A longitudinal study of immune responses to Plasmodium falciparum sexual stage antigens in Tanzanian adults.

Next to children, adults form a substantial part of the infectious reservoir that is responsible for the spread of malaria. In this longitudinal study, we determined sexual stage immune responses to Plasmodium falciparum and infectiousness to mosquitoes in adults from an area with intense malaria transmission. A cohort of 43 Tanzanian adults was followed for 18 months. Parasitological data were collected monthly; serum once every three months. Antibody prevalences were determined for sexual stage antigens Pfs230 and Pfs48/45 and circumsporozoite protein (NANP5)(n = 199). Functional transmission reducing activity (TRA) was assessed by standard membrane feeding assay (SMFA; n = 85). Cumulative parasite prevalence was 67.4% (29/43) for asexual stages and 34.9% (15/43) for gametocytes. Enrolment antibody prevalence was 95.3% (41/43) for NANP5, 18.9% (7/37) for Pfs230 and 7% (3/43) for Pfs48/45 epitope 3. TRA > 50% reduction was seen in 48.2% (41/85) and TRA > 90% reduction in 4.7% (4/85) of the samples. Our findings of low and inconsistent sexual stage immune responses are likely to be the result of a low exposure to gametocytes in this older age group. This may in turn be caused by effective asexual stage immunity. We conclude that the infectivity of older individuals is less likely to be affected by sexual stage immunity.