SURVEY OF POTABLE WATER-SUPPLIES FOR CRYPTOSPORIDIUM AND GIARDIA

The comparative occurrence of Cryptosporidium and Giardia was evaluated in 257 water samples from 17 states in the United States. Cryptosporidium oocysts were detected in 55% of the surface water samples at an average concentration of 43 oocysts/ 100 L, while Giardia cysts were found in 16% of the same samples at an average concentration of 3 cysts/100 L. Giardia and Cryptosporidium were more frequently detected in samples from waters receiving sewage and agricultural discharges as opposed to pristine waters. There was no correlation between the concentration of water quality indicator bacteria and either protozoa. Both protozoa were more frequently isolated in the fall than other seasons of the year. The concentrations of both organisms were significantly correlated in all waters. Cryptosporidium oocysts were detected in 17% of 36 drinking water samples (0.5-1.7 oocysts/100 L) while no Giardia cysts were detected. The widespread occurrence of cysts and oocysts in waters used as supplies of potable water suggests that there is a risk of waterborne transmission of Cryptosporidium and Giardia infections if the water is not adequately treated.