2005 Volume 3 Issue 1 Pages 33-40
Three turbid substances were added to suspensions of Cryptosporidium parvum oocysts, and changes in the degree of ultraviolet (UV) inactivation of the C. parvum oocysts were evaluated by in vitro excystation and animal infectivity. Because the dose distribution of UV irradiation delivered to oocysts in turbid water may vary according to the degree of movement of the water, experiments were performed under vigorously mixed (completely mixed) conditions and stationary conditions. The estimated kinetic inactivation rate in the completely mixed system of turbid water (190 ntu) was 4.058 cm2/mJ, which was only 15% less than that in pure water (4.690 cm2/mJ). The estimated relative infectivity in the stationary system differed markedly from that obtained by regression of the plot of UV irradiation dose versus relative infectivity in pure water. Nevertheless, values of the relative infectivity obtained in animal experiments were nearly in agreement with the calculated values. In other words, despite decreases in the dose of UV delivered to oocysts under the influence of turbid substances, the efficacy of UV in inactivation of oocysts in turbid water was almost equal to that in pure water when expressed as per unit dose. Consequently, it became evident consequently that if the dose of UV delivered to oocysts was known, the degree of expected inactivation could be estimated. In addition, we added substances that adhere to the surface of C. parvum oocysts (humin and kaoline) to suspensions of C. parvum oocysts and studied the degree of UV inactivation. Adhesion of humin or kaoline particles to the oocyst wall resulted in a reduction in UV inactivation. However, the magnitude of the log10 inactivation in a suspension containing humin or kaoline particles was only about 20% and 10%, respectively, less than that in pure water.
