The plasticity of Plasmodium falciparum gametocytaemia in relation to age in Burkina Faso

Abstract

BackgroundMalaria transmission depends on the presence of gametocytes in the peripheral blood. In this study, the age-dependency of gametocytaemia was examined by microscopy and molecular tools.MethodsA total of 5,383 blood samples from individuals of all ages were collected over six cross sectional surveys in Burkina Faso. One cross-sectional study used quantitative nucleic acid sequence based amplification (QT-NASBA) for parasite quantification (n = 412). The proportion of infections with concurrent gametocytaemia and median proportion of gametocytes among all parasites were calculated.ResultsAsexual parasite prevalence and gametocyte prevalence decreased with age. Gametocytes made up 1.8% of the total parasite population detected by microscopy in the youngest age group. This proportion gradually increased to 18.2% in adults (p < 0.001). Similarly, gametocytes made up 0.2% of the total parasite population detected by QT-NASBA in the youngest age group, increasing to 5.7% in adults (p < 0.001). This age pattern in gametocytaemia was also evident in the proportion of gametocyte positive slides without concomitant asexual parasites which increased from 13.4% (17/127) in children to 45.6% (52/114) in adults (OR 1.55, 95% CI 1.38-1.74, p < 0.001).ConclusionsThe findings of this study suggest that although gametocytes are most commonly detected in children, the proportion of asexual parasites that is committed to develop into gametocytes may increase with age. These findings underscore the importance of adults for the human infectious reservoir for malaria.

Epidemiology and Infectivity of Plasmodium falciparum and Plasmodium vivax Gametocytes in Relation to Malaria Control and Elimination

“…A recent study reported that gametocytes make up Ͻ2% of the total parasite population in the youngest children, but this proportion gradually increases to Ͼ15% in adults ( Fig. 3) (334). This finding was observed with both microscopy and quantitative molecular methods (334) and is in line with previously published patterns (120).…”

“…3) (334). This finding was observed with both microscopy and quantitative molecular methods (334) and is in line with previously published patterns (120).…”

“…In areas of high malaria transmission intensity, gametocyte carriage is most prevalent in younger age groups (Fig. 2), who also have the highest prevalences and densities of asexual malaria parasites (44,121,160,255,334,478). In areas of low transmission intensity, the association between gametocyte prevalence and age is weaker, and smaller proportions of all age groups carry gametocytes (1, 51,171,413).…”

“…In contrast to this overall trend, commitment to the sexual pathway may actually increase with acquired immunity. Both microscopic and submicroscopic densities of gametocytes relative to asexual parasites increase with age in cross-sectional surveys (49,120,334). While the proportion of gametocyte carriers among parasite carriers is highest for children, the proportion of parasites that are gametocytes may therefore be highest for those with acquired immunity (120,334).…”

“…Because of their larger representation in the population, higher likelihood of subpatent untreated infections (337) that are frequently also accompanied by gametocytes (337,413), and potentially higher level of commitment to the sexual pathway of low-density asexual infections (120,334), adults contribute considerably to the infectious reservoir. A study that determined the contributions of different age groups to the human infectious reservoir concluded that adults are probably responsible for 28 to 38% of mosquito infections (121), a proportion that could be greater if the larger body size and higher exposure of adults to mosquitoes (357) are taken into account.…”