Infection of immunosuppressed C57BL/6N adult mice with a single oocyst of Cryptosporidium parvum

The present study was designed to determine the minimum number of Cryptospo ridium parvum oocysts capable of producing pat ent infections in immunosupp ressed C57BL/6N adult mice. Sixty-four female mice were divided into 6 grou ps of 8 mice each, except group 1 that contained 24 mice. Mice in groups 1- 3 were immunosuppressed with dexamethasone and inoculated with 1, 5. and 10 oocysts per mouse, respectively. The accuracy of the inoculum size was mic roscopically confirmed. Mice in groups 4-6 served as controls: they receive d either only oocyst inoculation (group 4), or immunosuppression (group 5), or no treatments (group 6). Fecal oocyst shedding was monitored daily for each mouse using an indirect immunofluorescent assay. Parasite colonization in the terminal ileum of each mouse was evaluated histologically. Four of 24 mice in group 1 developed patent infections, with a prepatent period of approximately 6 days. All mice in groups 2 and 3 developed patent infection s, with prepatent periods ranging from 4 to 7 days. Mice in groups 4-6 rema ined uninfected. Parasite colonization was observed in the terminal ilea of all mice in groups 1-3 that shed fecal oocysts. The present study experime ntally demonstrates that a single viable oocyst can induce patent C. parvum infections in immunosuppressed C57BL/6N adult mice and indicates that this mouse model could be used for the parasite genotype or isolate cloning.