THE EARLY SPOROGONIC CYCLE OF PLASMODIUM-FALCIPARUM IN LABORATORY-INFECTED ANOPHELES-GAMBIAE - AN ESTIMATION OF PARASITE EFFICACY

This study investigated the successive losses in the parasite densitie s of Plasmodium falciparum stages during the early sporogony in labora tory-reared Anopheles gambiae infected by membrane feeding with blood from naturally infected gametocyte carriers (>50 gametocytes/mm(3)). T he developmental stages of P. falciparum Mm in the mosquito were studi ed 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 sporo gonic stages before, during and after passage of the midgut wall. Para site densities were determined within the entire blood meal at 3 h (zy gotes and macrogametes) and 24 h (ookinetes) post-infection. At 48 h a fter the mosquito blood meal, midguts were checked for the presence of early oocysts. For the mid-size oocysts count, classic microscopy exa mination was used at day 7 post-infection. The parasite efficacy was e stimated by following successive losses in parasite densities between different early stages of the sporogonic cycle in A, gambiae, Thirty-s even experimental infections were realized with high gametocyte densit ies, ranging from 64 to 2392 gametocytes/mm(3), All gametocyte carrier s showed infection with round forms 100%; ookinetes were found in 91.9 %. The prevalences of infections with oocysts were 48.6% at day 2 (you ng oocyst) and 37.8% at day 7 (mid-size oocyst). The mean densities pe r mosquito for each parasite stage were 12.6 round forms, 5.5 ookinete s, 1.8 young oocyst and 2 mid-size oocysts. Significant correlations w ere found between two consecutive parasite stages (round forms/ookinet es, ookinetes/young oocysts, young oocysts/mid-size oocysts) and betwe en round forms and mid-size oocysts. The mean parasite density signifi cantly decreased between round forms and ookinetes (yield Y1 = 41.6%) and between ookinetes and young oocysts (Y2 = 61.4%). By contrast, no significant decrease was observed between young oocysts and mid-size o ocysts (Y3 = 91.2%). The overall yield of the: early sporogonic cycle (from round form to oocyst at day 7) was equal to 25.7%, indicating th at almost 3/4 of the total parasites were lost during the early step o f the sporogonic cycle, from 3 h post-infection to day 7.