Infectivity and neutralization of Cryptosporidium parvum sporozoites.

Cryptosporidiosis, a diarrheal disease of calves and humans caused by the coccidian parasite Cryptosporidium parvum, is terminated in hosts with normal immune systems. To assess the mechanisms of immunity in cryptosporidiosis, it is necessary to isolate and quantitate sporozoites, the infective stage of Cryptosporidium spp. Here we report the (i) separation of infective C. parvum oocysts from calf feces by ether extraction, sieving, and hypochlorite treatment; (ii) separation of viable C. parvum sporozoites from intact and excysted oocysts by anion-exchange chromatography; and (iii) quantitation of sporozoite infectivity in vivo by direct intraintestinal injection of isolated sporozoites in 7-day-old BALB/c mice. When isolated sporozoites were incubated with heat-inactivated immune bovine serum, 25 times the 50% infective dose for 7-day-old mice was completely neutralized. Sporozoites incubated with preimmune bovine serum were infectious for 7-day-old mice.