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Applied and Environmental Microbiology, November 2002, p. 5387-5393, Vol. 68, No. 11
0099-2240/02/$04.00+0     DOI: 10.1128/AEM.68.11.5387-5393.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Efficacy of UV Irradiation in Inactivating Cryptosporidium parvum Oocysts

Shigemitsu Morita,1 Atsushi Namikoshi,1 Tsuyoshi Hirata,1* Kumiko Oguma,2 Hiroyuki Katayama,2 Shinichiro Ohgaki,2 Nobuyuki Motoyama,3 and Masahiro Fujiwara4

School of Environmental Health, Azabu University, Sagamihara, Kanagawa,1 Department of Urban Engineering, The University of Tokyo, Bunkyo-ku,2 Environmental Systems Engineering Department, Fuji Electric Co., Ltd., Shinagawa-ku,3 Japan Water Research Center, Minato-ku, Tokyo, Japan4

Received 16 April 2002/ Accepted 14 August 2002

To evaluate the effectiveness of UV irradiation in inactivating Cryptosporidium parvum oocysts, the animal infectivities and excystation abilities of oocysts that had been exposed to various UV doses were determined. Infectivity decreased exponentially as the UV dose increased, and the required dose for a 2-log10 reduction in infectivity (99% inactivation) was approximately 1.0 mWs/cm2 at 20°C. However, C. parvum oocysts exhibited high resistance to UV irradiation, requiring an extremely high dose of 230 mWs/cm2 for a 2-log10 reduction in excystation, which was used to assess viability. Moreover, the excystation ability exhibited only slight decreases at UV doses below 100 mWs/cm2. Thus, UV treatment resulted in oocysts that were able to excyst but not infect. The effects of temperature and UV intensity on the UV dose requirement were also studied. The results showed that for every 10°C reduction in water temperature, the increase in the UV irradiation dose required for a 2-log10 reduction in infectivity was only 7%, and for every 10-fold increase in intensity, the dose increase was only 8%. In addition, the potential of oocysts to recover infectivity and to repair UV-induced injury (pyrimidine dimers) in DNA by photoreactivation and dark repair was investigated. There was no recovery in infectivity following treatment by fluorescent-light irradiation or storage in darkness. In contrast, UV-induced pyrimidine dimers in the DNA were apparently repaired by both photoreactivation and dark repair, as determined by endonuclease-sensitive site assay. However, the recovery rate was different in each process. Given these results, the effects of UV irradiation on C. parvum oocysts as determined by animal infectivity can conclusively be considered irreversible.

* Corresponding author. Mailing address: Water Environment Laboratory, School of Environmental Health, Azabu University, 1-17-71 Fuchinobe, Sagamihara, Kanagawa 229-8501, Japan. Phone and fax: (81) 42-769-1894. E-mail: hiratat{at}azabu-u.ac.jp.

Applied and Environmental Microbiology, November 2002, p. 5387-5393, Vol. 68, No. 11
0099-2240/02/$04.00+0     DOI: 10.1128/AEM.68.11.5387-5393.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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