Higher gametocyte production and mosquito infectivity in chronic compared to incident Plasmodium falciparum infections

Dynamics of asymptomatic Plasmodium falciparum and Plasmodium vivax infections and their infectiousness to mosquitoes in a low transmission setting of Ethiopia: a longitudinal observational study

OBJECTIVE

A 15-month longitudinal study was conducted to determine the duration and infectivity of asymptomatic qPCR-detected Plasmodium falciparum and Plasmodium vivax infections in Ethiopia.

METHOD

Total parasite and gametocyte kinetics were determined by molecular methods; infectivity to Anopheles arabiensis mosquitoes by repeated membrane feeding assays. Infectivity results were contrasted with passively recruited symptomatic malaria cases.

RESULTS

For P. falciparum and P. vivax infections detected at enrolment, median durations of infection were 37 days (95% confidence interval [CI], 15-93) and 60 days (95% CI, 18-213), respectively. P. falciparum and P. vivax parasite densities declined over the course of infections. From 47 feeding assays on 22 asymptomatic P. falciparum infections, 6.4% (3/47) were infectious and these infected 1.8% (29/1579) of mosquitoes. No transmission was observed in feeding assays on asymptomatic P. vivax mono-infections (0/56); one mixed-species infection was highly infectious. Among the symptomatic cases, 4.3% (2/47) of P. falciparum and 73.3% (53/86) of P. vivax patients were infectious to mosquitoes.

CONCLUSION

The majority of asymptomatic infections were of short duration and low parasite density. Only a minority of asymptomatic individuals were infectious to mosquitoes. This contrasts with earlier findings and is plausibly due to the low parasite densities in this population.

Understanding and maximising the community impact of seasonal malaria chemoprevention in Burkina Faso (INDIE-SMC): study protocol for a cluster randomised evaluation trial

INTRODUCTION

Seasonal malaria chemoprevention (SMC) involves repeated administrations of sulfadoxine-pyrimethamine plus amodiaquine to children below the age of 5 years during the peak transmission season in areas of seasonal malaria transmission. While highly impactful in reducing Plasmodium falciparum malaria burden in controlled research settings, the impact of SMC on infection prevalence is moderate in real-life settings. It remains unclear what drives this efficacy decay. Recently, the WHO widened the scope for SMC to target all vulnerable populations. The Ministry of Health (MoH) in Burkina Faso is considering extending SMC to children below 10 years old. We aim to assess the impact of SMC on clinical incidence and parasite prevalence and quantify the human infectious reservoir for malaria in this population.

METHODS AND ANALYSIS

We will perform a cluster randomised trial in Saponé Health District, Burkina Faso, with three study arms comprising 62 clusters of three compounds: arm 1 (control): SMC in under 5-year-old children, implemented by the MoH without directly observed treatment (DOT) for the full course of SMC; arm 2 (intervention): SMC in under 5-year-old children, with DOT for the full course of SMC; arm 3 (intervention): SMC in under 10-year-old children, with DOT for the full course of SMC. The primary endpoint is parasite prevalence at the end of the malaria transmission season. Secondary endpoints include the impact of SMC on clinical incidence. Factors affecting SMC uptake, treatment adherence, drug concentrations, parasite resistance markers and transmission of parasites will be determined.

ETHICS AND DISSEMINATION

The London School of Hygiene & Tropical Medicine's Ethics Committee (29193) and the Burkina Faso National Medical Ethics Committee (Deliberation No 2023-05-104) approved this study. The findings will be presented to the community; disease occurrence data and study outcomes will also be shared with the Burkina Faso MoH. Findings will be published irrespective of their results.

TRIAL REGISTRATION NUMBER

NCT05878366.

Strong isolation by distance and evidence of population microstructure reflect ongoing Plasmodium falciparum transmission in Zanzibar

The Zanzibar archipelago of Tanzania has become a low-transmission area for Plasmodium falciparum. Despite being considered an area of pre-elimination for years, achieving elimination has been difficult, likely due to a combination of imported infections from mainland Tanzania, and continued local transmission. To shed light on these sources of transmission, we applied highly multiplexed genotyping utilizing molecular inversion probes to characterize the genetic relatedness of 282 P. falciparum isolates collected across Zanzibar and in Bagamoyo District on the coastal mainland from 2016-2018. Overall, parasite populations on the coastal mainland and Zanzibar archipelago remain highly related. However, parasite isolates from Zanzibar exhibit population microstructure due to rapid decay of parasite relatedness over very short distances. This, along with highly related pairs within shehias, suggests ongoing low level local transmission. We also identified highly related parasites across shehias that reflect human mobility on the main island of Unguja and identified a cluster of highly related parasites, suggestive of an outbreak, in the Micheweni district on Pemba island. Parasites in asymptomatic infections demonstrated higher complexity of infection than those in symptomatic infections, but have similar core genomes. Our data support importation as a main source of genetic diversity and contribution to the parasite population on Zanzibar, but they also show local outbreak clusters where targeted interventions are essential to block local transmission. These results highlight the need for preventive measures against imported malaria and enhanced control measures in areas that remain receptive for malaria reemergence due to susceptible hosts and competent vectors.

Transcriptomic approaches for identifying potential transmission blocking vaccine candidates in Plasmodium falciparum: a review of current knowledge and future directions

Utilizing transcriptomics, promising methods for identifying unique genes associated with Plasmodium gametocyte development offer a potential avenue for novel candidate targets in transmission blocking vaccine development. In this review, we identified 40 publicly available transcriptomic datasets related to parasite factors linked with sexual stage transmission, from which we analyzed two RNA-Seq datasets to identify potential genes crucial for the transmission of P. falciparum from humans to mosquito vectors. Differential expression analysis revealed 3500 (2489 upregulated and 1011 downregulated) common genes differentially expressed throughout sexual stage development of P. falciparum occurring in both humans (gametocyte stage II, V) and mosquitoes (ookinete). Among which 1283 (914 upregulated and 369 downregulated) and 826 (719 upregulated and 107 downregulated) genes were specific to female and male gametocytes, respectively. Also, 830 potential transition associated genes were identified that may be involved in the adaptation and survival of the parasite in between human and mosquito stages. Additionally, we reviewed the functional aspects of important genes highly expressed throughout the sexual stage pathway and evaluated their suitability as vaccine candidates. The review provides researchers with insight into the importance of publicly available transcriptomic datasets for identifying critical and novel gametocyte markers that may aid in the development of rational transmission blocking strategies.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03752-3.

Micro-heterogeneity of transmission shapes the submicroscopic malaria reservoir in coastal Tanzania

Background

Asymptomatic malaria may be patent (visible by microscopy) and detectable by rapid malaria diagnostic tests (RDTs), or it may be submicroscopic and only detectable by polymerase chain reaction (PCR).

Methods

To characterize the submicroscopic reservoir in an area of declining malaria transmission, asymptomatic persons >5 years of age in Bagamoyo District, Tanzania, were screened using RDT, microscopy, and PCR. We investigated the size of the submicroscopic reservoir across villages, determined factors associated with submicroscopic parasitemia, and assessed the natural history of submicroscopic malaria over four weeks.

Results

Among 6,076 participants, Plasmodium falciparum prevalence by RDT, microscopy, and PCR was 9%, 9%, and 28%, respectively, with roughly two-thirds of PCR-positive individuals harboring submicroscopic infection. Adult status, female gender, dry season months, screened windows, and bednet use were associated with submicroscopic carriage. Among 15 villages encompassing 80% of participants, the proportion of submicroscopic carriers increased with decreasing village-level malaria prevalence. Over four weeks, 23% (61/266) of submicroscopic carriers became RDT-positive and were treated, with half exhibiting symptoms. This occurred more frequently in villages with higher malaria prevalence.

Conclusions

Micro-heterogeneity in transmission impacts the size of the submicroscopic reservoir and the likelihood of submicroscopic carriers developing patent malaria in coastal Tanzania.

Extraordinary parasite multiplication rates in human malaria infections

For pathogenic organisms, faster rates of multiplication promote transmission success, the potential to harm hosts, and the evolution of drug resistance. Parasite multiplication rates (PMRs) are often quantified in malaria infections, given the relative ease of sampling. Using modern and historical human infection data, we show that established methods return extraordinarily - and implausibly - large PMRs. We illustrate how inflated PMRs arise from two facets of malaria biology that are far from unique: (i) some developmental ages are easier to sample than others; (ii) the distribution of developmental ages changes over the course of infection. The difficulty of accurately quantifying PMRs demonstrates a need for robust methods and a subsequent re-evaluation of what is known even in the well-studied system of malaria.

Infectivity of symptomatic Plasmodium vivax cases to different generations of wild-caught and laboratory-adapted Anopheles arabiensis using a membrane feeding assay, Ethiopia

When measuring human to mosquito transmission of Plasmodium spp., laboratory-adapted (colony) mosquitoes can be utilized. To connect transmission studies to the local epidemiology, it can be important to comprehend the relationship between infectivity in laboratory-adapted (colony) and wild-caught (wild) mosquitoes of the same species. Microscopically confirmed Plasmodium vivax cases were recruited from health facilities in Arba Minch town, and a nested polymerase chain reaction (nPCR) was used for subsequent confirmation. We performed paired membrane-feeding assays using colony An. arabiensis and three generations of wild origin An. arabiensis. Anopheles arabiensis aged 3-6 days were fed after being starved for 8-14 h. Microscopically, the oocyst development was evaluated at day 7 after feeding. Circumsporozoite proteins (CSPs) assay was carried out by enzyme-linked immunosorbent assay (ELISA). In 19 paired feeding experiments, the feeding efficiency was more than doubled in colony (median: 62.5%; interquartile range, IQR: 35-78%) than in wild mosquitoes (median: 28.5%; IQR: 17.5-40%; P < 0.001). Among the 19 P. vivax gametocyte-positive blood samples, 63.2% (n = 12) were infective to wild An. arabiensis and 73.7% (n = 14) were infective to colony An. arabiensis. The median infection rate was twice as high (26%) in the colony than in the wild (13%) An. arabiensis, although the difference was marginally insignificant (P = 0.06). Although the observed difference was not statistically significant (P = 0.19), the median number of oocysts per midgut was more than twice as high (17.8/midgut) in colony than in wild (7.2/midgut) An. arabiensis. The median feeding efficiency was 26.5% (IQR: 18-37%) in F1, 29.3% (IQR: 28-40%) in F2 and 31.2% (IQR: 30-37%) in F3 generations of wild An. arabiensis. Also, no significant difference was observed in oocyst infection rate and load between generations of wild An. arabiensis. CSP rate of P. vivax was 3.1% (3/97; 95% CI: 0.6-8.8%) in wild and 3.6% (3/84; 95% CI: 0.7-10.1%) in colony An. arabiensis. The results of the present study revealed that oocyst infection and load/midgut, and CSP rate were roughly comparable, indicating that colony mosquitoes can be employed for infectivity studies, while larger sample sizes may be necessary in future studies.

Assessment of the transmission blocking activity of antimalarial compounds by membrane feeding assays using natural Plasmodium falciparum gametocyte isolates from West-Africa

Antimalarial drugs that can block the transmission of Plasmodium gametocytes to mosquito vectors would be highly beneficial for malaria elimination efforts. Identifying transmission-blocking drugs currently relies on evaluation of their activity against gametocyte-producing laboratory parasite strains and would benefit from a testing pipeline with genetically diverse field isolates. The aims of this study were to develop a pipeline to test drugs against P. falciparum gametocyte field isolates and to evaluate the transmission-blocking activity of a set of novel compounds. Two assays were designed so they could identify both the overall transmission-blocking activity of a number of marketed and experimental drugs by direct membrane feeding assays (DMFA), and then also discriminate between those that are active against the gametocytes (gametocyte killing or sterilizing) or those that block development in the mosquito (sporontocidal). These DMFA assays used venous blood samples from naturally infected Plasmodium falciparum gametocyte carriers and locally reared Anopheles gambiae s.s. mosquitoes. Overall transmission-blocking activity was assessed following a 24 hour incubation of compound with gametocyte infected blood (TB-DMFA). Sporontocidal activity was evaluated following addition of compound directly prior to feeding, without incubation (SPORO-DMFA); Gametocyte viability was retained during 24-hour incubation at 37°C when gametocyte infected red blood cells were reconstituted in RPMI/serum. Methylene-blue, MMV693183, DDD107498, atovaquone and P218 showed potent transmission-blocking activity in the TB-DMFA, and both atovaquone and the novel antifolate P218 were potent inhibitors of sporogonic development in the SPORO-DMA. This work establishes a pipeline for the integral use of field isolates to assess the transmission-blocking capacity of antimalarial drugs to block transmission that should be validated in future studies.

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.

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.

Application of mathematical modelling to inform national malaria intervention planning in Nigeria

BACKGROUND

For their 2021-2025 National Malaria Strategic Plan (NMSP), Nigeria's National Malaria Elimination Programme (NMEP), in partnership with the World Health Organization (WHO), developed a targeted approach to intervention deployment at the local government area (LGA) level as part of the High Burden to High Impact response. Mathematical models of malaria transmission were used to predict the impact of proposed intervention strategies on malaria burden.

METHODS

An agent-based model of Plasmodium falciparum transmission was used to simulate malaria morbidity and mortality in Nigeria's 774 LGAs under four possible intervention strategies from 2020 to 2030. The scenarios represented the previously implemented plan (business-as-usual), the NMSP at an 80% or higher coverage level and two prioritized plans according to the resources available to Nigeria. LGAs were clustered into 22 epidemiological archetypes using monthly rainfall, temperature suitability index, vector abundance, pre-2010 parasite prevalence, and pre-2010 vector control coverage. Routine incidence data were used to parameterize seasonality in each archetype. Each LGA's baseline malaria transmission intensity was calibrated to parasite prevalence in children under the age of five years measured in the 2010 Malaria Indicator Survey (MIS). Intervention coverage in the 2010-2019 period was obtained from the Demographic and Health Survey, MIS, the NMEP, and post-campaign surveys.

RESULTS

Pursuing a business-as-usual strategy was projected to result in a 5% and 9% increase in malaria incidence in 2025 and 2030 compared with 2020, while deaths were projected to remain unchanged by 2030. The greatest intervention impact was associated with the NMSP scenario with 80% or greater coverage of standard interventions coupled with intermittent preventive treatment in infants and extension of seasonal malaria chemoprevention (SMC) to 404 LGAs, compared to 80 LGAs in 2019. The budget-prioritized scenario with SMC expansion to 310 LGAs, high bed net coverage with new formulations, and increase in effective case management rate at the same pace as historical levels was adopted as an adequate alternative for the resources available.

CONCLUSIONS

Dynamical models can be applied for relative assessment of the impact of intervention scenarios but improved subnational data collection systems are required to allow increased confidence in predictions at sub-national level.

Altitude, not potential larval habitat availability, explains pronounced variation in Plasmodium falciparum infection prevalence in the western Kenya highlands

Progress in malaria control has stalled over the recent years. Knowledge on main drivers of transmission explaining small-scale variation in prevalence can inform targeted control measures. We collected finger-prick blood samples from 3061 individuals irrespective of clinical symptoms in 20 clusters in Busia in western Kenya and screened for Plasmodium falciparum parasites using qPCR and microscopy. Clusters spanned an altitude range of 207 meters (1077-1284 m). We mapped potential mosquito larval habitats and determined their number within 250 m of a household and distances to households using ArcMap. Across all clusters, P. falciparum parasites were detected in 49.8% (1524/3061) of individuals by qPCR and 19.5% (596/3061) by microscopy. Across the clusters, prevalence ranged from 26% to 70% by qPCR. Three to 34 larval habitats per cluster and 0-17 habitats within a 250m radius around households were observed. Using a generalized linear mixed effect model (GLMM), a 5% decrease in the odds of getting infected per each 10m increase in altitude was observed, while the number of larval habitats and their proximity to households were not statistically significant predictors for prevalence. Kitchen located indoors, open eaves, a lower level of education of the household head, older age, and being male were significantly associated with higher prevalence. Pronounced variation in prevalence at small scales was observed and needs to be taken into account for malaria surveillance and control. Potential larval habitat frequency had no direct impact on prevalence.

Plasmodium falciparum Sexual Commitment Rate Variation among Clinical Isolates and Diverse Laboratory-Adapted Lines

Asexual blood-stage malaria parasites must produce sexual progeny to infect mosquitoes. It is important to understand the scope and causes of intraspecific variation in sexual commitment rates, particularly for the major human parasite P. falciparum. First, two alternative assay methods of measuring sexual commitment were compared to test a genetically modified P. falciparum line with elevated commitment rates inducible by overexpression of GDV1. The methods yielded correlated measurements with higher sensitivity and precision being achieved by one employing detection of the early gametocyte differentiation marker Pfs16. Thus, this was used to survey a diverse range of parasite lines and test each in multiple biological replicate assays in a serum-free medium supplemented with Albumax. There were differences among six recent clinical isolates from Ghana in their mean rates of sexual commitment per cycle, ranging from 3.3% to 12.2%. Among 13 diverse long-term laboratory-adapted lines, mean sexual commitment rates for most ranged from 4.7% to 13.4%, a few had lower rates with means from 0.3 to 1.6%, and one with a nonfunctional ap2-g gene always showed zero commitment. Among a subset of lines tested for the effects of exogenous choline to suppress commitment, there were significant differences. As expected, there was no effect in a line that had lost the gdv1 gene and that had generally low commitment, whereas the others showed quantitatively variable but significant responses to choline, suggesting potential trait variation. The results indicated the value of performing multiple replicate assays for understanding the variation of this key reproductive trait that likely affects transmission. IMPORTANCE Only sexual-stage malaria parasites are transmitted from human blood to mosquitoes. Thus, it is vital to understand variations in sexual commitment rates because these may be modifiable or susceptible to blocking. Two different methods of commitment rate measurement were first compared, demonstrating higher sensitivity and precision by the detection of an early differentiation marker, which was subsequently used to survey diverse lines. Clinical isolates from Ghana showed significant variation in mean per-cycle commitment rates and variation among biological replicates. Laboratory-adapted lines of diverse origins had a wider range with most being within the range observed for the clinical isolates, while a minority consistently had lower or zero rates. There was quantitative variation in the effects when adding choline to suppress commitment, indicating differing responsiveness of parasites to this environmental modification. Performing multiple assay replicates and comparisons of diverse isolates was important to understand this trait and its potential effects on transmission.

Persistence of Plasmodium falciparum HRP-2 antigenaemia after artemisinin combination therapy is not associated with gametocytes

BACKGROUND

In some settings, sensitive field diagnostic tools may be needed to achieve elimination of falciparum malaria. To this end, rapid diagnostic tests (RDTs) based on the detection of the Plasmodium falciparum protein HRP-2 are being developed with increasingly lower limits of detection. However, it is currently unclear how parasite stages that are unaffected by standard drug treatments may contribute to HRP-2 detectability and potentially confound RDT results even after clearance of blood stage infection. This study assessed the detectability of HRP-2 in periods of post-treatment residual gametocytaemia.

METHODS

A cohort of 100 P. falciparum infected, gametocyte positive individuals were treated with or without the gametocytocidal drug primaquine (PQ), alongside standard artemisinin-based combination therapy (ACT), in the context of a randomised clinical trial in Ouelessebougou, Mali. A quantitative ELISA was used to measure levels of HRP-2, and compared time to test negativity using a standard and ultra-sensitive RDT (uRDT) between residual gametocyte positive and negative groups.

RESULTS

Time to test negativity was longest by uRDT, followed by ELISA and then standard RDT. No significant difference in time to negativity was found between the treatment groups with and without residual gametocytes: uRDT (HR 0.79 [95% CI 0.52-1.21], p = 0.28), RDT (HR 0.77 [95% CI 0.51-1.15], p = 0.20) or ELISA (HR 0.88 [95% CI 0.59-1.32], p = 0.53). Similarly, no difference was observed when adjusting for baseline asexual parasite density. Quantified levels of HRP-2 over time were similar between groups, with differences attributable to asexual parasite densities. Furthermore, no difference in levels of HRP-2 was found between individuals who were or were not infectious to mosquitoes (OR 1.19 [95% CI 0.98-1.46], p = 0.077).

CONCLUSIONS

Surviving sexual stage parasites after standard ACT treatment do not contribute to the persistence of HRP-2 antigenaemia, and appear to have little impact on RDT results.

Basophil Depletion Alters Host Immunity, Intestinal Permeability, and Mammalian Host-to-Mosquito Transmission in Malaria

Malaria-induced bacteremia has been shown to result from intestinal mast cell (MC) activation. The appearance of MCs in the ileum and increased intestinal permeability to enteric bacteria are preceded by an early Th2-biased host immune response to infection, characterized by the appearance of IL-4, IL-10, mast cell protease (Mcpt)1 and Mcpt4, and increased circulating basophils and eosinophils. Given the functional similarities of basophils and MCs in the context of allergic inflammation and the capacity of basophils to produce large amounts of IL-4, we sought to define the role of basophils in increased intestinal permeability, in MC influx, and in the development of bacteremia in the context of malaria. Upon infection with nonlethal Plasmodium yoelii yoelii 17XNL, Basoph8 × ROSA-DTα mice or baso (-) mice that lack basophils exhibited increased intestinal permeability and increased ileal MC numbers, without any increase in bacterial 16S ribosomal DNA copy numbers in the blood, relative to baso (+) mice. Analysis of cytokines, chemokines, and MC-associated factors in the ileum revealed significantly increased TNF-α and IL-13 at day 6 postinfection in baso (-) mice compared with baso (+) mice. Moreover, network analysis of significantly correlated host immune factors revealed profound differences between baso (-) and baso (+) mice following infection in both systemic and ileal responses to parasites and translocated bacteria. Finally, basophil depletion was associated with significantly increased gametocytemia and parasite transmission to Anopheles mosquitoes, suggesting that basophils play a previously undescribed role in controlling gametocytemia and, in turn, mammalian host-to-mosquito parasite transmission.

School-Based Malaria Screening and Treatment Reduces Plasmodium falciparum Infection and Anemia Prevalence in Two Transmission Settings in Malawi

BACKGROUND

In areas highly endemic for malaria, Plasmodium falciparum infection prevalence peaks in school-age children, adversely affecting health and education. School-based intermittent preventive treatment reduces this burden but concerns about cost and widespread use of antimalarial drugs limit enthusiasm for this approach. School-based screening and treatment is an attractive alternative. In a prospective cohort study, we evaluated the impact of school-based screening and treatment on the prevalence of P. falciparum infection and anemia in 2 transmission settings.

METHODS

We screened 704 students in 4 Malawian primary schools for P. falciparum infection using rapid diagnostic tests (RDTs), and treated students who tested positive with artemether-lumefantrine. We determined P. falciparum infection by microscopy and quantitative polymerase chain reaction (qPCR), and hemoglobin concentrations over 6 weeks in all students.

RESULTS

Prevalence of infection by RDT screening was 37% (9%-64% among schools). An additional 9% of students had infections detected by qPCR. Following the intervention, significant reductions in infections were detected by microscopy (adjusted relative reduction [aRR], 48.8%; P < .0001) and qPCR (aRR, 24.5%; P < .0001), and in anemia prevalence (aRR, 30.8%; P = .003). Intervention impact was reduced by infections not detected by RDT and new infections following treatment.

CONCLUSIONS

School-based screening and treatment reduced P. falciparum infection and anemia. This approach could be enhanced by repeating screening, using more-sensitive screening tests, and providing longer-acting drugs.

CLINICAL TRIALS REGISTRATION

NCT04858087.

Declines in prevalence alter the optimal level of sexual investment for the malaria parasite Plasmodium falciparum

Like most human pathogens, the malaria parasite Plasmodium falciparum experiences strong selection pressure from public health interventions such as drug treatment. While most commonly studied in the context of drug targets and related pathways, parasite adaptation to control measures likely extends to phenotypes beyond drug resistance. Here, we use modeling to explore how control measures can reduce levels of within-host competition between P. falciparum genotypes and favor higher rates of sexual investment. We validate these predictions with longitudinally sampled genomic data from French Guiana during a period of malaria decline and find that the most strongly selected genes are enriched for transcription factors involved in commitment to and development of the parasite’s sexual gametocyte form.

Successful infectious disease interventions can result in large reductions in parasite prevalence. Such demographic change has fitness implications for individual parasites and may shift the parasite’s optimal life history strategy. Here, we explore whether declining infection rates can alter Plasmodium falciparum’s investment in sexual versus asexual growth. Using a multiscale mathematical model, we demonstrate how the proportion of polyclonal infections, which decreases as parasite prevalence declines, affects the optimal sexual development strategy: Within-host competition in multiclone infections favors a greater investment in asexual growth whereas single-clone infections benefit from higher conversion to sexual forms. At the same time, drug treatment also imposes selection pressure on sexual development by shortening infection length and reducing within-host competition. We assess these models using 148 P. falciparum parasite genomes sampled in French Guiana over an 18-y period of intensive intervention (1998 to 2015). During this time frame, multiple public health measures, including the introduction of new drugs and expanded rapid diagnostic testing, were implemented, reducing P. falciparum malaria cases by an order of magnitude. Consistent with this prevalence decline, we see an increase in the relatedness among parasites, but no single clonal background grew to dominate the population. Analyzing individual allele frequency trajectories, we identify genes that likely experienced selective sweeps. Supporting our model predictions, genes showing the strongest signatures of selection include transcription factors involved in the development of P. falciparum’s sexual gametocyte form. These results highlight how public health interventions impose wide-ranging selection pressures that affect basic parasite life history traits.

Genetic Diversity of Plasmodium falciparum and Distribution of Antimalarial Drug Resistance Mutations in Symptomatic and Asymptomatic Infections

Malaria control relies on passive case detection, and this strategy fails detecting asymptomatic infections. In addition, infections in endemic areas harbor multiple parasite genotypes that could affect case management and malaria epidemiology.

Quantifying Reductions in Plasmodium falciparum Infectivity to Mosquitos: A Sample Size Calculator to Inform Clinical Trials on Transmission-Reducing Interventions

Malaria transmission depends on the presence of mature Plasmodium transmission stages (gametocytes) that may render blood-feeding Anopheles mosquitos infectious. Transmission-blocking antimalarial drugs and vaccines can prevent transmission by reducing gametocyte densities or infectivity to mosquitos. Mosquito infection outcomes are thereby informative biological endpoints of clinical trials with transmission blocking interventions. Nevertheless, trials are often primarily designed to determine intervention safety; transmission blocking efficacy is difficult to incorporate in sample size considerations due to variation in infection outcomes and considerable inter-study variation. Here, we use clinical trial data from studies in malaria naive and naturally exposed study participants to present an online sample size calculator tool. This sample size calculator allows studies to be powered to detect reductions in the proportion of infected mosquitos or infection burden (oocyst density) in mosquitos. The utility of this online tool is illustrated using trial data with transmission blocking malaria drugs.

Systems biology of malaria explored with nonhuman primates

"The Primate Malarias" book has been a uniquely important resource for multiple generations of scientists, since its debut in 1971, and remains pertinent to the present day. Indeed, nonhuman primates (NHPs) have been instrumental for major breakthroughs in basic and pre-clinical research on malaria for over 50 years. Research involving NHPs have provided critical insights and data that have been essential for malaria research on many parasite species, drugs, vaccines, pathogenesis, and transmission, leading to improved clinical care and advancing research goals for malaria control, elimination, and eradication. Whilst most malaria scientists over the decades have been studying Plasmodium falciparum, with NHP infections, in clinical studies with humans, or using in vitro culture or rodent model systems, others have been dedicated to advancing research on Plasmodium vivax, as well as on phylogenetically related simian species, including Plasmodium cynomolgi, Plasmodium coatneyi, and Plasmodium knowlesi. In-depth study of these four phylogenetically related species over the years has spawned the design of NHP longitudinal infection strategies for gathering information about ongoing infections, which can be related to human infections. These Plasmodium-NHP infection model systems are reviewed here, with emphasis on modern systems biological approaches to studying longitudinal infections, pathogenesis, immunity, and vaccines. Recent discoveries capitalizing on NHP longitudinal infections include an advanced understanding of chronic infections, relapses, anaemia, and immune memory. With quickly emerging new technological advances, more in-depth research and mechanistic discoveries can be anticipated on these and additional critical topics, including hypnozoite biology, antigenic variation, gametocyte transmission, bone marrow dysfunction, and loss of uninfected RBCs. New strategies and insights published by the Malaria Host-Pathogen Interaction Center (MaHPIC) are recapped here along with a vision that stresses the importance of educating future experts well trained in utilizing NHP infection model systems for the pursuit of innovative, effective interventions against malaria.

African Plasmodium vivax malaria improbably rare or benign

The overwhelming dominance of Duffy blood group negativity among most people living in sub-Saharan Africa has been considered the basis of their protection from endemic Plasmodium vivax malaria. New evidence demonstrates widespread transmission of P. vivax in Duffy-negative Africa, though currently of unknown distribution, magnitude, or consequences. Other new evidence from outside of Africa demonstrates marked tropisms of P. vivax for extravascular tissues of bone marrow and spleen. Those establish states of proliferative infection with low-grade or undetectable parasitemia of peripheral blood causing acute and chronic disease. This review examines the plausibility of those infectious processes also operating in Duffy-negative Africans and causing harm of unrecognized origin.

Asymptomatic School-Aged Children Are Important Drivers of Malaria Transmission in a High Endemicity Setting in Uganda

Achieving malaria elimination requires a better understanding of the transmissibility of human infections in different transmission settings. This study aimed to characterize the human infectious reservoir in a high endemicity setting in eastern Uganda, using gametocyte quantification and mosquito feeding assays. In asymptomatic infections, gametocyte densities were positively associated with the proportion of infected mosquitoes (β = 1.60; 95% CI, 1.32-1.92; P < .0001). Combining transmissibility and abundance in the population, symptomatic and asymptomatic infections were estimated to contribute to 5.3% and 94.7% of the infectious reservoir, respectively. School-aged children (5-15 years old) contributed to 50.4% of transmission events and were important drivers of malaria transmission.

A cohort study on the duration of Plasmodium falciparum infections during the dry season in The Gambia

Background

In areas where Plasmodium falciparum malaria is seasonal, a dry season reservoir of blood-stage infection is essential for initiating transmission during the following wet season.

Methods

In The Gambia, a cohort of 42 individuals with quantitative polymerase chain reaction-positive P falciparum infections at the end of the transmission season (December) were followed monthly until the end of the dry season (May) to evaluate infection persistence. The influence of human host and parasitological factors was investigated.

Results

A large proportion of individuals infected at the end of the wet season had detectable infections until the end of the dry season (40.0%; 16 of 40). At the start of the dry season, the majority of these persistent infections (82%) had parasite densities >10 p/µL compared to only 5.9% of short-lived infections. Persistent infections (59%) were also more likely to be multiclonal than short-lived infections (5.9%) and were associated with individuals having higher levels of P falciparum-specific antibodies (P = .02).

Conclusions

Asymptomatic persistent infections were multiclonal with higher parasite densities at the beginning of the dry season. Screening and treating asymptomatic infections during the dry season may reduce the human reservoir of malaria responsible for initiating transmission in the wet season.

The Dynamic Change of Immune Responses Between Acute and Recurrence Stages of Rodent Malaria Infection

Malaria infections are persistent as frequent recrudescence of the disease may occur following the acute infection stage, but the different immune responses that control the acute and recrudescence stages are still largely unknown. Using single-cell RNA sequencing (scRNA-seq), we showed that the number of Th1 and plasma cells in the spleen was significantly reduced during the recurrence stage compared to the acute stage of Plasmodium chabaudi chabaudi AS (P. chabaudi) infection. Additionally, the ability of both CD4+ T cell responses and B cells to control P. chabaudi recurrence was significantly reduced compared to their roles in the control of acute infection. In contrast, the number of innate immune cells, including red pulp macrophages (RPMs), gamma delta (γδ) T cells, and Dendritic cells (DCs) were significantly increased during the recurrence stage and showed to be critical for P. chabaudi infection recurrence control. Thus, our data strongly suggest the complementary role of innate immune responses in controlling malaria recrudescence when adaptive immune responses are suppressed. These findings shed new light on the development of immune interventions against malaria.

'Bouncing Back' From Subclinical Malaria: Inflammation and Erythrocytosis After Resolution of P. falciparum Infection in Gambian Children

Recent malaria is associated with an increased risk of systemic bacterial infection. The aetiology of this association is unclear but malaria-related haemolysis may be one contributory factor. To characterise the physiological consequences of persistent and recently resolved malaria infections and associated haemolysis, 1650 healthy Gambian children aged 8-15 years were screened for P. falciparum infection (by 18sRNA PCR) and/or anaemia (by haematocrit) at the end of the annual malaria transmission season (t₁). P. falciparum-infected children and children with moderate or severe anaemia (haemoglobin concentration < 11g/dl) were age matched to healthy, uninfected, non-anaemic controls and screened again 2 months later (t₂). Persistently infected children (PCR positive at t₁ and t₂) had stable parasite burdens and did not differ significantly haematologically or in terms of proinflammatory markers from healthy, uninfected children. However, among persistently infected children, IL-10 concentrations were positively correlated with parasite density suggesting a tolerogenic response to persistent infection. By contrast, children who naturally resolved their infections (positive at t₁ and negative at t₂) exhibited mild erythrocytosis and concentrations of pro-inflammatory markers were raised compared to other groups of children. These findings shed light on a 'resetting' and potential overshoot of the homeostatic haematological response following resolution of malaria infection. Interestingly, the majority of parameters tested were highly heterogeneous in uninfected children, suggesting that some may be harbouring cryptic malaria or other infections.

Cross-sectional survey of asymptomatic malaria in Dak Nong province in the Central Highlands of Vietnam for the malaria elimination roadmap

Asymptomatic parasite carriers represent a “silent” infective reservoir for malaria transmission and contributes to malaria persistence. However, limited data are available on asymptomatic malaria in Vietnam. Between November 2018 and March 2019, we conducted a malaria epidemiological survey of asymptomatic people (children ≥ 10 years old and adults ≥18 years old, n = 2,809) residing in three communes in Tuy Duc district, Dak Nong province in the Central Highlands of Vietnam. Based on the national stratification of malaria risk, Dak Buk So, Dak Ngo and Quang Truc communes were classified by the National Malaria Control Programme as low, moderate and high malaria endemic areas, respectively. Using participants’ finger prick blood samples, malaria parasites were detected by one-step reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The median age (Interquartile Range) for adults and children were 35 years (26–50) and 12 years (11–14), respectively. The prevalence of asymptomatic malaria was 1.7% (22/1,328), 3.5% (31/890) and 12.2% (72/591) for participants from Dak Buk So, Dak Ngo and Quang Truc, respectively. The prevalence of asymptomatic malaria was lower in children compared to adults: 2.6% (9/352) versus 4.7% (116/2,457) (Odds Ratio 0.53, 95% Confidence Interval 0.28 to1.02). Ownership of long-lasting insecticide-treated bed nets and hammocks was 97.1%, 99.0% and 94.7% for participants in Dak Buk So, Dak Ngo and Quang Truc, respectively, however, only 66.0%, 57.3% and 42.8% of the participants reported using bed nets every night. Of the several risk factors examined, going to the forest two weeks prior to enrolment into the study and sleeping in the forest had a significant association with participants being infected with asymptomatic malaria in Quang Truc, but not in the other two communes. Knowledge of the prevalence and distribution of asymptomatic malaria will help design and evaluate future intervention strategies for malaria elimination in Vietnam.

Infectivity of patent Plasmodium falciparum gametocyte carriers to mosquitoes: establishing capacity to investigate the infectious reservoir of malaria in a low-transmission setting in The Gambia

Background

Understanding the human malaria infectious reservoir is important for elimination initiatives. Here, we implemented mosquito membrane feeding experiments to prepare for larger studies to quantify the transmission potential and relative contribution of the human infectious reservoir.

Methods

Patients with clinical malaria attending four health facilities with at least 16 Plasmodium falciparum gametocytes per μL were recruited during the 2018 transmission season. Infectiousness to mosquitoes was assessed by direct membrane feeding assay (DMFA). We compared our results with a Bayesian predictive model to investigate the relationship between infectiousness and gametocyte density and explore the impact of fever on gametocyte infectivity.

Results

A total of 3177 suspected malaria cases were screened; 43.3% (1376) had microscopically patent P. falciparum parasites and 3.6% (114) of them had gametocytes. Out of 68 DMFAs, 38 (55.9%) resulted in at least one infected mosquito, with a total of 15.4% (1178/7667) of mosquitoes infected with 1–475 oocysts per gut. The relationship between mosquito infection prevalence and gametocytaemia was similar to other African settings and negatively associated with fever (OR: 0.188, 95% CI 0.0603 to 0.585, p=0.0039).

Conclusions

Among symptomatic malaria patients, fever is strongly associated with transmission failure. Future studies can use DMFA to better understand the human malaria reservoir in settings of low endemicity in The Gambia and inform malaria elimination initiatives.