The shape of the iceberg: quantification of submicroscopic Plasmodium falciparum and Plasmodium vivax parasitaemia and gametocytaemia in five low endemic settings in Ethiopia

Abstract

BackgroundThe widespread presence of low-density asymptomatic infections with concurrent gametocytes may be a stumbling block for malaria elimination. This study investigated the asymptomatic reservoir of Plasmodium falciparum and Plasmodium vivax infections in schoolchildren from five settings in northwest Ethiopia.MethodsTwo cross-sectional surveys were conducted in June and November 2015, enrolling 551 students from five schools and 294 students from three schools, respectively. Finger prick whole blood and plasma samples were collected. The prevalence and density of P. falciparum and P. vivax parasitaemia and gametocytaemia were determined by 18S rRNA quantitative PCR (qPCR) and pfs25 and pvs25 reverse transcriptase qPCR. Antibodies against blood stage antigens apical membrane antigen-1 (AMA-1) and merozoite surface protein-1 (MSP-119) were measured for both species.ResultsWhilst only 6 infections were detected by microscopy in 881 slides (0.7%), 107 of 845 blood samples (12.7%) were parasite positive by (DNA-based) qPCR. qPCR parasite prevalence between sites and surveys ranged from 3.8 to 19.0% for P. falciparum and 0.0 to 9.0% for P. vivax. The median density of P. falciparum infections (n = 85) was 24.4 parasites/µL (IQR 18.0–34.0) and the median density of P. vivax infections (n = 28) was 16.4 parasites/µL (IQR 8.8–55.1). Gametocyte densities by (mRNA-based) qRT-PCR were strongly associated with total parasite densities for both P. falciparum (correlation coefficient = 0.83, p = 0.010) and P. vivax (correlation coefficient = 0.58, p = 0.010). Antibody titers against P. falciparum AMA-1 and MSP-119 were higher in individuals who were P. falciparum parasite positive in both surveys (p < 0.001 for both comparisons).DiscussionThis study adds to the available evidence on the wide-scale presence of submicroscopic parasitaemia by quantifying submicroscopic parasite densities and concurrent gametocyte densities. There was considerable heterogeneity in the occurrence of P. falciparum and P. vivax infections and serological markers of parasite exposure between the examined low endemic settings in Ethiopia.

Global & Temporal Patterns of Submicroscopic Plasmodium falciparum Malaria Infection

“…Previous reviews lacked sufficient data in low and very low transmission areas to 4 explore this paradox sufficiently and moreover, identified extensive unexplained variation in the size 5 of the submicroscopically infected population across settings, suggesting the existence of other 6 important factors that determine the size of the submicroscopic reservoir. For example, although the 7 burden of submicroscopic infection is highly heterogeneous across different locations 9,24 , it remains 8 unclear whether this represents systematic variation according to geography or is reflective of other 9 underlying location-specific characteristics. 10…”

Spatial distribution of asymptomatic Plasmodium vivax and Plasmodium falciparum infections in low, moderate and high endemic settings in Ethiopia

“…For the community surveys, we expected an overall prevalence of 6.8% asymptomatic Plasmodium infections based on previous observations [26][27][28][29][30][31][32][33][34][35][36][37] with a precision of 5%. Based on previous experience, a minimum of 75 samples for the school surveys and 114 for the community samples was targeted across the study sites [21].…”

“…School based surveys were conducted at North Achefer, Bahir Dar Zuria, and Jawi districts in 2017. For the school-based surveys, students were randomly selected from elementary school students strati ed by age as described before [21] following protocols developed by Brooker and colleagues [22].…”

Immune effector mechanisms in malaria: An update focusing on human immunity

“…Many immunologic studies of malaria have revolved around understanding the mediators of the immunopathology associated with this disease and have identified the cell sources and responsible mediators. Yet, the global burden of P. falciparum today rests within those who are chronically infected and serve as reservoirs that perpetuate the transmission cycle . Therefore, more recent studies of malaria have interrogated the impact of chronic infections thereby increasing our understanding of immune regulators influenced by this parasite.…”

“…This phenotypic plasticity has been illustrated by CHMI studies 20 within those who are chronically infected 27,28 and serve as reservoirs that perpetuate the transmission cycle. 29,30 Therefore, more recent studies of malaria have interrogated the impact of chronic infections thereby increasing our understanding of immune regulators influenced by this parasite.…”