Effect of sewage treatment plants and diffuse pollution on the occurrence of protozoal parasites in the course of a small river

In Germany, protozoal parasites are detected frequently in surface waters. This study aims to assess the parasitological pollution in a river course and two of its tributaries under regular weather conditions. Cryptosporidium was detected in 67% of all samples. The median concentration was 4 oocysts/100 l. Giardia lamblia were detected in 90% of the samples. The median concentration of G. lamblia was 22 cysts/100 l and increased significantly following the river course. A statistically significant correlation was found between both parasites and the faecal indicators Clostridium perfringens and Escherichia coli. The intermittent occurrence of Cryptosporidium in the streams investigated was ascribed primarily to diffuse pollution, particularly during rainfall events. The relatively frequent detection of G. lamblia derived mainly from sewage treatment plants. The pathogen concentrations observed present a relevant public health risk as the river is used for recreational activities and agricultural purposes.