Evaluating Cryptosporidium and Giardia concentrations in combined sewer overflow.
Arnone, R.D. and Walling, J.P. (2006) Journal of Water & Health, 4(2); 157-65.

This study was conducted to expand the database on the expected occurrence and concentration of Cryptosporidium and Giardia in combined sewer overflows (CSOs). CSOs are discharges of mixed untreated sewage and stormwater released directly into receiving waters during rainfall.

CSOs from three outfall sites in two US states were sampled and analysed for Cryptosporidium and Giardia. All three sites discharge to receiving waters that are a source for potable water. A total of six samples were collected, from two overflow events at each site. Analysis required modifying Method 1623 by eliminating the filtration step and replacing it with centrifugation due to the probability of high particulate matter concentration found in CSO clogging the filter. Several physical and chemical properties, five bacterial indicators (total coliform, faecal coliform, E. coli, Enterococcus and faecal streptococcus) and endospore concentrations were also measured in the CSO samples.

The microbial, physical and chemical properties measured were found to be consistent with values for CSO samples reported in the literature. Two of the samples collected were from an Atlanta, Georgia outfall and four were from two Louisville, Kentucky outfalls. Cryptosporidium was found in 3 samples, one from the Louisville site and both samples from one Atlanta site. Analysis was performed in quadruplicate and at most 2 oocysts were found per volume examined. In contrast, the CSO samples did contain significant Giardia concentrations. Giardia was detected in 96% of the samples. Analysis was also performed in quadruplicate. The number of Giardia cysts found in the sample volume examined ranged from non-detectable to 186. Extrapolated results per 100L range from 200 to 30,000 cysts per 100l. As Cryptosporidium was rarely found a meaningful relationship could not be tested with bacterial indicators or endospore concentrations. Bacterial indicator and endospore concentrations were not found to be well correlated with Giardia. The best correlated indicators were found to be Enterococcus and faecal streptococcus. The Method 1623 met quality control requirements for most samples. These results suggest that CSO from urban areas is not a significant source of Cryptosporidium but is a significant source of Giardia.

Comment Clinical evidence indicates that Giardia infections are more common than Cryptosporidium infections in the general population. There is also evidence of a relatively high rate of asymptomatic carriage of Giardia compared to Cryptosporidium.