Effect of Chlorine on Giardia lamblia Cyst Viability

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Abstract
The effect of Cl concentration on G. lamblia cyst viability was tested under a variety of conditions. The ability of Giardia cysts to undergo excystation was used as the criterion of viability. The experimental variables included temperature (25, 15 and 5.degree. C), pH (6, 7 and 8), Cl-cyst contact time (10, 30 and 60 min) and Cl concentration (1-8 mg/l). In the pH range studied, cyst survival generally increased as buffer pH increased. Water temperature coupled with chlorination proved to be important in cyst survival. At 25.degree. C, exposure to 1.5 mg/l for 10 min killed all cysts at pH 6, 7 and 8. At 15.degree. C, 2.5 mg of Cl/l for 10 min killed all cysts at pH 6, but at pH 7 and 8, small numbers of cysts remained viable after 30 min but not after 60 min. At 5.degree. C, 1 mg of Cl/l for 60 min failed to kill all the cysts at any pH tested. At this temperature, 2 mg of Cl/l killed all cysts after 60 min at pH 6 and 7, but not at pH 8. A Cl concentration of 4 mg/l killed all the cysts at all 3 pH values after 60 min, but not after 30 min. A cl concentration of 8 mg/l killed all Giardia cysts at pH 6 and 7 after contact for 10 min, and at pH 8 after 30 min. This study points up the role of temperature, pH and Cl demand in the halogen treatment of drinking water to destroy cysts. It also raises an epidemiological problem, i.e., low water temperatures, where killing of Giardia requires relatively high Cl concentrations and long contact times, are to be expected in many areas where epidemic waterborne giardiasis [human] has been reported because such conditions are particularly conducive to the long-term survival of Giardia cysts.