Seasonal Variation in the Epidemiology of Asymptomatic Plasmodium falciparum Infections across Two Catchment Areas in Bongo District, Ghana

Abstract

Understanding the epidemiology of asymptomatic Plasmodium falciparum infections is critical for countries to move toward malaria elimination. Using different methods for parasite detection, we evaluated how seasonality, spatial location, and other factors affect the age-specific epidemiology of asymptomatic malaria in Bongo District, Ghana. Asymptomatic prevalence by microscopy decreased significantly from 42.5% at the end of the wet to 27.5% at the end of the dry season (P < 0.001). Using the 18S rRNA polymerase chain reactions (PCRs), all microscopy-negative samples were screened and prevalence of submicroscopic infections also decreased significantly from the wet (55.4%) to the dry (20.7%) season (P < 0.001). Combining detection methods, 74.4% and 42.5% of the population in the wet and dry seasons, respectively, had evidence of a P. falciparum infection. Interestingly in those > 20 years of age, we found evidence of infection in 64.3% of the population in the wet and 27.0% in the dry season. Using both microscopy and PCR, the asymptomatic P. falciparum reservoir peaks at the end of the wet season and infections in all age groups constitute the reservoir of malaria infection. At the end of the wet season, spatial heterogeneity in the prevalence and density of P. falciparum infections was observed between the two catchment areas surveyed in Bongo District. These results indicate that if elimination is to succeed, interventions will need to target not just P. falciparum infections in children but also in adults, and be implemented toward the end of the dry season in this area of West Africa.

Figure 1.

The distribution of the compounds in Bongo District (BD) included in this study (red points) and the sections (blue lines) within each catchment area: Vea/Gowrie (lower left) and Soe (upper right). The local topography, major centers, road network (yellow and white lines) and the Ghanaian boarder with Burkina Faso (solid white line) are also indicated. A dam and irrigation area is proximal to the Vea/Gowrie catchment area. The location of the study site within Ghana is presented in the insert map (upper left). Note: The population of BD resides in rural communities made up of small farm settlements scattered throughout the district. For the purposes of this study not all compounds/households in BD were geolocated and therefore are not included on the map. This figure appears in color at www.ajtmh.org.

Figure 2.

Asymptomatic Plasmodium falciparum prevalence as determined by microscopy and 18S rRNA polymerase chain reactions (PCR) in relation to age group and survey/season. Hatched bars indicate individuals with microscopically detectable P. falciparum parasites; and solid bars, submicroscopic P. falciparum parasites detected by 18S rRNA PCR. The darker hatched and solid bars represent the data for the total number of participants surveyed.(A) Prevalence among all participants surveyed in Bongo District. (B) Prevalence among participants surveyed in Vea/Gowrie. (C) Prevalence among participants surveyed in Soe. Error bars represent the upper limits of the 95% confidence interval (CI).