Detection of falciparum malarial forms in naturally infected anophelines in Cameroon using a fluorescent anti-25-kD monoclonal antibody.

Anopheles gambiae s.s. and An. funestus were sampled in houses located in a Plasmodium falciparum-holoendemic site in southern Cameroon. The midguts of female mosquitoes in half-gravid or gravid stages of blood digestion were incubated with a fluorescent monoclonal antibody directed against the P. falciparum zygote/ookinete surface protein Pfs25 and examined using a fluorescent light microscope. Malarial forms were detected in 11.6% of the half-gravid mosquitoes and in 0.0% of the gravid ones (P = 0.012). No difference in infections or the occurrence of malarial forms between An. gambiae and An. funestus was observed. Overall, 127 malarial forms were counted and distributed among round forms, retorts, and ookinetes in 77.2%, 9.5%, and 13.4%, respectively. Round forms include macrogametes, activating microgametocytes, and zygotes. The mean number of malarial forms per infected midgut was 2.16 and the maximum number observed was 13. In four anophelines, round forms, retorts, and ookinetes were simultaneously observed. Sporozoite rates were 5.7% for An. gambiae and 3.8% for An. funestus. In the human population, the gametocyte index for P. falciparum was 38% with a mean density of 1.11 gametocytes per microliter of blood. Differences concerning malarial forms in mosquito midguts were observed between houses (range percentage = 4.7--21.3%; mean range of forms per positive anopheline = 1.1--3.1). In each house, relationships existed between infected vectors and the gametocyte reservoir of their inhabitants. The role in transmission of people with very low gametocytemia, approximately one per microliter, as a reservoir of falciparum malaria in highly endemic areas, is emphasized.