Report No DWI0739

Aug 1988


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 been 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 20ºC, whereas at 5ºC a free chlorine level of 10 mg/l was necessary.

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