A Molecular Assay to Quantify Male and Female Plasmodium falciparum Gametocytes: Results From 2 Randomized Controlled Trials Using Primaquine for Gametocyte Clearance

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.

Longitudinal impact of asymptomatic malaria/HIV-1 co-infection on Plasmodium falciparum gametocyte transcript expression and transmission to Anopheles mosquitoes

Despite significant developments towards malaria reduction, parasite transmission in the common context of HIV-1 co-infection and treatment for one or both infections has not been fully characterized. This is particularly important given that HIV-1 and malaria chemotherapies have the potential to alter gametocyte burden and mosquito infectivity. In this study, we examined 782 blood samples collected from a longitudinal cohort of 300 volunteers with asymptomatic parasitemia seeking HIV testing or treatment in the endemic region of Kisumu, Kenya, to define the impacts of HIV-1-malaria co-infection, antiretroviral therapy (ART) plus trimethoprim-sulfamethoxazole (TS) and the antimalarials artemether/lumefantrine (AL) on Plasmodium falciparum gametocyte transcript prevalence and parasite transmission to the African malaria mosquito Anopheles gambiae. Volunteers were assigned to three distinct HIV-1 groups: HIV-1 positive on treatment, HIV-1 positive newly diagnosed, and HIV-1 negative. Volunteers were monitored monthly over the course of six months. Using our highly sensitive digital droplet PCR (ddPCR) assay of three gametocyte specific transcript markers, we detected gametocyte transcripts in 51.1% of 18S positive volunteers across all study groups and time points. After correcting for multiple comparisons, the factors of HIV-1 status, time, CD4+ T-cell levels and hematocrit were not predictive of gametocyte prevalence or transmission. However, among those volunteers who were newly diagnosed with HIV-1 and malaria positive by rapid diagnostic test (RDT) at enrollment, the initiation of ART/TS and AL treatment was associated with a significant reduction in gametocyte transcript prevalence in the subsequent month when compared to HIV-1 negative volunteers treated with AL. To assess gametocyte transmissibility, volunteer blood samples were used in standard membrane feeding assays (SFMA) with laboratory-reared A. gambiae, with evidence of transmission confirmed by at least one of 25 dissected mosquitoes per sample positive for at least one midgut oocyst. HIV-1 status, CD4+ T-cell levels and hematocrit were not significantly associated with successful transmission to A. gambiae. Analysis of SMFA blood samples revealed that 50% of transmission-positive blood samples failed to test positive by Plasmodium-specific 18S ribosomal RNA quantitative PCR (qPCR) and 35% failed to test positive for any gametocyte specific transcript marker by droplet digital (ddPCR), documenting that transmission occurred in the absence of molecular parasite/gametocyte detection. Overall, these findings highlight the complexity of HIV-1 malaria co-infection and the need to further define the unpredictable role of asymptomatic parasitemia in transmission to mosquitoes.

The Plasmodium falciparum CCCH Zinc Finger Protein ZNF4 Plays an Important Role in Gametocyte Exflagellation through the Regulation of Male Enriched Transcripts

CCCH zinc finger proteins (ZFPs) function mainly as RNA-binding proteins (RBPs) and play a central role in the mRNA metabolism. Over twenty seven CCCH-ZFPs are encoded in the genome of the human malaria parasite Plasmodium falciparum, the causative agent of malaria tropica. However, little is known about their functions. In this study, we characterize one member of the PfCCCH-ZFP named ZNF4. We show that ZNF4 is highly expressed in mature gametocytes, where it predominantly localizes to the cytoplasm. Targeted gene disruption of ZNF4 showed no significant effect in asexual blood stage replication and gametocyte development while male gametocyte exflagellation was significantly impaired, leading to reduced malaria transmission in the mosquito. Comparative transcriptomics between wildtype (WT) and the ZNF4-deficient line (ZNF4-KO) demonstrated the deregulation of about 473 genes (274 upregulated and 199 downregulated) in mature gametocytes. Most of the downregulated genes show peak expression in mature gametocyte with male enriched genes associated to the axonemal dynein complex formation, and cell projection organization is highly affected, pointing to the phenotype in male gametocyte exflagellation. Upregulated genes are associated to ATP synthesis. Our combined data therefore indicate that ZNF4 is a CCCH zinc finger protein which plays an important role in male gametocyte exflagellation through the regulation of male gametocyte-enriched genes.

Evaluation of the effect of targeted Mass Drug Administration and Reactive Case Detection on malaria transmission and elimination in Eastern Hararghe zone, Oromia, Ethiopia: a cluster randomized control trial

Background

Reactive and proactive case detection measures are widely implemented by national malaria elimination programs globally. Ethiopia decided to include Reactive Case Detection (RCD) and targeted Mass Drug Administration (tMDA) approaches as part of their elimination strategy along with rigorous evaluation. The purpose of this study is to compare the impact of RCD and tMDA on malaria elimination over the 2-year study period, by looking at the annual parasite incidence before and after the intervention.

Methods

The study will be conducted in the East Hararghe zone of Ethiopia. Malaria transmission in the area is low to moderate. This study will deploy a community-based, three-arm, cluster-randomized control trial implemented over 2 years. Forty-eight clusters (16 clusters per arm) will be selected based on the annual number of confirmed malaria cases seen in the cluster. All clusters will receive the current standard of care in terms of malaria elimination interventions provided by the national malaria control program. In addition, following the identification of malaria parasite infection, individuals who reside within a 100-m radius of the index case will receive a diagnosis for malaria and treatment if positive in the RCD arm or presumptive treatment in the tMDA arm. The primary effectiveness endpoint will be measured at baseline and endline for each intervention arm and compared to the control arm using a difference in difference approach.

Discussion

This randomized controlled trial will provide evidence of the impact of the proposed intervention approaches for malaria elimination.

Trial registration

ClinicalTrials.gov NCT04241705. Registration date: January 27, 2020.

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.

Histamine Ingestion by Anopheles stephensi Alters Important Vector Transmission Behaviors and Infection Success with Diverse Plasmodium Species

An estimated 229 million people worldwide were impacted by malaria in 2019. The vectors of malaria parasites (Plasmodium spp.) are Anopheles mosquitoes, making their behavior, infection success, and ultimately transmission of great importance. Individuals with severe malaria can exhibit significantly increased blood concentrations of histamine, an allergic mediator in humans and an important insect neuromodulator, potentially delivered to mosquitoes during blood-feeding. To determine whether ingested histamine could alter Anopheles stephensi biology, we provisioned histamine at normal blood levels and at levels consistent with severe malaria and monitored blood-feeding behavior, flight activity, antennal and retinal responses to host stimuli and lifespan of adult female Anopheles stephensi. To determine the effects of ingested histamine on parasite infection success, we quantified midgut oocysts and salivary gland sporozoites in mosquitoes infected with Plasmodium yoelii and Plasmodium falciparum. Our data show that provisioning An. stephensi with histamine at levels consistent with severe malaria can enhance mosquito behaviors and parasite infection success in a manner that would be expected to amplify parasite transmission to and from human hosts. Such knowledge could be used to connect clinical interventions by reducing elevated histamine to mitigate human disease pathology with the delivery of novel lures for improved malaria control.

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

Plasmodium falciparum gametocyte kinetics and infectivity may differ between chronic and incident infections. In the current study, we assess parasite kinetics and infectivity to mosquitoes among children (aged 5-10 years) from Burkina Faso with (a) incident infections following parasite clearance (n = 48) and (b) chronic asymptomatic infections (n = 60). In the incident infection cohort, 92% (44/48) of children develop symptoms within 35 days, compared to 23% (14/60) in the chronic cohort. All individuals with chronic infection carried gametocytes or developed them during follow-up, whereas only 35% (17/48) in the incident cohort produce gametocytes before becoming symptomatic and receiving treatment. Parasite multiplication rate (PMR) and the relative abundance of ap2-g and gexp-5 transcripts are positively associated with gametocyte production. Antibody responses are higher and PMR lower in chronic infections. The presence of symptoms and sexual stage immune responses are associated with reductions in gametocyte infectivity to mosquitoes. We observe that most incident infections require treatment before the density of mature gametocytes is sufficient to infect mosquitoes. In contrast, chronic, asymptomatic infections represent a significant source of mosquito infections. Our observations support the notion that malaria transmission reduction may be expedited by enhanced case management, involving both symptom-screening and infection detection.

G6PD Polymorphisms and Hemolysis After Antimalarial Treatment With Low Single-Dose Primaquine: A Pooled Analysis of Six African Clinical Trials

Primaquine (PQ) is an antimalarial drug with the potential to reduce malaria transmission due to its capacity to clear mature Plasmodium falciparum gametocytes in the human host. However, the large-scale roll-out of PQ has to be counterbalanced by the additional risk of drug-induced hemolysis in individuals suffering from Glucose-6-phospate dehydrogenase (G6PD) deficiency, a genetic condition determined by polymorphisms on the X-linked G6PD gene. Most studies on G6PD deficiency and PQ-associated hemolysis focused on the G6PD A- variant, a combination of the two single nucleotide changes G202A (rs1050828) and A376G (rs1050829), although other polymorphisms may play a role. In this study, we tested the association of 20 G6PD single nucleotide polymorphisms (SNPs) with hemolysis measured seven days after low single dose of PQ given at the dose of 0.1 mg/kg to 0.75 mg/kg in 957 individuals from 6 previously published clinical trials investigating the safety and efficacy of this drug spanning five African countries. After adjusting for inter-study effects, age, gender, baseline hemoglobin level, PQ dose, and parasitemia at screening, our analysis showed putative association signals from the common G6PD mutation, A376G [-log10(p-value) = 2.44] and two less-known SNPs, rs2230037 [-log10(p-value] = 2.60), and rs28470352 [-log10(p-value) = 2.15]; A376G and rs2230037 were in very strong linkage disequilibrium with each other (R 2 = 0.978). However, when the effects of these SNPs were included in the same regression model, the subsequent associations were in the borderline of statistical significance. In conclusion, whilst a role for the A- variant is well established, we did not observe an important additional role for other G6PD polymorphisms in determining post-treatment hemolysis in individuals treated with low single-dose PQ.

Targeting Gametocytes of the Malaria Parasite Plasmodium falciparum in a Functional Genomics Era: Next Steps

Mosquito transmission of the deadly malaria parasite Plasmodium falciparum is mediated by mature sexual forms (gametocytes). Circulating in the vertebrate host, relatively few intraerythrocytic gametocytes are picked up during a bloodmeal to continue sexual development in the mosquito vector. Human-to-vector transmission thus represents an infection bottleneck in the parasite's life cycle for therapeutic interventions to prevent malaria. Even though recent progress has been made in the identification of genetic factors linked to gametocytogenesis, a plethora of genes essential for sexual-stage development are yet to be unraveled. In this review, we revisit P. falciparum transmission biology by discussing targetable features of gametocytes and provide a perspective on a forward-genetic approach for identification of novel transmission-blocking candidates in the future.

HIV-1 Infection Is Associated With Increased Prevalence and Abundance of Plasmodium falciparum Gametocyte-Specific Transcripts in Asymptomatic Adults in Western Kenya

As morbidity and mortality due to malaria continue to decline, the identification of individuals with a high likelihood of transmitting malaria is needed to further reduce the prevalence of malaria. In areas of holoendemic malaria transmission, asymptomatically infected adults may be infected with transmissible gametocytes. The impact of HIV-1 on gametocyte carriage is unknown, but co-infection may lead to an increase in gametocytemia. In this study, a panel of qPCR assays was used to quantify gametocyte stage-specific transcripts present in dried blood spots obtained from asymptomatic adults seeking voluntary HIV testing in Kombewa, Kenya. A total of 1,116 Plasmodium-specific 18S-positive samples were tested and 20.5% of these individuals had detectable gametocyte-specific transcripts. Individuals also infected with HIV-1 were 1.82 times more likely to be gametocyte positive (P<0.0001) and had significantly higher gametocyte copy numbers when compared to HIV-negative individuals. Additionally, HIV-1 positivity was associated with higher gametocyte prevalence in men and increased gametocyte carriage with age. Overall, these data suggest that HIV-positive individuals may have an increased risk of transmitting malaria parasites in regions with endemic malaria transmission and therefore should be at a higher priority for treatment with gametocidal antimalarial drugs.

Defining the Antimalarial Activity of Cipargamin in Healthy Volunteers Experimentally Infected with Blood-Stage Plasmodium falciparum

The spiroindolone cipargamin, a new antimalarial compound that inhibits Plasmodium ATP4, is currently in clinical development. This study aimed to characterize the antimalarial activity of cipargamin in healthy volunteers experimentally infected with blood-stage Plasmodium falciparum.

The spiroindolone cipargamin, a new antimalarial compound that inhibits Plasmodium ATP4, is currently in clinical development. This study aimed to characterize the antimalarial activity of cipargamin in healthy volunteers experimentally infected with blood-stage Plasmodium falciparum. Eight subjects were intravenously inoculated with parasite-infected erythrocytes and received a single oral dose of 10 mg cipargamin 7 days later. Blood samples were collected to monitor the development and clearance of parasitemia and plasma cipargamin concentrations. Parasite regrowth was treated with piperaquine monotherapy to clear asexual parasites, while allowing gametocyte transmissibility to mosquitoes to be investigated. An initial rapid decrease in parasitemia occurred in all participants following cipargamin dosing, with a parasite clearance half-life of 3.99 h. As anticipated from the dose selected, parasite regrowth occurred in all 8 subjects 3 to 8 days after dosing and allowed the pharmacokinetic/pharmacodynamic relationship to be determined. Based on the limited data from the single subtherapeutic dose cohort, a MIC of 11.6 ng/ml and minimum parasiticidal concentration that achieves 90% of maximum effect of 23.5 ng/ml were estimated, and a single 95-mg dose (95% confidence interval [CI], 50 to 270) was predicted to clear 10⁹ parasites/ml. Low gametocyte densities were detected in all subjects following piperaquine treatment, which did not transmit to mosquitoes. Serious adverse liver function changes were observed in three subjects, which led to premature study termination. The antimalarial activity characterized in this study supports the further clinical development of cipargamin as a new treatment for P. falciparum malaria, although the hepatic safety profile of the compound warrants further evaluation. (This study has been registered at ClinicalTrials.gov under identifier NCT02543086.)

qRT-PCR versus IFA-based Quantification of Male and Female Gametocytes in Low-Density Plasmodium falciparum Infections and Their Relevance for Transmission

Background

Accurate quantification of female and male gametocytes and sex ratios in asymptomatic low-density malaria infections are important for assessing their transmission potential. Gametocytes often escape detection even by molecular methods, therefore ultralow gametocyte densities were quantified in large blood volumes.

Methods

Female and male gametocytes were quantified in 161 PCR-positive Plasmodium falciparum infections from a cross-sectional survey in Papua New Guinea. Ten-fold concentrated RNA from 800 µL blood was analyzed using female-specific pfs25 and male-specific pfmget or mssp qRT-PCR. Gametocyte sex ratios from qRT-PCR were compared with those from immunofluorescence assays (IFA).

Results

Gametocytes were identified in 58% (93/161) P. falciparum-positive individuals. Mean gametocyte densities were frequently below 1 female and 1 male gametocyte/µL by qRT-PCR. The mean proportion of males was 0.39 (95% confidence interval, 0.33–0.44) by pfs25/pfmget qRT-PCR; this correlated well with IFA results (Pearsons r² = 0.91; P < .001). A Poisson model fitted to our data predicted 16% P. falciparum-positive individuals that are likely to transmit, assuming at least 1 female and 1 male gametocyte per 2.5 µL mosquito bloodmeal.

Conclusions

Based on model estimates of female and male gametocytes per 2.5 µL blood, P. falciparum-positive individuals detected exclusively by ultrasensitive diagnostics are negligible for human-to-mosquito transmission.

Estimating the transmission potential of ultralow-density malaria infections informs interventions. Almost all infections with ≥1 female and male gametocyte per 2.5 µL mosquito bloodmeal, and thus with highest likelihood of contributing to human-to-mosquito transmission, were detectable by standard molecular diagnostics.

Transmission-blocking Effects of Primaquine and Methylene Blue Suggest Plasmodium falciparum Gametocyte Sterilization Rather Than Effects on Sex Ratio

Gametocyte density and sex ratio can predict the proportion of mosquitoes that will become infected after feeding on blood of patients receiving nongametocytocidal drugs. Because primaquine and methylene blue sterilize gametocytes before affecting their density and sex ratio, mosquito feeding experiments are required to demonstrate their early transmission-blocking effects.

Novel Method for the Separation of Male and Female Gametocytes of the Malaria Parasite Plasmodium falciparum That Enables Biological and Drug Discovery

The protozoan Plasmodium falciparum causes the most severe form of human malaria. The development of sexual forms (so-called gametocytes) is crucial for disease transmission. However, knowledge of these forms is severely hampered by the paucity of sex-specific markers and the inability to extract single sex gametocytes in high purity. Moreover, the identification of compounds that specifically affect one sex is difficult due to the female bias of the gametocytes. We have developed a system that allows for the separation of male and female gametocytes from the same population. Applying our system, we show that male and female parasites mature at different rates, which might have implications for transmission. We also identified new sex-specific genes that can be used as sex markers or to unravel sex-specific functions. Our system will not only aid in the discovery of much needed gametocidal compounds, but it also represents a valuable tool for exploring malaria transmission biology.

We developed a flow-cytometry-based method to separate and collect cocultured male and female Plasmodium falciparum gametocytes responsible for malaria transmission. The purity of the collected cells was estimated at >97% using flow cytometry, and sorted cells were observed by Giemsa-stained thin-smear and live-cell fluorescence microscopy. The expression of validated sex-specific markers corroborated the sorting strategy. Collected male and female gametocytes were used to confirm three novel sex-specific markers by quantitative real-time PCR that were more enriched in sorted male and female gametocyte populations than existing sex-specific markers. We also applied the method as a proof-of-principle drug screen that allows the identification of drugs that kill gametocytes in a sex-specific manner. Since the developed method allowed for the separation of male and female parasites from the same culture, we observed for the first time a difference in development time between the sexes: females developed faster than males. Hence, the ability to separate male and female gametocytes opens the door to a new field of sex-specific P. falciparum gametocyte biology to further our understanding of malaria transmission.

IMPORTANCE The protozoan Plasmodium falciparum causes the most severe form of human malaria. The development of sexual forms (so-called gametocytes) is crucial for disease transmission. However, knowledge of these forms is severely hampered by the paucity of sex-specific markers and the inability to extract single sex gametocytes in high purity. Moreover, the identification of compounds that specifically affect one sex is difficult due to the female bias of the gametocytes. We have developed a system that allows for the separation of male and female gametocytes from the same population. Applying our system, we show that male and female parasites mature at different rates, which might have implications for transmission. We also identified new sex-specific genes that can be used as sex markers or to unravel sex-specific functions. Our system will not only aid in the discovery of much needed gametocidal compounds, but it also represents a valuable tool for exploring malaria transmission biology.

Assays for quantification of male and female gametocytes in human blood by qRT-PCR in the absence of pure sex-specific gametocyte standards

Background

Malaria transmission from humans to Anopheles mosquitoes requires the presence of gametocytes in human peripheral circulation, and the dynamics of transmission are determined largely by the density and sex ratio of the gametocytes. Molecular methods are thus employed to measure gametocyte densities, particularly when assessing transmission epidemiology and the efficacy of transmission-blocking interventions. However, accurate quantification of male and female gametocytes with molecular methods requires pure male and female gametocytes as reference standards, which are not widely available.

Methods

qRT-PCR assays were used to quantify levels of sex-specific mRNA transcripts in Plasmodium falciparum female and male gametocytes (pfs25 and pfMGET, respectively) using synthetic complimentary RNA standards and in vitro cultured gametocytes. Assays were validated and assay performance was investigated in blood samples of clinical trial participants using these standards and compared to absolute quantification by droplet digital PCR (ddPCR).

Results

The number of transcript copies per gametocyte were determined to be 279.3 (95% CI 253.5–307.6) for the female-specific transcript pfs25, and 12.5 (95% CI 10.6–14.9) for the male-specific transcript pfMGET. These numbers can be used to convert from transcript copies/mL to gametocyte/mL. The reportable range was determined to be 5.71 × 10⁶ to 5.71 female gametocytes/mL for pfs25, and 1.73 × 10⁷ to 1.73 × 10¹ male gametocytes/mL for pfMGET. The limit of detection was 3.9 (95% CI 2.5–8.2) female gametocytes/mL for pfs25, and 26.9 (95% CI 19.3–51.7) male gametocytes/mL for PfMGET. Both assays showed minimal intra-assay and inter-assay variability with coefficient of variation < 3%. No cross-reactivity was observed in both assays in uninfected human blood samples. Comparison of results from ddPCR to qRT-PCR assays on clinical blood samples indicated a high-level agreement (ICC = 0.998 for pfs25 and 0.995 for pfMGET).

Conclusions

This study reports the validation of qRT-PCR assays that are able to accurately quantify female and male P. falciparum gametocytes at sub-microscopic densities. The assays showed excellent reproducibility, sensitivity, precision, specificity, and accuracy. The methodology will enable the estimation of gametocyte density in the absence of pure female and male gametocyte standards, and will facilitate clinical trials and epidemiological studies.

Plasmodium falciparum Gametocyte Enrichment in Peripheral Blood Samples by Magnetic Fractionation: Gametocyte Yields and Possibilities to Reuse Columns

Gametocytes are sexual stage malaria parasites responsible for transmission to mosquitoes. Multiple gametocyte-producing clones may be present in natural infections, but the molecular characterization of gametocytes is challenging. Because of their magnetic properties, gametocyte enrichment can be achieved by magnetic fractionation. This increases detection sensitivity and allows specific genotyping of clones that contribute to malaria transmission. Here, we determined the percentage of Plasmodium falciparum gametocytes successfully bound to magnetic activated cell sorting (MACS) LS columns during magnetic fractionation and assessed whether columns can be reused without risking contamination or affecting column binding efficiency. Bound column fractions were quantified using multiplex quantitative reverse transcription polymerase chain reaction (qRT-PCR) for male (pfMGET) and female (CCp4) gametocytes and ring-stage asexual parasites (SBP1). To investigate cross contamination between columns, parasite strain identity was determined by merozoite surface protein 2 genotyping followed by capillary electrophoresis fragment sizing. A reproducible high percentage of gametocytes was bound to MACS LS columns with < 5% gametocytes appearing in the flow-through and < 0.6% asexual ring-stage parasites appearing in the gametocyte fraction. A high yield (> 94%) of gametocyte enrichment was achieved when columns were used up to five times with lower binding success after eight times (79%). We observed no evidence for cross contamination between columns.

Plasmodium falciparum Gametocyte Density and Infectivity in Peripheral Blood and Skin Tissue of Naturally Infected Parasite Carriers in Burkina Faso

Background

Plasmodium falciparum transmission depends on mature gametocytes that can be ingested by mosquitoes taking a blood meal on human skin. Although gametocyte skin sequestration has long been hypothesized as important contributor to efficient malaria transmission, this has never been formally tested.

Methods

In naturally infected gametocyte carriers from Burkina Faso, we assessed infectivity to mosquitoes by direct skin feeding and membrane feeding. We directly quantified male and female gametocytes and asexual parasites in finger-prick and venous blood samples, skin biopsy samples, and in of mosquitoes that fed on venous blood or directly on skin. Gametocytes were visualized in skin tissue with confocal microscopy.

Results

Although more mosquitoes became infected when feeding directly on skin then when feeding on venous blood (odds ratio, 2.01; 95% confidence interval, 1.21–3.33; P = .007), concentrations of gametocytes were not higher in the subdermal skin vasculature than in other blood compartments; only sparse gametocytes were observed in skin tissue.

Discussion

Our data strongly suggest that there is no significant skin sequestration of P. falciparum gametocytes. Gametocyte densities in peripheral blood are thus informative for predicting onward transmission potential to mosquitoes and can be used to target and monitor malaria elimination initiatives.

A Single-Dose Combination Study with the Experimental Antimalarials Artefenomel and DSM265 To Determine Safety and Antimalarial Activity against Blood-Stage Plasmodium falciparum in Healthy Volunteers

Artefenomel and DSM265 are two new compounds that have been shown to be well tolerated and effective when administered as monotherapy malaria treatment. This study aimed to determine the safety, pharmacokinetics, and pharmacodynamics of artefenomel and DSM265 administered in combination to healthy subjects in a volunteer infection study using the Plasmodium falciparum-induced blood-stage malaria model. Thirteen subjects were inoculated with parasite-infected erythrocytes on day 0 and received a single oral dose of artefenomel and DSM265 on day 7. Cohort 1 (n = 8) received 200 mg artefenomel plus 100 mg DSM265, and cohort 2 (n = 5) received 200 mg artefenomel plus 50 mg DSM265. Blood samples were collected to measure parasitemia, gametocytemia, and artefenomel-DSM265 plasma concentrations. There were no treatment-related adverse events. The pharmacokinetic profiles of artefenomel and DSM265 were similar to those of the compounds when administered as monotherapy, suggesting no pharmacokinetic interactions. A reduction in parasitemia occurred in all subjects following treatment (log₁₀ parasite reduction ratios over 48 h [PRR₄₈] of 2.80 for cohort 1 and 2.71 for cohort 2; parasite clearance half-lives of 5.17 h for cohort 1 and 5.33 h for cohort 2). Recrudescence occurred in 5/8 subjects in cohort 1 between days 19 and 28 and in 5/5 subjects in cohort 2 between days 15 and 22. Low-level gametocytemia (1 to 330 female gametocytes/ml) was detected in all subjects from day 14. The results of this single-dosing combination study support the further clinical development of the use of artefenomel and DSM265 in combination as a treatment for falciparum malaria. (This study has been registered at ClinicalTrials.gov under identifier NCT02389348.).

Gametocyte clearance in children, from western Kenya, with uncomplicated Plasmodium falciparum malaria after artemether-lumefantrine or dihydroartemisinin-piperaquine treatment

BACKGROUND

The efficacy and safety of artemether-lumefantrine (AL) and dihydroartemisinin-piperaquine (DP) against asexual parasites population has been documented. However, the effect of these anti-malarials on sexual parasites is still less clear. Gametocyte clearance following treatment is essential for malaria control and elimination efforts; therefore, the study sought to determine trends in gametocyte clearance after AL or DP treatment in children from a malaria-endemic site in Kenya.

METHODS

Children aged between 0.5 and 12 years from Busia, western Kenya with uncomplicated Plasmodium falciparum malaria were assigned randomly to AL or DP treatment. A total of 334 children were enrolled, and dried blood spot samples were collected for up to 6 weeks after treatment during the peak malaria transmission season in 2016 and preserved. Plasmodium falciparum gametocytes were detected by qRT-PCR and gametocyte prevalence, density and mean duration of gametocyte carriage were determined.

RESULTS

At baseline, all the 334 children had positive asexual parasites by microscopy, 12% (40/334) had detectable gametocyte by microscopy, and 83.7% (253/302) children had gametocytes by RT-qPCR. Gametocyte prevalence by RT-qPCR decreased from 85.1% (126/148) at day 0 to 7.04% (5/71) at day 42 in AL group and from 82.4% (127/154) at day 0 to 14.5% (11/74) at day 42 in DP group. The average duration of gametocyte carriage as estimated by qRT-PCR was slightly shorter in the AL group (4.5 days) than in the DP group (5.1 days) but not significantly different (p = 0.301).

CONCLUSION

The study identifies no significant difference between AL and DP in gametocyte clearance. Gametocytes persisted up to 42 days post treatment in minority of individuals in both treatment arms. A gametocytocidal drug, in combination with artemisinin-based combination therapy, will be useful in blocking malaria transmission more efficiently.

CYP2D6 Polymorphisms and the Safety and Gametocytocidal Activity of Single-Dose Primaquine for Plasmodium falciparum

Single-dose primaquine (PQ) clears mature gametocytes and reduces the transmission of Plasmodium falciparum after artemisinin combination therapy. Genetic variation in CYP2D6, the gene producing the drug-metabolizing enzyme cytochrome P450 2D6 (CYP2D6), influences plasma concentrations of PQ and its metabolites and is associated with PQ treatment failure in Plasmodium vivax malaria.

Single-dose primaquine (PQ) clears mature gametocytes and reduces the transmission of Plasmodium falciparum after artemisinin combination therapy. Genetic variation in CYP2D6, the gene producing the drug-metabolizing enzyme cytochrome P450 2D6 (CYP2D6), influences plasma concentrations of PQ and its metabolites and is associated with PQ treatment failure in Plasmodium vivax malaria. Using blood and saliva samples of varying quantity and quality from 8 clinical trials across Africa (n = 1,076), we were able to genotype CYP2D6 for 774 samples (72%). We determined whether genetic variation in CYP2D6 has implications for PQ efficacy in individuals with gametocytes at the time of PQ administration (n = 554) and for safety in glucose-6-phosphate dehydrogenase (G6PD)-deficient individuals treated with PQ (n = 110). Individuals with a genetically inferred CYP2D6 poor/intermediate metabolizer status had a higher gametocyte prevalence on day 7 or 10 after PQ than those with an extensive/ultrarapid CYP2D6 metabolizer status (odds ratio [OR] = 1.79 [95% confidence interval {CI}, 1.10, 2.90]; P = 0.018). The mean minimum hemoglobin concentrations during follow-up for G6PD-deficient individuals were 11.8 g/dl for CYP2D6 extensive/ultrarapid metabolizers and 12.1 g/dl for CYP2D6 poor/intermediate metabolizers (P = 0. 803). CYP2D6 genetically inferred metabolizer status was also not associated with anemia following PQ treatment (P = 0.331). We conclude that CYP2D6 poor/intermediate metabolizer status may be associated with prolonged gametocyte carriage after treatment with single-low-dose PQ but not with treatment safety.

Evolutionary sex allocation theory explains sex ratios in natural Plasmodium falciparum infections

Malaria transmission is achieved by sexual stages, called gametocytes, and the proportion of gametocytes that are male versus female (sex ratio) influences transmission success. In malaria model systems, variation in gametocyte sex ratios can be explained by the predictions of evolutionary sex allocation theory. We test these predictions using natural Plasmodium falciparum infections. The predicted negative correlation between sex ratio and gametocyte density holds: the sex ratio increases when gametocyte densities decrease, and this is most apparent in single genotype infections and in the dry season. We do not observe higher gametocyte sex ratios in mixed compared with single genotype infections.

Safety and tolerability of single low-dose primaquine in a low-intensity transmission area in South Africa: an open-label, randomized controlled trial

Background

To reduce onward falciparum malaria transmission, the World Health Organization recommends adding single low-dose (SLD) primaquine to artemisinin-based combination treatment in low transmission areas. However, uptake of this recommendation has been relatively slow given concerns about whether individual risks justify potential community benefit. This study was undertaken to generate comprehensive local data on the risk–benefit profile of SLD primaquine deployment in a pre-elimination area in South Africa.

Methods

This randomized, controlled open-label trial investigated adding a single low primaquine dose on day 3 to standard artemether–lumefantrine treatment for uncomplicated falciparum malaria. Efficacy, safety and tolerability of artemether–lumefantrine and primaquine treatment were assessed on days 3, 7, 14, 28 and 42. Lumefantrine concentrations were assayed from dried blood spot samples collected on day 7.

Results

Of 217 patients screened, 166 were enrolled with 140 randomized on day 3, 70 to each study arm (primaquine and no primaquine). No gametocytes were detected by either microscopy or PCR in any of the follow-up samples collected after randomization on day 3, precluding assessment of primaquine efficacy. Prevalence of the CYP2D6*4, CYP2D6*10 and CYP2D6*17 mutant alleles was low with allelic frequencies of 0.02, 0.11 and 0.16, respectively; none had the CYP2D6*4/*4 variant associated with null activity. Among 172 RDT-positive patients G6PD-genotyped, 24 (14%) carried the G6PD deficient (A−) variant. Median haemoglobin concentrations were similar between treatment arms throughout follow-up. A third of participants had a haemoglobin drop > 2 g/dL; this was not associated with primaquine treatment but may be associated with G6PD genotype [52.9% (9/17) with A− genotype vs. 31% (36/116) with other genotypes (p = 0.075)]. Day 7 lumefantrine concentrations and the number and nature of adverse events were similar between study arms; only one serious adverse event occurred (renal impairment in the no primaquine arm). The artemether–lumefantrine PCR-corrected adequate clinical and parasitological response rate was 100%, with only one re-infection found among the 128 patients who completed 42-day follow-up.

Conclusions

Safety, tolerability, CYP2D6 and G6PD variant data from this study support the deployment of the WHO-recommended SLD primaquine without G6PD testing to advance malaria elimination in South African districts with low-intensity residual transmission.

Trial registration Pan African Clinical Trial Registry, PACTR201611001859416. Registered 11 November 2016, https://pactr.samrc.ac.za/TrialDisplay.aspx?TrialID=1859

Plasmodium falciparum sexual differentiation in malaria patients is associated with host factors and GDV1-dependent genes

Plasmodium sexual differentiation is required for malaria transmission, yet much remains unknown about its regulation. Here, we quantify early gametocyte-committed ring (gc-ring) stage, P. falciparum parasites in 260 uncomplicated malaria patient blood samples 10 days before maturation to transmissible stage V gametocytes using a gametocyte conversion assay (GCA). Seventy six percent of the samples have gc-rings, but the ratio of gametocyte to asexual-committed rings (GCR) varies widely (0–78%). GCR correlates positively with parasitemia and is negatively influenced by fever, not hematocrit, age or leukocyte counts. Higher expression levels of GDV1-dependent genes, ap2-g, msrp1 and gexp5, as well as a gdv1 allele encoding H²¹⁷ are associated with high GCR, while high plasma lysophosphatidylcholine levels are associated with low GCR in the second study year. The results provide a view of sexual differentiation in the field and suggest key regulatory roles for clinical factors and gdv1 in gametocytogenesis in vivo.

Here, the authors quantify early gametocyte-committed ring (gc-ring) stage Plasmodium falciparum parasites in 260 malaria patients 10 days before maturation to transmissible stage V gametocytes, and show that the ratio of circulating gc-rings is positively correlated with parasitemia and negatively correlated with body temperature.

DSM265 at 400 Milligrams Clears Asexual Stage Parasites but Not Mature Gametocytes from the Blood of Healthy Subjects Experimentally Infected with Plasmodium falciparum

DSM265 is a novel antimalarial drug in clinical development that acts as a selective inhibitor of Plasmodium dihydroorotate dehydrogenase. In a previous phase 1b study, a single 150-mg dose of DSM265 showed partial efficacy against experimentally induced blood-stage Plasmodium falciparum malaria (IBSM).

DSM265 is a novel antimalarial drug in clinical development that acts as a selective inhibitor of Plasmodium dihydroorotate dehydrogenase. In a previous phase 1b study, a single 150-mg dose of DSM265 showed partial efficacy against experimentally induced blood-stage Plasmodium falciparum malaria (IBSM). Pharmacokinetic/pharmacodynamic modeling predicted a human efficacious dose of 340 mg. The primary objectives of the current study were to determine the safety and efficacy of a single oral 400-mg dose of DSM265 against P. falciparum in the IBSM model. Eight healthy participants were inoculated intravenously with 2,800 parasites and treated with DSM265 7 days later. Unexpectedly, one participant did not develop parasitemia during the study. All other participants developed parasitemia, with the complete clearance of asexual parasites occurring following DSM265 treatment. All seven subjects also became gametocytemic. The secondary objectives were to investigate the gametocytocidal and transmission-blocking activity of a second 400-mg dose of DSM265, which was administered 23 days after inoculation. Gametocytes were not cleared by the second dose of DSM265, and transmission-blocking activity could not be determined due to low gametocyte densities. Three DSM265-related adverse events occurred, including a cutaneous rash in one subject on the day of the second DSM265 dose. The results obtained in this study support the prediction of the efficacious dose of DSM265 and provide further evidence that DSM265 is generally safe and well tolerated. In addition, this study confirms preclinical data indicating that DSM265 permits the development and maturation of gametocytes and does not clear mature circulating gametocytes. (This study has been registered at ClinicalTrials.gov under identifier NCT02573857.)

Biology of Plasmodium falciparum gametocyte sex ratio and implications in malaria parasite transmission

While significant advances have been made in understanding Plasmodium falciparum gametocyte biology and its relationship with malaria parasite transmission, the gametocyte sex ratio contribution to this process still remains a relevant research question. The present review discusses the biology of sex determination in P. falciparum, the underlying host and parasite factors, the sex specific susceptibility to drugs, the effect of sex ratio dynamics on malaria parasite transmission and the development of gametocyte sex specific diagnosis tools. Despite the inherent differences across several studies and approaches, the emerging picture highlights a potentially relevant contribution of the P. falciparum gametocyte sex ratio in the modulation of malaria parasite transmission. The increasing availability of molecular methods to measure gametocyte sex ratio will enable evaluation of important parameters, such as the impact of drug treatment on gametocyte sex ratio in vitro and in vivo as well as the changes of gametocyte sex ratios in natural infections, key steps towards elucidating how these parameters affect parasite infectiousness to the mosquito vectors.

Gametocyte Sex Ratio: The Key to Understanding Plasmodium falciparum Transmission?

A mosquito needs to ingest at least one male and one female gametocyte to become infected with malaria. The sex of Plasmodium falciparum gametocytes can be determined microscopically but recent transcriptomics studies paved the way for the development of molecular methods that allow sex-ratio assessments at much lower gametocyte densities. These sex-specific gametocyte diagnostics were recently used to examine gametocyte dynamics in controlled and natural infections as well as the impact of different antimalarial drugs. It is currently unclear to what extent sex-specific gametocyte diagnostics obviate the need for mosquito feeding assays to formally assess transmission potential. Here, we review recent and historic assessments of gametocyte sex ratio in relation to host and parasite characteristics, treatment, and transmission potential.

Recent RNA sequencing studies have uncovered a number of P. falciparum gametocyte sex-specific targets and provided new insights in gametocyte biology.

After decades when gametocyte sex-ratio research was restricted to nonhuman malarias or in vitro experiments, molecular tools for assessing gametocyte sex ratio are now increasingly available for use in natural P. falciparum infections.

Evidence that gametocyte sex ratio is influenced by total gametocyte density and antimalarial treatment, and improves predictions of transmission potential, highlight the relevance of understanding the gametocyte sex ratio during natural infections.

The finding that the most widely used P. falciparum gametocyte marker Pfs25 is expressed predominantly by female gametocytes and has non-negligible levels of background expression in asexual parasites necessitates a re-evaluation of existing gametocyte data.

Critical examination of approaches exploited to assess the effectiveness of transmission-blocking drugs for malaria

In the absence of clinically proven vaccines and emerging resistance to common antimalarials and insecticides, the onus of interrupting the life cycle of Plasmodium falciparum, is upon the transmission-blocking drugs. Current transmission-blocking drug primaquine finds its use restricted because of associated hemolytic toxicity issues in Glucose-6-Phosphate-Dehydrogenase deficient individuals. This article provides an extensive review of the assays used by the investigators to evaluate the transmission-blocking activity of drugs. Furthermore, limitations in existing transmission-blocking assessment approaches/studies are also covered in detail. This review is expected to help in the identification of lacunae in current understanding of transmission-blocking strategies, which are hindering our efforts to develop sustainable and effective transmission-blocking interventions.

A multiplex assay for the sensitive detection and quantification of male and female Plasmodium falciparum gametocytes

Background

The transmission of malaria to mosquitoes depends on the presence of gametocytes that circulate in the peripheral blood of infected human hosts. Sensitive estimates of the densities of female gametocytes (FG) and male gametocytes (MG) may allow the prediction of infectivity to mosquitoes and thus a molecular estimate of the human infectious reservoir for transmission.

Methods

A novel multiplex qRT-PCR assay with intron-spanning primers was developed for the parallel quantification of FG and MG. CCp4 (PF3D7_0903800) transcripts specific for FG and PfMGET (PF3D7_1469900) transcripts specific for MG were quantified in total nucleic acids. The assay was validated on sex-sorted gametocytes from culture material and on samples from clinical trials with gametocytocidal drugs. Synthetic RNA standards were generated for the two targets genes and calibrated against known gametocyte quantities.

Results

The limit of detection was determined at 0.1 male and 0.1 female gametocyte/µL, which was equal to the limit of quantification (LOQ) for MG, while the LOQ for FG was 1 FG/µL. Results from previously reported clinical trials that used separate gametocyte qRT-PCR assays for FG (targeting Pfs25) and MG (targeting PfMGET) were reproduced with the multiplex assay. High levels of agreement between separate assays and the multiplex approach were observed (R² = 0.9473, 95% CI 0.9314–0.9632, for FG measured by transcript levels of Pfs25 in qRT-PCR or CCp4 in multiplex; R² = 0.8869, 95% CI 0.8541–0.9197, for MG measured by PfMGET in either single or multiplex qRT-PCR). FG and MG transcripts were detected in pure ring stage parasites at 10,000- and 100,000-fold reduced frequency for CCp4 and PfMGET, respectively. The CCp4 and PfMGET transcripts were equally stable under suboptimal storage conditions.

Conclusions

Gametocyte densities and their sex ratios can be determined in the presented one-step multiplex assay with higher throughput than single assays. The interpretation of low gametocyte densities at asexual parasite densities above 1000 parasites/µL requires caution to avoid false positive gametocyte signals from spurious transcript levels in ring stage parasites.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2584-y) contains supplementary material, which is available to authorized users.

Plasmodium falciparum gametocyte dynamics after pyronaridine-artesunate or artemether-lumefantrine treatment

BACKGROUND

Artemisinin-based combinations differ in their impact on gametocyte prevalence and density. This study assessed female and male gametocyte dynamics after treating children with uncomplicated Plasmodium falciparum malaria with either pyronaridine-artesunate (PA) or artemether-lumefantrine (AL).

METHODS

Kenyan children with uncomplicated Plasmodium falciparum malaria were included and randomly assigned to PA or AL treatment. Filter paper blood samples were collected as a source of RNA for quantitative reverse-transcription PCR (qRT-PCR) and nucleic acid sequence based amplification (QT-NASBA) to detect female gametocytes (targeting Pfs25 mRNA). Male gametocytes were detected by qRT-PCR (targeting PfMGET mRNA). Duration of gametocyte carriage, the female and male gametocyte response and the agreement between qRT-PCR and QT-NASBA were determined.

RESULTS

The mean duration of female gametocyte carriage was significantly longer for PA (4.9 days) than for AL (3.8 days) as estimated by QT-NASBA (P = 0.036), but this difference was less clear when determined by Pfs25 qRT-PCR (4.5 days for PA and 3.7 for AL, P = 0.166). qRT-PCR based female gametocyte prevalence decreased from 100% (75/75) at baseline to 6.06% (4/66) at day 14 in the AL group and from 97.7% (83/85) to 13.9% (11/79) in the PA group. Male gametocyte prevalence decreased from 41.3% (31/75) at baseline to 19.7% (13/66) at day 14 in the AL group and from 35.3% (30/85) to 22.8% (18/79) in the PA group. There was good agreement between Pfs25 qRT-PCR and QT-NASBA female gametocyte prevalence (0.85, 95% CI 0.82-0.87).

CONCLUSIONS

This study indicates that female gametocyte clearance may be slightly faster after AL compared to PA. Male gametocytes showed similar post-treatment clearance between study arms. Future studies should further address potential differences between the post-treatment transmission potential after PA compared to AL. Trial registration This study is registered at clinicaltrials.gov under NCT02411994. Registration date: 8 April 2015. https://clinicaltrials.gov/ct2/show/NCT02411994?term=pyronaridine-artesunate&cond=Malaria&cntry=KE&rank=1.

Efficacy and safety of primaquine and methylene blue for prevention of Plasmodium falciparum transmission in Mali: a phase 2, single-blind, randomised controlled trial

BACKGROUND

Primaquine and methylene blue are gametocytocidal compounds that could prevent Plasmodium falciparum transmission to mosquitoes. We aimed to assess the efficacy and safety of primaquine and methylene blue in preventing human to mosquito transmission of P falciparum among glucose-6-phosphate dehydrogenase (G6PD)-normal, gametocytaemic male participants.

METHODS

This was a phase 2, single-blind, randomised controlled trial done at the Clinical Research Centre of the Malaria Research and Training Centre (MRTC) of the University of Bamako (Bamako, Mali). We enrolled male participants aged 5-50 years with asymptomatic P falciparum malaria. G6PD-normal participants with gametocytes detected by blood smear were randomised 1:1:1:1 in block sizes of eight, using a sealed-envelope design, to receive either sulfadoxine-pyrimethamine and amodiaquine, sulfadoxine-pyrimethamine and amodiaquine plus a single dose of 0·25 mg/kg primaquine, dihydroartemisinin-piperaquine, or dihydroartemisinin-piperaquine plus 15 mg/kg per day methylene blue for 3 days. Laboratory staff, investigators, and insectary technicians were masked to the treatment group and gametocyte density of study participants. The study pharmacist and treating physician were not masked. Participants could request unmasking. The primary efficacy endpoint, analysed in all infected patients with at least one infectivity measure before and after treatment, was median within-person percentage change in mosquito infectivity 2 and 7 days after treatment, assessed by membrane feeding. This study is registered with ClinicalTrials.gov, number NCT02831023.

FINDINGS

Between June 27, 2016, and Nov 1, 2016, 80 participants were enrolled and assigned to the sulfadoxine-pyrimethamine and amodiaquine (n=20), sulfadoxine-pyrimethamine and amodiaquine plus primaquine (n=20), dihydroartemisinin-piperaquine (n=20), or dihydroartemisinin-piperaquine plus methylene blue (n=20) groups. Among participants infectious at baseline (54 [68%] of 80), those in the sulfadoxine-pyrimethamine and amodiaquine plus primaquine group (n=19) had a median 100% (IQR 100 to 100) within-person reduction in mosquito infectivity on day 2, a larger reduction than was noted with sulfadoxine-pyrimethamine and amodiaquine alone (n=12; -10·2%, IQR -143·9 to 56·6; p<0·0001). The dihydroartemisinin-piperaquine plus methylene blue (n=11) group had a median 100% (IQR 100 to 100) within-person reduction in mosquito infectivity on day 2, a larger reduction than was noted with dihydroartemisinin-piperaquine alone (n=12; -6·0%, IQR -126·1 to 86·9; p<0·0001). Haemoglobin changes were similar between gametocytocidal arms and their respective controls. After exclusion of blue urine, adverse events were similar across all groups (59 [74%] of 80 participants had 162 adverse events overall, 145 [90%] of which were mild).

INTERPRETATION

Adding a single dose of 0·25 mg/kg primaquine to sulfadoxine-pyrimethamine and amodiaquine or 3 days of 15 mg/kg per day methylene blue to dihydroartemisinin-piperaquine was highly efficacious for preventing P falciparum transmission. Both primaquine and methylene blue were well tolerated.

FUNDING

Bill & Melinda Gates Foundation, European Research Council.

Drugs in Development for Malaria

The last two decades have seen a surge in antimalarial drug development with product development partnerships taking a leading role. Resistance of Plasmodium falciparum to the artemisinin derivatives, piperaquine and mefloquine in Southeast Asia means new antimalarials are needed with some urgency. There are at least 13 agents in clinical development. Most of these are blood schizonticides for the treatment of uncomplicated falciparum malaria, under evaluation either singly or as part of two-drug combinations. Leading candidates progressing through the pipeline are artefenomel-ferroquine and lumefantrine-KAF156, both in Phase 2b. Treatment of severe malaria continues to rely on two parenteral drugs with ancient forebears: artesunate and quinine, with sevuparin being evaluated as an adjuvant therapy. Tafenoquine is under review by stringent regulatory authorities for approval as a single-dose treatment for Plasmodium vivax relapse prevention. This represents an advance over standard 14-day primaquine regimens; however, the risk of acute haemolytic anaemia in patients with glucose-6-phosphate dehydrogenase deficiency remains. For disease prevention, several of the newer agents show potential but are unlikely to be recommended for use in the main target groups of pregnant women and young children for some years. Latest predictions are that the malaria burden will continue to be high in the coming decades. This fact, coupled with the repeated loss of antimalarials to resistance, indicates that new antimalarials will be needed for years to come. Failure of the artemisinin-based combinations in Southeast Asia has stimulated a reappraisal of current approaches to combination therapy for malaria with incorporation of three or more drugs in a single treatment under consideration.

Predicting the likelihood and intensity of mosquito infection from sex specific Plasmodium falciparum gametocyte density

Understanding the importance of gametocyte density on human-to-mosquito transmission is of immediate relevance to malaria control. Previous work (Churcher et al., 2013) indicated a complex relationship between gametocyte density and mosquito infection. Here we use data from 148 feeding experiments on naturally infected gametocyte carriers to show that the relationship is much simpler and depends on both female and male parasite density. The proportion of mosquitoes infected is primarily determined by the density of female gametocytes though transmission from low gametocyte densities may be impeded by a lack of male parasites. Improved precision of gametocyte quantification simplifies the shape of the relationship with infection increasing rapidly before plateauing at higher densities. The mean number of oocysts per mosquito rises quickly with gametocyte density but continues to increase across densities examined. The work highlights the importance of measuring both female and male gametocyte density when estimating the human reservoir of infection.

Plasmodium-associated changes in human odor attract mosquitoes

Malaria parasites (Plasmodium) can change the attractiveness of their vertebrate hosts to Anopheles vectors, leading to a greater number of vector-host contacts and increased transmission. Indeed, naturally Plasmodium-infected children have been shown to attract more mosquitoes than parasite-free children. Here, we demonstrate Plasmodium-induced increases in the attractiveness of skin odor in Kenyan children and reveal quantitative differences in the production of specific odor components in infected vs. parasite-free individuals. We found the aldehydes heptanal, octanal, and nonanal to be produced in greater amounts by infected individuals and detected by mosquito antennae. In behavioral experiments, we demonstrated that these, and other, Plasmodium-induced aldehydes enhanced the attractiveness of a synthetic odor blend mimicking "healthy" human odor. Heptanal alone increased the attractiveness of "parasite-free" natural human odor. Should the increased production of these aldehydes by Plasmodium-infected humans lead to increased mosquito biting in a natural setting, this would likely affect the transmission of malaria.

Plasmodium Gametocytes in Field Studies: Do We Measure Commitment to Transmission or Detectability?

The proportion of Plasmodium spp. infections carrying gametocytes, and gametocyte densities, are often reported as surrogate markers for transmission potential. It remains unclear whether parasites under natural conditions adjust commitment to transmission depending on external factors. Population-based surveys comprising mostly asymptomatic low-density infections are always impacted by the sensitivity of the assays used to diagnose infections and detect gametocytes. Asexual parasite density is an important predictor for the probability of detecting gametocytes, and in many cases it can explain patterns in gametocyte carriage without the need for an adjustment of the gametocyte conversion rate. When reporting gametocyte data, quantification of blood-stage parasitemia and its inclusion as a confounder in multivariable analyses is essential.

A controlled human malaria infection model enabling evaluation of transmission-blocking interventions

BACKGROUND. Drugs and vaccines that can interrupt the transmission of Plasmodium falciparum will be important for malaria control and elimination. However, models for early clinical evaluation of candidate transmission-blocking interventions are currently unavailable. Here, we describe a new model for evaluating malaria transmission from humans to Anopheles mosquitoes using controlled human malaria infection (CHMI).

METHODS. Seventeen healthy malaria-naive volunteers underwent CHMI by intravenous inoculation of P. falciparum–infected erythrocytes to initiate blood-stage infection. Seven to eight days after inoculation, participants received piperaquine (480 mg) to attenuate asexual parasite replication while allowing gametocytes to develop and mature. Primary end points were development of gametocytemia, the transmissibility of gametocytes from humans to mosquitoes, and the safety and tolerability of the CHMI transmission model. To investigate in vivo gametocytocidal drug activity in this model, participants were either given an experimental antimalarial, artefenomel (500 mg), or a known gametocytocidal drug, primaquine (15 mg), or remained untreated during the period of gametocyte carriage.

RESULTS. Male and female gametocytes were detected in all participants, and transmission to mosquitoes was achieved from 8 of 11 (73%) participants evaluated. Compared with results in untreated controls (n = 7), primaquine (15 mg, n = 5) significantly reduced gametocyte burden (P = 0.01), while artefenomel (500 mg, n = 4) had no effect. Adverse events (AEs) were mostly mild or moderate. Three AEs were assessed as severe — fatigue, elevated alanine aminotransferase, and elevated aspartate aminotransferase — and were attributed to malaria infection. Transaminase elevations were transient, asymptomatic, and resolved without intervention.

CONCLUSION. We report the safe and reproducible induction of P. falciparum gametocytes in healthy malaria-naive volunteers at densities infectious to mosquitoes, thereby demonstrating the potential for evaluating transmission-blocking interventions in this model.

TRIAL REGISTRATION. ClinicalTrials.gov NCT02431637 and NCT02431650.

FUNDING. Bill & Melinda Gates Foundation.

On a mission to block transmission

The controlled infection of volunteers with Plasmodium falciparum parasites could provide a platform to evaluate new drugs and vaccines aimed at blocking malaria transmission.

A randomized feasibility trial comparing four antimalarial drug regimens to induce Plasmodium falciparum gametocytemia in the controlled human malaria infection model

Background

Malaria elimination strategies require a thorough understanding of parasite transmission from human to mosquito. A clinical model to induce gametocytes to understand their dynamics and evaluate transmission-blocking interventions (TBI) is currently unavailable. Here, we explore the use of the well-established Controlled Human Malaria Infection model (CHMI) to induce gametocyte carriage with different antimalarial drug regimens.

Methods

In a single centre, open-label randomised trial, healthy malaria-naive participants (aged 18–35 years) were infected with Plasmodium falciparum by bites of infected Anopheles mosquitoes. Participants were randomly allocated to four different treatment arms (n = 4 per arm) comprising low-dose (LD) piperaquine (PIP) or sulfadoxine-pyrimethamine (SP), followed by a curative regimen upon recrudescence. Male and female gametocyte densities were determined by molecular assays.

Results

Mature gametocytes were observed in all participants (16/16, 100%). Gametocytes appeared 8.5–12 days after the first detection of asexual parasites. Peak gametocyte densities and gametocyte burden was highest in the LD-PIP/SP arm, and associated with the preceding asexual parasite biomass (p=0.026). Male gametocytes had a mean estimated circulation time of 2.7 days (95% CI 1.5–3.9) compared to 5.1 days (95% CI 4.1–6.1) for female gametocytes. Exploratory mosquito feeding assays showed successful sporadic mosquito infections. There were no serious adverse events or significant differences in the occurrence and severity of adverse events between study arms (p=0.49 and p=0.28).

Conclusions

The early appearance of gametocytes indicates gametocyte commitment during the first wave of asexual parasites emerging from the liver. Treatment by LD-PIP followed by a curative SP regimen, results in the highest gametocyte densities and the largest number of gametocyte-positive days. This model can be used to evaluate the effect of drugs and vaccines on gametocyte dynamics, and lays the foundation for fulfilling the critical unmet need to evaluate transmission-blocking interventions against falciparum malaria for downstream selection and clinical development.

Funding

Funded by PATH Malaria Vaccine Initiative (MVI).

Clinical trial number

NCT02836002.

Variation in natural exposure to anopheles mosquitoes and its effects on malaria transmission

Variation in biting frequency by Anopheles mosquitoes can explain some of the heterogeneity in malaria transmission in endemic areas. In this study in Burkina Faso, we assessed natural exposure to mosquitoes by matching the genotype of blood meals from 1066 mosquitoes with blood from residents of local households. We observed that the distribution of mosquito bites exceeded the Pareto rule (20/80) in two of the three surveys performed (20/85, 76, and 96) and, at its most pronounced, is estimated to have profound epidemiological consequences, inflating the basic reproduction number of malaria by 8-fold. The distribution of bites from sporozoite-positive mosquitoes followed a similar pattern, with a small number of individuals within households receiving multiple potentially infectious bites over the period of a few days. Together, our findings indicate that heterogeneity in mosquito exposure contributes considerably to heterogeneity in infection risk and suggest significant variation in malaria transmission potential.

Probabilistic data integration identifies reliable gametocyte-specific proteins and transcripts in malaria parasites

Plasmodium gametocytes are the sexual forms of the malaria parasite essential for transmission to mosquitoes. To better understand how gametocytes differ from asexual blood-stage parasites, we performed a systematic analysis of available 'omics data for P. falciparum and other Plasmodium species. 18 transcriptomic and proteomic data sets were evaluated for the presence of curated "gold standards" of 41 gametocyte-specific versus 46 non-gametocyte genes and integrated using Bayesian probabilities, resulting in gametocyte-specificity scores for all P. falciparum genes. To illustrate the utility of the gametocyte score, we explored newly predicted gametocyte-specific genes as potential biomarkers of gametocyte carriage and exposure. We analyzed the humoral immune response in field samples against 30 novel gametocyte-specific antigens and found five antigens to be differentially recognized by gametocyte carriers as compared to malaria-infected individuals without detectable gametocytes. We also validated the gametocyte-specificity of 15 identified gametocyte transcripts on culture material and samples from naturally infected individuals, resulting in eight transcripts that were >1000-fold higher expressed in gametocytes compared to asexual parasites and whose transcript abundance allowed gametocyte detection in naturally infected individuals. Our integrated genome-wide gametocyte-specificity scores provide a comprehensive resource to identify targets and monitor P. falciparum gametocytemia.

Concentration of Plasmodium falciparum gametocytes in whole blood samples by magnetic cell sorting enhances parasite infection rates in mosquito feeding assays

Background

Mosquito-feeding assays are important tools to guide the development and support the evaluation of transmission-blocking interventions. These functional bioassays measure the sporogonic development of gametocytes in blood-fed mosquitoes. Measuring the infectivity of low gametocyte densities has become increasingly important in malaria elimination scenarios. This will pose challenges to the sensitivity and throughput of existing mosquito-feeding assay protocols. Here, different gametocyte concentration methods of blood samples were explored to optimize conditions for detection of positive mosquito infections.

Methods

Mature gametocytes of Plasmodium falciparum were diluted into whole blood samples of malaria-naïve volunteers. Standard centrifugation, Percoll gradient, magnetic cell sorting (MACS) enrichment were compared using starting blood volumes larger than the control (direct) feed.

Results

MACS gametocyte enrichment resulted in the highest infection intensity with statistically significant increases in mean oocyst density in 2 of 3 experiments (p = 0.0003; p ≤ 0.0001; p = 0.2348). The Percoll gradient and standard centrifugation procedures resulted in variable infectivity. A significant increase in the proportion of infected mosquitoes and oocyst density was found when larger volumes of gametocyte-infected blood were used with the MACS procedure.

Conclusions

The current study demonstrates that concentration methods of P. falciparum gametocyte-infected whole blood samples can enhance transmission in mosquito-feeding assays. Gametocyte purification by MACS was the most efficient method, allowing the assessment of gametocyte infectivity in low-density gametocyte infections, as can be expected in natural or experimental conditions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-017-1959-9) contains supplementary material, which is available to authorized users.