Plasmodium simium and Saimiri boliviensis as a model system for testing candidate vaccines against Plasmodium vivax

Observations on Plasmodium simium infections in Saimiri boliviensis boliviensis monkeys suggest that this host-parasite combination would be a suitable model for the testing of candidate vaccines against Plasmodium vivax. To evaluate the normal course of infections, parasitemia in 52 splenectomized S. boliviensis boliviensis monkeys infected with P. simium were analyzed. The mean maximum parasite count for 31 monkeys after injection with trophozoite-infected erythrocytes was 77,580/microL. Twenty-one monkeys were infected via sporozoites, and prepatent periods ranged from 14 to 24 days with a median of 15 days. The mean maximum parasite count was 29,234/microL. The mean maximum parasite count for monkeys previously infected with Old World P. vivax was 26,337/microL versus 56,362/microL for those previously infected with New World P. vivax, possibly suggesting a closer antigenic relationship between P. simium and the Old World parasites.