Objective 1: To determine gametocyte commitment and maturation after controlled human malaria infections (CHMI)
Under this objective, we aimed to provide the first conclusive evidence on gametocyte commitment and maturation during the first rounds of parasite multiplication in primary infections. We analysed samples from previously completed controlled human malaria infections (CHMI) for the presence of mature gametocytes by molecular methods. In a CHMI study that was specifically designed to induce gametocytes using subcurative doses of sulphadoxine-pyrimethamine and piperaquine , we successfully induced gametocytes in all volunteers who were inoculated by malaria-infected mosquito bites. Infectious male and female gametocytes were observed in a subset of volunteers. The first appearance of male and female gametocytes (8.5-12 days after the first asexual parasites), indicates that gametocyte commitment appears very early in infections, during the first wave of asexual parasites.
Objective 2: To determine commitment and maturation of gametocytes in natural infections
Under this objective we analyzed gametocyte prevalence, density and infectivity in populations of clinical malaria patients and asymptomatically infected individuals. In settings across Africa, we observed a very modest contribution of clinical malaria patients to mosquito infections compared to asymptomatic infections. To examine the timing of gametocyte production and infectivity in relation to the time since infection and time of occurrence of symptoms, we designed a cohort study in Burkina Faso to directly compare incident and chronic infections in partially immune children where we prospectively monitored acute and chronic infections to study the dynamics of gametocyte commitment, maturation, sex-ratio and infectivity. We observed higher gametocyte production and infectivity in chronic infections as compared to acute, incident infections.
Objective 3: To quantify ongoing gametocyte production and release from the bone marrow following antimalarial treatment
Under this objective, we aimed to assess in detail the dynamics of gametocyte decay and production following antimalarial treatment. We hypothesized that low levels of asexual parasites persist after apparently successful antimalarial treatment and may give rise to gametocyte production. We observed that conventional antimalarials have differential effect on gametocyte development and clearance. In trials in Burkina Faso and Mali, we demonstrated the potency of the gametocytocidal drugs primaquine and methylene blue in reducing gametocyte circulation times and preventing malaria transmission to mosquitoes. Next, we determined submicroscopic parasite persistence in three clinical trials conducted in Uganda, Burkina Faso and Mali. These trials provided evidence for our hypothesis that low densities of ring-stage parasites may survive treatment and that this ring-stage survival depends on the schizonticidal drug given.
Objective 4: To study clustering and infectivity of mature gametocytes in the subdermal vasculature
Under this objective, we proposed to test a long-standing hypothesis on subdermal clustering of mature gametocytes as contributor to the apparent transmission success of P. falciparum. Skin biopsy samples and venous and finger-prick blood samples were collected repeatedly and processed for immunohistochemistry and molecular parasite detection. In a separate study, a comparison of mosquito infection rates by direct skin feeding and membrane feeding was performed in adult gametocyte carriers using laboratory reared An. coluzzii mosquitoes. We observed no significant clustering of gametocytes in skin tissue.
