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 differences in the number of Giardia cysts measured in river water were due to the method itself, the analyst, or real differences in the distribution of these cysts in water. To minimize the methodological differences, centrifugation only was used as the primary concentration method. Differences were observed between results from different analysts 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 differences were observed among samples collected sequentially at the same time, as well as for samples collected on different days or at different times on the same day. The differences reported in the literature in the number of Giardia cysts detected in water samples from the same site could be an artifact of the methods more than true differences in the counts.