Effects of the Norwegian winter environment on Giardia cysts and Cryptosporidium oocysts

The structural integrity of Cryptosporidium oocysts and Giardia cysts in the Norwegian winter environment was investigated. During winter 2001/2002, Cryptosporidium oocysts and Giardia cysts were placed in the upper layers of soil in different matrices contained in chambers and exposed to the Norwegian climate. Morphological characteristics and inclusion/exclusion of vital dyes were monitored and compared to refrigerated controls. Reduction in parasite numbers was recorded for all parasites, geographical locations, and matrices. Shear forces generated during freeze-thaw cycles are postulated to have disintegrated the parasites exposed to the Norwegian winter and retrospective laboratory studies support this theory. Increased dye inclusion, possibly indicative of viability loss, was also noted. The refrigerated control parasites exhibited no decline in numbers, and alteration in dye inclusion characteristics for refrigerated parasites was slower. Cryptosporidium oocysts were apparently more robust than Giardia cysts; differences between isolates were also noted. These results suggest Cryptosporidium oocysts and Giardia cysts do not persist in the Norwegian terrestrial environment over winter, and when detected, will have been excreted since the previous winter. Differences in the morphological characteristics, matrix effects, and the possible relationship of the dye data to parasite survival are discussed in relation to further studies.