Asymptomatic malaria infections: detectability, transmissibility and public health relevance

Evidence of Plasmodium vivax circulation in western and eastern regions of Senegal: implications for malaria control

BACKGROUND

Malaria elimination in Senegal requires accurate diagnosis of all Plasmodium species. Plasmodium falciparum is the most prevalent species in Senegal, although Plasmodium malariae, Plasmodium ovale, and recently Plasmodium vivax have also been reported. Nonetheless, most malaria control tools, such as Histidine Rich Protein 2 rapid diagnosis test (PfHRP2-RDT,) can only diagnose P. falciparum. Thus, PfHRP2-RDT misses non-falciparum species and P. falciparum infections that fall below the limit of detection. These limitations can be addressed using highly sensitive Next Generation Sequencing (NGS). This study assesses the burden of the four different Plasmodium species in western and eastern regions of Senegal using targeted PCR amplicon sequencing.

METHODS

Three thousand samples from symptomatic and asymptomatic individuals in 2021 from three sites in Senegal (Sessene, Diourbel region; Parcelles Assainies, Kaolack region; Gabou, Tambacounda region) were collected. All samples were tested using PfHRP2-RDT and photoinduced electron transfer polymerase chain reaction (PET-PCR), which detects all Plasmodium species. Targeted sequencing of the nuclear 18S rRNA and the mitochondrial cytochrome B genes was performed on PET-PCR positive samples.

RESULTS

Malaria prevalence by PfHRP2-RDT showed 9.4% (94/1000) and 0.2% (2/1000) in Diourbel (DBL) and Kaolack (KL), respectively. In Tambacounda (TAM) patients who had malaria symptoms and had a negative PfHRP2-RDT were enrolled. The PET-PCR had a positivity rate of 23.5% (295/1255) overall. The PET-PCR positivity rate was 37.6%, 12.3%, and 22.8% in Diourbel, Kaolack, and Tambacounda, respectively. Successful sequencing of 121/295 positive samples detected P. falciparum (93%), P. vivax (2.6%), P. malariae (4.4%), and P. ovale wallikeri (0.9%). Plasmodium vivax was co-identified with P. falciparum in thirteen samples. Sequencing also detected two PfHRP2-RDT-negative mono-infections of P. vivax in Tambacounda and Kaolack.

CONCLUSION

The findings demonstrate the circulation of P. vivax in western and eastern Senegal, highlighting the need for improved malaria control strategies and accurate diagnostic tools to better understand the prevalence of non-falciparum species countrywide.

Dual CRISPR/Cas13a Cascade Strand Displacement-Triggered Transcription for Point-of-Care Detection of Plasmodium in Asymptomatic Malaria

Asymptomatic infections of Plasmodium parasites are major obstacles to malaria control and elimination. A sensitive, specific, and user-friendly method is urgently needed for point-of-care (POC) Plasmodium diagnostics in asymptomatic malaria, especially in resource-limited settings. In this work, we present a POC method (termed Cas13a-SDT) based on the cascade sequence recognition and signal amplification of dual Cas13a trans-cleavage and strand displacement-triggered transcription (SDT). Cas13a-SDT not only achieves exceptional specificity in discriminating the target RNA from nontarget RNAs with any cross-interaction but also meets the sensitivity criterion set by the World Health Organization (WHO) for effective malaria detection. Remarkably, this novel method was successfully applied to screen malaria in asymptomatic infections from clinical samples. The proposed method provides a user-friendly and visually interpretable output mode while maintaining high accuracy and reliability comparable to RT-PCR. These excellent features demonstrate the significant potential of Cas13a-SDT for POC diagnosis of Plasmodium infections, laying a vital foundation for advancing malaria control and elimination efforts.

Cooperation for malaria control and elimination in the Guiana Shield

The Guiana Shield, a small region of South America, is currently one of the main hotspots of malaria transmission on the continent. This Amazonian area is characterised by remarkable socioeconomic, cultural, health, and political heterogeneity and a high degree of regional and cross-border population mobility, which has contributed to the increase of malaria in the region in the past few years. In this context, regional cooperation to control malaria represents both a challenge and an indispensable initiative. This Viewpoint advocates for the creation of a regional cooperative mechanism for the elimination of malaria in the Guiana Shield. This strategy would help address operational and political obstacles to successful technical cooperation in the region and could contribute to reversing the regional upsurge in malaria incidence through creating a functional international control and elimination partnership.

Changing Clinical Epidemiology of Plasmodium vivax Malaria as Transmission Decreases: Population-Based Prospective Panel Survey in the Brazilian Amazon

BACKGROUND

Malarial infections are often missed by microscopy, and most parasite carriers are asymptomatic in low-endemicity settings. Whether parasite detectability and its ability to elicit symptoms change as transmission declines remains unclear.

METHODS

We performed a prospective panel survey with repeated measurements on the same participants over 12 months to investigate whether Plasmodium vivax detectability by microscopy and risk of symptoms upon infection varied during a community-wide larviciding intervention in the Amazon basin of Brazil that markedly reduced vector density. We screened 1096 to 1400 residents in the intervention site for malaria by microscopy and quantitative TaqMan assays at baseline and twice during intervention.

RESULTS

We found that more P vivax infections than expected from their parasite densities measured by TaqMan assays were missed by microscopy as transmission decreased. At lower transmission, study participants appeared to tolerate higher P vivax loads without developing symptoms. We hypothesize that changes in the ratio between circulating parasites and those that accumulate in the bone marrow and spleen, by avoiding peripheral blood microscopy detection, account for decreased parasite detectability and lower risk of symptoms under low transmission.

CONCLUSIONS

P vivax infections are more likely to be subpatent and remain asymptomatic as malaria transmission decreases.

Genotyping Plasmodium falciparum gametocytes using amplicon deep sequencing

BACKGROUND

Understanding the dynamics of gametocyte production in polyclonal Plasmodium falciparum infections requires a genotyping method that detects distinct gametocyte clones and estimates their relative frequencies. Here, a marker was identified and evaluated to genotype P. falciparum mature gametocytes using amplicon deep sequencing.

METHODS

A data set of polymorphic regions of the P. falciparum genome was mined to identify a gametocyte genotyping marker. To assess marker resolution, the number of unique haplotypes in the marker region was estimated from 95 Malawian P. falciparum whole genome sequences. Specificity of the marker for detection of mature gametocytes was evaluated using reverse transcription-polymerase chain reaction of RNA extracted from NF54 mature gametocytes and rings from a non-gametocyte-producing strain of P. falciparum. Amplicon deep sequencing was performed on experimental mixtures of mature gametocytes from two distinct parasite clones, as well as gametocyte-positive P. falciparum field isolates to evaluate the quantitative ability and determine the limit of detection of the genotyping approach.

RESULTS

A 400 bp region of the pfs230 gene was identified as a gametocyte genotyping marker. A larger number of unique haplotypes was observed at the pfs230 marker (34) compared to the sera-2 (18) and ama-1 (14) markers in field isolates from Malawi. RNA and DNA genotyping accurately estimated gametocyte and total parasite clone frequencies when evaluating agreement between expected and observed haplotype frequencies in gametocyte mixtures, with concordance correlation coefficients of 0.97 [95% CI: 0.92-0.99] and 0.92 [95% CI: 0.83-0.97], respectively. The detection limit of the genotyping method for male gametocytes was 0.41 pfmget transcripts/µl [95% CI: 0.28-0.72] and for female gametocytes was 1.98 ccp4 transcripts/µl [95% CI: 1.35-3.68].

CONCLUSIONS

A region of the pfs230 gene was identified as a marker to genotype P. falciparum gametocytes. Amplicon deep sequencing of this marker can be used to estimate the number and relative frequency of parasite clones among mature gametocytes within P. falciparum infections. This gametocyte genotyping marker will be an important tool for studies aimed at understanding dynamics of gametocyte production in polyclonal P. falciparum infections.

Naturally acquired immunity to Plasmodium pitheci in Bornean orangutans (Pongo pygmaeus)

Naturally acquired immunity to the different types of malaria in humans occurs in areas of endemic transmission and results in asymptomatic infection of peripheral blood. The current study examined the possibility of naturally acquired immunity in Bornean orangutans, Pongo pygmaeus, exposed to endemic Plasmodium pitheci malaria. A total of 2140 peripheral blood samples were collected between January 2017 and December 2022 from a cohort of 135 orangutans housed at a natural forested Rescue and Rehabilitation Centre in West Kalimantan, Indonesia. Each individual was observed for an average of 4.3 years during the study period. Blood samples were examined by microscopy and polymerase chain reaction for the presence of plasmodial parasites. Infection rates and parasitaemia levels were measured among age groups and all 20 documented clinical malaria cases were reviewed to estimate the incidence of illness and risk ratios among age groups. A case group of all 17 individuals that had experienced clinical malaria and a control group of 34 individuals having an event of >2000 parasites μL−1 blood but with no outward or clinical sign of illness were studied. Immature orangutans had higher-grade and more frequent parasitaemia events, but mature individuals were more likely to suffer from clinical malaria than juveniles. The case orangutans having patent clinical malaria were 256 times more likely to have had no parasitaemia event in the prior year relative to asymptomatic control orangutans. The findings are consistent with rapidly acquired immunity to P. pitheci illness among orangutans that wanes without re-exposure to the pathogen.

Child Health and Infection with Low Density (CHILD) malaria: a protocol for a randomised controlled trial to assess the long-term health and socioeconomic impacts of testing and treating low-density malaria infection among children in Tanzania

INTRODUCTION

As malaria declines, low-density malaria infections (LMIs) represent an increasing proportion of infections and may have negative impacts on child health and cognition, necessitating development of targeted and effective solutions. This trial assesses the health, cognitive and socioeconomic impact of two strategies for detecting and treating LMI in a low transmission setting.

METHODS AND ANALYSIS

The study is a 3-arm open-label individually randomised controlled trial enrolling 600 children aged 6 months to 10 years in Bagamoyo district, Tanzania. Children are randomised to one of three arms: active case detection with molecular (ACDm) testing by high volume quantitative PCR (qPCR), passive case detection also with molecular testing (PCDm) and a control of standard PCD using rapid diagnostics tests (RDTs). Over the 2-year trial, ACDm participants receive malaria testing using RDT and qPCR three times annually, and malaria testing by RDT only when presenting with fever. PCDm and PCD participants receive malaria testing by RDT and qPCR or RDT only, respectively, when presenting with fever. RDT or qPCR positive participants with uncomplicated malaria are treated with artemether lumefantrine. The primary outcome is cumulative incidence of all-cause sick visits. Secondary outcomes include fever episodes, clinical failure after fever episodes, adverse events, malaria, non-malarial infection, antibiotic use, anaemia, growth faltering, cognition and attention, school outcomes, immune responses, and socioeconomic effects. Outcomes are assessed through monthly clinical assessments and testing, and baseline and endline neurodevelopmental testing. The trial is expected to provide key evidence and inform policy on health, cognitive and socioeconomic impact of interventions targeting LMI in children.

ETHICS AND DISSEMINATION

Study is approved by Tanzania NatHREC and institutional review boards at University of California San Francisco and Ifakara Health Institute. Findings will be reported on ClinicalTrials.gov, in peer-reviewed journals and through stakeholder meetings.

TRIAL REGISTRATION NUMBER

NCT05567016.

Malaria infection among adults residing in a highly endemic region from the Democratic Republic of the Congo

BACKGROUND

Adults infected with Plasmodium spp. in endemic areas need to be re-evaluated in light of global malaria elimination goals. They potentially undermine malaria interventions but remain an overlooked aspect of public health strategies.

METHODS

This study aimed to estimate the prevalence of Plasmodium spp. infections, to identify underlying parasite species, and to assess predicting factors among adults residing in an endemic area from the Democratic Republic of Congo (DRC). A community-based cross-sectional survey in subjects aged 18 years and above was therefore carried out. Study participants were interviewed using a standard questionnaire and tested for Plasmodium spp. using a rapid diagnostic test and a nested polymerase chain reaction assay. Logistic regression models were fitted to assess the effect of potential predictive factors for infections with different Plasmodium spp.

RESULTS

Overall, 420 adults with an estimated prevalence of Plasmodium spp. infections of 60.2% [95% CI 55.5; 64.8] were included. Non-falciparum species infected 26.2% [95% CI 22.2; 30.5] of the study population. Among infected participants, three parasite species were identified, including Plasmodium falciparum (88.5%), Plasmodium malariae (39.9%), and Plasmodium ovale (7.5%) but no Plasmodium vivax. Mixed species accounted for 42.3% of infections while single-species infections predominated with P. falciparum (56.5%) among infected participants. All infected participants were asymptomatic at the time of the survey. Adults belonging to the "most economically disadvantaged" households had increased risks of infections with any Plasmodium spp. (adjusted odds ratio, aOR = 2.87 [95% CI 1.66, 20.07]; p < 0.001), compared to those from the "less economically disadvantaged" households. Conversely, each 1 year increase in age reduced the risk of infections with any Plasmodium spp. (aOR = 0.99 [95% CI 0.97, 0.99]; p = 0.048). Specifically for non-falciparum spp., males had increased risks of infection than females (aOR = 1.83 [95% CI 1.13, 2.96]; p = 0.014).

CONCLUSION

Adults infected with malaria constitute a potentially important latent reservoir for the transmission of the disease in the study setting. They should specifically be taken into account in public health measures and translational research.

Malaria blood stage infection suppresses liver stage infection via host-induced interferons but not hepcidin

Malaria-causing Plasmodium parasites first replicate as liver stages (LS), which then seed symptomatic blood stage (BS) infection. Emerging evidence suggests that these stages impact each other via perturbation of host responses, and this influences the outcome of natural infection. We sought to understand whether the parasite stage interplay would affect live-attenuated whole parasite vaccination, since the efficacy of whole parasite vaccines strongly correlates with their extend of development in the liver. We thus investigated the impact of BS infection on LS development of genetically attenuated and wildtype parasites in female rodent malaria models and observed that for both, LS infection suffered severe suppression during concurrent BS infection. Strikingly and in contrast to previously published studies, we find that the BS-induced iron-regulating hormone hepcidin is not mediating suppression of LS development. Instead, we demonstrate that BS-induced host interferons are the main mediators of LS developmental suppression. The type of interferon involved depended on the BS-causing parasite species. Our study provides important mechanistic insights into the BS-mediated suppression of LS development. This has direct implications for understanding the outcomes of live-attenuated Plasmodium parasite vaccination in malaria-endemic areas and might impact the epidemiology of natural malaria infection.

Malaria Prevalence in Asymptomatic and Symptomatic Children Living in Rural, Semi-Urban and Urban Areas in Eastern Gabon

BACKGROUND

Malaria remains a major public health issue in the world despite a decline in the disease burden. However, though symptomatic malaria is diagnosed and treated, asymptomatic infections remain poorly known and support transmission. This study assessed the prevalence of symptomatic and asymptomatic Plasmodium spp. infections in three areas in Gabon to monitor and evaluate the impact of malaria.

METHODS AND RESULTS

A cross-sectional study was conducted in three areas of Gabon. Febrile and afebrile children aged 6 months to 15 years were included in this study. Malaria prevalence was determined by microscopy of and using rapid diagnostic test (RDT). Plasmodium spp. species were identified by PCR according to the Snounou method. The data were recorded in Excel, and the statistical analyses were performed using the software R version R 64 × 3.5.0. A total of 2381(333 asymptomatic and 107 symptomatic) children were included. The overall prevalence of malaria was 40% (952/2381), with the majority (77% symptomatic and 98% asymptomatic) of infections caused by Plasmodium falciparum. A high prevalence of malaria was found in infected children in rural and semi-rural areas. In these two areas, a higher prevalence of Plasmodium malariae was observed in asymptomatic. Furthermore, mixed infections were more prevalent in asymptomatic children than in symptomatic.

CONCLUSION

This study showed that the prevalence of Plasmodium spp. infection varied according to the regions. The main species was Plasmodium falciparum, but in asymptomatic children the prevalence of Plasmodium malariae was high in rural areas. To help fight malaria more effectively asymptomatic infections should be taken into account and treated.

Comparative analysis of peripheral whole blood transcriptome from asymptomatic carriers reveals upregulation of subsets of surface proteins implicated in Plasmodium falciparum phenotypic plasticity

The molecular mechanism underlying Plasmodium falciparum's persistence in the asymptomatic phase of infection remains largely unknown. However, large-scale shifts in the parasites' gene expression during asymptomatic infections may enhance phenotypic plasticity, maximizing their fitness and leading to the persistence of the asymptomatic infections. To uncover these mechanisms, we aimed to identify parasite genetic factors implicated in asymptomatic infections through whole transcriptome analysis. We analyzed publicly available transcriptome datasets containing asymptomatic malaria (ASM), uncomplicated malaria (SM), and malaria-naïve (NSM) samples from 35 subjects for differentially expressed genes (DEGs) and long noncoding RNAs. Our analysis identified 755 and 1773 DEGs in ASM vs SM and NSM, respectively. These DEGs revealed sets of genes coding for proteins of unknown functions (PUFs) upregulated in ASM vs SM and ASM, suggesting their role in underlying fundamental molecular mechanisms during asymptomatic infections. Upregulated genes in ASM vs SM revealed a subset of 24 clonal variant genes (CVGs) involved in host-parasite and symbiotic interactions and modulation of the symbiont of host erythrocyte aggregation pathways. Moreover, we identified 237 differentially expressed noncoding RNAs in ASM vs SM, of which 11 were found to interact with CVGs, suggesting their possible role in regulating the expression of CVGs. Our results suggest that P. falciparum utilizes phenotypic plasticity as an adaptive mechanism during asymptomatic infections by upregulating clonal variant genes, with long noncoding RNAs possibly playing a crucial role in their regulation. Thus, our study provides insights into the parasites' genetic factors that confer a fitness advantage during asymptomatic infections.

Cryogenic enrichment of Plasmodium falciparum gametocytes from spiked whole blood

Each individual cell type typically requires a unique set of conditions for optimal cryopreservation outcome, which relates to its specific response to cryoprotective agent (CPA) toxicity, osmotic behavior and sensitivity to ice crystallization. Cryopreservation of heterogenous cell populations is therefore exceedingly difficult as it requires separate and often conflicting conditions for each cell type. Conversely, these contrasting conditions could be utilized to favor cryogenic preference of a single cell population within a heterogenous sample, leading to its enrichment by elimination of remaining cells. To establish proof-of-concept for this overall approach, a protocol was developed for the cryogenic enrichment of Plasmodium falciparum gametocytes from whole blood. To accomplish this goal, we evaluated the effects of CPAs and cooling conditions during cryopreservation of whole blood samples spiked with P. falciparum gametocytes. We identified that cooling to -80 °C at a rate of -1 °C/min in the presence of 11 % glycerol selectively favors recovery of gametocytes. This protocol eliminates 95.3 ± 1.7 % of total blood cells and recovers 43.2 ± 6.5 % of parasites, leading to a 19-fold enrichment as assessed by microscopic examination of blood smears. This protocol is tunable, where gametocyte enrichment 900-fold may be feasible, however there is an apparent tradeoff in overall parasite recovery. Although translation of this protocol for point-of-care testing for malaria presents many challenges, the overall approach of cryogenic purification may prove useful for alternative diagnostic applications.

Microfluidic systems for infectious disease diagnostics

Microorganisms, encompassing both uni- and multicellular entities, exhibit remarkable diversity as omnipresent life forms in nature. They play a pivotal role by supplying essential components for sustaining biological processes across diverse ecosystems, including higher host organisms. The complex interactions within the human gut microbiota are crucial for metabolic functions, immune responses, and biochemical signalling, particularly through the gut-brain axis. Viruses also play important roles in biological processes, for example by increasing genetic diversity through horizontal gene transfer when replicating inside living cells. On the other hand, infection of the human body by microbiological agents may lead to severe physiological disorders and diseases. Infectious diseases pose a significant burden on global healthcare systems, characterized by substantial variations in the epidemiological landscape. Fast spreading antibiotic resistance or uncontrolled outbreaks of communicable diseases are major challenges at present. Furthermore, delivering field-proven point-of-care diagnostic tools to the most severely affected populations in low-resource settings is particularly important and challenging. New paradigms and technological approaches enabling rapid and informed disease management need to be implemented. In this respect, infectious disease diagnostics taking advantage of microfluidic systems combined with integrated biosensor-based pathogen detection offers a host of innovative and promising solutions. In this review, we aim to outline recent activities and progress in the development of microfluidic diagnostic tools. Our literature research mainly covers the last 5 years. We will follow a classification scheme based on the human body systems primarily involved at the clinical level or on specific pathogen transmission modes. Important diseases, such as tuberculosis and malaria, will be addressed more extensively.

Hyper-diverse antigenic variation and resilience to transmission-reducing intervention in falciparum malaria

Intervention against falciparum malaria in high transmission regions remains challenging, with relaxation of control efforts typically followed by rapid resurgence. Resilience to intervention co-occurs with incomplete immunity, whereby children eventually become protected from severe disease but not infection and a large transmission reservoir results from high asymptomatic prevalence across all ages. Incomplete immunity relates to the vast antigenic variation of the parasite, with the major surface antigen of the blood stage of infection encoded by the multigene family known as var. Recent deep sampling of var sequences from individual isolates in northern Ghana showed that parasite population structure exhibited persistent features of high-transmission regions despite the considerable decrease in prevalence during transient intervention with indoor residual spraying (IRS). We ask whether despite such apparent limited impact, the transmission system had been brought close to a transition in both prevalence and resurgence ability. With a stochastic agent-based model, we investigate the existence of such a transition to pre-elimination with intervention intensity, and of molecular indicators informative of its approach. We show that resurgence ability decreases sharply and nonlinearly across a narrow region of intervention intensities in model simulations, and identify informative molecular indicators based on var gene sequences. Their application to the survey data indicates that the transmission system in northern Ghana was brought close to transition by IRS. These results suggest that sustaining and intensifying intervention would have pushed malaria dynamics to a slow-rebound regime with an increased probability of local parasite extinction.

Sickle cell allele HBB-rs334(T) is associated with decreased risk of childhood Burkitt lymphoma in East Africa

Burkitt lymphoma (BL) is an aggressive B-cell lymphoma that significantly contributes to childhood cancer burden in sub-Saharan Africa. Plasmodium falciparum, which causes malaria, is geographically associated with BL, but the evidence remains insufficient for causal inference. Inference could be strengthened by demonstrating that mendelian genes known to protect against malaria-such as the sickle cell trait variant, HBB-rs334(T)-also protect against BL. We investigated this hypothesis among 800 BL cases and 3845 controls in four East African countries using genome-scan data to detect polymorphisms in 22 genes known to affect malaria risk. We fit generalized linear mixed models to estimate odds ratios (OR) and 95% confidence intervals (95% CI), controlling for age, sex, country, and ancestry. The ORs of the loci with BL and P. falciparum infection among controls were correlated (Spearman's ρ = 0.37, p = .039). HBB-rs334(T) was associated with lower P. falciparum infection risk among controls (OR = 0.752, 95% CI 0.628-0.9; p = .00189) and BL risk (OR = 0.687, 95% CI 0.533-0.885; p = .0037). ABO-rs8176703(T) was associated with decreased risk of BL (OR = 0.591, 95% CI 0.379-0.992; p = .00271), but not of P. falciparum infection. Our results increase support for the etiological correlation between P. falciparum and BL risk.

Comparative transcriptomics reveal differential gene expression among Plasmodium vivax geographical isolates and implications on erythrocyte invasion mechanisms

The documentation of Plasmodium vivax malaria across Africa especially in regions where Duffy negatives are dominant suggests possibly alternative erythrocyte invasion mechanisms. While the transcriptomes of the Southeast Asian and South American P. vivax are well documented, the gene expression profile of P. vivax in Africa is unclear. In this study, we examined the expression of 4,404 gene transcripts belong to 12 functional groups and 43 erythrocyte binding gene candidates in Ethiopian isolates and compared them with the Cambodian and Brazilian P. vivax transcriptomes. Overall, there were 10-26% differences in the gene expression profile amongst geographical isolates, with the Ethiopian and Cambodian P. vivax being most similar. Majority of the gene transcripts involved in protein transportation, housekeeping, and host interaction were highly transcribed in the Ethiopian isolates. Members of the reticulocyte binding protein PvRBP2a and PvRBP3 expressed six-fold higher than Duffy binding protein PvDBP1 and 60-fold higher than PvEBP/DBP2 in the Ethiopian isolates. Other genes including PvMSP3.8, PvMSP3.9, PvTRAG2, PvTRAG14, and PvTRAG22 also showed relatively high expression. Differential expression patterns were observed among geographical isolates, e.g., PvDBP1 and PvEBP/DBP2 were highly expressed in the Cambodian but not the Brazilian and Ethiopian isolates, whereas PvRBP2a and PvRBP2b showed higher expression in the Ethiopian and Cambodian than the Brazilian isolates. Compared to Pvs25, gametocyte genes including PvAP2-G, PvGAP (female gametocytes), and Pvs47 (male gametocytes) were highly expressed across geographical samples.

Immune mechanisms targeting malaria transmission: opportunities for vaccine development

INTRODUCTION

Malaria continues to remain a major global health problem with nearly a quarter of a billion clinical cases and more than 600,000 deaths in 2022. There has been significant progress toward vaccine development, however, poor efficacy of approved vaccines requiring multiple immunizing doses emphasizes the need for continued efforts toward improved vaccines. Progress to date, nonetheless, has provided impetus for malaria elimination.

AREAS COVERED

In this review we will focus on diverse immune mechanisms targeting gametocytes in the human host and gametocyte-mediated malaria transmission via the mosquito vector.

EXPERT OPINION

To march toward the goal of malaria elimination it will be critical to target the process of malaria transmission by mosquitoes, mediated exclusively by the sexual stages, i.e. male, and female gametocytes, ingested from infected vertebrate host. Studies over several decades have established antigens in the parasite sexual stages developing in the mosquito midgut as attractive targets for the development of transmission blocking vaccines (TBVs). Immune clearance of gametocytes in the vertebrate host can synergize with TBVs and directly aid in maintaining effective transmission reducing immune potential.

Is qPCR always the most sensitive method for malaria diagnostic quality surveillance?

In many studies to evaluate the quality of malaria diagnosis, microscopy or rapid diagnostic tests (RDT) are compared to PCR. Depending on the method for sample collection and storage (whole blood or dried blood spot), volume of blood used for extraction, volume of DNA used as PCR template, and choice of PCR target (single vs. multi-copy gene), the limit of detection (LOD) of PCR might not exceed the LOD of expert microscopy or RDT. One should not assume that PCR always detects the highest number of infections.

Prevalence of G6PD deficiency and submicroscopic malaria parasites carriage in malaria hotspot area in Northwest, Tanzania

BACKGROUND

The use of primaquine for mass drug administration (MDA) is being considered as a key strategy for malaria elimination. In addition to being the only drug active against the dormant and relapsing forms of Plasmodium vivax, primaquine is the sole potent drug against mature/infectious Plasmodium falciparum gametocytes. It may prevent onward transmission and help contain the spread of artemisinin resistance. However, higher dose of primaquine is associated with the risk of acute haemolytic anaemia in individuals with a deficiency in glucose-6-phosphate dehydrogenase. In many P. falciparum endemic areas there is paucity of information about the distribution of individuals at risk of primaquine-induced haemolysis at higher dose 45 mg of primaquine.

METHODS

A retrospective cross-sectional study was carried out using archived samples to establish the prevalence of G6PD deficiency in a malaria hotspot area in Misungwi district, located in Mwanza region, Tanzania. Blood samples collected from individuals recruited between August and November 2010 were genotyped for G6PD deficiency and submicroscopic parasites carriage using polymerase chain reaction.

RESULTS

A total of 263 individuals aged between 0 and 87 were recruited. The overall prevalence of the X-linked G6PD A- mutation was 83.7% (220/263) wild type, 8% (21/263) heterozygous and 8.4% (22/263) homozygous or hemizygous. Although, assessment of the enzymatic activity to assign the phenotypes according to severity and clinical manifestation as per WHO was not carried out, the overall genotype and allele frequency for the G6PD deficiency was 16.4% and 13. 2%, respectively. There was no statistically significant difference in among the different G6PD genotypes (p > 0.05). Out of 248 samples analysed for submicroscopic parasites carriage, 58.1% (144/248) were P. falciparum positive by PCR. G6PD heterozygous deficiency were associated with carriage of submicroscopic P. falciparum (p = 0.029).

CONCLUSIONS

This study showed that 16.4% of the population in this part of North-western Tanzania carry the G6PD A- mutation, within the range of 15-32% seen in other parts of Africa. G6PD gene mutation is widespread and heterogeneous across the study area where primaquine would be valuable for malaria control and elimination. The maps and population estimates presented here reflect potential risk of higher dose of primaquine being associated with the risk of acute haemolytic anaemia (AHA) in individuals with a deficiency in glucose-6-phosphate dehydrogenase and call further research on mapping of G6PD deficiency in Tanzania. Therefore, screening and education programmes for G6PD deficiency is warranted in a programme of malaria elimination using a higher primaquine dose.

Monitoring the density of Plasmodium spp. gametocytes in isolates from patient samples in the region of Porto Velho, Rondônia

Gametocytes are the forms of the malaria parasite that are essential for the continuation of the transmission cycle to the vector Anopheles. This study aimed to evaluate the parasite density of Plasmodium spp gametocytes in samples from patients in the region of Porto Velho, Rondônia. Slides containing patient samples were selected from users who sought out care at the Center for Research in Tropical Medicine (CEPEM) during the period from January to December 2016. Samples of Plasmodium vivax and Plasmodium falciparum were selected for analysis of their respective gametocytes. In parallel, monitoring was performed in cultures of NF54 strain P. falciparum gametocytes. Of 248 thick smear slides (EG) evaluated in double blind, 142 (57.2%) were detected with P. vivax, of this total 47 (18.9%) had gametocytes, 1 (0.4%) with LVC negative diagnosis for gametocytes and 1 (0.4%) Pv + Pf (mixed malaria). Regarding P. falciparum, the total number of samples analyzed was 106 (42.7%), of which 20 (8.0%) had gametocytes detected, 6 (2.4%) LVC negative for gametocyte forms, and 3 (1.2%) Pv + Pf (mixed malaria), Plasmodium malariae species was not detected among the samples. The results showed that P. vivax gametocytes were present in the first days of symptoms, with a higher prevalence in patients with two crosses, a fact that was also observed in patients with P. falciparum regarding the prevalence of gametocytes. Faced with this problem, it is necessary to monitor the fluctuation of gametocytes, since these forms are responsible for continuing the malaria cycle within the mosquito vector.

Diagnostic performance of ultrasensitive rapid diagnostic test for the detection of Plasmodium falciparum infections in asymptomatic individuals in Kisangani, Northeast Democratic Republic of Congo

BACKGROUND

Ultrasensitive rapid diagnostic test (usRDT) was recently developed to improve the detection of low-density Plasmodium falciparum infections. However, its diagnostic performance has not been evaluated in the Democratic Republic of Congo (DRC). This study aims to determine the performance of the usRDT in malaria diagnosis in asymptomatic individuals under field condition in Kisangani, Northeast of DRC.

METHODS

A community-based cross-sectional study was carried out from June to August 2022 on 312 asymptomatic individuals residing in the city of Kisangani. Capillary blood samples were collected by finger prick for microscopic examination of thick and thin blood film, RDTs, and nested polymerase chain reaction (PCR). Alere™ Malaria Ag P.f usRDT and conventional RDT (cRDT/SD Bioline Malaria Ag P.f) kits were used for the detection of Plasmodium histidine rich protein 2 (HRP2) antigen as a proxy for the presence of P. falciparum. The diagnostic performance of the usRDT was compared with cRDT, microscopy and PCR.

RESULTS

The prevalence of asymptomatic P. falciparum malaria was 40.4%, 42.0%, 47.1% and 54.2% by cRDT, microscopy, usRDT and PCR, respectively. By using PCR as a reference, usRDT had sensitivity and specificity of 87.0% (95% CI 81.4-91.7) and 100.0% (95% CI 97.5-100.0), respectively, whereas the cRDT had sensitivity and specificity of 74.6% (95% CI 67.3-80.9) and 100% (95% CI 97.1-100.0), respectively. By using microscopy as a reference, usRDT had sensitivity and specificity of 96.9% (95% CI 92.4-99.2) and 89.0% (95% CI 83.5-93.1), respectively, while the cRDT had sensitivity and specificity of 96.2% (95% CI 92.3-98.7) and 100% (95% CI 97.9-100.0), respectively.

CONCLUSION

The usRDT showed better diagnostic performance with higher sensitivity than the cRDT which is currently in use as point-of-care test. Further research is necessary to assess the access and cost-effectiveness of the usRDTs to use for malaria surveillance.

Schoolchildren with asymptomatic malaria are potential hotspot for malaria reservoir in Ethiopia: implications for malaria control and elimination efforts

BACKGROUND

Schoolchildren with asymptomatic malaria infections often go undiagnosed and untreated, serving as reservoirs for infection that hamper malaria control and elimination efforts. In this context, little is known about the magnitude of asymptomatic malaria infections in apparently healthy schoolchildren in Ethiopia. This study was aimed at determining the prevalence of asymptomatic malaria infection and its associated factors in apparently healthy schoolchildren in Ethiopia.

METHODS

From September 2021 to January 2022, a school-based cross-sectional study was conducted on 994 apparently healthy schoolchildren (aged 6-15 years) selected from 21 primary schools in the Gomma district, of Jimma zone, southwestern Oromia, Ethiopia. A multi-stage sampling technique was used to select schools and participants. After allocating the total sample proportionally to each school and then to each grade, participants were selected using the lottery method from a list of student records (rosters). Finger-pricked blood samples were collected for microscopy blood film preparation and malaria rapid diagnostic test (RDT) (SD Bioline Malaria Ag Pf/Pv). Moreover, dry blood spots (DBSs) were prepared onto filter papers for quantitative real time polymerase chain reaction (qPCR) analysis.

RESULTS

As determined by RDT and microscopy, the prevalence of asymptomatic malaria was 2.20% and 1.51%, respectively. Using qPCR, the overall prevalence was 5.03% (50/994). Of this, Plasmodium falciparum, Plasmodium vivax and mixed infections accounted for 90%, 6% and 4%, respectively. Submicroscopic asymptomatic malaria infection was also accounted for 70% (35/50) of the overall prevalence. Household head age, nighttime outdoor activities of household heads, family history of malaria, absence of insecticide-treated nets (ITN), and presence of stagnant water around the houses are all significantly associated with asymptomatic malaria infections among schoolchildren.

CONCLUSIONS

This study found that both RDT and microscopy underestimated the prevalence of asymptomatic malaria in schoolchildren. However, qPCR was able to detect even low levels of parasitaemia and revealed a higher prevalence of asymptomatic submicroscopic malaria infections. The findings imply that schoolchildren with asymptomatic malaria infection are potential hotspot for malaria reservoir that fuels ongoing transmission. Therefore, it is imperative to include schoolchildren and schools in malaria intervention package and equally important is the adoption of more advanced and sensitive diagnostic tools, which would be crucial for successful malaria control and elimination efforts. Targeted interventions for asymptomatic malaria-infected schoolchildren can provide invaluable support to the National Malaria Control Programme in controlling and eventually eliminating the disease.

Development and evaluation of PlasmoPod: A cartridge-based nucleic acid amplification test for rapid malaria diagnosis and surveillance

Malaria surveillance is hampered by the widespread use of diagnostic tests with low sensitivity. Adequate molecular malaria diagnostics are often only available in centralized laboratories. PlasmoPod is a novel cartridge-based nucleic acid amplification test for rapid, sensitive, and quantitative detection of malaria parasites. PlasmoPod is based on reverse-transcription quantitative polymerase chain reaction (RT-qPCR) of the highly abundant Plasmodium spp. 18S ribosomal RNA/DNA biomarker and is run on a portable qPCR instrument which allows diagnosis in less than 30 minutes. Our analytical performance evaluation indicates that a limit-of-detection as low as 0.02 parasites/μL can be achieved and no cross-reactivity with other pathogens common in malaria endemic regions was observed. In a cohort of 102 asymptomatic individuals from Bioko Island with low malaria parasite densities, PlasmoPod accurately detected 83 cases, resulting in an overall detection rate of 81.4%. Notably, there was a strong correlation between the Cq values obtained from the reference RT-qPCR assay and those obtained from PlasmoPod. In an independent cohort, using dried blood spots from malaria symptomatic children living in the Central African Republic, we demonstrated that PlasmoPod outperforms malaria rapid diagnostic tests based on the PfHRP2 and panLDH antigens as well as thick blood smear microscopy. Our data suggest that this 30-minute sample-to-result RT-qPCR procedure is likely to achieve a diagnostic performance comparable to a standard laboratory-based RT-qPCR setup. We believe that the PlasmoPod rapid NAAT could enable widespread accessibility of high-quality and cost-effective molecular malaria surveillance data through decentralization of testing and surveillance activities, especially in elimination settings.

Prevalence and predictors of asymptomatic malaria infection in Boricha District, Sidama Region, Ethiopia: implications for elimination strategies

BACKGROUND

Malaria remains a major public health threat in Ethiopia despite the tremendous progress made towards the 2030 elimination targets. The silent transmission of asymptomatic infection is one of the factors that enhance the persistence of the disease as a public health issue and impedes efforts to eliminate malaria. Thus, this study aimed at investigating the prevalence and risk factors of asymptomatic malaria infection in Boricha district, Sidama region of Ethiopia.

METHODS

A community-based cross-sectional study was conducted in eight selected kebeles (smallest administrative unit) in Boricha district. Representative households were chosen using a multi-stage sampling technique. A total of 573 participants were included in the study. Malaria diagnosis was performed using rapid diagnostic test (RDT) and microscopy. A structured questionnaire was administered to collect socio-demographic information. Epi data 3.1 was employed for data entry, and SPSS version 25 was used for analysis.

RESULTS

Of the 573 asymptomatic participants tested, 6.1% were found to be positive by RDT and 4.0% by microscopy. Participants aged under 5 years (AOR = 1.57, 95% CI 0.46-5.39) and 5-14 years old (AOR = 2.42, 95% CI 1.08-5.40), Insecticide-treated net utilization (AOR = 8.41; 95% CI 1.09-65.08), travel history (AOR = 6.85, 95% CI 2.32-20.26) and living in a house with windows (AOR = 2.11, 95% CI 1.02-4.36) were significantly associated with the asymptomatic malaria infection.

CONCLUSION

The findings of this study revealed that prevalence of asymptomatic malaria infection was higher in the study area. As a result, rigorous implementation of existing interventions, such as vector control and anti-malaria drugs, is strongly recommended. In addition, devising new ones that are suited to the contextual situations is highly suggested.

The question of the early diagnosis of asymptomatic and subpatent malaria in pregnancy: Implications for diagnostic tools in a malaria endemic area

Objectives

Malaria in pregnancy (MIP) is a major healthcare challenge in low-income countries with high malaria endemicity. Early but accurate diagnosis and appropriate treatment is the hallmark of preventing disease progression/adverse outcomes in the mother, foetus and neonates. We assessed the comparative diagnostic performance of Malaria Rapid Diagnostic Test (mRDT), microscopy and PCR for malaria diagnosis in pregnant women for early detection of asymptomatic malaria in pregnant women.

Study design

Five hundred and twenty Pregnant women attending study clinics within Ikene and Remo North LGAs with gestational age between 16 and 29 weeks, willing and consented; were enrolled into the study. Blood samples collected via venepuncture were screened for malaria using microscopy, mRDTs kits, and PCR techniques on their first visit (V1) and at delivery. The parasite positivity rates, sensitivity and specificity were calculated and compared for each technique using PCR as the standard. Data was entered into REDCap® online database and analysis done using Stata and MedCalc®.

Results and conclusions

Average age of enrolled women was 28.8 years and mean gestational age was 21.0 weeks. The parasite positivity rates were 4.3%, 8.8% and 25.0% for microscopy, mRDT and PCR at V1 and was 2.4%, 3.4% and 43.4% at delivery, respectively. Sensitivity for microscopy and mRDT was 11.2% and 30.3% respectively at V1, while specificity was 98.2% and 98.5%. At delivery, the sensitivity reduced to 1.6% and 4.9%; while specificity was 96.9% and 97.6% respectively. Only 2.3% cases correlated with all three diagnostic methods. Our data showed a decrease in sensitivity of the diagnostic methods as pregnancy progressed, which may be due to very low parasitaemia, but high specificity. Our study demonstrated a high rate of subpatent parasitaemia amongst pregnant women. This finding therefore raises the question of the effect of subpatent parasitaemia on the health of the mother and foetus.

Novel bioinformatic methods and machine learning approaches reveal candidate biomarkers of the intensity and timing of past exposure to Plasmodium falciparum

Accurately quantifying the burden of malaria over time is an important goal of malaria surveillance efforts and can enable effective targeting and evaluation of interventions. Malaria surveillance methods capture active or recent infections which poses several challenges to achieving malaria surveillance goals. In high transmission settings, asymptomatic infections are common and therefore accurate measurement of malaria burden demands active surveillance; in low transmission regions where infections are rare accurate surveillance requires sampling large subsets of the population; and in any context monitoring malaria burden over time necessitates serial sampling. Antibody responses to Plasmodium falciparum parasites persist after infection and therefore measuring antibodies has the potential to overcome several of the current obstacles to accurate malaria surveillance. Identifying which antibody responses are markers of the timing and intensity of past exposure to P. falciparum remains challenging, particularly among adults who tend to be re-exposed multiple times over the course of their lifetime and therefore have similarly high antibody responses to many Plasmodium antigens. A previous analysis of 479 serum samples from individuals in three regions in southern Africa with different historical levels of P. falciparum malaria transmission (high, intermediate, and low) revealed regional differences in antibody responses to P. falciparum antigens among children under 5 years of age. Using a novel bioinformatic pipeline optimized for protein microarrays that minimizes between-sample technical variation, we used antibody responses to Plasmodium antigens as predictors in random forest models to classify samples from adults into these three regions of differing historical malaria transmission with high accuracy (AUC = 0.99). Many of the most important antigens for classification in these models do not overlap with previously published results and are therefore novel candidate markers for the timing and intensity of past exposure to P. falciparum. Measuring antibody responses to these antigens could lead to improved malaria surveillance.

Epidemiology of malaria, schistosomiasis, and geohelminthiasis amongst children 3-15 years of age during the dry season in Northern Cameroon

BACKGROUND

The double burden of malaria and helminthiasis in children poses an obvious public health challenge, particularly in terms of anemia morbidity. While both diseases frequently geographically overlap, most studies focus on mono-infection and general prevalence surveys without molecular analysis. The current study investigated the epidemiological determinants of malaria, schistosomiasis, and geohelminthiasis transmission among children in the North Region of Cameroon.

METHODOLOGY

School and pre-school children aged 3-15 year-of-age were enrolled from three communities in March 2021 using a community cross-sectional design. Capillary-blood samples were obtained, and each was examined for malaria parasites using rapid-diagnostic-test (RDT), microscopy, and PCR while hemoglobin level was measured using a hemoglobinometer. Stool samples were analyzed for Schistosoma mansoni, S. guineensis, and soil-transmitted-helminthiasis (STH) infections using the Kato Katz method, and urine samples were assessed for the presence of S. haematobium eggs (including hybrids) using the standard urine filtration technique.

RESULT

A malaria prevalence of 56% (277/495) was recorded by PCR as opposed to 31.5% (156/495) by microscopy and 37.8% (186/495) by RDT. Similarly, schistosomiasis was observed at prevalence levels of up to 13.3% (66/495) overall [S. haematobium (8.7%); S. mansoni (3.8%); mixed Sh/Sm (0.6%); mixed Sh/Sm/Sg (0.2%). Both infections were higher in males and the 3-9 year-of-age groups. A high frequency of PCR reported P. falciparum mono-infection of 81.9% (227/277) and mixed P. falciparum/P. malariae infection of 17.3% (48/277) was observed. Malaria-helminths co-infections were observed at 13.1% (65/495) with marked variation between P. falciparum/S. haematobium (50.8%, 33/65); P. falciparum/S. mansoni (16.9%, 11/65) and P. falciparum/Ascaris (9.2%, 6/65) (χ2 = 17.5, p = 0.00003). Anemia prevalence was 32.9% (163/495), categorically associated with P. falciparum (45.8%, 104/227), Pf/Sh (11.5%, 26/227), and Pf/Sm (3.9%, 9/227) polyparasitism.

CONCLUSION

Polyparasitism with malaria and helminth infections is common in school-aged children despite periodic long-lasting insecticide-treated nets (LLINs) distribution and regular school-based praziquantel (for schistosomiasis) and albendazole (for STH) campaigns. Co-existence of Plasmodium parasites and helminths infections notably Schistosoma species among children may concurrently lead to an increase in Plasmodium infection with an enhanced risk of anemia, highlighting the necessity of an integrated approach for disease control interventions.

Malaria prevalence in Commune 5 in Tumaco (Nariño, Colombia)

Background Urban malaria is a public health problem in Colombia and there is still lack of knowledge about its epidemiological characteristics, which are key to the implementation of control measures. The presence of urban malaria cases and disease diagnosis are some of the challenges faced by malaria elimination programs. The objective of this research was to estimate malaria prevalence, explore associated factors and detect pfhrp 2/3 genes, in the urban area of Tumaco between July and December 2019. Methods A prevalence study was conducted by using a stratified random probability sample. Structured surveys were administered and blood samples were taken and examined through optical microscopy, rapid diagnostic tests (RDT) and polymerase chain reaction (PCR). A logistic regression model was used to explore associated factors. Results 1,504 people living in 526 households were surveyed. The overall prevalence was 2.97% (95% CI: 2.1 - 4.3%). It was higher in males, in the 10-19 age group and in asymptomatic cases. The prevalence of pfhrp2 amplification was 2.16% (95% CI: 1.6 - 2.9%). Households with three or more people had a higher risk of malaria infection (adjusted odds ratio (ORa) 4.05; 95% confidence interval (CI) 1.57-10.43). All cases were due to P. falciparum. Conclusions The prevalence of urban malaria was low. Strategies to eliminate malaria in urban areas should be adjusted considering access to early diagnosis, asymptomatic infection, and the RDTs used to detect the presence of the pfhrp2 gene.

Drug resistance profiling of asymptomatic and low-density Plasmodium falciparum malaria infections on Ngodhe island, Kenya, using custom dual-indexing next-generation sequencing

Malaria control initiatives require rapid and reliable methods for the detection and monitoring of molecular markers associated with antimalarial drug resistance in Plasmodium falciparum parasites. Ngodhe island, Kenya, presents a unique malaria profile, with lower P. falciparum incidence rates than the surrounding region, and a high proportion of sub-microscopic and low-density infections. Here, using custom dual-indexing and Illumina next generation sequencing, we generate resistance profiles on seventy asymptomatic and low-density P. falciparum infections from a mass drug administration program implemented on Ngodhe island between 2015 and 2016. Our assay encompasses established molecular markers on the Pfcrt, Pfmdr1, Pfdhps, Pfdhfr, and Pfk13 genes. Resistance markers for sulfadoxine-pyrimethamine were identified at high frequencies, including a quintuple mutant haplotype (Pfdhfr/Pfdhps: N51I, C59R, S108N/A437G, K540E) identified in 62.2% of isolates. The Pfdhps K540E biomarker, used to inform decision making for intermittent preventative treatment in pregnancy, was identified in 79.2% of isolates. Several variants on Pfmdr1, associated with reduced susceptibility to quinolones and lumefantrine, were also identified (Y184F 47.1%; D1246Y 16.0%; N86 98%). Overall, we have presented a low-cost and extendable approach that can provide timely genetic profiles to inform clinical and surveillance activities, especially in settings with abundant low-density infections, seeking malaria elimination.

Molecular detection of human Plasmodium species using a multiplex real time PCR

Molecular detection methods have revealed higher sensitivity and specificity than conventional microscopy or rapid diagnostic tests for malaria diagnosis. In this study, we implemented, evaluated and validated according to the ISO 15,189 requirements, a multiplex real-time PCR assay to detect and identify the five human malaria parasites. DNA samples were extracted from whole blood or dried blood spots drawn from patients. Based on the External Quality Assessment (whole blood), this method shows 100% sensitivity and specificity. This PCR detected P. vivax up to 0.25 p/µl, P. falciparum and P. knowlesi up to 0.5 p/µl, P. ovale up to 1 p/µl and P. malariae up to 5 p/µl of blood. From blood spots (extraction from four punches), it detected P. vivax at 5 p/µl, P. falciparum, P. ovale and P. knowlesi at 20 p/µl and P. malariae at 125 p/µl. In conclusion, this quantitative PCR shows excellent performance, is easy to use and DNA saver. It is especially useful to actively screen large population groups and identify the five human malaria parasites in a context of low malaria transmission.

Protocol for a cluster randomised placebo-controlled trial of adjunctive ivermectin mass drug administration for malaria control on the Bijagós Archipelago of Guinea-Bissau: the MATAMAL trial

INTRODUCTION

As malaria declines, innovative tools are required to further reduce transmission and achieve elimination. Mass drug administration (MDA) of artemisinin-based combination therapy (ACT) is capable of reducing malaria transmission where coverage of control interventions is already high, though the impact is short-lived. Combining ACT with ivermectin, an oral endectocide shown to reduce vector survival, may increase its impact, while also treating ivermectin-sensitive co-endemic diseases and minimising the potential impact of ACT resistance in this context.

METHODS AND ANALYSIS

MATAMAL is a cluster-randomised placebo-controlled trial. The trial is being conducted in 24 clusters on the Bijagós Archipelago, Guinea-Bissau, where the peak prevalence of Plasmodium falciparum (Pf) parasitaemia is approximately 15%. Clusters have been randomly allocated to receive MDA with dihydroartemisinin-piperaquine and either ivermectin or placebo. The primary objective is to determine whether the addition of ivermectin MDA is more effective than dihydroartemisinin-piperaquine MDA alone in reducing the prevalence of P. falciparum parasitaemia, measured during peak transmission season after 2 years of seasonal MDA. Secondary objectives include assessing prevalence after 1 year of MDA; malaria incidence monitored through active and passive surveillance; age-adjusted prevalence of serological markers indicating exposure to P. falciparum and anopheline mosquitoes; vector parous rates, species composition, population density and sporozoite rates; prevalence of vector pyrethroid resistance; prevalence of artemisinin resistance in P. falciparum using genomic markers; ivermectin's impact on co-endemic diseases; coverage estimates; and the safety of combined MDA.

ETHICS AND DISSEMINATION

The trial has been approved by the London School of Hygiene and Tropical Medicine's Ethics Committee (UK) (19156) and the Comite Nacional de Eticas de Saude (Guinea-Bissau) (084/CNES/INASA/2020). Results will be disseminated in peer-reviewed publications and in discussion with the Bissau-Guinean Ministry of Public Health and participating communities.

TRIAL REGISTRATION NUMBER

NCT04844905.

Origin and diversity of malaria parasites and other Haemosporida

Symbionts, including parasites, are ubiquitous in all world ecosystems. Understanding the diversity of symbiont species addresses diverse questions, from the origin of infectious diseases to inferring processes shaping regional biotas. Here, we review the current approaches to studying Haemosporida's species diversity and evolutionary history. Despite the solid knowledge of species linked to diseases, such as the agents of human malaria, studies on haemosporidian phylogeny, diversity, ecology, and evolution are still limited. The available data, however, indicate that Haemosporida is an extraordinarily diverse and cosmopolitan clade of symbionts. Furthermore, this clade seems to have originated with their vertebrate hosts, particularly birds, as part of complex community level processes that we are still characterizing.

Malaria rapid diagnostic tests reliably detect asymptomatic Plasmodium falciparum infections in school-aged children that are infectious to mosquitoes

BACKGROUND

Asymptomatic malaria infections (Plasmodium falciparum) are common in school-aged children and represent a disease transmission reservoir as they are potentially infectious to mosquitoes. To detect and treat such infections, convenient, rapid and reliable diagnostic tools are needed. In this study, malaria rapid diagnostic tests (mRDT), light microscopy (LM) and quantitative polymerase chain reaction (qPCR) were used to evaluate their performance detecting asymptomatic malaria infections that are infectious to mosquitoes.

METHODS

One hundred seventy asymptomatic school-aged children (6-14 years old) from the Bagamoyo district in Tanzania were screened for Plasmodium spp. infections using mRDT (SD BIOLINE), LM and qPCR. In addition, gametocytes were detected using reverse transcription quantitative polymerase chain reaction (RT-qPCR) for all qPCR-positive children. Venous blood from all P. falciparum positive children was fed to female Anopheles gambiae sensu stricto mosquitoes via direct membrane feeding assays (DMFAs) after serum replacement. Mosquitoes were dissected for oocyst infections on day 8 post-infection.

RESULTS

The P. falciparum prevalence in study participants was 31.7% by qPCR, 18.2% by mRDT and 9.4% by LM. Approximately one-third (31.2%) of asymptomatic malaria infections were infectious to mosquitoes in DMFAs. In total, 297 infected mosquitoes were recorded after dissections, from which 94.9% (282/297) were derived from infections detected by mRDT and 5.1% (15/297) from subpatent mRDT infections.

CONCLUSION

The mRDT can be used reliably to detect children carrying gametocyte densities sufficient to infect high numbers of mosquitoes. Subpatent mRDT infections contributed marginally to the pool of oocyts-infected mosquitoes.

Prevalence and proportion estimate of asymptomatic Plasmodium infection in Asia: a systematic review and meta-analysis

Asymptomatic Plasmodium infection raises a problem for the persistent transmission of malaria in low-endemic areas such as Asia. This systematic review was undertaken to estimate the prevalence and proportion of asymptomatic Plasmodium infection in Asia. The systematic review was registered at PROSPERO (ID: CRD42022373664). The research followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses. A comprehensive search of five databases, Ovid, Scopus, MEDLINE, PubMed, and Embase, was conducted to identify studies of asymptomatic Plasmodium infection in Asian countries. The pooled prevalence of asymptomatic Plasmodium infection, the pooled proportion of asymptomatic Plasmodium infection among all parasitised individuals, and the associated 95% confidence intervals were estimated using a random-effects model. A total of 916 articles were retrieved, and 87 articles that met the criteria were included in the systematic review. The pooled prevalence of asymptomatic Plasmodium infection among enrolled participants in Southeast Asia, South Asia, and Western Asia was 5.8%, 9.4%, and 8.4%, respectively. The pooled proportion of asymptomatic Plasmodium infection among all parasitised individuals in Southeast Asia, South Asia, and Western Asia was 89.3%, 87.2%, and 64.8%, respectively. There was a low prevalence of asymptomatic Plasmodium infection, but there was a high proportion of asymptomatic Plasmodium infection per all parasitised individuals in different parts of Asia. These results may support and facilitate elimination and control programs for asymptomatic Plasmodium infection in Asia.

An active and targeted survey reveals asymptomatic malaria infections among high-risk populations in Mondulkiri, Cambodia

BACKGROUND

Malaria is a mosquito-borne disease that is one of the most serious public health issues globally and a leading cause of mortality in many developing countries worldwide. Knowing the prevalence of both symptomatic and asymptomatic malaria on a subnational scale allows for the estimation of the burden of parasitaemia present in the transmission system, enabling targeting and tailoring of resources towards greater impact and better use of available capacity. This study aimed to determine the PCR-based point prevalence of malaria infection, by parasite species, among three high-risk populations in Mondulkiri province, Cambodia: forest rangers, forest dwellers, and forest goers.

METHODS

A cross-sectional survey was performed during the transmission season in November and December 2021. Blood samples collected on filter paper from participants (n = 1301) from all target groups were screened for Plasmodium spp using PCR.

RESULTS

Malaria prevalence among all study participants was 6.7% for any Plasmodium species. Malaria prevalence in the forest ranger group was 8.1%, was 6.8% in forest goers, and 6.4% in forest dwellers; all infections were asymptomatic. Plasmodium vivax was detected in all participant groups, while the few Plasmodium falciparum infections were found in goers and dwellers. 81% of all infections were due to P. vivax, 9% were due to P. falciparum, 3% due to Plasmodium cynomolgi, and the rest (7%) remained undefined. Gender was associated with malaria infection prevalence, with male participants having higher odds of malaria infection than female participants (OR = 1.69, 95% CI 1.08-2.64). Passively collected malaria incidence data from the Cambodian government were also investigated. Health facility-reported malaria cases, based on rapid diagnostic tests, for the period Jan-Dec 2021 were 521 Plasmodium vivax (0.89% prevalence), 34 P. falciparum (0.06%) and four P. falciparum + mixed (0.01%)-a total of 559 cases (0.95%) for all of Mondulkiri.

CONCLUSION

This reservoir of asymptomatic parasitaemia may be perpetuating low levels of transmission, and thus, new strategies are required to realize the goal of eliminating malaria in Cambodia by 2025.

Genome-wide genetic variation and molecular surveillance of drug resistance in Plasmodium falciparum isolates from asymptomatic individuals in Ouélessébougou, Mali

Sequence analysis of Plasmodium falciparum parasites is informative in ensuring sustained success of malaria control programmes. Whole-genome sequencing technologies provide insights into the epidemiology and genome-wide variation of P. falciparum populations and can characterise geographical as well as temporal changes. This is particularly important to monitor the emergence and spread of drug resistant P. falciparum parasites which is threatening malaria control programmes world-wide. Here, we provide a detailed characterisation of genome-wide genetic variation and drug resistance profiles in asymptomatic individuals in South-Western Mali, where malaria transmission is intense and seasonal, and case numbers have recently increased. Samples collected from Ouélessébougou, Mali (2019-2020; n = 87) were sequenced and placed in the context of older Malian (2007-2017; n = 876) and African-wide (n = 711) P. falciparum isolates. Our analysis revealed high multiclonality and low relatedness between isolates, in addition to increased frequencies of molecular markers for sulfadoxine-pyrimethamine and lumefantrine resistance, compared to older Malian isolates. Furthermore, 21 genes under selective pressure were identified, including a transmission-blocking vaccine candidate (pfCelTOS) and an erythrocyte invasion locus (pfdblmsp2). Overall, our work provides the most recent assessment of P. falciparum genetic diversity in Mali, a country with the second highest burden of malaria in West Africa, thereby informing malaria control activities.

Teles CB, Medeiros JF, Pereira DB, Vinetz JM, Gazzinelli RT, Araújo MS. Transmission-blocking activity of antimalarials for Plasmodium vivax malaria in Anopheles darlingi

Malaria is caused by parasite of the genus Plasmodium and is still one of the most important infectious diseases in the world. Several biological characteristics of Plasmodium vivax contribute to the resilience of this species, including early gametocyte production, both of which lead to efficient malaria transmission to mosquitoes. This study evaluated the impact of currently used drugs on the transmission of P. vivax. Participants received one of the following treatments for malaria: i) chloroquine [10 mg/kg on day 1 and 7.5 mg/kg on day 2 and 3] co-administered with Primaquine [0.5 mg/kg/day for 7 days]; ii) Chloroquine [10 mg/kg on day 1 and 7.5 mg/kg on day 2 and 3] co-administered with one-dose of Tafenoquine [300 mg on day 1]; and iii) Artesunate and Mefloquine [100 mg and 200 mg on day 1, 2 and 3] co-administered with Primaquine [0.5 mg/kg/day for 14 days]. Patient blood was collected before treatment and 4 h, 24 h, 48 h and 72 h after treatment. The blood was used to perform a direct membrane feeding assay (DMFA) using Anopheles darlingi mosquitoes. The results showed 100% inhibition of the mosquito infection after 4 h using ASMQ+PQ, after 24 h for the combination of CQ+PQ and 48 h using CQ+TQ. The density of gametocytes declined over time in all treatment groups, although the decline was more rapid in the ASMQ+PQ group. In conclusion, it was possible to demonstrate the transmission-blocking efficacy of the malaria vivax treatment and that ASMQ+PQ acts faster than the two other treatments.

Influence of the sickle cell trait on Plasmodium falciparum infectivity from naturally infected gametocyte carriers

BACKGROUND

Sickle cell trait (SCT) refers to the carriage of one abnormal copy of the β-globin gene, the HbS allele. SCT offers protection against malaria, controlling parasite density and preventing progression to symptomatic malaria. However, it remains unclear whether SCT also affects transmission stages and mosquito infection parameters. Deciphering the impact of the SCT on human to mosquito malaria transmission is key to understanding mechanisms that maintain the trait in malaria endemic areas.

METHODS

The study was conducted from June to July 2017 among asymptomatic children living in the locality of Mfou, Cameroon. Blood samples were collected from asymptomatic children to perform malaria diagnosis by microscopy, Plasmodium species by PCR and hemoglobin typing by RFLP. Infectiousness of gametocytes to mosquitoes was assessed by membrane feeding assays using blood from gametocyte carriers of HbAA and HbAS genotypes. A zero-inflated model was fitted to predict distribution of oocysts in mosquitoes according to hemoglobin genotype of the gametocyte source.

RESULTS

Among the 1557 children enrolled in the study, 314 (20.16%) were of the HbAS genotype. The prevalence of children with P. falciparum gametocytes was 18.47% in HbAS individuals and 13.57% in HbAA, and the difference is significant (χ² = 4.61, P = 0.032). Multiplicity of infection was lower in HbAS gametocyte carriers (median = 2 genotypes/carrier in HbAS versus 3.5 genotypes/carrier in HbAA, Wilcoxon sum rank test = 188, P = 0.032). Gametocyte densities in the blood donor significantly influenced mosquito infection prevalence in both HbAS and HbAA individuals. The HbAS genotype had no significant effect on mosquito infection outcomes when using immune or naïve serum in feeding assays. In AB replacement feeding experiments, the odds ratio of mosquito infection for HbAA blood as compared to HbAS was 0.56 (95% CI 0.29-1.10), indicating a twice higher risk of infection in mosquitoes fed on gametocyte-containing blood of HbAS genotype.

CONCLUSION

Plasmodium transmission stages were more prevalent in SCT individuals. This may reflect the parasite's enhanced investment in the sexual stage to increase their survival rate when asexual replication is impeded. The public health impact of our results points the need for intensive malaria control interventions in areas with high prevalence of HbAS. The similar infection parameters in feeding experiments where mosquitoes received the original serum from the blood donor indicated that immune responses to gametocyte surface proteins occur in both HbAS and HbAA individuals. The higher risk of infection in mosquitoes fed on HbAS blood depleted of immune factors suggests that changes in the membrane properties in HbAS erythrocytes may impact on the maturation process of gametocytes within circulating red blood cells.

Prevalence of Plasmodium falciparum gametocytaemia in asymptomatic school children before and after treatment with dihydroartemisinin-piperaquine (DP)

Background

Asymptomatic Plasmodium carriers form the majority of malaria-infected individuals in most endemic areas. A proportion of these asymptomatically infected individuals carry gametocytes, the transmissible stages of malaria parasites, that sustain human to mosquito transmission. Few studies examine gametocytaemia in asymptomatic school children who may form an important reservoir for transmission. We assessed the prevalence of gametocytaemia before antimalarial treatment and monitored clearance of gametocytes after treatment in asymptomatic malaria children.

Methods

A total of 274 primary school children were screened for P. falciparum parasitaemia by microscopy. One hundred and fifty-five (155) parasite positive children were treated under direct observation with dihydroartemisinin-piperaquine (DP). Gametocyte carriage was determined by microscopy seven days prior to treatment, day 0 before treatment, and on days 7, 14 and 21 post initiation of treatment.

Results

The prevalence of microscopically-detectable gametocytes at screening (day -7) and enrolment (day 0) were 9% (25/274) and 13.6% (21/155) respectively. Following DP treatment, gametocyte carriage dropped to 4% (6/135), 3% (5/135) and 6% (10/151) on days 7, 14 and 21 respectively. Asexual parasites persisted in a minority of treated children, resulting in microscopically detectable parasites on days 7 (9%, 12/135), 14 (4%, 5/135) and 21 (7%, 10/151). Gametocyte carriage was inversely correlated with the age of the participants (p = 0.05) and asexual parasite density (p = 0.08). In a variate analysis, persistent gametocytaemia 7 or more days after treatment was significantly associated with post-treatment asexual parasitaemia at day 7 (P = 0.027) and presence of gametocytes on the day of treatment (P < 0.001).

Conclusions

Though DP provides both excellent cure rates for clinical malaria and a long prophylactic half-life, our findings suggest that after treatment of asymptomatic infections, both asexual parasites and gametocytes may persist in a minority of individuals during the first 3 weeks after treatment. This indicates DP may be unsuitable for use in mass drug administration strategies towards malaria elimination in Africa.

Plasmodium falciparum gametocyte carriage in longitudinally monitored incident infections is associated with duration of infection and human host factors

Malaria transmission depends on the presence of Plasmodium gametocytes that are the only parasite life stage that can infect mosquitoes. Gametocyte production varies between infections and over the course of infections. Infection duration is highly important for gametocyte production but poorly quantified. Between 2017 and 2019 an all-age cohort of individuals from Tororo, eastern Uganda was followed by continuous passive and routine assessments. We longitudinally monitored 104 incident infections from 98 individuals who were sampled once every 28 days and on any day of symptoms. Among infections that lasted ≥ 3 months, gametocyte appearance was near-universal with 96% of infections having detectable gametocytes prior to clearance. However, most infections were of much shorter duration; 55.7% of asymptomatic infections were detected only once. When considering all asymptomatic infections, regardless of their duration, only 36.3% had detectable gametocytes on at least one time-point prior to parasite clearance. Infections in individuals with sickle-cell trait (HbAS) were more likely to have gametocytes detected (Hazard Rate (HR) = 2.68, 95% CI 1.12, 6.38; p = 0.0231) and had gametocytes detected at higher densities (Density Ratio (DR) = 9.19, 95% CI 2.79, 30.23; p = 0.0002) compared to infections in wildtype (HbAA) individuals. Our findings suggest that a large proportion of incident infections is too short in duration and of too low density to contribute to onward transmission.

Modeling of adaptive immunity uncovers disease tolerance mechanisms

When an organism is challenged with a pathogen a cascade of events unfolds. The innate immune system rapidly mounts a preliminary nonspecific defense, while the acquired immune system slowly develops microbe-killing specialists. These responses cause inflammation, and along with the pathogen cause direct and indirect tissue damage, which anti-inflammatory mediators seek to temper. This interplay of systems is credited for maintaining homeostasis but may produce unexpected results such as disease tolerance. Tolerance is characterized by the persistence of pathogen and damage mitigation, where the relevant mechanisms are poorly understood. In this work we develop an ordinary differential equations model of the immune response to infection in order to identify key components in tolerance. Bifurcation analysis uncovers health, immune- and pathogen-mediated death clinical outcomes dependent on pathogen growth rate. We demonstrate that decreasing the inflammatory response to damage and increasing the strength of the immune system gives rise to a region in which limit cycles, or periodic solutions, are the only biological trajectories. We then describe areas of parameter space corresponding to disease tolerance by varying immune cell decay, pathogen removal, and lymphocyte proliferation rates.

Imported malaria from land bordering countries in China: A challenge in preventing the reestablishment of malaria transmission

OBJECTIVE

No indigenous malaria cases have been reported since 2017 in China, but a large number of imported cases are still reported every year, including those from the land bordering countries. To characterize their epidemiological profiles will provide evidence for the development of appropriate strategies to effectively address the challenges of border malaria in the post-elimination phase.

METHODS

Individual-level data of imported malaria cases from the land bordering countries were collected from 2017 to 2021 in China via the web-based surveillance systems, and analyzed by SPSS, ArcGIS and WPS software, to explore their epidemiological profiles.

RESULTS

A total of 1170 malaria cases imported into China from six of the fourteen land bordering countries were reported between 2017 and 2021 with a decline trend. Overall, cases were widely distributed in 31-97 counties from 11 to 21 provinces but mainly in Yunnan. Moreover, these imported cases were mainly infected with P. vivax (94.8%), and a total of 68 recurrent cases were reported in 6-14 counties from 4 to 8 provinces. In addition, nearly 57.1% of the total reported cases could seek healthcare within 2 days of getting sick, and 71.3% of the reported cases could be confirmed as malaria on the day they sought medical care.

CONCLUSIONS

China still needs to attach great importance to the risk and challenge of the imported malaria from bordering countries particularly from Myanmar in preventing reestablishment of malaria transmission in the post-elimination phase. It is necessary not only to strengthen collaboration and cooperation with the bordering countries, but also coordinate multiple departments at home to improve malaria surveillance and response system and prevent the reestablishment of malaria transmission in China.

Using antenatal care as a platform for malaria surveillance data collection: study protocol

BACKGROUND

While many malaria-endemic countries have health management information systems that can measure and report malaria trends in a timely manner, these routine systems have limitations. Periodic community cross-sectional household surveys are used to estimate malaria prevalence and intervention coverage but lack geographic granularity and are resource intensive. Incorporating malaria testing for all women at their first antenatal care (ANC) visit (i.e., ANC1) could provide a more timely and granular source of data for monitoring trends in malaria burden and intervention coverage. This article describes a protocol designed to assess if ANC-based surveillance could be a pragmatic tool to monitor malaria.

METHODS

This is an observational, cross-sectional study conducted in Benin, Burkina Faso, Mozambique, Nigeria, Tanzania, and Zambia. Pregnant women attending ANC1 in selected health facilities will be tested for malaria infection by rapid diagnostic test and administered a brief questionnaire to capture key indicators of malaria control intervention coverage and care-seeking behaviour. In each location, contemporaneous cross-sectional household surveys will be leveraged to assess correlations between estimates obtained using each method, and the use of ANC data as a tool to track trends in malaria burden and intervention coverage will be validated.

RESULTS

This study will assess malaria prevalence at ANC1 aggregated at health facility and district levels, and by gravidity relative to current pregnancy (i.e., gravida 1, gravida 2, and gravida 3 +). ANC1 malaria prevalence will be presented as monthly trends. Additionally, correlation between ANC1 and household survey-derived estimates of malaria prevalence, bed net ownership and use, and care-seeking will be assessed.

CONCLUSION

ANC1-based surveillance has the potential to provide a cost-effective, localized measure of malaria prevalence that is representative of the general population and useful for tracking monthly changes in parasite prevalence, as well as providing population-representative estimates of intervention coverage and care-seeking behavior. This study will evaluate the representativeness of these measures and collect information on operational feasibility, usefulness for programmatic decision-making, and potential for scale-up of malaria ANC1 surveillance.

Laboratory evaluation of the miniature direct-on-blood PCR nucleic acid lateral flow immunoassay (mini-dbPCR-NALFIA), a simplified molecular diagnostic test for Plasmodium

BACKGROUND

Point-of-care diagnosis of malaria is currently based on microscopy and rapid diagnostic tests. However, both techniques have their constraints, including poor sensitivity for low parasitaemias. Hence, more accurate diagnostic tests for field use and routine clinical settings are warranted. The miniature direct-on-blood PCR nucleic acid lateral flow immunoassay (mini-dbPCR-NALFIA) is an innovative, easy-to-use molecular assay for diagnosis of malaria in resource-limited settings. Unlike traditional molecular methods, mini-dbPCR-NALFIA does not require DNA extraction and makes use of a handheld, portable thermal cycler that can run on a solar-charged power pack. Result read-out is done using a rapid lateral flow strip enabling differentiation of Plasmodium falciparum and non-falciparum malaria infections. A laboratory evaluation was performed to assess the performance of the mini-dbPCR-NALFIA for diagnosis of pan-Plasmodium and P. falciparum infections in whole blood.

METHODS

Diagnostic accuracy of the mini-dbPCR-NALFIA was determined by testing a set of Plasmodium-positive blood samples from returned travellers (n = 29), and Plasmodium-negative blood samples from travellers with suspected malaria (n = 23), the Dutch Blood Bank (n = 19) and intensive care patients at the Amsterdam University Medical Centers (n = 16). Alethia Malaria (LAMP) with microscopy for species differentiation were used as reference. Limit of detection for P. falciparum was determined by 23 measurements of a dilution series of a P. falciparum culture. A fixed sample set was tested three times by the same operator to evaluate the repeatability, and once by five different operators to assess the reproducibility.

RESULTS

Overall sensitivity and specificity of the mini-dbPCR-NALFIA were 96.6% (95% CI, 82.2%-99.9%) and 98.3% (95% CI, 90.8%-100%). Limit of detection for P. falciparum was 10 parasites per microlitre of blood. The repeatability of the assay was 93.7% (95% CI, 89.5%-97.8%) and reproducibility was 84.6% (95% CI, 79.5%-89.6%).

CONCLUSIONS

Mini-dbPCR-NALFIA is a sensitive, specific and robust method for molecular diagnosis of Plasmodium infections in whole blood and differentiation of P. falciparum. Incorporation of a miniature thermal cycler makes the assay well-adapted to resource-limited settings. A phase-3 field trial is currently being conducted to evaluate the potential implementation of this tool in different malaria transmission areas.

Asymptomatic Plasmodium falciparum carriage and clinical disease: a 5-year community-based longitudinal study in The Gambia

BACKGROUND

Carriers of persistent asymptomatic Plasmodium falciparum infections constitute an infectious reservoir that maintains malaria transmission. Understanding the extent of carriage and characteristics of carriers specific to endemic areas could guide use of interventions to reduce infectious reservoir.

METHODS

In eastern Gambia, an all-age cohort from four villages was followed up from 2012 to 2016. Each year, cross-sectional surveys were conducted at the end of the malaria transmission season (January) and just before the start of the next one (June) to determine asymptomatic P. falciparum carriage. Passive case detection was conducted during each transmission season (August to January) to determine incidence of clinical malaria. Association between carriage at the end of the season and at start of the next one and the risk factors for this were assessed. Effect of carriage before start of the season on risk of clinical malaria during the season was also examined.

RESULTS

A total of 1403 individuals-1154 from a semi-urban village and 249 from three rural villages were enrolled; median age was 12 years (interquartile range [IQR] 6, 30) and 12 years (IQR 7, 27) respectively. In adjusted analysis, asymptomatic P. falciparum carriage at the end of a transmission season and carriage just before start of the next one were strongly associated (adjusted odds ratio [aOR] = 19.99; 95% CI 12.57-31.77, p < 0.001). The odds of persistent carriage (i.e. infected both in January and in June) were higher in rural villages (aOR = 13.0; 95% CI 6.33-26.88, p < 0.001) and in children aged 5-15 years (aOR = 5.03; 95% CI 2.47-10.23, p =  < 0.001). In the rural villages, carriage before start of the season was associated with a lower risk of clinical malaria during the season (incidence risk ratio [IRR] 0.48, 95% CI 0.27-0.81, p = 0.007).

CONCLUSIONS

Asymptomatic P. falciparum carriage at the end of a transmission season strongly predicted carriage just before start of the next one. Interventions that clear persistent asymptomatic infections when targeted at the subpopulation with high risk of carriage may reduce the infectious reservoir responsible for launching seasonal transmission.

Sub-microscopic Plasmodium falciparum parasitaemia, dihydropteroate synthase (dhps) resistance mutations to sulfadoxine-pyrimethamine, transmission intensity and risk of malaria infection in pregnancy in Mount Cameroon Region

BACKGROUND

Plasmodium falciparum resistance to intermittent preventive treatment with sulfadoxine-pyrimethamine (IPTp-SP) continues to spread throughout sub-Saharan Africa. This study assessed the occurrence of microscopic and sub-microscopic P. falciparum parasitaemia, dihydropteroate synthase mutations associated with resistance to SP and maternal anaemia in the Mount Cameroon area.

METHODS

Consenting pregnant women living in semi-rural and semi-urban/urbanized settings were enrolled in this cross-sectional study. Socio-demographic, antenatal and clinical data were documented. Microscopic and sub-microscopic parasitaemia were diagnosed using peripheral blood microscopy and nested polymerase chain reaction (PCR) respectively. The dhps mutations were genotyped by restriction fragment length polymorphism analysis. The presence of A437G, K540E, and A581G was considered a marker for high-level resistance. Haemoglobin levels and anaemia status were determined.

RESULTS

Among the women, the prevalence of microscopic and sub-microscopic P. falciparum infection were 7.7% (67/874) and 18.6% (93/500) respectively. Predictors of microscopic infection were younger age (< 21 years) (AOR = 2.89; 95% CI 1.29-6.46) and semi-rural settings (AOR = 2.27; 95% CI 1.31-3.96). Determinants of sub-microscopic infection were the rainy season (AOR, 3.01; 95% CI 1.77-5.13), primigravidity (AOR = 0.45; 95% CI 0.21-0.94) and regular ITN usage (AOR = 0.49; 95% CI 0.27-0.90). Of the145 P. falciparum isolates genotyped, 66.9% (97) carried mutations associated with resistance to SP; 33.8% (49), 0%, 52.4% (76) and 19.3% (28) for A437G, K540E, A581G and A437G + A581G respectively. The A581G mutation was associated with ≥ 3 SP doses evident only among sub-microscopic parasitaemia (P = 0.027) and multigravidae (P = 0.009). Women with microscopic infection were more likely from semi-rural settings (AOR = 7.09; 95% CI 2.59-19.42), to report history of fever (AOR = 2.6; 95% CI 1.07-6.31), to harbour parasites with double resistant mutations (AOR = 6.65; 95% CI 1.85-23.96) and were less likely to have received 2 SP doses (AOR = 0.29; 95% CI 1.07-6.31). Microscopic infection decreased Hb levels more than sub-microscopic infection.

CONCLUSION

The occurrence of sub-microscopic P. falciparum parasites resistant to SP and intense malaria transmission poses persistent risk of malaria infection during pregnancy in the area. ITN usage and monitoring spread of resistance are critical.

Exploring how space, time, and sampling impact our ability to measure genetic structure across Plasmodium falciparum populations

A primary use of malaria parasite genomics is identifying highly related infections to quantify epidemiological, spatial, or temporal factors associated with patterns of transmission. For example, spatial clustering of highly related parasites can indicate foci of transmission and temporal differences in relatedness can serve as evidence for changes in transmission over time. However, for infections in settings of moderate to high endemicity, understanding patterns of relatedness is compromised by complex infections, overall high forces of infection, and a highly diverse parasite population. It is not clear how much these factors limit the utility of using genomic data to better understand transmission in these settings. In particular, further investigation is required to determine which patterns of relatedness we expect to see with high quality, densely sampled genomic data in a high transmission setting and how these observations change under different study designs, missingness, and biases in sample collection. Here we investigate two identity-by-state measures of relatedness and apply them to amplicon deep sequencing data collected as part of a longitudinal cohort in Western Kenya that has previously been analysed to identify individual-factors associated with sharing parasites with infected mosquitoes. With these data we use permutation tests, to evaluate several hypotheses about spatiotemporal patterns of relatedness compared to a null distribution. We observe evidence of temporal structure, but not of fine-scale spatial structure in the cohort data. To explore factors associated with the lack of spatial structure in these data, we construct a series of simplified simulation scenarios using an agent based model calibrated to entomological, epidemiological and genomic data from this cohort study to investigate whether the lack of spatial structure observed in the cohort could be due to inherent power limitations of this analytical method. We further investigate how our hypothesis testing behaves under different sampling schemes, levels of completely random and systematic missingness, and different transmission intensities.

Comparative transcriptomics reveal differential gene expression in Plasmodium vivax geographical isolates and implications on erythrocyte invasion mechanisms

Plasmodium vivax uses Duffy binding protein (PvDBP1) to bind to the Duffy Antigen-Chemokine Receptor (DARC) to invade human erythrocytes. Individuals who lack DARC expression (Duffy-negative) are thought to be resistance to P. vivax. In recent years, P. vivax malaria is becoming more prevalent in Africa with a portion of these cases detected in Duffy-negatives. Apart from DBP1, members of the reticulocyte binding protein (RBP) and tryptophan-rich antigen (TRAg) families may also play a role in erythrocyte invasion. While the transcriptomes of the Southeast Asian and South American P. vivax are well documented, the gene expression profile of P. vivax in Africa and more specifically the expression level of several erythrocyte binding gene candidates as compared to DBP1 are largely unknown. This paper characterized the first P. vivax transcriptome in Africa and compared with those from the Southeast Asian and South American isolates. The expression of 4,404 gene transcripts belong to 12 functional groups including 43 specific erythrocyte binding gene candidates were examined. Overall, there were 10-26% differences in the gene expression profile amongst the geographical isolates, with the Ethiopian and Cambodian P. vivax being most similar. Majority of the gene transcripts involved in protein transportation, housekeeping, and host interaction were highly transcribed in the Ethiopian P. vivax. Erythrocyte binding genes including PvRBP2a and PvRBP3 expressed six-fold higher than PvDBP1and 60-fold higher than PvEBP/DBP2. Other genes including PvRBP1a, PvMSP3.8, PvMSP3.9, PvTRAG2, PvTRAG14, and PvTRAG22 also showed relatively high expression. Differential expression was observed among geographical isolates, e.g., PvDBP1 and PvEBP/DBP2 were highly expressed in the Cambodian but not the Brazilian and Ethiopian isolates, whereas PvRBP2a and PvRBP2b showed higher expression in the Ethiopian and Cambodian than the Brazilian isolates. Compared to Pvs25, the standard biomarker for detecting female gametocytes, PvAP2-G (PVP01_1440800), GAP (PVP01_1403000), and Pvs47 (PVP01_1208000) were highly expressed across geographical samples. These findings provide an important baseline for future comparisons of P. vivax transcriptomes from Duffy-negative infections and highlight potential biomarkers for improved gametocyte detection.

Prevalence of asymptomatic malaria infection by microscopy and its determinants among residents of Ido-Ekiti, Southwestern Nigeria

BACKGROUND

Asymptomatic malaria infections have received less attention than symptomatic malaria infections in major studies. Few epidemiological studies on asymptomatic malaria infections have often focused on pregnant women and children under-five years of age as the most vulnerable groups. However, there is limitation on data regarding asymptomatic infections among the old adult populations, particularly in the study area. Therefore, this study determined the prevalence of asymptomatic malaria infection by microscopy and its determinants among residents of Ido- Ekiti, Southwestern Nigeria.

METHODS

A hospital-based cross-sectional study was conducted between July and September 2021 among 232 consenting apparently healthy individuals aged 40 years and above who were recruited during a free health screening program using a standardised interviewer-administered questionnaire. The questionnaire sought information on respondents' socio-demographics, presence and types of co-morbidity, and the prevention methods being adopted against malaria infection. Venous blood samples were collected and processed for asymptomatic infections using Giemsa-stained blood smear microscopy. Data were analysed using SPSS version 21. Multivariate logistic regression was used to identify factors associated with asymptomatic infections.

RESULTS

Of the total 232 respondents, 19.0% (48/232) were confirmed to be infected with Plasmodium falciparum (95% confidence interval (CI): 14.1% - 24.6%). Lack of formal education (Adjusted odds ratio (AOR): 5.298, 95% (CI): 2.184-13.997), being diabetic (AOR: 4.681, 95% CI: 1.669-16.105), and not sleeping under Long Lasting Insecticide Net (LLINs) (AOR: 4.594, 95% CI: 1.194-14.091), were the determinants of asymptomatic Plasmodium falciparum infection.

CONCLUSION

The prevalence of asymptomatic Plasmodium falciparum was 19%. Lack of formal education, being diabetic, and not sleeping under LLINs were the determinants of asymptomatic infections.