Cryptosporidium parvum represents a challenge to the water industry and a threat to public health. In this study, we developed a cell culture-quantitative PCR assay to evaluate the inactivation of C. parvum with disinfectants. The assay was validated by using a range of disinfectants in common use in the water industry, including low-pressure UV light (LP-UV), ozone, mixed oxidants (MIOX), and chlorine. The assay was demonstrated to be reliable and sensitive, with a lower detection limit of a single infectious oocyst. Effective oocyst inactivation was achieved (>2 log 10 units) with LP-UV (20 mJ/cm 2 ) or 2 mg of ozone/liter (for 10 min). MIOX and chlorine treatments of oocysts resulted in minimal effective disinfection, with <0.1 log 10 unit being inactivated. These results demonstrate the inability of MIOX to inactivate Cryptosporidium. The assay is a valuable tool for the evaluation of disinfection systems for drinking water and recycled water.Cryptosporidium is recognized as a frequent cause of waterborne disease in humans (3). The disease has been documented worldwide, with speculation that many undiagnosed cases of gastroenteritis may have been caused by this parasite (21,38). Two means of transmission have been shown to be contaminated drinking water in distribution systems (25,28) and swimming pools (17). Cryptosporidium oocysts are resistant to standard chlorine-based disinfection procedures that are a key barrier in the transmission of other waterborne pathogens in potable water. Although some water treatment processes remove oocysts, any oocysts that break through represent a potential threat to human health due to their relative chlorine resistance. Various methods of disinfection have been investigated by a number of research groups; these methods include UV light (4, 10, 11, 13, 14, 23, 35), ozone (5, 18, 22, 29), chlorine dioxide (8,12,22,30,34), mixed oxidants (MIOX) (6, 40), and chlorine (7,22,17). Many disinfection studies have used animal infectivity or surrogate in vitro assays to determine the viability of oocysts after disinfection. Animal bioassays are considered the "gold standard" for assessing Cryptosporidium oocyst infectivity, and the neonatal mouse model has been used extensively in the assessment of oocyst disinfection. However, this model has limited applications for the assessment of waterborne oocysts because type 1 (human) Cryptosporidium parvum cannot infect mice. The human genotype can be cultured in gnotobiotic pigs (41), and this model has been used to assess drug efficacy (37).Significant developments in determining oocyst infectivity have included cell culture (CC) assays for type 1 oocysts (20) and for type 2 oocysts (14,15,20,31,36,39). These methods have used culturing of oocysts in HCT-8 (human ileocecal adenocarcinoma) cells. Evaluation of these methods has shown the CC assay with the HCT-8 cell line to be equivalent to the gold standard neonatal mouse infectivity assay (32,35). Shim et al. (35) demonstrated that CC assays provide a level of sensitivity similar to t...

Aims:  To determine the effect of solar radiation on Cryptosporidium parvum in tap and environmental waters. Methods and Results:  Outdoor tank experiments and a cell culture infectivity assay were used to measure solar inactivation of C. parvum oocysts in different waters. Experiments conducted on days with different levels of solar insolation identified rapid inactivation of oocysts in tap water (up to 90% inactivation within the first hour). Increased dissolved organic carbon content in environmental waters decreased solar inactivation. The role of solar ultraviolet (UV) in inactivation was confirmed by long‐pass filter experiments, where UV‐B was identified as the most germicidal wavelength. Reductions in oocyst infectivity following solar radiation were not related to a loss of excystation capacity. Conclusions:  Solar UV can rapidly inactivate C. parvum in environmental waters. Significance and Impact of the Study:  This is the first study to assess natural sunlight inactivation of C. parvum oocysts in surface waters and drinking water using an infectivity measure and determines the wavelengths of light responsible for the inactivation. The findings presented here provide valuable information for determining the relative risks associated with Cryptosporidium oocysts in aquatic environments and identify solar radiation as a critical process affecting the oocyst survival in the environment.

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