Giardiasis, caused by the sporozoan parasite

Giardia lamblia, is worldwide the most frequent

parasitic gastro-enteritis of man. In Britain, no

outbreak of waterborne giardiasis had b&en reported

until 1985, when more than 108 persons were

affected. Although no cause was ever found the

incident led to a recommendation that accepted

practices for disinfection of new and repaired

water mains by chlorination should be reviewed.

It was the aim of this project to produce optimal

methods for recovering G lamblia cysts from faeces,

for estimating viability of these cysts and for

determining the effects of free available chlorine

concentration, water temperature pH value and

exposure duration in destroying viability.

The efficiency of continuous and discontinuous

sucrose gradients, for purification of g lamblia

cysts, was evaluated in terms of cyst recovery

efficiency and cleanness of cysts. The inclusion

of 1% Tween 20 into the central layer of

discontinuous gradients produced good recoveries

and clean cysts, and an excystation percentage of

31.2% was obtained. Fluorogenic dye inclusion and

exclusion tests using fluorescein diacetate and

propidium iodide failed to correlate with in vitro -- excystment as an indicator of cyst viability. The

cysticidal effects of free chlorine were both

temperature and pH dependent. A free chlorine

level of 3 mg/l killed all cysts after a contact

time of 10 minutes at 20 OC, whereas at 5 OC a free

chlorine level of 10 mg/l was necessary.