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Utility of Surrogates for Measuring Cryptosporidium Oocysts Infectivity

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Abstract:The water industry must assess whether Cryptosporidium oocysts detected in source and finished water are viable and/or infectious. Initial approaches measuring the infectious nature of C. parvum oocysts have focused on in vitro excystation and in vitro vital dye staining. Recently there has been increased interest in animal models and a Cryptosporidium cell culture assay. Studies comparing in vitro assays to over estimate the infectious nature of oocysts exposed to disinfectants. While human volunteer studies have been conducted to determine the infectious dose of various Cryptosporidium isolates, human volunteers have never been used for disinfection studies. Besides being extremely expensive, it is difficult to get human subjects approval to do such infectivity studies. Animal models have been used with some success, but they are only able to evaluate C. parvum genotype 2 isolated from bovids and humans and not C. parvum genotype 1 isolated only from humans. In addition, results from both human volunteers and animal models are extremely variable, as they are dependent on the strain and age of the host, how long it has been since the oocysts were shed, the strain and isolate of C. parvum, the methods used to enumerate the oocysts, the procedures used to inoculate the oocysts, and the inter- and intra-strain variation in animal susceptibility to infection. Limited side by side studies comparing the Cryptosporidium cell culture assay with the animal model have shown the two to produce comparable results in the case of C. parvum genotype 2 oocysts. Unlike the animal model, the Cryptosporidium cell culture assay is able to estimate infectivity of both C. parvum genotype 1 and genotype 2 oocysts.

Of the approaches used to determine the infectivity of Cryptosporidum oocysts, the Cryptosporidium cell culture assay currently appears to be the most promising. However, at present there are a number of knowledge gaps associated with the Cryptosporidium cell culture assay which must be overcome before it can be used on a reliable, routine basis. The assay is not standardized as to the optimal cell line and culture medium formulation to be used. The sensitivity and specificity of the assay have yet to be determined in an impartial manner. Since a number of Cryptosporidium spp. including C. felis, C. canis, C. meleagridis, C. parvum genotype 1 and C. parvum genotype 2 now are known to infect humans the question is, will the Cryptosporidium cell culture assay work equally well for each of these different species? Once a standardized assay is available, then a laboratory round robin validation must be conducted to determine how robust the assay is. Incorporation of the Cryptosporidium cell culture assay into a complete method which would allow concentrating and purifying ocysts from various environmental matrices of varying turbidity must also be accomplished.
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Citation:Schaefer III, F. W. Utility of Surrogates for Measuring Cryptosporidium Oocysts Infectivity. Presented at Drinking Water Inspectorate Research Conference, Tadley Court, UK, June 10-11, 2002.
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Contact: Mary P. O'Bryant - (919)-541-4871 or obriant.mary@epa.gov
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Division: Microbiological & Chemical Exposure Assessment Division
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Branch: Biohazard Assessment Research Branch
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Product Type: Sympos/Conf
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Presented: 06/10/2002
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Related Entries:
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Bullet Item Detecting Ccl-Related, Emerging and Regulated Waterborne Human Protozoa for Exposure Assessment
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Last Updated on Monday, October 22, 2007
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