Effect of xanthurenic acid on infectivity of Plasmodium falciparum to Anopheles stephensi.

Terminally differentiated malarial gametocytes remain in the vertebrate circulation in a developmentally arrested state until they are taken up by the mosquito. The gametocytes then undergo gametogenesis in the mosquito mid-gut within minutes after ingestion of the infected blood meal. The male gametogenesis (exflagellation) can be triggered by the combination of a decrease in temperature of at least 5 degrees C and a simultaneous increase in pH between 8.0 and 8.3. Xanthurenic acid, which is present in mosquito mid-gut as well as in mosquito head, had been shown to induce exflagellation in vitro at a non-permissible pH. Here we report for the first time that with the increasing concentration of exogenous xanthurenic acid, there is a gradual increase in the number of oocysts in the mid-gut of infected mosquitoes. The concentration of xanthurenic acid for optimum infection in the membrane feeding assay was determined to be 100 microM. Three different strains of Plasmodium falciparum, viz. 3D7, 7G8 and W2 were tested in different experiments and similar findings hold true for all of them. These results demonstrate that xanthurenic acid not only induces exflagellation of male gametocytes but also promotes infectivity of Plasmodium falciparum to mosquito vectors.