Quantitative flow cytometric evaluation of maximal Cryptosporidium parvum oocyst infectivity in a neonate mouse model

The importance of waterborne transmission of Cryptosporidium parvum to huma ns has been highlighted by recent outbreaks of cryptosporidiosis, The first step in a survey of contaminated water currently consists of counting C. p arvum oocysts. Data suggest that an accurate risk evaluation should include a determination of viability and infectivity of counted oocysts in water. In this study, oocyst infectivity was addressed by using a suckling mouse m odel. Four-day-old NMRI (Naval Medical Research Institute) mice were inocul ated per os with 1 to 1,000 oocysts in saline. Seven days later, the number of oocysts present in the entire small intestine was counted by flow cytom etry using a fluorescent, oocyst-specific monoclonal antibody. The number o f intestinal oocysts was directly related to the number of inoculated oocys ts. For each dose group, infectivity of oocysts, expressed as the percentag e of infected animals, was 100% for challenge doses between 25 and 1,000 oo cysts and about 70% for doses ranging from 1 to 10 oocysts/animal. Immunofl uorescent flow cytometry was useful in enhancing the detection sensitivity in the highly susceptible NMRI suckling mouse model and so was determined t o be suitable for the evaluation of maximal infectivity risk.