Giardia and Cryptosporidium spp. in filtered drinking water supplies

Abstract

Giardia and Cryptosporidium levels were determined by using a combined immunofluorescence test for ifitered drinking water samples collected from 66 surface water treatment plants in 14 states and 1 Canadian province. Giardia cysts were detected in 17% of the 83 filtered water effluents. Cryptosporidium oocysts, were observed in 27% of the drinking water samples. Overall, cysts or oocysts were found in 39% of the treated effluent samples. Despite the frequent detection of parasites in drinking water, microscopic observations of the cysts and oocysts suggested that most of the organisms were nonviable. Compliance with the filtration criteria outlined by the Surface Water Treatment Rule of the U.S. Environmental Protection Agency did not ensure that treated water was free of cysts and oocysts. The average plant effluent turbidity for sites which were parasite positive was 0.19 nephelometric turbidity units. Of sites that were positive for Giardia or Cryptosporidium spp., 78% would have been able to meet the turbidity regulations of the Surface Water Temperature Rule. Evaluation of the data by using a risk assessment model developed for Giardia spp. showed that 24% of the utilities examined would not meet a 1/10,000 annual risk of Giardia infection. For cold water conditions (0.5°C), 46% of the plants would not achieve the 1/10,000 risk level.

Developments in microbiological risk assessment for drinking water

“…Current routine methods for the enumeration of Cryptosporidium in water are not only inef®cient but also do not provide an indication of which species has been recovered nor of the viability or infectivity of the oocysts. Not surprisingly, most 100±1000 l samples record zero pathogens under non-outbreak conditions (Payment et al 1985;LeChevallier et al 1991). Indeed, LeChevallier et al (1991) reported 73% of 100-l volumes from ®ltered drinking water supplies to contain zero oocysts of Cryptosporidium.…”

“…It is apparent that oocyst densities in drinking water supplies during non-outbreak conditions show large variation. While most samples contain zero oocysts, a few contain high counts (LeChevallier et al 1991). This re¯ects both spatial/temporal variations within different parts of the same supply and systematic differences between different supplies (e.g.…”

“…It could be argued that oocysts are too dilute during nonoutbreak conditions for this to happen. However, oocyst counts of 48 and 286 in 100 l have been reported (LeChevallier et al 1991;Craun et al 1998). Furthermore, other samples with still higher oocyst counts could have been missed (Christian and Pipes 1983) and would only have been detected by more intensive sampling.…”

Detection of Infectious Cryptosporidium in Filtered Drinking Water

“…During this study, it was not feasible to concurrently measure source water Cryptosporidium levels for all participating water treatment plants, but previous studies have documented surface water oocyst levels (LeChevallier & Norton, 1995;LeChevallier et al, 1991). Although these results from the studies cited were generated using older techniques that do not match method 1622 results (e.g., they should not be used to establish bin levels required by the LT2ESWTR), the Cryptosporidium levels can be used to rank the relative level of source water pollution.…”

“…On the basis of the published human dose-response data (Messner et al, 2001), it is possible to use the results of the current study to determine the risk of Cryptosporidium infection from conventionally treated drinking water. To calculate the risk, it was necessary to assume that each positive sample was identified as such because of the detection of a single oocyst, because previous IFA studies typically detected a single oocyst in filtered drinking water (LeChevallier et al, 1995;LeChevallier et al, 1991).…”