We have established a simple procedure for the enumeration of Giardia cysts in environmental waters of Hong Kong. The procedure involves concentrating water or sewage samples by the flocculation-percoll/sucrose gradient method followed by a modified tube method using immunofluorescent staining containing fluorescein isothiocyanate labelled anti-Giardia monoclonal antibodies. Enumeration of cysts was carried out using fluorescent microscopy. This procedure has the highest recovery rate (61%) compared with other conventional procedures tested (0-4%). This method was successfully applied to the detection and enumeration of Giardia cysts in water samples collected from two selected rivers of Hong Kong, Shing Mun River and Lam Tsuen River. Preliminary results indicate that water of both rivers has high counts of Giardia cysts.

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