The levels of Giardia cysts and Cryptosporidium oocysts in 650 environmental water samples were investigated. Cysts and oocysts were found in all types of water tested. The presence of Giardia cysts exceeded Cryptosporidium oocysts both in the number per sample and the number of positive samples. Almost 50% of sewage samples studied contained Giardia cysts and 30% contained both Giardia cysts and Cryptosporidium oocysts. Treatment of sewage resulted in a reduction in the percentage of samples containing cysts and/or oocysts (30% of treated effluent samples were positive for Giardia and 25% had both cysts and oocysts). Higher numbers of Giardia cysts were found in surface water samples than in either sewage or treated effluents (55% of surface water samples were positive). However, the number of cysts isolated per surface water sample was lower on average. Most water purification plants showed effective removal of cysts and oocysts. However, 13% of potable water samples contained protozoan parasites, indicating occasional failure of the purification processes and the need for monitoring final treated water.

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