Quantification of the infectivity of Cryptosporidium parvum by monitoring the oocyst discharge from SCID mice.

Doses of 1-10(5) oocysts of Cryptosporidium parvum HNJ-1 were inoculated into severe combined immunodeficient (SCID) mice, and the discharge of oocysts was monitored for 30 days post inoculation. None of the mice discharged any oocysts after oral inoculation of one oocyst. Only one of five SCID mice discharged oocysts after oral inoculation of 10 oocysts, and the prepatent period was 17 days. The other four mice did not discharge any oocysts. All the SCID mice discharged oocysts after oral inoculation of 10(2)-10(5) oocysts. The prepatent periods were 13-17, 8-10, 8, and 10 days in SCID mice inoculated with 10(2), 10(3), 10(4), and 10(5) oocysts, respectively. A proportional correlation was observed between inoculation doses of oocysts ranging from 10 to 10(4) oocysts and the corresponding prepatent periods. The prepatent period can be used to evaluate the infectivity of C. parvum oocysts.