Sources of variation in isolation rate of Giardia lamblia cysts and their homogeneous distribution in river water entering a water treatment plant.

The objective of this work was to determine if differencesin the number of Giardiacysts measured in river water were due to the method itself, the analyst, or real differencesin the distribution of these cysts in water. To minimize the methodological differences, centrifugation only was used as the primary concentration method. Differenceswere observed between results from differentanalysts and they were identified as technical errors. Once the method had been well established, cysts were found to be distributed homogeneously in the river water tested. Small differenceswere observed among samples collected sequentially at the same time, as well as for samples collected on differentdays or at differenttimes on the same day. The differencesreported in the literature in the number of Giardiacysts detected in water samples from the same site could be an artifact of the methods more than true differencesin the counts.Key words: water, Giardia, parasite, methods.