Occurrence of Cryptosporidium Oocysts and Giardia Cysts in Sewage in Norway

Samples of sewage influent from 40 sewage treatment works (STW) throughout Norway were examined forCryptosporidium oocysts andGiardia duodenalis cysts. Both parasites were detected frequently (80% of STW wereCryptosporidium positive; 93% of STW wereGiardia positive) and at maximum concentrations of >20,000 parasites/liter. The data suggest giardiasis is more widespread, and/or occurs with greater infection intensity, than cryptosporidiosis in Norway. STW serving higher person equivalents were more likely to be positive and had higher parasite concentrations. Parasite concentrations were used to estimate the proportion of contributing populations that could be clinically infected. ForCryptosporidium , the highest estimates were up to 5 per 100,000 individuals for two populations in eastern Norway. ForGiardia , the highest estimate was 40 infected per 100,000 persons (approximately five times the usual national annual average) contributing to an STW in western Norway. As this population experienced a large waterborne giardiasis outbreak 6 months after sampling, it can be speculated that regular challenge withGiardia may occur here. MostGiardia isolates in sewage influent were assemblage A, although some assemblage B isolates were detected. There was substantial heterogeneity, but most samples contained isolates similar to genotype A3. Removal efficiencies at two STW with secondary treatment processes were estimated to be approximately 50% forCryptosporidium and >80% forGiardia . An STW with minimal treatment had negligible removal of both parasites. Many STW in Norway have minimal treatment and discharge effluent into rivers and lakes, thus, risk of contamination of water courses byCryptosporidium andGiardia is considerable.