Assessment of the risk of infection by Cryptosporidium or Giardia in drinking water from a surface water source

The significance of the presence in drinking water of the protozoan microparasites Cryptosporidium parvum and Giardia lamblia for public health may be analyzed by means of risk assessment. This requires quantitative knowledge of all the contributing factors, from the concentration of these organisms in the source water to the dose-response relation for the probability of infection or disease in a human host. The major contributing factors are: the concentration of cysts or occysts in raw water, the recovery of the detection method, the viability of recovered cysts or oocysts, the removal of organisms in the treatment process, and the daily consumption of unboiled tap water. To enable analysis of the uncertainty in the calculated risk of infection, each of these factors is treated as a stochastic variable, for which a suitable distribution is proposed. A frequency distribution for the probability of infection is then constructed with standard sampling techniques. This first evaluation of the calculation of the risk of infection due to exposure to Cryptosporidium oocysts and Giardia cysts via drinking water, shows that the uncertainty in the estimated removal efficiency of the treatment process dominates over uncertainities in other contributing factors.