Abstract
BackgroundNatural exposure to gametocytes can result in the development of immunity against the gametocyte by the host as well as genetic diversity in the gametocyte. This study evaluated the quantity and quality of natural immune responses against a gametocyte antigen, Pfs230 as well as the prevalence and diversity of gametocytes circulating in children living in two communities in southern Ghana.MethodsWhole blood (2.5 ml) was collected from 137 non-febrile school children aged between 6 and 12 years old quarterly for a 6-month period. A drop of blood was used to prepare thick and thin blood films for parasite prevalence and density estimation. Subsequently, stored plasma samples were used in ELISAs assays to measure antibody responses and avidity against Pfs230. RNA was extraction from Trizol preserved packed cells and subsequently converted to complementary DNA (cDNA) which was used for reverse transcriptase PCR (RT-PCR) to determine gametocytes prevalence and diversity.ResultsGametocyte carriage in the peak season (July) determined by Pfg377 RT-PCR was 49.2% in Obom and 22.2% in Abura, and was higher than that determined by microscopy. Gametocyte diversity was low and predominated by the same allele at both sites. The relative avidity index for antibodies measured in Abura was higher than that recorded in Obom at all time points although Pfs230 IgG concentrations were significantly high (P < 0.0001) in Obom than in Abura at all time points. The IgG responses in Obom were significantly higher than that in Abura during the peak season.ConclusionNaturally induced antibody responses against Pfs230 in children living in both high perennial and low seasonal malaria transmission settings reduced significantly in moving from the peak to the off-peak season. The relative avidity of antibodies against Pfs230 in Abura was significantly higher than those measured in Obom, despite having lower IgG levels. Very limited diversity was identified in the gametocytes circulating in both Obom and Abura.
Highlights
Natural exposure to gametocytes can result in the development of immunity against the gametocyte by the host as well as genetic diversity in the gametocyte
Only samples collected from 65 children living in Obom and 72 children living in Abura who were present for all three sample collections (July 2015, October 2015 and January 2016) were analysed in this study
Gametocyte prevalence in the peak season did not correlate with asexual parasite density in either of the two study sites (p > 0.05 for all comparisons)
Summary
Natural exposure to gametocytes can result in the development of immunity against the gametocyte by the host as well as genetic diversity in the gametocyte. Malaria associated morbidity and mortality in Ghana has steadily declined [1, 2] This is attributed to numerous control interventions targeting vectors and parasites [1]. Despite all these interventions, the high rate of asymptomatic Plasmodium parasite carriage is likely to enhance malaria transmission [3, 4]. During Plasmodium falciparum infections, the human host develops immune responses against exposed parasite antigens, which increases after repeated exposure. These immune responses are directed against both the asexual as well as the sexual (gametocyte) transmissible forms of the parasite [5,6,7]. Though microscopy is the gold standard for parasite detection, the low densities of the gametocytes warrants more sensitive molecular tools including reverse transcriptase (RT) or real time reverse transcriptase (q) polymerase chain reaction (PCR) to make measurements more accurate [8, 9]
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