A novel method for detection of viable Giardia cysts in water samples

Assessment of Giardia viability is a major requirement for public health purveyors and the water industry. Several indicators of viability such as stains, excystation and animal infectivity have been used to enumerate cysts with varying degrees of success. A combined detection-viability method for use in water samples would be useful for detecting and determining the viability of cysts in raw and drinking waters and the efficacy of disinfection at treatment plants. Distilled water samples were seeded with purified Giardia cysts and incubated with fluorescein diacetate (FDA) initially to stain viable cysts followed by tetramethyl red labelled anti-Giardia monoclonal antibodies (TMR) for confirmation of identity. As a result of FDA staining, green fluorescence of intact viable cysts was observed microscopically using a 450–490 nm exciter filter while nonviable cysts were not stained. Giardia cysts reacted positively with TMR and glowed red using a triple band microscope filter with excitations of 400/450/570 nm. At this wavelength a combination of FDA and TMR stained viable cysts green internally with a red wall while nonviable cysts only stained red. This simple, reliable and quick method allowed differentiation of Giardia cysts in water samples while simultaneously determining their viability.