Giardia is the most commonly reported intestinal parasite in North America and in the world (Farthing 1989; Adam 1991). Giardiasis has been shown to be endemic in man and in over 40 species of animals with prevalence rates ranging from 1 to 90+%. The prevalence rate among humans in Canada is typically 5-10% but accurate rates are difficult to estimate because of the large number of asymptomatic cases and inadequacy of reporting. Over 9,000 confirmed cases of giardiasis are reported each year to the Laboratory Centre for Disease Control in Ottawa. Gyorkos (1983;1987) reported 4.11% of 112,714 or 4110 per 100,000 stool samples to be positive for Giardia in 1984. The number of cases actually notified in 1984 was actually only 27.8 per 100,000 suggesting that the real incidence of giardiasis is grossly under reported.
The beaver has often been implicated in waterborne outbreaks of giardiasis (Lopez et al. 1980; Kirner et al. 1978; Lippy 1981). It is now clear, however, that other mammals can also be responsible for the introduction of cysts to surface water that is used for drinking including dogs, muskrat, and cattle. As population pressures increase and more human related activity occurs in catchment areas the potential for faecal contamination 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.
Despite the obvious potential for zoonotic transmission in Canada, most waterborne outbreaks have been traced back to human sewage contamination (eg. Temagami, ON) although aquatic mammals have also been implicated as a source (eg. Creston BC, Corner Brook NF) by circumstantial evidence. It is known that beaver and muskrat can be infected with human source Giardia (Erlandsen et al. 1988) and these animals are certainly exposed to raw or partially treated sewage frequently in Canada. If aquatic mammals are contaminating lakes and rivers with human infective Giardia it is very likely that they acquired the infection themselves from sewage contaminated water. Beaver and muskrat undoubtably play an important role in amplifying the contamination of water with Giardia cysts when infected animals live in close proximity to water intakes but human sewage is most commonly the source (Erlandsen 1994). Watershed management to control both sewage inputs and the populations of aquatic mammals in the vicinity of water intakes is just as important to disease prevention as adequate water treatment. It should also be remembered that giardiasis can be transmitted person-to-person via poor hygiene, food and sexual transmission.
Giardia cysts are commonly found in sewage and raw surface waters. Jakubowski et al. (1991) found that the concentration of cysts in raw sewage from 11 US cities ranged from 683 to 3750 cysts/L. Wallis et al. (1995) found that 56.2% of 162 raw sewage samples contained Giardia cysts ranging from 1 to 88,000/L and 10% of 1215 raw and treated drinking water samples contained 0.001 to 2 cysts/L. Additional data from Quebec collected by the Ministry of the Environment and Wildlife showed that 45% of polluted and 34% of pristine water sources were contaminated with Giardia cysts (total number of samples was 71), most of which were from rivers (Barthe and Brassard, unpub. data). Hibler (1988) reported that 34% of 301 U.S. municipal sites were found to contain Giardia cysts in either raw or treated water from 1979 to 1986 and that there was a trend to more positive samples in the winter months. Isaac-Renton et al. (1987; 1993; 1994) has reported Giardia cyst contamination from a number of sites in British Columbia, some of which experienced waterborne outbreaks of giardiasis. Similar results have been reported from the Yukon (Roach et al. 1993) and the Maritimes (Wallis et al. 1995).
Very few data are available from Ontario but an outbreak of giardiasis occurred at Temagami in the spring of 1994 that was characterized by an attack rate of 30% and cyst concentrations up to 2/L in treated water (Wallis et al. 1995). LeChevallier et al. (1991) found Giardia cysts in 81% of 83 raw water samples at concentrations ranging from 0.05 to 242 cysts/L in the northeastern United States. One Canadian sample from Alberta was included in the survey and a concentration of 4.94 cysts/L was reported. LeChevallier et al. (1991) also found significant correlations between cyst concentration and turbidity and total and faecal coliform densities although data from previous studies do not support these associations, possibly because of early reports of waterborne giardiasis from places like Colorado which normally experience very low turbidities in raw water (Karlin and Hopkins 1983). LeChevallier et al. (1991) concluded that cyst contamination could be modeled in terms of watershed characteristics and that water reuse and sewage contamination were important factors in predicting cyst concentrations.
The viability of Giardia cysts found in water is commonly assumed to be high but monitoring experience suggests otherwise. Cysts found in surface waters are often dead as shown by propidium iodide dye exclusion (approximately 50% viability was observed using this technique during the Temagami outbreak) and water and sewage isolates only infected 9.4% of gerbils inoculated in the Canadian survey reported by Wallis et al. (1995). It is possible that not all of the Giardia isolates tested were actually infective to gerbils but it is common to observe cysts that are non-refractile under phase microscopy and have obviously damaged cyst walls. The immunofluorescent technique commonly used for detection is very sensitive and frequently reveals the presence of empty cysts ("ghosts"), particularly in sewage.
Confirmed outbreaks of waterborne giardiasis in Canada have occurred in British Columbia, Alberta, Ontario, Quebec, New Brunswick and Newfoundland. Waterborne outbreaks of giardiasis have been suspected but not proven in a number of other communities. Many of these communities were relying on surface water with only chlorination for water treatment but others had filtration plants that were not functioning properly. In the United States, outbreaks have been reported from 24 states (W. Jakubowski, pers. comm. fax May 20, 1994), especially Colorado and New England. During the period 1965 from to 1992, 115 outbreaks were reported that resulted in 26,530 known cases of giardiasis in the United States (Moore et al. 1993). A number of these outbreaks occurred in Colorado including epidemics at Vail, Estes Park, Lookout Mt., Boulder and Aspen. Karlin and Hopkins (1983) reported that consumption of surface water, a lack of complete conventional treatment, and improper operation or malfunction of equipment were common causes. These authors concluded that strict adherence to the multiple barrier concept of water treatment is essential. Craun (1979), in an earlier study, identified reliance on surface water, minimal treatment (usually only chlorination) and inadequate treatment facilities as common causes of waterborne giardiasis. Small water treatment systems that used otherwise good quality surface water of low turbidity seemed to be most commonly affected. A useful review of some of the best known American outbreaks has been compiled by Lin (1985). These include the very large outbreaks at Rome, New York (over 5000 cases were reported) and at Berlin, New Hampshire (7000 cases out of a total population of 15,000). Lin concluded that these and other outbreaks had been caused by lack of filtration, improper filter operations, inadequate chlorination, cross connections to sewers, and drinking contaminated surface waters.
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