Plasmodium falciparum malaria challenge by the bite of aseptic Anopheles stephensi mosquitoes: results of a randomized infectivity trial

Abstract

Background: Experimental infection of malaria-naïve volunteers by the bite of Plasmodium falciparum-infected mosquitoes is a preferred means to test the protective effect of malaria vaccines and drugs. The standard model relies on the bite of five infected mosquitoes to induce malaria. We examined the efficacy of malaria transmission using mosquitoes raised aseptically in compliance with current Good Manufacturing Practices (cGMPs).

Methods and findings: Eighteen adults aged 18-40 years were randomized to receive 1, 3 or 5 bites of Anopheles stephensi mosquitoes infected with the chloroquine-sensitive NF54 strain of P. falciparum. Seventeen participants developed malaria; fourteen occurring on Day 11. The mean prepatent period was 10.9 days (9-12 days). The geometric mean parasitemia was 15.7 parasites/µL (range: 4-70) by microscopy. Polymerase chain reaction (PCR) detected parasites 3.1 (range: 0-4) days prior to microscopy. The geometric mean sporozoite load was 16,753 sporozoites per infected mosquito (range: 1,000-57,500). A 1-bite participant withdrew from the study on Day 13 post-challenge and was PCR and smear negative.

Conclusions: The use of aseptic, cGMP-compliant P. falciparum-infected mosquitoes is safe, is associated with a precise prepatent period compared to the standard model and appears more efficient than the standard approach, as it led to infection in 100% (6/6) of volunteers exposed to three mosquito bites and 83% (5/6) of volunteers exposed to one mosquito bite.

Trial registration: ClinicalTrials.gov NCT00744133.

Figure 1. Study flow diagram.

Figure 2. Time to infection of volunteers measured by blood smear and PCR.

The time to infection of all volunteers (pooled exposed to 1, 3 and 5 infectious bites) measured by blood smear (solid line, median time to infection  = 11 days) and PCR (dashed line, median time to infection  = 8 days) was significantly different (p≤0.001, Mantel-Cox test).

Figure 3. Dynamics of parasite growth in volunteers after challenge.

Each line shows the parasite density in an individual volunteer as measured by PCR after being bitten by 1 (top), 3 (middle) or 5 (bottom) Plasmodium falciparum-infected Anopheles stephensi mosquitoes. All volunteers were treated on day 11 (vertical dashed line on each panel) when parasites were detected by blood smear, except for one volunteer in the 1 bite group (blue dashed line) who was positive and treated on day 12, one volunteer (grey dashed line) in the 3 bite group who was positive and treated on day 10, and one volunteer (red dashed line) in the 5 bite group who was positive and treated on day 9. Data are presented until last positive identification of parasites in the blood by PCR. Values shown as 1 on the log scale were negative.