*These authors contributed equally to this work
There are no preclinical in vivo models for human malaria capable of completing the entire Plasmodium life cycle in immune competent animals. Here, we have utilized our access to stable colonies of Aotus nancymaae owl monkeys and laboratory-adapted Anopheles darlingi mosquitos to test several strains of P. vivax for their ability to support a tractable P. vivax challenge model initiated by sporozoite infection in immune competent monkeys.
We previously tested 3 P. vivax monkey-adapted strains (Vietnam IV, Brazil-I and Sal-I) for their utility towards this end. Each strain had variable parasitemia in splenectomized monkeys, yet only Brazil-I and Sal-I produced low-level salivary gland sporozoites. These sporozoites failed to demonstrate successful transition blood stage parasitemia after injection into splenectomized monkeys. We next generated a mixed blood stage infection in a single splenectomized monkey with the hypothesis that this could create hybrids with increased fitness. Specifically, we infected a splenectomized monkey with Brazil-I, Sal-I and ONG—a monkey-adapted strain reported to generate salivary gland sporozoites. Infected blood from this monkey yielded a low level of salivary gland sporozoite infection (205 spz/mosquito). Another splenectomized monkey was infected with 24000 of these sporozoites and this monkey became patent at day 12. This blood was passaged into a new splenectomized animal and this infected blood produced low salivary gland sporozoites infection in mosquitos (132 spz/mosquito). These sporozoites were used to infect a new group of three spleen-intact monkeys at a dose of 5000 sporozoites each. Only one of three animals became patent at day 15 with peak parasitemia at day 51 (565 par/uL). Whole genome sequencing is underway to determine the presence and composition of hybrid parasites. In addition, we are continuing to select and adapt this parasite line to achieve higher sporozoite yields and greater infection in spleen-intact animals.