California Sea Grant College Program

Toxoplasma gondii, a ubiquitous parasitic protozoan, is emerging as an aquatic biological pollutant. Infections can result from ingesting environmentally resistant oocysts in drinking water. However, recommendations regarding water treatment for oocyst inactivation have not been established. As part of this doctoral thesis, the efficacies of two commonly used chemicals, sodium hypochlorite and ozone, as well as physical methods of ultraviolet (UV) radiation and radiofrequency (RF) power were evaluated for their ability to inactivate T. gondii oocysts in water. With chemical treatments, oocysts were exposed to either 100 mg/L of chlorine for 30 min, 2 hr, 4 hr, 8 hr, 16 hr, and 24 hr or 6 mg/L of ozone for 1, 2, 4, 8, and 12 min. Oocysts were exposed to pulsed and continuous UV exposure doses ranging from 20 to 1000 ml/cm2 and to various RF energy levels to induce 50, 55, 60, 70 and 80°C temperatures. Post-treatment, oocyst viability was determined by mouse bioassay with serology, immunohistochemistry, and in vitro parasite isolation as the test methods to confirm T. gondii infection in mice. Initially, mouse bioassay experiments were conducted to compare the analytical sensitivity of these 3 detection methods prior to completing the chemical inactivation experiments. Toxoplasma gondii infection was confirmed by at least 1 of the 3 detection methods in mice inoculated with all doses (105 -10°) of oocysts. Results of the chemical exposure experiments indicated that neither sodium hypochlorite nor ozone effectively inactivated T. gondii oocysts, even when used at high concentrations. Both pulsed and continuous UV irradiation results indicated that some T. gondii oocysts were inactivated when exposed to UV radiation, but not with 100% efficiency over the entire range of UV doses evaluated. Oocysts were inactivated with RF-induced thermal levels of > 60°C in an initial experiment, but not in subsequent experiments conducted under similar conditions. These results indicated that UV treatment may be useful to sanitize drinking water and sewage effluent but the impact of factors such as water heating time, cooling time, and the volume of water treated must be considered when evaluating any method for oocyst inactivation.