Effect of weekly fever-screening and treatment and monthly RDT testing and treatment on the infectious reservoir of malaria parasites in Burkina Faso: a cluster-randomised trial.

BACKGROUND: The majority of Plasmodium spp infections in endemic countries are asymptomatic and a source of onward transmission to mosquitoes. We aimed to examine whether Plasmodium falciparum transmission and malaria burden could be reduced by improving early detection and treatment of infections with active screening approaches. METHODS: In this 18-month cluster randomised study in Sapone, Burkina Faso, households were enrolled and randomly assigned (1:1:1) to one of three groups: group 1 (control) received standard of care only, group 2 received active weekly, at home, fever screening by a community health worker regardless of symptoms, participants with a fever received a rapid diagnostic test (RDT) and treatment if RDT positive, and group 3 received active weekly fever screening (as in group 2) plus a monthly RDT regardless of symptoms, and treatment if RDT positive. Eligible households had a minimum of three eligible residents, one in each age group (<5 years, 5-15 years, and >15 years). The primary outcome was parasite prevalence by quantitative PCR (qPCR) in the end-of-study cross-sectional survey. Secondary outcomes included parasite and gametocyte prevalence and density in all three end-of-season cross-sectional surveys, incidence of infection, and the transmissibility of infections to mosquitoes. This trial was registered at ClinicalTrials.gov (NCT03705624) and is completed. FINDINGS: A total of 906 individuals from 181 households were enrolled during two phases, and participated in the study. 412 individuals were enrolled between Aug 9 and 17, 2018, and participated in phase 1 and 494 individuals were enrolled between Jan 10 and 31, 2019, in phase 2. In the end-of-study cross-sectional survey (conducted between Jan 13 and 21, 2020), Pfalciparum prevalence by qPCR was significantly lower in group 3 (29·26%; 79 of 270), but not in group 2 (45·66%; 121 of 265), when compared with group 1 (48·72%; 133 of 273; risk ratio 0·65 [95% CI 0·52-0·81]; p=0·0001). Total parasite and gametocyte prevalence and density were also significantly lower in group 3 in all surveys. The largest differences were seen at the end of the dry season, with gametocyte prevalence 78·4% and predicted transmission potential 98·2% lower in group 3 than in group 1. INTERPRETATION: Active monthly RDT testing and treatment can reduce parasite carriage and the infectious reservoir of P falciparum to less than 2% when used during the dry season. This insight might inform approaches for malaria control and elimination. FUNDING: Bill & Melinda Gates Foundation, European Research Council, and The Netherlands Organization for Scientific Research.

Plasmodium falciparum gametocyte production correlates with genetic markers of parasite replication but is not influenced by experimental exposure to mosquito biting.

BACKGROUND: Plasmodium blood-stage parasites balance asexual multiplication with gametocyte development. Few studies link these dynamics with parasite genetic markers in vivo; even fewer in longitudinally monitored infections. Environmental influences on gametocyte formation, such as mosquito exposure, may influence the parasite's investment in gametocyte production. METHODS: We investigated gametocyte production and asexual multiplication in two Plasmodium falciparum infected populations; a controlled human malaria infection (CHMI) study and a 28-day observational study in naturally infected individuals in Burkina Faso with controlled mosquito exposure. We measured gene transcript levels previously related to gametocyte formation (ap2-g, surfin1.2, surfin13.1, gexp-2) or inhibition of asexual multiplication (sir2a) and compared transcript levels to ring-stage parasite and mature gametocyte densities. FINDINGS: Three of the five markers (ap2-g, surfin1.2, surfin13.1) predicted peak gametocytaemia in the CHMI study. An increase in all five markers in natural infections was associated with an increase in mature gametocytes 14 days later; the effect of sir2a on future gametocytes was strongest (fold change = 1.65, IQR = 1.22-2.24, P = 0.004). Mosquito exposure was not associated with markers of gametocyte formation (ap2-g P = 0.277; sir2a P = 0.499) or carriage of mature gametocytes (P = 0.379). INTERPRETATION: All five parasite genetic markers predicted gametocyte formation over a single cycle of gametocyte formation and maturation in vivo; sir2a and ap2-g were most closely associated with gametocyte growth dynamics. We observed no evidence to support the hypothesis that exposure to Anopheles mosquito bites stimulates gametocyte formation. FUNDING: This work was funded by the Bill & Melinda Gates Foundation (INDIE OPP1173572), the European Research Council fellowship (ERC-CoG 864180) and UKRI Medical Research Council (MR/T016272/1) and Wellcome Center (218676/Z/19/Z).

Artemether-lumefantrine with or without single-dose primaquine and sulfadoxine-pyrimethamine plus amodiaquine with or without single-dose tafenoquine to reduce Plasmodium falciparum transmission: a phase 2, single-blind, randomised clinical trial in Ouelessebougou, Mali.

BACKGROUND: Artemether-lumefantrine is widely used for uncomplicated Plasmodium falciparum malaria; sulfadoxine-pyrimethamine plus amodiaquine is used for seasonal malaria chemoprevention. We aimed to determine the efficacy of artemether-lumefantrine with and without primaquine and sulfadoxine-pyrimethamine plus amodiaquine with and without tafenoquine for reducing gametocyte carriage and transmission to mosquitoes. METHODS: In this phase 2, single-blind, randomised clinical trial conducted in Ouelessebougou, Mali, asymptomatic individuals aged 10-50 years with P falciparum gametocytaemia were recruited from the community and randomly assigned (1:1:1:1) to receive either artemether-lumefantrine, artemether-lumefantrine with a single dose of 0·25 mg/kg primaquine, sulfadoxine-pyrimethamine plus amodiaquine, or sulfadoxine-pyrimethamine plus amodiaquine with a single dose of 1·66 mg/kg tafenoquine. All trial staff other than the pharmacist were masked to group allocation. Participants were not masked to group allocation. Randomisation was done with a computer-generated randomisation list and concealed with sealed, opaque envelopes. The primary outcome was the median within-person percent change in mosquito infection rate in infectious individuals from baseline to day 2 (artemether-lumefantrine groups) or day 7 (sulfadoxine-pyrimethamine plus amodiaquine groups) after treatment, assessed by direct membrane feeding assay. All participants who received any trial drug were included in the safety analysis. This study is registered with ClinicalTrials.gov, NCT05081089. FINDINGS: Between Oct 13 and Dec 16, 2021, 1290 individuals were screened and 80 were enrolled and randomly assigned to one of the four treatment groups (20 per group). The median age of participants was 13 (IQR 11-20); 37 (46%) of 80 participants were female and 43 (54%) were male. In individuals who were infectious before treatment, the median percentage reduction in mosquito infection rate 2 days after treatment was 100·0% (IQR 100·0-100·0; n=19; p=0·0011) with artemether-lumefantrine and 100·0% (100·0-100·0; n=19; p=0·0001) with artemether-lumefantrine with primaquine. Only two individuals who were infectious at baseline infected mosquitoes on day 2 after artemether-lumefantrine and none at day 5. By contrast, the median percentage reduction in mosquito infection rate 7 days after treatment was 63·6% (IQR 0·0-100·0; n=20; p=0·013) with sulfadoxine-pyrimethamine plus amodiaquine and 100% (100·0-100·0; n=19; p<0·0001) with sulfadoxine-pyrimethamine plus amodiaquine with tafenoquine. No grade 3-4 or serious adverse events occurred. INTERPRETATION: These data support the effectiveness of artemether-lumefantrine alone for preventing nearly all mosquito infections. By contrast, there was considerable post-treatment transmission after sulfadoxine-pyrimethamine plus amodiaquine; therefore, the addition of a transmission-blocking drug might be beneficial in maximising its community impact. FUNDING: Bill & Melinda Gates Foundation.

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.

Safety of single-dose primaquine as a Plasmodium falciparum gametocytocide: a systematic review and meta-analysis of individual patient data.

BACKGROUND: In 2012, the World Health Organization (WHO) recommended single low-dose (SLD, 0.25 mg/kg) primaquine to be added as a Plasmodium (P.) falciparum gametocytocide to artemisinin-based combination therapy (ACT) without glucose-6-phosphate dehydrogenase (G6PD) testing, to accelerate malaria elimination efforts and avoid the spread of artemisinin resistance. Uptake of this recommendation has been relatively slow primarily due to safety concerns. METHODS: A systematic review and individual patient data (IPD) meta-analysis of single-dose (SD) primaquine studies for P. falciparum malaria were performed. Absolute and fractional changes in haemoglobin concentration within a week and adverse effects within 28 days of treatment initiation were characterised and compared between primaquine and no primaquine arms using random intercept models. RESULTS: Data comprised 20 studies that enrolled 6406 participants, of whom 5129 (80.1%) had received a single target dose of primaquine ranging between 0.0625 and 0.75 mg/kg. There was no effect of primaquine in G6PD-normal participants on haemoglobin concentrations. However, among 194 G6PD-deficient African participants, a 0.25 mg/kg primaquine target dose resulted in an additional 0.53 g/dL (95% CI 0.17-0.89) reduction in haemoglobin concentration by day 7, with a 0.27 (95% CI 0.19-0.34) g/dL haemoglobin drop estimated for every 0.1 mg/kg increase in primaquine dose. Baseline haemoglobin, young age, and hyperparasitaemia were the main determinants of becoming anaemic (Hb < 10 g/dL), with the nadir observed on ACT day 2 or 3, regardless of G6PD status and exposure to primaquine. Time to recovery from anaemia took longer in young children and those with baseline anaemia or hyperparasitaemia. Serious adverse haematological events after primaquine were few (9/3, 113, 0.3%) and transitory. One blood transfusion was reported in the primaquine arms, and there were no primaquine-related deaths. In controlled studies, the proportions with either haematological or any serious adverse event were similar between primaquine and no primaquine arms. CONCLUSIONS: Our results support the WHO recommendation to use 0.25 mg/kg of primaquine as a P. falciparum gametocytocide, including in G6PD-deficient individuals. Although primaquine is associated with a transient reduction in haemoglobin levels in G6PD-deficient individuals, haemoglobin levels at clinical presentation are the major determinants of anaemia in these patients. TRIAL REGISTRATION: PROSPERO, CRD42019128185.

The acquisition of humoral immune responses targeting Plasmodium falciparum sexual stages in controlled human malaria infections.

Individuals infected with P. falciparum develop antibody responses to intra-erythrocytic gametocyte proteins and exported gametocyte proteins present on the surface of infected erythrocytes. However, there is currently limited knowledge on the immunogenicity of gametocyte antigens and the specificity of gametocyte-induced antibody responses. In this study, we assessed antibody responses in participants of two controlled human malaria infection (CHMI) studies by ELISA, multiplexed bead-based antibody assays and protein microarray. By comparing antibody responses in participants with and without gametocyte exposure, we aimed to disentangle the antibody response induced by asexual and sexual stage parasites. We showed that after a single malaria infection, a significant anti-sexual stage humoral response is induced in malaria-naïve individuals, even after exposure to relatively low gametocyte densities (up to ~1,600 gametocytes/mL). In contrast to antibody responses to well-characterised asexual blood stage antigens that were detectable by day 21 after infection, responses to sexual stage antigens (including transmission blocking vaccine candidates Pfs48/45 and Pfs230) were only apparent at 51 days after infection. We found antigens previously associated with early gametocyte or anti-gamete immunity were highly represented among responses linked with gametocyte exposure. Our data provide detailed insights on the induction and kinetics of antibody responses to gametocytes and identify novel antigens that elicit antibody responses exclusively in individuals with gametocyte exposure. Our findings provide target identification for serological assays for surveillance of the malaria infectious reservoir, and support vaccine development by describing the antibody response to leading vaccine antigens after primary infection.

Plasmodium falciparum serology: A comparison of two protein production methods for analysis of antibody responses by protein microarray.

The evaluation of protein antigens as putative serologic biomarkers of infection has increasingly shifted to high-throughput, multiplex approaches such as the protein microarray. In vitro transcription/translation (IVTT) systems-a similarly high-throughput protein expression method-are already widely utilised in the production of protein microarrays, though purified recombinant proteins derived from more traditional whole cell based expression systems also play an important role in biomarker characterisation. Here we have performed a side-by-side comparison of antigen-matched protein targets from an IVTT and purified recombinant system, on the same protein microarray. The magnitude and range of antibody responses to purified recombinants was found to be greater than that of IVTT proteins, and responses between targets from different expression systems did not clearly correlate. However, responses between amino acid sequence-matched targets from each expression system were more closely correlated. Despite the lack of a clear correlation between antigen-matched targets produced in each expression system, our data indicate that protein microarrays produced using either method can be used confidently, in a context dependent manner, though care should be taken when comparing data derived from contrasting approaches.

Heterologous Expression and Evaluation of Novel Plasmodium falciparum Transmission Blocking Vaccine Candidates.

Malaria transmission blocking vaccines (TBV) aim to induce antibodies that can interrupt Plasmodium falciparum development in the mosquito midgut and thereby prevent onward malaria transmission. A limited number of TBV candidates have been identified and only three (Pfs25, Pfs230 and Pfs48/45) have entered clinical testing. While one of these candidates may emerge as a highly potent TBV candidate, it is premature to determine if they will generate sufficiently potent and sustained responses. It is therefore important to explore novel candidate antigens. We recently analyzed sera from naturally exposed individuals and found that the presence and/or intensity of antibodies against 12 novel putative surface expressed gametocyte antigens was associated with transmission reducing activity. In this study, protein fragments of these novel TBV candidates were designed and heterologously expressed in Drosophila melanogaster S2 cells and Lactococcus lactis. Eleven protein fragments, covering seven TBV candidates, were successfully produced. All tested antigens were recognized by antibodies from individuals living in malaria-endemic areas, indicating that native epitopes are present. All antigens induced antigen-specific antibody responses in mice. Two antigens induced antibodies that recognized a native protein in gametocyte extract, and antibodies elicited by four antigens recognized whole gametocytes. In particular, we found that antigen Pf3D7_0305300, a putative transporter, is abundantly expressed on the surface of gametocytes. However, none of the seven novel TBV candidates expressed here induced an antibody response that reduced parasite development in the mosquito midgut as assessed in the standard membrane feeding assay. Altogether, the antigen fragments used in this study did not prove to be promising transmission blocking vaccine constructs, but led to the identification of two gametocyte surface proteins that may provide new leads for studying gametocyte biology.

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.

Single low-dose tafenoquine combined with dihydroartemisinin-piperaquine to reduce Plasmodium falciparum transmission in Ouelessebougou, Mali: a phase 2, single-blind, randomised clinical trial.

BACKGROUND: Tafenoquine was recently approved as a prophylaxis and radical cure for Plasmodium vivax infection, but its Plasmodium falciparum transmission-blocking efficacy is unclear. We aimed to establish the efficacy and safety of three single low doses of tafenoquine in combination with dihydroartemisinin-piperaquine for reducing gametocyte density and transmission to mosquitoes. METHODS: In this four-arm, single-blind, phase 2, randomised controlled trial, participants were recruited at the Clinical Research Unit of the Malaria Research and Training Centre of the University of Bamako in Mali. Eligible participants were aged 12-50 years, with asymptomatic P falciparum microscopy-detected gametocyte carriage, had a bodyweight of 80 kg or less, and had no clinical signs of malaria defined by fever. Participants were randomly assigned (1:1:1:1) to standard treatment with dihydroartemisinin-piperaquine, or dihydroartemisinin-piperaquine plus a single dose of tafenoquine (in solution) at a final dosage of 0·42 mg/kg, 0·83 mg/kg, or 1·66 mg/kg. Randomisation was done with a computer-generated randomisation list and concealed with sealed, opaque envelopes. Dihydroartemisinin-piperaquine was administered as oral tablets over 3 days (day 0, 1, and 2), as per manufacturer instructions. A single dose of tafenoquine was administered as oral solution on day 0 in parallel with the first dose of dihydroartemisinin-piperaquine. Tafenoquine dosing was based on bodyweight to standardise efficacy and risk variance. The primary endpoint, assessed in the per-protocol population, was median percentage change in mosquito infection rate 7 days after treatment compared with baseline. Safety endpoints included frequency and incidence of adverse events. The final follow-up visit was on Dec 23, 2021; the trial is registered with ClinicalTrials.gov, NCT04609098. FINDINGS: From Oct 29 to Nov 25, 2020, 1091 individuals were screened for eligibility, 80 of whom were enrolled and randomly assigned (20 per treatment group). Before treatment, 53 (66%) individuals were infectious to mosquitoes, infecting median 12·50% of mosquitoes (IQR 3·64-35·00). Within-group reduction in mosquito infection rate on day 7 was 79·95% (IQR 57·15-100; p=0·0005 for difference from baseline) following dihydroartemisinin-piperaquine only, 100% (98·36-100; p=0·0005) following dihydroartemisinin-piperaquine plus tafenoquine 0·42 mg/kg, 100% (100-100; p=0·0001) following dihydroartemisinin-piperaquine plus tafenoquine 0·83 mg/kg, and 100% (100-100; p=0·0001) following dihydroartemisinin-piperaquine plus tafenoquine 1·66 mg/kg. 55 (69%) of 80 participants had a total of 94 adverse events over the course of the trial; 86 (92%) adverse events were categorised as mild, seven (7%) as moderate, and one (1%) as severe. The most common treatment-related adverse event was mild or moderate headache, which occurred in 15 (19%) participants (dihydroartemisinin-piperaquine n=2; dihydroartemisinin-piperaquine plus tafenoquine 0·42 mg/kg n=6; dihydroartemisinin-piperaquine plus tafenoquine 0·83 mg/kg n=3; and dihydroartemisinin-piperaquine plus tafenoquine 1·66 mg/kg n=4). No serious adverse events occurred. No significant differences in the incidence of all adverse events (p=0·73) or treatment-related adverse events (p=0·62) were observed between treatment groups. INTERPRETATION: Tafenoquine was well tolerated at all doses and accelerated P falciparum gametocyte clearance. All tafenoquine doses showed improved transmission reduction at day 7 compared with dihydroartemisinin-piperaquine alone. These data support the case for further research on tafenoquine as a transmission-blocking supplement to standard antimalarials. FUNDING: Bill & Melinda Gates Foundation. TRANSLATIONS: For the French, Portuguese, Spanish and Swahili translations of the abstract see Supplementary Materials section.

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.

Efficacy of Single-Dose Primaquine With Artemisinin Combination Therapy on Plasmodium falciparum Gametocytes and Transmission: An Individual Patient Meta-Analysis.

BACKGROUND: Since the World Health Organization recommended single low-dose (0.25 mg/kg) primaquine (PQ) in combination with artemisinin-based combination therapies (ACTs) in areas of low transmission or artemisinin-resistant Plasmodium falciparum, several single-site studies have been conducted to assess efficacy. METHODS: An individual patient meta-analysis to assess gametocytocidal and transmission-blocking efficacy of PQ in combination with different ACTs was conducted. Random effects logistic regression was used to quantify PQ effect on (1) gametocyte carriage in the first 2 weeks post treatment; and (2) the probability of infecting at least 1 mosquito or of a mosquito becoming infected. RESULTS: In 2574 participants from 14 studies, PQ reduced PCR-determined gametocyte carriage on days 7 and 14, most apparently in patients presenting with gametocytemia on day 0 (odds ratio [OR], 0.22; 95% confidence interval [CI], .17-.28 and OR, 0.12; 95% CI, .08-.16, respectively). Rate of decline in gametocyte carriage was faster when PQ was combined with artemether-lumefantrine (AL) compared to dihydroartemisinin-piperaquine (DP) (P = .010 for day 7). Addition of 0.25 mg/kg PQ was associated with near complete prevention of transmission to mosquitoes. CONCLUSIONS: Transmission blocking is achieved with 0.25 mg/kg PQ. Gametocyte persistence and infectivity are lower when PQ is combined with AL compared to DP.

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.

Immunity against sexual stage Plasmodium falciparum and Plasmodium vivax parasites.

The efficient spread of malaria from infected humans to mosquitoes is a major challenge for malaria elimination initiatives. Gametocytes are the only Plasmodium life stage infectious to mosquitoes. Here, we summarize evidence for naturally acquired anti-gametocyte immunity and the current state of transmission blocking vaccines (TBV). Although gametocytes are intra-erythrocytic when present in infected humans, developing Plasmodium falciparum gametocytes may express proteins on the surface of red blood cells that elicit immune responses in naturally exposed individuals. This immune response may reduce the burden of circulating gametocytes. For both P. falciparum and Plasmodium vivax, there is a solid evidence that antibodies against antigens present on the gametocyte surface, when co-ingested with gametocytes, can influence transmission to mosquitoes. Transmission reducing immunity, reducing the burden of infection in mosquitoes, is a well-acknowledged but poorly quantified phenomenon that forms the basis for the development of TBV. Transmission enhancing immunity, increasing the likelihood or intensity of transmission to mosquitoes, is more speculative in nature but is convincingly demonstrated for P. vivax. With the increased interest in malaria elimination, TBV and monoclonal antibodies have moved to the center stage of malaria vaccine development. Methodologies to prioritize and evaluate products are urgently needed.

Investigating the impact of enhanced community case management and monthly screening and treatment on the transmissibility of malaria infections in Burkina Faso: study protocol for a cluster-randomised trial.

INTRODUCTION: A large proportion of malaria-infected individuals in endemic areas do not experience symptoms that prompt treatment-seeking. These asymptomatically infected individuals may retain their infections for many months during which sexual-stage parasites (gametocytes) are produced that may be transmissible to mosquitoes. Reductions in malaria transmission could be achieved by detecting and treating these infections early. This study assesses the impact of enhanced community case management (CCM) and monthly screening and treatment (MSAT) on the prevalence and transmissibility of malaria infections. METHODS AND ANALYSIS: This cluster-randomised trial will take place in Sapone, an area of intense, highly seasonal malaria in Burkina Faso. In total, 180 compounds will be randomised to one of three interventions: arm 1 - current standard of care with passively monitored malaria infections; arm 2 - standard of care plus enhanced CCM, comprising active weekly screening for fever, and detection and treatment of infections in fever positive individuals using conventional rapid diagnostic tests (RDTs); or arm 3 - standard of care and enhanced CCM, plus MSAT using RDTs. The study will be conducted over approximately 18 months covering two high-transmission seasons and the intervening dry season. The recruitment strategy aims to ensure that overall transmission and force of infection is not affected so we are able to continuously evaluate the impact of interventions in the context of ongoing intense malaria transmission. The main objectives of the study are to determine the impact of enhanced CCM and MSAT on the prevalence and density of parasitaemia and gametocytaemia and the transmissibility of infections. This will be achieved by molecular detection of infections in all study participants during start and end season cross-sectional surveys and routine sampling of malaria-positive individuals to assess their infectiousness to mosquitoes. ETHICS AND DISSEMINATION: The study has been reviewed and approved by the London School of Hygiene and Tropical Medicine (LSHTM) (Review number: 14724) and The Centre National de Recherche et de Formation sur le Paludisme institutional review board (IRB) (Deliberation N° 2018/000002/MS/SG/CNRFP/CIB) and Burkina Faso national medical ethics committees (Deliberation N° 2018-01-010).Findings of the study will be shared with the community via local opinion leaders and community meetings. Results may also be shared through conferences, seminars, reports, theses and peer-reviewed publications; disease occurrence data and study outcomes will be shared with the Ministry of Health. Data will be published in an online digital repository. TRIAL REGISTRATION NUMBER: NCT03705624.

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. 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.

Naturally acquired immunity against immature Plasmodium falciparum gametocytes.

The recent decline in global malaria burden has stimulated efforts toward Plasmodium falciparum elimination. Understanding the biology of malaria transmission stages may provide opportunities to reduce or prevent onward transmission to mosquitoes. Immature P. falciparum transmission stages, termed stages I to IV gametocytes, sequester in human bone marrow before release into the circulation as mature stage V gametocytes. This process likely involves interactions between host receptors and potentially immunogenic adhesins on the infected red blood cell (iRBC) surface. Here, we developed a flow cytometry assay to examine immune recognition of live gametocytes of different developmental stages by naturally exposed Malawians. We identified strong antibody recognition of the earliest immature gametocyte-iRBCs (giRBCs) but not mature stage V giRBCs. Candidate surface antigens (n = 30), most of them shared between asexual- and gametocyte-iRBCs, were identified by mass spectrometry and mouse immunizations, as well as correlations between responses by protein microarray and flow cytometry. Naturally acquired responses to a subset of candidate antigens were associated with reduced asexual and gametocyte density, and plasma samples from malaria-infected individuals were able to induce immune clearance of giRBCs in vitro. Infected RBC surface expression of select candidate antigens was validated using specific antibodies, and genetic analysis revealed a subset with minimal variation across strains. Our data demonstrate that humoral immune responses to immature giRBCs and shared iRBC antigens are naturally acquired after malaria exposure. These humoral immune responses may have consequences for malaria transmission potential by clearing developing gametocytes, which could be leveraged for malaria intervention.

Two-Faced Immunity? The Evidence for Antibody Enhancement of Malaria Transmission.

Plasmodium gametocytes can induce an immune response in humans that interferes with the development of sexual-stage parasites in the mosquito gut. Many early studies of the sexual-stage immune response noted that mosquito infection could be enhanced as well as reduced by immune sera. For Plasmodium falciparum, these reports are scarce, and the phenomenon is generally regarded as a methodological artefact. Plasmodium transmission enhancement (TE) remains contentious, but the clinical development of transmission-blocking vaccines based on sexual-stage antigens requires that it is further studied. In this essay, we review the early literature on the sexual-stage immune response and transmission-modulating immunity. We discuss hypotheses for the mechanism of TE, suggest experiments to prove or disprove its existence, and discuss its possible implications.

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.

Assessing Plasmodium falciparum transmission in mosquito-feeding assays using quantitative PCR.

BACKGROUND: Evaluating the efficacy of transmission-blocking interventions relies on mosquito-feeding assays, with transmission typically assessed by microscopic identification of oocysts in mosquito midguts; however, microscopy has limited throughput, sensitivity and specificity. Where low prevalence and intensity mosquito infections occur, as observed during controlled human malaria infection studies or natural transmission, a reliable method for detection and quantification of low-level midgut infection is required. Here, a semi-automated, Taqman quantitative PCR (qPCR) assay sufficiently sensitive to detect a single-oocyst midgut infection is described. RESULTS: Extraction of genomic DNA from Anopheles stephensi midguts using a semi-automated extraction process was shown to have equivalent extraction efficiency to manual DNA extraction. An 18S Plasmodium falciparum qPCR assay was adapted for quantitative detection of P. falciparum midgut oocyst infection using synthetic DNA standards. The assay was validated for sensitivity and specificity, and the limit of detection was 0.7 genomes/µL (95% CI 0.4-1.6 genomes/µL). All microscopy-confirmed oocyst infected midgut samples were detected by qPCR, including all single-oocyst positive midguts. The genome number per oocyst was assessed 8-9 days after feeding assay using both qPCR and droplet digital PCR and was 3722 (IQR: 2951-5453) and 3490 (IQR: 2720-4182), respectively. CONCLUSIONS: This semi-automated qPCR method enables accurate detection of low-level P. falciparum oocyst infections in mosquito midguts, and may improve the sensitivity, specificity and throughput of assays used to evaluate candidate transmission-blocking interventions.