抄録
Experiments to determine the efficacy of radioactive irradiation in inactivating Cryptosporidium parvum oocysts were conducted. The infectivity decreased exponentially as radiation dose increased. The gamma ray and electron beam doses resulting in a 2 log10 reduction in infectivity for mice were 94 Gy and 92 Gy, respectively. Although gamma rays had been reported to be twice as potent as beta rays in inactivating E. coli, no marked differences were found between gamma ray and an electron beam in terms of efficacy in inactivating C. parvum oocysts. The required dose increase to yield a 2 log10 reduction in infectivity was only 5% for every 10°C decrease in water temperature and 2% for a 10-fold increase in intensity. The gamma ray and electron beam doses required for a 2 log10 reduction in viability, as evaluated by in vitro excystation experiments, were 13 kGy and 12 kGy, respectively. The large differences in the radioactive doses required to reduce infectivity versus those required to reduce excystation suggest that exposed oocysts are still viable following a low dose of radiation, but not infective. The potential to recover infectivity was also investigated. There was no recovery in infectivity following fluorescent-light irradiation or storage in darkness. Moreover, the degree of inactivation can be estimated if radiation dose delivered to oocysts is known.
