The prevalence of submicroscopic Plasmodium falciparum gametocyte carriage and multiplicity of infection in children, pregnant women and adults in a low malaria transmission area in Southern Ghana

Abstract

Background: The gametocyte stage of Plasmodium falciparum is considered an important target for disrupting malaria transmission. Indications are that various demographic groups, such as children and pregnant women may differ in risk of harbouring gametocytes, which may be crucial for targeted control. In this study, the relationship between the prevalence and multiplicity of P. falciparum, asexual parasite infections and gametocytaemia was assessed in three different demographic groups in an area of southern Ghana with low malaria endemicity. Levels of antibody responses to Pfs230 were also assessed as a proxy for the presence of gametocytes.

Methods: The study involved multiple cross-sectional sampling of children (N = 184, aged 2-15 years), male and non-pregnant female adults (N = 154, aged 16-65 years) and pregnant women (N = 125, aged 18-45 years) from Asutsuare in the Shai Osudoku District of Greater Accra Region in Ghana. Asexual parasitaemia was detected by microscopy and PCR, and gametocytaemia was assessed by Pfs25-real time PCR. Multiclonal P. falciparum infections were estimated by msp2 genotyping and an indirect ELISA was used to measure plasma IgG antibodies to Pfs230 antigen.

Results: Overall, children and pregnant women had higher prevalence of submicroscopic gametocytes (39.5% and 29.7%, respectively) compared to adults (17.4%). Multiplicity of infection observed amongst children (3.1) and pregnant women (3.9) were found to be significantly higher (P = 0.006) compared with adults (2.7). Risk of gametocyte carriage was higher in individuals infected with P. falciparum having both Pfmsp2 3D7 and FC27 parasite types (OR = 5.92, 95% CI 1.56-22.54, P = 0.009) compared with those infected with only 3D7 or FC27 parasite types. In agreement with the parasite prevalence data, anti-Pfs230 antibody levels were lower in gametocyte positive adults (β = - 0.57, 95% CI - 0.81, - 0.34, P < 0.001) compared to children.

Conclusions: These findings suggest that children and pregnant women are particularly important as P. falciparum submicroscopic gametocyte reservoirs and represent important focus groups for control interventions. The number of clones increased in individuals carrying gametocytes compared to those who did not carry gametocytes. The higher anti-gametocyte antibody levels in children suggests recent exposure and may be a marker of gametocyte carriage.

Keywords: Gametocyte prevalence; Ghana; Multiplicity of infection; Pfs230; Plasmodium falciparum; Seroprevalence; Submicroscopic infections.

Fig. 1

Site Map of Asutsuare in the Shai Osudoku District of Greater Accra Region, Ghana, showing the study sites

Fig. 2

Asexual parasite prevalence among the study groups at the various time points. Enrolment (November 2013) and follow-up time points. Follow-up 1 (February 2014), Follow-up 2 (May 2014) and Follow-up 3 (September 2014) among the groups: Children, Adults and Pregnant women detected by both microscopy and PCR

Fig. 3

Anti-Pfs230 median antibody levels among the different study populations. a Antibody levels for Children’s group, b is antibody levels for Adults; c antibody levels for pregnant women. P value ≤ 0.05 is significant by Kruskal–Wallis test and Bonferroni test. Error bars indicate the upper and lower limits of the 95% confidence interval around the proportion

Fig. 4

IgG Seropositives among the three different groups. Antibody seropositive responses for Pfs230 was done using the mixture model, which shows a bimodal distribution into seropositive and seronegative populations. Cut-off for seropositivity was defined as the mean of the seronegative individuals from the Gaussian distribution plus three times the standard deviation. a Baseline, November 2013; Children (5/159), Adults (35/133), Pregnant women (8/123); b follow-up 1, February 2014: Children (14/175), Adults (81/91), Pregnant women (10/79); c follow-up 2, May 2014; Children (5/85), Adults (11/34), Pregnant women (27/58); d follow-up 3, September 2014 Children (43/164), Adults (33/108), Pregnant women (32/54)

Fig. 5

Anti-Pfs230 levels and gametocyte positive and negative individuals among the groups. Association was determined by linear regression