Aging of Cryptosporidium parvum oocysts in river water and their susceptibility to disinfection by chlorine and monochloramine.

Cryptosporidium parvum oocysts were aged in waters from both the St. Lawrence River and the Ottawa River. In situ survival experiments were carried out by incubating the oocysts in either dialysis cassettes or microtubes floated into an overflow tank. A significant portion of the oocysts survived in the test waters for several weeks. Oocyst survival in the St. Lawrence River was better in membrane-filtered (0.2 microm-pore diameter) water than in unfiltered water, suggesting that biological antagonism may play a role in the environmental fate of the parasite. Oocysts aged in river waters under in situ conditions and control oocysts kept refrigerated in synthetic water (100 ppm as CaCO3); pH 7.0) were subjected to the same disinfection protocol. Aged oocysts were at least as resistant as, if not more resistant than, the control oocysts to disinfection. This indicates that the oocysts surviving in the water environment may be just as difficult to inactivate by potable water disinfection as freshly shed oocysts. Therefore, water treatment should not be based on the assumption that environmental oocysts may be more easily inactivated than freshly shed oocysts. First-order kinetics die-off rates varied from one river to another (from 0.013 to 0.039 log(10).day(-1)) and from one experiment to another with water from the same river collected at different times. Calculation of the die-off rates based on either in vitro excystation or in vitro excystation in combination with total counts (overall die-off rates) showed that the assessment of oocyst viability by microscopic methods must account for the total oocyst loss observed during long-term inactivation assays of river waters.