The viability of 4 human isolates of Giardia intestinalis
cysts using either the fluorogenic vital
dyes fluorescein diacetate (FDA) and
propidium iodide (PI) or in vitro excystation was assessed. Whereas viable
cysts, as defined by in vitro excystation were present in each of the 4 isolates,
cysts from only 3 of the 4 isolates took up the vital
dyes. FDA consistently over-estimated
cyst viability whilst PI under-estimated non-viable
cysts when compared with in vitro excystation. Following in vitro excystation, both FDA and PI stained a proportion of unexcysted
cysts indicating that FDA stained
cysts which were incapable of excystation, whereas PI did not
stain all
cysts which were incapable of excystation. One human
cyst isolate, which underwent in vitro excystation, could not be stained with either FDA or PI. In the absence of currently more specific fluorescent indicators of viability, PI alone could be used to determine the lower limit of nonviability in positive water-related samples, where small numbers of
cysts are to be expected.