Plasmodium vivax Sporozoite Production in Anopheles albimanus Mosquitoes for Vaccine Clinical Trials

Abstract

Abstract. Vaccine development for Plasmodium vivax malaria is underway. A model to assess the protective efficacy of vaccine candidates in humans is urgently needed. Given the lack of continuous P. vivax cultures, we developed a system to infect Anopheles albimanus mosquitoes using blood from P. vivax-infected patients and determined parameters for challenge of malaria-naive volunteers by mosquito bite. Absence of co-infections in parasitized blood was confirmed by tests consistent with blood bank screening. A total of 119 experiments were conducted using batches of 900-4,500 mosquitoes fed by an artificial membrane feeding method. Optimal conditions for mosquito probing and infection were determined. Presence of oocyst and sporozoites were assessed on Days 7-8 and 14-15, respectively, and conditions to choose batches of infected mosquitoes for sporozoite challenge were established. Procedures to infect volunteers took a 2-hour period including verification of inoculum dose. Anopheles albimanus mosquitoes represent a valuable resource for P. vivax sporozoite challenge of volunteers.

Consistent Safety and Infectivity in Sporozoite Challenge Model of Plasmodium vivax in Malaria-Naive Human Volunteers

“…stephensi and An. dirus that are more robust vectors; 15,18,[27][28][29] however the number of sporozoites inoculated by a single mosquito bite appears to be relatively standard (1-50 spz/bite) independently of the mosquito sporozoite load. [30][31][32] On the basis of the range of mosquito bites estimated in endemic areas, we believe that the two mosquito bites that have shown to be reproducible sufficient for infection of naive volunteers in studies 18 is the maximal amount desirable, and therefore the five mosquito bites required to infect naive volunteers with P. falciparum might correspond to an overwhelming simultaneous sporozoite dose to assess the efficacy of P. vivax malaria vaccines.…”

“…For the infectious challenge we used screen-meshed boxes (7 × 7 × 7 cm) filled with four mosquitoes, to have a better chance of achieving the targeted mosquito dose in a single biting round. 18 Participants who did not complete the minimal targeted dose (two bites) within the first biting round were subjected to a second one to complete the target dose. A mosquito bite was considered infectious if the mosquito was scored with at least 1+ of spz load (1-10 spz).…”

“…albimanus engorgement. 18 After biting, all mosquitoes were dissected to confirm the presence of blood meal and sporozoites. Study participants were followed up at 1, 8, and 24 hr after challenge to assess their response to mosquito bites and parasite challenge.…”

“…The TBS were stained by Giemsa staining and were read by experienced microscopists as described elsewhere. 18 The PCR was performed later for retrospective analysis. As soon as parasites became detected by TBS, participants were treated with chloroquine (1,500 mg chloroquine base provided orally in divided doses: 600 mg initially followed by 450 mg given 24 and 48 hr later) and primaquine (30 mg daily for 14 d), administered directly by the medical team.…”

“…From the 18 mosquito lots exposed to infected blood, 7 lots were discarded because of blood co-infection, and 4 lots because of low sporozoite infection; the other 7 lots with > 50% of the mosquitoes showing oocysts were considered useful for challenge. 18 On the day before challenge, 3 mosquito lots that were arbitrarily selected from 7 eligible mosquito lots, were examined microscopically and showed the presence of sporozoites in 94.7%, 84.2%, and 55.3% of the mosquitoes in each lot, respectively.…”