DETERMINATION OF GIARDIA CYST VIABILITY IN ENVIRONMENTAL AND FECAL SAMPLES BY IMMUNOFLUORESCENCE, FLUOROGENIC DYE STAINING AND DIFFERENTIALINTERFERENCE CONTRAST MICROSCOPY

Amongst the techniques suggested for the determination of Giardia cyst viability, the use of the fluorogenic dyes, fluorescein diacetate (FD A) and propidium iodide (PI) is the most often recommended, even thoug h it appears to overestimate the number of viable cysts. In the presen t study, the replacement of FDA with 4',6-diamidino-2-phenylindole (DA PI) allowed simultaneous direct immunofluorescence with monoclonal ant ibody labelled with fluorescein isothiocyanate (MAb-FITC). Under these conditions, it was possible both to quantify the cysts according to t he immunofluorescence technique, and to appreciate their viability by using fluorogenic dye staining (DAPI and PI) and differential interfer ence contrast (DIC) microscopy. This method proved to be significantly better than the counting methods normally suggested. The technique ha s been applied to Giardia cysts recovered from faeces and wastewater s ludge.