) predominates; however, cattle are susceptible to infection with human-infective (zoonotic) genotypes of Giardia. It is likely that cattle acquire zoonotic genotypes of Giardia from their handlers and/or from contaminated water sources. Given that calves infected with Giardia commonly shed between 10⁵ and 10⁶ cysts per gram of faeces, they could play an important role in the transmission of Giardia.

The role that wildlife plays in the zoonotic transmission of Giardia is also unclear. Although wildlife, including beavers, can become infected with human-source G. lamblia (Davies and Hibler, 1979; Hewlett et al., 1982; Erlandsen and Bemrick, 1988; Erlandsen et al., 1988; Traub et al., 2004, 2005; Eligio-García et al., 2005) and have been associated with waterborne outbreaks of giardiasis (Kirner et al., 1978; Lopez et al., 1980; Lippy, 1981; Isaac-Renton et al., 1993), the epidemiological and molecular data do not support zoonotic transmission via wildlife as a significant risk for human infections (Hoque et al., 2003; Stuart et al., 2003; Berrilli et al., 2004; Thompson, 2004; Hunter and Thompson, 2005; Ryan et al., 2005a). The data do, however, suggest that wildlife acquire human-infective genotypes of Giardia from sources contaminated by human sewage. As population pressures increase and as more human-related activity occurs in watersheds, the potential for faecal contamination of source waters becomes greater, and the possibility of contamination with human sewage must always be considered. Erlandsen and Bemrick (1988) concluded that Giardia cysts in water may be derived from multiple sources and that epidemiological studies that focus on beavers may be missing important sources of cyst contamination. Some waterborne outbreaks have been traced back to human sewage contamination (Wallis et al., 1998). Ongerth et al. (1995) showed that there is a statistically significant relationship between increased human use of water for domestic and recreational purposes and the prevalence of Giardia in animals and surface water. It is known that beaver and muskrat can be infected with human-source Giardia (Erlandsen et al., 1988), and these animals are frequently exposed to raw or partially treated sewage in Canada. The application of genotyping procedures has provided further proof of this linkage. Thus, it is likely that wildlife and other animals can act as a reservoir of human-infective Giardia from sewage-contaminated water and, in turn, amplify concentrations of Giardia cysts in water. If infected animals live upstream and/or in close proximity to drinking water treatment plant intakes, then they could play an important role in the waterborne transmission of Giardia. Thus, watershed management, to control both sewage inputs and the populations of aquatic mammals in the vicinity of water intakes, is important to disease prevention.

As is the case for livestock and wildlife animals, it is unclear what role domestic animals play in the zoonotic transmission of Giardia. Although dogs and cats are susceptible to infection with zoonotic genotypes of Giardia, few studies have provided direct evidence of transmission between them and humans (Eligio-García et al., 2005; Shukla et al., 2006; Thompson et al., 2008).

5.1.4 Waterborne illness

Giardia is the most commonly reported intestinal protozoan in North America and worldwide (Farthing, 1989; Adam, 1991). The World Health Organization (WHO, 1996) estimates its worldwide incidence at 200 million cases per year. In Canada, just over 4000 confirmed cases of giardiasis were reported in 2004. This represents a significant decline from the 9543 cases that were reported in 1989. Incidence rates have similarly declined over this period (from 34.98 to 13.08 cases per 100 000 persons) (PHAC, 2007).

Giardia is a common cause of waterborne infectious disease outbreaks in Canada and elsewhere (Hrudey and Hrudey, 2004). Between 1974 and 2001, Giardia was the most commonly reported causative agent associated with infectious disease outbreaks related to drinking water in Canada (Schuster et al., 2005). Giardia was linked to 51 of the 138 outbreaks for which causative agents were identified. The majority (38/51; 75%) of these Giardia outbreaks were associated with public drinking water systems; a selection of these outbreaks can be found in Appendix E. Contamination of source waters from human sewage and inadequate treatment (e.g., poor or no filtration, relying solely on chlorination) appear to have been major contributing factors (Schuster et al., 2005). Most of these outbreaks could have been prevented through the adoption and implementation of adequate source water protection strategies (e.g., wastewater management) and appropriate treatment based on source water characterization. No outbreaks have been reported since 2001. This is in large part due to the lessons that were learned by all Canadian jurisdictions following the Walkerton and North Battleford contamination events and recommendations from their subsequent inquiries. Comprehensive approaches, including source water protection strategies, were adopted by provinces and territories based on the source-to-tap approach developed collaboratively by the Canadian Council of Ministers of the Environment and the Federal-Provincial-Territorial Committee on Drinking Water (CCME, 2004).

In the United States, outbreaks have been reported in 48 states (Craun, 1979; Lin, 1985; Moore et al., 1993; Jakubowski, 1994; CDC, 2004; Craun et al., 2010). Giardia was the most frequently identified etiological agent associated with waterborne outbreaks in the United States between 1971 and 2006, accounting for 16% of outbreaks (Craun et al., 2010). In a worldwide review of waterborne protozoan outbreaks, G. lamblia accounted for 40.6% of the 325 outbreaks reported between 1954 and 2002 (Karanis et al., 2007). The largest reported Giardia drinking water-related outbreak occurred in 2004, in Norway (Robertson et al., 2006).

5.2 Cryptosporidium

5.2.1 Sources

Humans and other animals, especially cattle, are important reservoirs for Cryptosporidium. Human cryptosporidiosis has been reported in more than 90 countries over six continents (Fayer et al., 2000; Dillingham et al., 2002). Reported prevalence rates of human cryptosporidiosis range from 1% to 20% (Table 5), with higher rates reported in developing countries (Caprioli et al., 1989; Zu et al., 1992; Mølbak et al., 1993; Nimri and Batchoun, 1994; Dillingham et al., 2002; Cacciò and Pozio, 2006). Livestock, especially cattle, are a significant source of C. parvum (Table 5). In a survey of Canadian farm animals, Cryptosporidium was detected in faecal samples from cattle (20%), sheep (24%), hogs (11%) and horses (17%) (Olson et al., 1997). Oocysts were more prevalent in calves than in adult animals; conversely, they were more prevalent in mature pigs and horses than in young animals. Infected calves can excrete up to 10⁷ oocysts per gram of faeces (Smith and Rose, 1990) and represent an important source of Cryptosporidium in surface waters (refer to Section 5.2.2). Wild ungulates (hoofed animals) and rodents are not a significant source of human-infectious Cryptosporidium (Roach et al., 1993; Ong et al., 1996).

Oocysts are easily disseminated in the environment and are transmissible via the faecal-oral route. Person-to-person transmission is one of the most common routes of transmission of Cryptosporidium. Contaminated drinking water, recreational water and food are also important mechanisms for transmission of Cryptosporidium. Contact with animals, especially livestock, also appears to be a major pathway for transmission. A more detailed discussion of zoonotic transmission is provided in Section 5.2.3.

Cryptosporidium oocysts are commonly found in sewage and surface waters and occasionally in treated water. In a cross-Canada survey of 72 municipalities performed between 1991 and 1995, Wallis et al. (1996) found that 6.1%, 4.5% and 3.5% of raw sewage, raw water and treated water samples, respectively, contained Cryptosporidium oocysts. Table 6 highlights a selection of studies that have investigated the occurrence of Cryptosporidium in surface waters in Canada. Typically, Cryptosporidium concentrations in surface waters ranged from 1 to 100 oocysts/100 L. Concentrations as high as 10 300 cysts/100 L were reported and were associated with a record spring runoff, highlighting the importance of event-based sampling (see Section 7.0) (Gammie et al., 2000).

An international review of source water quality data demonstrated that concentrations of Cryptosporidium in source waters across North America and Europe vary greatly (Dechesne and Soyeux, 2007). Cryptosporidium concentrations ranged from 0.006 to 250 oocysts/L. As part of this initiative, source water quality monitoring data were gathered for nine European (France, Germany, the Netherlands, Sweden and the United Kingdom) water sources and one Australian source. Overall, Cryptosporidium was frequently detected at relatively low concentrations, and levels ranged from 0.05 to 4.6 oocysts/L. In an earlier survey, Medema et al. (2003) reported concentrations of oocysts in raw and treated domestic wastewater (i.e., secondary effluent) ranging from 1000 to 10 000 oocysts/L and from 10 to 1000 oocysts/L, respectively.

Little is known about the occurrence of Cryptosporidium in groundwaters in Canada. Studies in the Unites States and elsewhere have reported the presence of oocysts in groundwaters, although at low frequencies, and at low concentrations (Hancock et al., 1998; Moulton-Hancock et al., 2000; Gaut et al., 2008).

The presence of Cryptosporidium in treated water in Canada is rarely assessed. When testing has been conducted, oocysts are typically not present or are present in very low numbers (Payment and Franco, 1993; Ong et al., 1996; Wallis et al., 1996; EPCOR, 2005; Douglas, 2009), with some exceptions (Gammie et al., 2000). Oocysts have been detected in treated water derived from unfiltered surface water supplies (Wallis et al., 1996) and after extreme contamination events. For example, in 1997, a heavy spring runoff event in Edmonton, Alberta, resulted in the presence of 80 oocysts/1000 L in treated water (Gammie et al., 2000).

5.2.2 Survival

Cryptosporidium oocysts have been shown to survive in cold waters (4°C) in the laboratory for up to 18 months (AWWA, 1988). Robertson et al. (1992) reported that C. parvum oocysts could withstand a variety of environmental stresses, including freezing (viability greatly reduced) and exposure to seawater. In general, oocyst survival time decreases as temperature increases (Pokorny et al., 2002; Li et al., 2010).

Although it is commonly assumed that the majority of oocysts in water are viable, monitoring experience suggests otherwise. Smith et al. (1993) found that oocyst viability in surface waters is often very low. A more recent study by LeChevallier et al. (2003) reported that 37% of oocysts detected in natural waters were infectious. It should, however, be emphasized that although low concentrations of viable oocysts are commonly found in raw water, they may not represent an immediate public health risk; rather, it is the sudden and rapid influx of large numbers of oocysts into source waters that is likely to overwhelm drinking water treatment barriers and be responsible for the increased risk of infection associated with transmission through drinking water. Environmental events such as flooding or high precipitation can lead to a rapid rise in oocyst concentration within a defined area of a watershed.

Low oocyst viability has also been reported in filtered water. A survey by LeChevallier et al. (1991b) found that, in filtered waters, 21 of 23 oocysts had "non-viable-type" morphology (i.e., absence of sporozoites and distorted or shrunken cytoplasm).

5.2.3 Exposure

Direct contact with livestock and indirect contact through faecally contaminated waters are major pathways for transmission of Cryptosporidium (Fayer et al., 2000; Robertson et al., 2002; Stantic-Pavlinic et al., 2003; Roy et al., 2004; Hunter and Thompson, 2005). Cattle are a significant source of C. parvum in surface waters. For example, a weekly examination of creek samples upstream and downstream of a cattle ranch in the B.C. interior during a 10-month period revealed that the downstream location had significantly higher levels of Cryptosporidium oocysts (geometric mean 13.3 oocysts/100 L, range 1.4-300 oocysts/100 L) compared with the upstream location (geometric mean 5.6/100 L, range 0.5-34.4 oocysts/100 L) (Ong et al., 1996). A pronounced spike was observed in downstream samples following calving in late February. During a confirmed waterborne outbreak of cryptosporidiosis in British Columbia, oocysts were detected in 70% of the cattle faecal specimens collected in the watershed close to the reservoir intake (Ong et al., 1997).

Waterfowl can also act as a source of Cryptosporidium. Graczyk et al. (1998) demonstrated that Cryptosporidium oocysts retain infectivity in mice following passage through ducks. However, histological examination of the avian respiratory and digestive systems at 7 days post-inoculation revealed that the protozoa were unable to infect birds. In an earlier study (Graczyk et al., 1996), the authors found that faeces from migratory Canada geese collected from seven of nine sites on Chesapeake Bay contained Cryptosporidium oocysts. Oocysts from three of the sites were infectious to mice. Based on these and other studies (Graczyk et al., 2008; Quah et al., 2011), it appears that waterfowl can pick up infectious Cryptosporidium oocysts from their habitat and can carry and deposit them in the environment, including drinking water supplies.

5.2.4 Waterborne illness

Cryptosporidium is one of the most commonly reported enteric protozoans in North America and worldwide. In Canada, over 550 confirmed cases of cryptosporidiosis were reported in 2004; a similar number of cases (i.e., 623 cases) was reported in 2000. Incidence rates increased over this period from 1.85 (2000) to 2.67 (2004) cases per 100 000 persons (PHAC, 2007).

Cryptosporidium parvum and C. hominis are the major species associated with human cryptosporidiosis, although C. hominis appears to be more prevalent in North and South America, Australia and Africa, whereas C. parvum is responsible for more infections in Europe (McLauchlin et al., 2000; Guyot et al., 2001; Lowery et al., 2001b; Yagita et al., 2001; Ryan et al., 2003; Learmonth et al., 2004).

Waterborne outbreaks of cryptosporidiosis have been reported in many countries, including Canada (Fayer, 2004; Joachim, 2004; Smith et al., 2006). Between 1974 and 2001, Cryptosporidium was the third most reported causative agent associated with infectious disease outbreaks related to drinking water in Canada, representing 12 of the 138 outbreaks for which causative agents were identified (Schuster et al., 2005). The majority (11/12; 92%) of these Cryptosporidium outbreaks were associated with public drinking water systems; a selection of these outbreaks can be found in Appendix E (Table E.1). Contamination of source waters from human sewage and inadequate treatment (e.g., having poor or no filtration, relying solely on chlorination) appear to be major contributing factors (Schuster et al., 2005). Most of these outbreaks could have been prevented through the adoption and implementation of adequate source water protection strategies (e.g., wastewater management) and appropriate treatment based on source water characterization. No outbreaks have been reported since 2001. This is in large part due to the lessons that were learned by all Canadian jurisdictions following the Walkerton and North Battleford contamination events and their subsequent inquiries. Comprehensive approaches, including source water protection strategies, were adopted by provinces and territories based on the source-to-tap approach developed collaboratively by the Canadian Council of Ministers of the Environment and the Federal-Provincial-Territorial Committee on Drinking Water (CCME, 2004).

In the United States between 1984 and 2000, 10 outbreaks were associated with the presence of Cryptosporidium in drinking water; 421 000 cases of illness were reported, most of which (403 000) were associated with the Milwaukee outbreak in 1993 (U.S. EPA, 2006a). Between 2001 and 2002, the U.S. Centers for Disease Control and Prevention reported 17 waterborne disease outbreaks associated with drinking water; only one of these outbreaks was linked to Cryptosporidium (CDC, 2004). Cryptosporidium was the second most frequently identified infectious agent associated with waterborne outbreaks in the United States between 1991 and 2002, accounting for 7% of outbreaks (Craun et al., 2006). Nineteen outbreaks were reported in the United Kingdom (Craun et al., 1998). In a worldwide review of waterborne protozoan outbreaks, Cryptosporidium accounted for 50.6% of the 325 outbreaks reported between 1954 and 2002 (Karanis et al., 2007). Attack rates were typically high, ranging from 26% to 40%, and many thousands of people were affected. In addition, there have been several outbreaks associated with swimming pools, wave pools and lakes.

5.3 Relationship to indicator organisms

The indicator organisms routinely monitored in Canada as part of the multi-barrier, source-to-tap approach for assessing water quality are E. coli and total coliforms. The presence of E. coli in water indicates faecal contamination and thus the strong potential for a health risk, regardless of whether specific pathogens such as enteric protozoa are observed. However, its absence does not necessarily indicate that enteric protozoa are also absent. Total coliforms 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 water quality management can be found in the guideline technical documents on E. coli and total coliforms (Health Canada, 2006a,b).

5.3.1 Treated drinking water

Compared with protozoans, E. coli and members of the coliform group do not survive as long in the environment (Edberg et al., 2000) and are more susceptible to many of the disinfectants commonly used in the drinking water industry. As a result, although the presence of E. coli indicates recent faecal contamination and thus the potential for pathogens such as enteric protozoa to also be present, the absence of E. coli does not necessarily indicate that enteric protozoa are also absent. As evidence of this, Giardia and Cryptosporidium (oo)cysts have been detected in filtered, treated drinking water meeting existing regulatory standards and have been linked to waterborne disease outbreaks (LeChevallier et al., 1991b; Craun et al., 1997; Marshall et al., 1997; Rose et al., 1997; Nwachuku et al., 2002; Aboytes et al., 2004).

Thus, to control risks from enteric protozoa, a multi-barrier, source-to-tap approach is needed. When each treatment 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 an important part of the verification process.  These bacteriological indicators, when used in conjunction with information on treatment  performance (e.g., filter performance, appropriate concentration × time [CT] values [see Section 7.1.3.2] for inactivation of Giardia, UV fluence), are a confirmation that the water has been adequately treated and is therefore of an acceptable microbiological quality.

5.3.2 Surface water sources

Several studies have investigated the relationship between indicator organisms and the presence or absence of enteric protozoa in surface water sources. In general, studies have reported little (Medema et al, 1997; Atherholt et al., 1998; Payment et al., 2000) or no (Rose at al., 1988, 1991; Chauret et al., 1995; Stevens et al., 2001; Hörman et al., 2004; Dorner et al., 2007; Sunderland et al., 2007) correlation between protozoa and faecal indicators, including E. coli. In the cases where a correlation has been reported, it is with Giardia and at very high indicator levels. A review of 40 years of published data on indicator-pathogen correlations found that neither Cryptosporidium (odds ratio 0.41, 95% confidence interval 0.25-0.69) nor Giardia  (odds ratio 0.65, 95% confidence interval 0.36-1.15) is likely to be correlated with faecal indicator organisms (Wu et al., 2011). This overall lack of correlation is likely due to a variety of factors, including differential survival rates in the environment, sampling location, and methodological differences related to the analysis of water (Payment and Pintar, 2006). Watershed characteristics, including sources and levels of faecal contamination, and geochemical factors, may also influence the correlation between faecal indicators and protozoa, leading to site-specific differences (Chauret et al., 1995).

These observations have raised significant questions regarding the appropriateness of using E. coli as an indicator of protozoan contamination in surface waters, and highlighted the need for targeted protozoa monitoring of surface waters to gain a better understanding of public health risk.

5.3.3 Groundwater sources

Only a few studies have reported the presence of enteric protozoa, specifically Cryptosporidium, in groundwater (see Section 5.2.1). As such, the usefulness of E. coli as an indicator of enteric protozoa contamination of groundwater sources has not been assessed.

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