The effect of
chlorine concentration on Giardia 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 employed included temperature (25, 15, and 5 degrees C), pH (6, 7, and 8),
chlorine-
cyst contact time (10, 30, and 60 min), and
chlorine concentration (1 to 8 mg/liter). In the pH range studied,
cyst survival generally was observed to increase as
buffer pH increased. Water temperature coupled with chlorination proved to be important in
cyst survival. Results of these experiments at the three temperatures studied can be summarized as follows: at 25 degrees C, exposure to 1.5 mg/liter for 10 min killed all
cysts at pH 6, 7, and 8. At 15 degrees C, 2.5 mg of
chlorine per liter 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 degrees C, 1 mg of
chlorine per liter for 60 min failed to kill all the
cysts at any pH tested. At this temperature, 2 mg of
chlorine per liter killed all
cysts after 60 min at pH 6 and 7, but not at pH 8. A
chlorine concentration of 4 mg/liter killed all the
cysts at all three pH values after 60 min, but not after 30 min. A
chlorine concentration of 8 mg/liter 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
chlorine demand in the
halogen treatment of
drinking water to destroy
cysts. It also raises an epidemiological problem, namely: low water temperatures, where killing of Giardia requires relatively high
chlorine concentrations and long contact times, are (i) to be expected in many areas where epidemic waterborne
giardiasis has been reported and (ii) particularly conducive to the long-term survival of Giardia
cysts.