The early sporogonic cycle of Plasmodium falciparum in laboratory‐infected Anopheles gambiae: an estimation of parasite efficacy

Abstract

SummaryThis study investigated the successive losses in the parasite densities of Plasmodium falciparum stages during the early sporogony in laboratory-reared Anopheles gambiae infected by membrane feeding with blood from naturally infected gametocyte carriers (Ͼ50 gametocytes/mm 3 ). The developmental stages of P. falciparum in the mosquito were studied from zygote to oocyst, by immunofluorescent method using monoclonal antibodies against the Pfs25 protein present on the surface of newly formed gametes. This method allows for assessment of the various sporogonic stages before, during and after passage of the midgut wall. Parasite densities were determined within the entire blood meal at 3 h (zygotes and macrogametes) and 24 h (ookinetes) post-infection. At 48 h after the mosquito blood meal, midguts were checked for the presence of early oocysts. For the mid-size oocysts count, classic microscopy examination was used at day 7 postinfection. The parasite efficacy was estimated by following successive losses in parasite densities between different early stages of the sporogonic cycle in A. gambiae. Thirty-seven experimental infections were realized with high gametocyte densities, ranging from 64 to 2392 gametocytes/mm 3 . All gametocyte carriers showed infection with round forms 100%; ookinetes were found in 91.9%. The prevalences of infections with oocysts were 48.6% at day 2 (young oocyst) and 37.8% at day 7 (mid-size oocyst). The mean densities per mosquito for each parasite stage were 12.6 round forms, 5.5 ookinetes, 1.8 young oocyst and 2 mid-size oocysts. Significant correlations were found between two consecutive parasite stages (round forms/ookinetes, ookinetes/young oocysts, young oocysts/mid-size oocysts) and between round forms and mid-size oocysts. The mean parasite density significantly decreased between round forms and ookinetes (yield Y1 ϭ 41.6%) and between ookinetes and young oocysts (Y2 ϭ 61.4%). By contrast, no significant decrease was observed between young oocysts and mid-size oocysts (Y3 ϭ 91.2%). The overall yield of the early sporogonic cycle (from round form to oocyst at day 7) was equal to 25.7%, indicating that almost 3/4 of the total parasites were lost during the early step of the sporogonic cycle, from 3 h post-infection to day 7.

Comparison of direct and membrane feeding methods to infect Anopheles arabiensis with Plasmodium falciparum.

“…This percentage is slightly higher than those found in similar studies (53% in Burkina Faso; 7 38% or 62% in Cameroon; 8,10 45% in Papua New Guinea). 15 This difference may be related to the large proportion of recent malaria attacks in our gametocyte carrier sample.…”

“…The results have two implications: first, studies of experimental infections performed by membrane feeding [7][8][9][10] and by direct feeding [11][12][13] have epidemiological relevance and second, trials to evaluate the efficacy of a transmission-blocking vaccine can use either method. Volunteers appear to favor providing blood by venipuncture for the membrane feeding method rather than directly to mosquitoes feeding on their skins.…”

Increased melanizing activity in Anopheles gambiae does not affect development of Plasmodium falciparum

“…Recent detailed laboratory-based analysis of the population dynamics of avian, murine and human malaria parasites in different vector species has identified a strong developmental bottleneck during the transition of ookinetes to oocysts coinciding with the transition of the parasite through the midgut epithelium (2). This bottleneck was also observed by using field isolates of Plasmodium falciparum (3).…”

Anti-mosquito midgut antibodies block development of Plasmodium falciparum and Plasmodium vivax in multiple species of Anopheles mosquitoes and reduce vector fecundity and survivorship

“…Several studies with rodent and human malaria parasites have revealed that the ookinete-to-oocyst transition is the most vulnerable link in sporogonic development of malaria parasites (1)(2)(3). Thus, the developing parasite stages in the mosquito midgut provide an ideal target for transmission-blocking vaccines.…”