The global public health significance of Plasmodium vivax

Another decade of antimalarial drug discovery: New targets, tools and molecules

Malaria remains a devastating but preventable infectious disease that disproportionately affects the African continent. Emerging resistance to current frontline therapies means that not only are new treatments urgently required, but also novel validated antimalarial targets to circumvent cross-resistance. Fortunately, tremendous efforts have been made by the global drug discovery community over the past decade. In this chapter, we will highlight some of the antimalarial drug discovery and development programmes currently underway across the globe, charting progress in the identification of new targets and the development of new classes of drugs to prosecute them. These efforts have been complemented by the development of valuable tools to accelerate target validation such as the NOD scid gamma (NSG) humanized mouse efficacy model and progress in predictive modelling and open-source software. Among the medicinal chemistry programmes that have been conducted over the past decade are those targeting Plasmodium falciparum ATPase4 (ATP4) and acetyl-CoA synthetase (AcAS) as well as proteins disrupting parasite protein translation such as the aminoacyl-tRNA synthetases (aaRSs) and eukaryotic elongation factor 2 (eEF2). The benefits and challenges of targeting Plasmodium kinases will be examined, with a focus on Plasmodium cyclic GMP-dependent protein kinase (PKG), cyclin-dependent-like protein kinase 3 (CLK3) and phosphatidylinositol 4-kinase (PI4K). The chapter concludes with a survey of incipient drug discovery centres in Africa and acknowledges the value of recent international meetings in galvanizing and uniting the antimalarial drug discovery community.

Plasmodium vivax tryptophan-rich antigen reduces type I collagen secretion via the NF-κBp65 pathway in splenic fibroblasts

BACKGROUND

The spleen plays a critical role in the immune response against malaria parasite infection, where splenic fibroblasts (SFs) are abundantly present and contribute to immune function by secreting type I collagen (collagen I). The protein family is characterized by Plasmodium vivax tryptophan-rich antigens (PvTRAgs), comprising 40 members. PvTRAg23 has been reported to bind to human SFs (HSFs) and affect collagen I levels. Given the role of type I collagen in splenic immune function, it is important to investigate the functions of the other members within the PvTRAg protein family.

METHODS

Protein structural prediction was conducted utilizing bioinformatics analysis tools and software. A total of 23 PvTRAgs were successfully expressed and purified using an Escherichia coli prokaryotic expression system, and the purified proteins were used for co-culture with HSFs. The collagen I levels and collagen-related signaling pathway protein levels were detected by immunoblotting, and the relative expression levels of inflammatory factors were determined by quantitative real-time PCR.

RESULTS

In silico analysis showed that P. vivax has 40 genes encoding the TRAg family. The C-terminal region of all PvTRAgs is characterized by the presence of a domain rich in tryptophan residues. A total of 23 recombinant PvTRAgs were successfully expressed and purified. Only five PvTRAgs (PvTRAg5, PvTRAg16, PvTRAg23, PvTRAg30, and PvTRAg32) mediated the activation of the NF-κBp65 signaling pathway, which resulted in the production of inflammatory molecules and ultimately a significant reduction in collagen I levels in HSFs.

CONCLUSIONS

Our research contributes to the expansion of knowledge regarding the functional role of PvTRAgs, while it also enhances our understanding of the immune evasion mechanisms utilized by parasites.

Revolutionizing Malaria Vector Control: The Importance of Accurate Species Identification through Enhanced Molecular Capacity

Many factors, such as the resistance to pesticides and a lack of knowledge of the morphology and molecular structure of malaria vectors, have made it more challenging to eradicate malaria in numerous malaria-endemic areas of the globe. The primary goal of this review is to discuss malaria vector control methods and the significance of identifying species in vector control initiatives. This was accomplished by reviewing methods of molecular identification of malaria vectors and genetic marker classification in relation to their use for species identification. Due to its specificity and consistency, molecular identification is preferred over morphological identification of malaria vectors. Enhanced molecular capacity for species identification will improve mosquito characterization, leading to accurate control strategies/treatment targeting specific mosquito species, and thus will contribute to malaria eradication. It is crucial for disease epidemiology and surveillance to accurately identify the Plasmodium spp. that are causing malaria in patients. The capacity for disease surveillance will be significantly increased by the development of more accurate, precise, automated, and high-throughput diagnostic techniques. In conclusion, although morphological identification is quick and achievable at a reduced cost, molecular identification is preferred for specificity and sensitivity. To achieve the targeted malaria elimination goal, proper identification of vectors using accurate techniques for effective control measures should be prioritized.

Malaria prevalence in Pakistan: A systematic review and meta-analysis (2006-2021)

Malaria is one of the major public health issues globally. Malaria infection spreads through mosquito bites from infected female Anopheles mosquitoes. This study aims to conduct a systematic review and meta-analysis on malaria prevalence in Pakistan from 2006 to 2021. We searched PubMed, Science Direct, EMBASE, EMCare, and Google Scholar to acquire data on the prevalence of malaria infections. We performed a meta-analysis with a random-effects model to obtain the pooled prevalence of malaria, Plasmodium vivax, and Plasmodium falciparum. Meta-analysis was computed using R 4.1.2 Version statistical software. I² and time series analysis were performed to identify a possible source of heterogeneity across studies. A funnel plot and the Freeman-Tukey Double Arcsine Transformed Proportion were used to evaluate the presence of publication bias. Out of the 315 studies collected, only 45 full-text articles were screened and included in the final measurable meta-analysis. Pooled malaria prevalence in Pakistan was 23.3%, with Plasmodium vivax, Plasmodium falciparum, and mixed infection rates of 79.13%, 16.29%, and 3.98%, respectively. Similarly, the analysis revealed that the maximum malaria prevalence was 99.79% in Karachi and the minimum was 1.68% in the Larkana district. Amazingly, this systematic review and meta-analysis detected a wide variation in malaria prevalence in Pakistan. Pakistan's public health department and other competent authorities should pay close attention to the large decrease in mosquito populations to curb the infection rate.

Defining species-specific and conserved interactions of apical membrane protein 1 during erythrocyte invasion in malaria to inform multi-species vaccines

Plasmodium falciparum and P. vivax are the major causes of human malaria, and P. knowlesi is an important additional cause in SE Asia. Binding of apical membrane antigen 1 (AMA1) to rhoptry neck protein 2 (RON2) was thought to be essential for merozoite invasion of erythrocytes by Plasmodium spp. Our findings reveal that P. falciparum and P. vivax have diverged and show species-specific binding of AMA1 to RON2, determined by a β-hairpin loop in RON2 and specific residues in AMA1 Loop1E. In contrast, cross-species binding of AMA1 to RON2 is retained between P. vivax and P. knowlesi. Mutation of specific amino acids in AMA1 Loop1E in P. falciparum or P. vivax ablated RON2 binding without impacting erythrocyte invasion. This indicates that the AMA1-RON2-loop interaction is not essential for invasion and additional AMA1 interactions are involved. Mutations in AMA1 that disrupt RON2 binding also enable escape of invasion inhibitory antibodies. Therefore, vaccines and therapeutics will need to be broader than targeting only the AMA1-RON2 interaction. Antibodies targeting AMA1 domain 3 had greater invasion-inhibitory activity when RON2-loop binding was ablated, suggesting this domain is a promising additional target for vaccine development. Targeting multiple AMA1 interactions involved in invasion may enable vaccines that generate more potent inhibitory antibodies and address the capacity for immune evasion. Findings on specific residues for invasion function and species divergence and conservation can inform novel vaccines and therapeutics against malaria caused by three species, including the potential for cross-species vaccines.

Population-based genomic study of Plasmodium vivax malaria in seven Brazilian states and across South America

Background

Brazil is a unique and understudied setting for malaria, with complex foci of transmission associated with human and environmental conditions. An understanding of the population genomic diversity of P. vivax parasites across Brazil can support malaria control strategies.

Methods

Through whole genome sequencing of P. vivax isolates across 7 Brazilian states, we use population genomic approaches to compare genetic diversity within country (n = 123), continent (6 countries, n = 315) and globally (26 countries, n = 885).

Findings

We confirm that South American isolates are distinct, have more ancestral populations than the other global regions, with differentiating mutations in genes under selective pressure linked to antimalarial drugs (pvmdr1, pvdhfr-ts) and mosquito vectors (pvcrmp3, pvP45/48, pvP47). We demonstrate Brazil as a distinct parasite population, with signals of selection including ABC transporter (PvABCI3) and PHIST exported proteins.

Interpretation

Brazil has a complex population structure, with evidence of P. simium infections and Amazonian parasites separating into multiple clusters. Overall, our work provides the first Brazil-wide analysis of P. vivax population structure and identifies important mutations, which can inform future research and control measures.

Funding

AI is funded by an MRC LiD PhD studentship. TGC is funded by the Medical Research Council (Grant no. MR/M01360X/1, MR/N010469/1, MR/R025576/1, MR/R020973/1 and MR/X005895/1). SC is funded by Medical Research Council UK grants (MR/M01360X/1, MR/R025576/1, MR/R020973/1 and MR/X005895/1) and Bloomsbury SET (ref. CCF17-7779). FN is funded by The Shloklo Malaria Research Unit - part of the Mahidol Oxford Research Unit, supported by the Wellcome Trust (Grant no. 220211). ARSB is funded by São Paulo Research Foundation - FAPESP (Grant no. 2002/09546-1). RLDM is funded by Brazilian National Council for Scientific and Technological Development - CNPq (Grant no. 302353/2003-8 and 471605/2011-5); CRFM is funded by FAPESP (Grant no. 2020/06747-4) and CNPq (Grant no. 302917/2019-5 and 408636/2018-1); JGD is funded by FAPESP fellowships (2016/13465-0 and 2019/12068-5) and CNPq (Grant no. 409216/2018-6).

International Center of Excellence for Malaria Research for South Asia and Broader Malaria Research in India

The Malaria Evolution in South Asia (MESA) International Center of Excellence for Malaria Research (ICEMR) conducted research studies at multiple sites in India to record blood-slide positivity over time, but also to study broader aspects of the disease. From the Southwest of India (Goa) to the Northeast (Assam), the MESA-ICEMR invested in research equipment, operational capacity, and trained personnel to observe frequencies of Plasmodium falciparum and Plasmodium vivax infections, clinical presentations, treatment effectiveness, vector transmission, and reinfections. With Government of India partners, Indian and U.S. academics, and trained researchers on the ground, the MESA-ICEMR team contributes information on malaria in selected parts of India.

Malaria-Transmitting Vectors Microbiota: Overview and Interactions With Anopheles Mosquito Biology

Malaria remains a vector-borne infectious disease that is still a major public health concern worldwide, especially in tropical regions. Malaria is caused by a protozoan parasite of the genus Plasmodium and transmitted through the bite of infected female Anopheles mosquitoes. The control interventions targeting mosquito vectors have achieved significant success during the last two decades and rely mainly on the use of chemical insecticides through the insecticide-treated nets (ITNs) and indoor residual spraying (IRS). Unfortunately, resistance to conventional insecticides currently being used in public health is spreading in the natural mosquito populations, hampering the long-term success of the current vector control strategies. Thus, to achieve the goal of malaria elimination, it appears necessary to improve vector control approaches through the development of novel environment-friendly tools. Mosquito microbiota has by now given rise to the expansion of innovative control tools, such as the use of endosymbionts to target insect vectors, known as "symbiotic control." In this review, we will present the viral, fungal and bacterial diversity of Anopheles mosquitoes, including the bacteriophages. This review discusses the likely interactions between the vector microbiota and its fitness and resistance to insecticides.

The changing malaria trend and control efforts in Oromia Special zone, Amhara Regional State, North-East Ethiopia

Background

Countries in malaria endemic regions are determinedly making an effort to achieve the global malaria elimination goals. In Ethiopia, too, all concerned bodies have given attention to this mission as one of their priority areas so that malaria would be eradicated from the country. Despite the success stories from some areas in the country, however, malaria is still a major public health concern in most parts of Ethiopia. Therefore, this study is aimed at analysing the changing malaria trend and assessing the impact of malaria control efforts in one of the malaria endemic regions of Ethiopia.

Methods

Five years data on clinical malaria cases diagnosed and treated at all health facilities (including 28 Health Centres, 105 Health Posts and 2 Hospitals) in Oromia Special zone, Amhara Regional State, Ethiopia, were reviewed for the period from June 2014 to June 2019. Data on different interventional activities undertaken in the zone during the specified period were obtained from the Regional Health Bureau.

Results

The cumulative malaria positivity rate documented in the zone was 12.5% (n = 65,463/524,722). Plasmodium falciparum infection was the dominant malaria aetiology and accounted for 78.9% (n = 51,679). The age group with the highest malaria burden was found to be those aged above 15 years (54.14%, n = 35,443/65,463). The malaria trend showed a sharp decreasing pattern from 19.33% (in 2015) to 5.65% (in 2018), although insignificant increment was recorded in 2019 (8.53%). Distribution of long-lasting insecticidal nets (LLIN) and indoor residual spraying (IRS) were undertaken in the zone once a year only for two years, specifically in 2014 and 2017. In 2014, a single LLIN was distributed per head of households, which was not sufficient for a family size of more than one family member. Number of houses sprayed with indoor residual spray in 2014 and 2017 were 33,314 and 32,184 houses, respectively, leading to the assumption that, 151,444 (25.9%) and 141,641 (24.2%) population were protected in year 2014 and 2017, respectively. The analysis has shown that P. falciparum positivity rate was significantly decreased following the interventional activities by 3.3% (p = 0.009), but interventional efforts did not appear to have significant effect on vivax malaria, as positivity rate of this parasite increased by 1.49% (p = 0.0218).

Conclusion

Malaria burden has shown a decreasing pattern in the study area, although the pattern was not consistent throughout all the years and across the districts in the study area. Therefore, unremitting surveillance along implementation of interventional efforts should be considered taking into account the unique features of Plasmodium species, population dynamics in the zone, seasonality, and malaria history at different districts of the zone should be in place to achieve the envisaged national malaria elimination goal by 2030.

GIP: an open-source computational pipeline for mapping genomic instability from protists to cancer cells

Genome instability has been recognized as a key driver for microbial and cancer adaptation and thus plays a central role in many diseases. Genome instability encompasses different types of genomic alterations, yet most available genome analysis software are limited to just one type of mutation. To overcome this limitation and better understand the role of genetic changes in enhancing pathogenicity we established GIP, a novel, powerful bioinformatic pipeline for comparative genome analysis. Here, we show its application to whole genome sequencing datasets of Leishmania, Plasmodium, Candida and cancer. Applying GIP on available data sets validated our pipeline and demonstrated the power of our tool to drive biological discovery. Applied to Plasmodium vivax genomes, our pipeline uncovered the convergent amplification of erythrocyte binding proteins and identified a nullisomic strain. Re-analyzing genomes of drug adapted Candida albicans strains revealed correlated copy number variations of functionally related genes, strongly supporting a mechanism of epistatic adaptation through interacting gene-dosage changes. Our results illustrate how GIP can be used for the identification of aneuploidy, gene copy number variations, changes in nucleic acid sequences, and chromosomal rearrangements. Altogether, GIP can shed light on the genetic bases of cell adaptation and drive disease biomarker discovery.

4-Arylthiosemicarbazide Derivatives as Toxoplasmic Aromatic Amino Acid Hydroxylase Inhibitors and Anti-inflammatory Agents

Approximately one-third of the human population is infected with the intracellular cosmopolitan protozoan Toxoplasma gondii (Tg), and a specific treatment for this parasite is still needed. Additionally, the increasing resistance of Tg to drugs has become a challenge for numerous research centers. The high selectivity of a compound toward the protozoan, along with low cytotoxicity toward the host cells, form the basis for further research, which aims at determining the molecular targets of the active compounds. Thiosemicarbazide derivatives are biologically active organic compounds. Previous studies on the initial preselection of 58 new 4-arylthiosemicarbazide derivatives in terms of their anti-Tg activity and selectivity made it possible to select two promising derivatives for further research. One of the important amino acids involved in the proliferation of Tg and the formation of parasitophorous vacuoles is tyrosine, which is converted by two unique aromatic amino acid hydroxylases to levodopa. Enzymatic studies with two derivatives (R: para-nitro and meta-iodo) and recombinant aromatic amino acid hydroxylase (AAHs) obtained in the E. coli expression system were performed, and the results indicated that toxoplasmic AAHs are a molecular target for 4-arylthiosemicarbazide derivatives. Moreover, the drug affinity responsive target stability assay also confirmed that the selected compounds bind to AAHs. Additionally, the anti-inflammatory activity of these derivatives was tested using THP1-Blue™ NF-κB reporter cells due to the similarity of the thiosemicarbazide scaffold to thiosemicarbazone, both of which are known NF-κB pathway inhibitors.

Significant decline of malaria incidence in a low socioeconomic area in the southeast of Iran: 10 years field assessment during malaria elimination programme

Although malaria burden and its active foci have sharply declined after the implementation of elimination programme since 2010, it is still considered as a major public health problem in southeast Iran. This descriptive-analytical study aimed to determine 10-years of malaria epidemiological trends in Bashagard County. Data were collected from 7 selected malarious region of the county based on active and passive surveillance of clinical cases. For diagnosis of malaria, the examination of microscopic slides and rapid diagnostic test, were used. In total, 237 malaria cases were found from 2009 to 2018. Plasmodium vivax was the dominant parasite species and identified in 232 (97.9%) individuals. Males were infected more than females and the majority of malaria cases (67.4%) were recorded from rural areas. Although about 98% of malaria cases were indigenous, they have decreased form 200 cases in 2009 to zero indigenous transmission in 2018. During the study period, malaria cases had decreased significantly by about 99% and the incidence rate had declined from 5.47/1000 cases in 2009 to 0.002/1000 in 2018. The incidence of malaria, especially indigenous cases, in Bashagard County has decreased dramatically in the past 10 years. However, there is still probability of malaria re-introduction and outbreak in the county due to climatical and geographical conditions. Therefore, it is necessary to implement an active surveillance system to detect and treat malaria cases quickly, during the elimination phase in this county.

The global burden of Plasmodium vivax malaria is obscure and insidious

J. Kevin Baird and colleagues, examine and discuss the estimated global burden of vivax malaria and it's biological, clinical, and public health complexity.

Species Composition, Parous Rate, and Infection Rate of Anopheles Mosquitoes (Diptera: Culicidae) in Bahir Dar City Administration, Northwest Ethiopia

Anopheles mosquitoes are the main vectors of malaria. There is little information on the current entomological aspects of Anopheles mosquitoes in Amhara region of northwestern Ethiopia. Therefore, the aim of this study was to assess the prevailing species composition, parous rate, and infection rate of Anopheles mosquitoes in the Bahir Dar city administration. A community-based cross-sectional study was conducted from January through July 2020. For this, six Centers for Disease Control and Prevention light traps (three traps indoor and three traps outdoor) were used to collect adult female Anopheles mosquitoes. The species were morphologically identified, and the parous and infection rates were determined via dissection of ovaries and salivary gland, respectively. A total of 378 adult female Anopheles mosquitoes comprised of three species (Anopheles d'thali, Anopheles rhodesiensis, and Anopheles gambiae complex) were collected and identified at the study sites. Anopheles rhodesiensis was the predominant species accounting for 90% of all collections at the Zenzelima site, followed by An. gambiae complex (6.5%). In contrast, An. gambiae complex was the predominant species at the Tis Abay site, comprising 94% of captures. The overall parous and infection rates were 35 (62.5%) and 1 (2.9%), respectively.

Humoral and cellular immune response to Plasmodium vivax VIR recombinant and synthetic antigens in individuals naturally exposed to P. vivax in the Republic of Korea

BACKGROUND

Plasmodium vivax proteins with variant interspersed repeats (VIR) are the key proteins used by the parasite to escape from the host immune system through the creation of antigenic variations. However, few studies have been done to elucidate their role as targets of immunity. Thus, this study evaluated the naturally-acquired immune response against VIR proteins in vivax malaria-infected individuals in the Republic of Korea (ROK).

METHODS

Seven recombinant VIR proteins and two synthetic peptides previously studied in other countries that elicited a robust immune response were used to investigate the antibody and cellular immune response in 681 P. vivax-infected people in ROK. The expression of IgM, IgG, and IgG subclasses against each VIR antigen or against PvMSP1-19 was analysed by ELISA. PvMSP1-19, known as a promising vaccine candidate of P. vivax, was used as the positive control for immune response assessment. Furthermore, the cellular immune response to VIR antigens was evaluated by in vitro proliferative assay, cellular activation assay, and cytokine detection in mononuclear cells of the P. vivax-infected population.

RESULTS

IgM or IgG were detected in 52.4% of the population. Among all the VIR antigens, VIR25 elicited the highest humoral immune response in the whole population with IgG and IgM prevalence of 27.8% and 29.2%, respectively, while PvMSP1-19 elicited even higher prevalence (92%) of IgG in the population. As for the cellular immune response, VIR-C2, PvLP2, and PvMSP1-19 induced high cell activation and secretion of IL-2, IL-6, IL-10, and G-CSF in mononuclear cells from the P. vivax-infected population, comparable with results from PvMSP1-19. However, no significant proliferation response to these antigens was observed between the malaria-infected and healthy groups.

CONCLUSION

Moderate natural acquisition of antibody and cellular responses in P. vivax-infected Korean malaria patients presented here are similar to that in other countries. It is interesting that the immune response to VIR antigens is conserved among malaria parasites in different countries, considering that VIR genes are highly polymorphic. This thus warrants further studies to elucidate molecular mechanisms by which human elicit immune response to the malaria parasite VIR antigens.

Repositioning and Characterization of 1-(Pyridin-4-yl)pyrrolidin-2-one Derivatives as Plasmodium Cytoplasmic Prolyl-tRNA Synthetase Inhibitors

Prolyl-tRNA synthetase (PRS) is a clinically validated antimalarial target. Screening of a set of PRS ATP-site binders, initially designed for human indications, led to identification of 1-(pyridin-4-yl)pyrrolidin-2-one derivatives representing a novel antimalarial scaffold. Evidence designates cytoplasmic PRS as the drug target. The frontrunner 1 and its active enantiomer 1-S exhibited low-double-digit nanomolar activity against resistant Plasmodium falciparum (Pf) laboratory strains and development of liver schizonts. No cross-resistance with strains resistant to other known antimalarials was noted. In addition, a similar level of growth inhibition was observed against clinical field isolates of Pf and P. vivax. The slow killing profile and the relative high propensity to develop resistance in vitro (minimum inoculum resistance of 8 × 10⁵ parasites at a selection pressure of 3 × IC₅₀) constitute unfavorable features for treatment of malaria. However, potent blood stage and antischizontal activity are compelling for causal prophylaxis which does not require fast onset of action. Achieving sufficient on-target selectivity appears to be particularly challenging and should be the primary focus during the next steps of optimization of this chemical series. Encouraging preliminary off-target profile and oral efficacy in a humanized murine model of Pf malaria allowed us to conclude that 1-(pyridin-4-yl)pyrrolidin-2-one derivatives represent a promising starting point for the identification of novel antimalarial prophylactic agents that selectively target Plasmodium PRS.

CYP2D6 Genetic Variation and Its Implication for Vivax Malaria Treatment in Madagascar

Plasmodium vivax is one of the five human malaria parasite species, which has a wide geographical distribution and can cause severe disease and fatal outcomes. It has the ability to relapse from dormant liver stages (hypnozoites), weeks to months after clearance of the acute blood-stage infection. An 8-aminoquinoline drug primaquine (PQ) can clear the hypnozoites, and thus can be used as an anti-relapse therapeutic agent. Recently, a number of studies have found that its efficacy is compromised by polymorphisms in the cytochrome P450 2D6 (CYP2D6) gene; decreased or absence of CYP2D6 activity contributes to PQ therapeutic failure. The present study sought to characterize CYP2D6 genetic variation in Madagascar, where populations originated from admixture between Asian and African populations, vivax malaria is endemic, and PQ can be deployed soon to achieve national malaria elimination. In a total of 211 samples collected from two health districts, CYP2D6 decreased function alleles CYP2D6*10, *17, *29, *36+*10, and *41 were observed at frequencies of 3.55-17.06%. In addition, nonfunctional alleles were observed, the most common of which were CYP2D6*4 (2.13%), *5 (1.66%), and the *4x2 gene duplication (1.42%). Given these frequencies, 34.6% of the individuals were predicted to be intermediate metabolizers (IM) with an enzyme activity score (AS) ≤ 1.0; both the IM phenotype and AS ≤ 1.0 have been found to be associated with PQ therapeutic failure. Furthermore, the allele and genotype frequency distributions add to the archaeological and genomic evidence of Malagasy populations constituting a unique, Asian-African admixed origin. The results from this exploratory study provide fresh insights about genomic characteristics that could affect the metabolism of PQ into its active state, and may enable optimization of PQ treatment across human genetic diversity, which is critical for achieving P. vivax elimination.

Amino Acid Metabolism in Apicomplexan Parasites

Obligate intracellular pathogens have coevolved with their host, leading to clever strategies to access nutrients, to combat the host's immune response, and to establish a safe niche for intracellular replication. The host, on the other hand, has also developed ways to restrict the replication of invaders by limiting access to nutrients required for pathogen survival. In this review, we describe the recent advancements in both computational methods and high-throughput -omics techniques that have been used to study and interrogate metabolic functions in the context of intracellular parasitism. Specifically, we cover the current knowledge on the presence of amino acid biosynthesis and uptake within the Apicomplexa phylum, focusing on human-infecting pathogens: Toxoplasma gondii and Plasmodium falciparum. Given the complex multi-host lifecycle of these pathogens, we hypothesize that amino acids are made, rather than acquired, depending on the host niche. We summarize the stage specificities of enzymes revealed through transcriptomics data, the relevance of amino acids for parasite pathogenesis in vivo, and the role of their transporters. Targeting one or more of these pathways may lead to a deeper understanding of the specific contributions of biosynthesis versus acquisition of amino acids and to design better intervention strategies against the apicomplexan parasites.

Insights into the molecular diversity of Plasmodium vivax merozoite surface protein-3γ (pvmsp3γ), a polymorphic member in the msp3 multi-gene family

Plasmodium vivax merozoite surface protein 3 (PvMSP3) is encoded by a multi-gene family. Of these, PvMSP3α, PvMSP3β and PvMSP3γ, are considered to be vaccine targets. Despite comprehensive analyses of PvMSP3α and PvMSP3β, little is known about structural and sequence diversity in PvMSP3γ. Analysis of 118 complete pvmsp3γ sequences from diverse endemic areas of Thailand and 9 reported sequences has shown 86 distinct haplotypes. Based on variation in insert domains, pvmsp3γ can be classified into 3 types, i.e. Belem, Salvador I and NR520. Imperfect nucleotide repeats were found in six regions of the gene; none encoded tandem amino acid repeats. Predicted coiled-coil heptad repeats were abundant in the protein and displayed variation in length and location. Interspersed phase shifts occurred in the heptad arrays that may have an impact on protein structure. Polymorphism in pvmsp3γ seems to be generated by intragenic recombination and driven by natural selection. Most P. vivax isolates in Thailand exhibit population structure, suggesting limited gene flow across endemic areas. Phylogenetic analysis has suggested that insert domains could have been subsequently acquired during the evolution of pvmsp3γ. Sequence and structural diversity of PvMSP3γ may complicate vaccine design due to alteration in predicted immunogenic epitopes among variants.

Plasmodium vivax drug resistance markers: Genetic polymorphisms and mutation patterns in isolates from Malaysia

Transmission of Plasmodium vivax still persist in Malaysia despite the government's aim to eliminate malaria in 2020. High treatment failure rate of chloroquine monotherapy was reported recently. Hence, parasite drug susceptibility should be kept under close monitoring. Mutation analysis of the drug resistance markers is useful for reconnaissance of anti-malarial drug resistance. Hitherto, information on P. vivax drug resistance marker in Malaysia are limited. This study aims to evaluate the mutations in four P. vivax drug resistance markers pvcrt-o (putative), pvmdr1 (putative), pvdhfr and pvdhps in 44 isolates from Malaysia. Finding indicates that 27.3%, 100%, 47.7%, and 27.3% of the isolates were carrying mutant allele in pvcrt-o, pvmdr1, pvdhfr and pvdhps genes, respectively. Most of the mutant isolates had multiple point mutations rather than single point mutation in pvmdr1 (41/44) and pvdhfr (19/21). One novel point mutation V111I was detected in pvdhfr. Allelic combination analysis shows significant strong association between mutations in pvcrt-o and pvmdr1 (X² = 9.521, P < 0.05). In the present study, 65.9% of the patients are non-Malaysians, with few of them arrived in Malaysia 1-2 weeks before the onset of clinical manifestations, or had previous history of malaria infection. Besides, few Malaysian patients had travel history to vivax-endemic countries, suggesting that these patients might have acquired the infections during their travel. All these possible imported cases could have placed Malaysia in a risk to have local transmission or outbreak of malaria. Six isolates were found to have mutations in all four drug resistance markers, suggesting that the multiple-drugs resistant P. vivax strains are circulating in Malaysia.

Plasmodium vivax Malaria Viewed through the Lens of an Eradicated European Strain

The protozoan Plasmodium vivax is responsible for 42% of all cases of malaria outside Africa. The parasite is currently largely restricted to tropical and subtropical latitudes in Asia, Oceania, and the Americas. Though, it was historically present in most of Europe before being finally eradicated during the second half of the 20th century. The lack of genomic information on the extinct European lineage has prevented a clear understanding of historical population structuring and past migrations of P. vivax. We used medical microscope slides prepared in 1944 from malaria-affected patients from the Ebro Delta in Spain, one of the last footholds of malaria in Europe, to generate a genome of a European P. vivax strain. Population genetics and phylogenetic analyses placed this strain basal to a cluster including samples from the Americas. This genome allowed us to calibrate a genomic mutation rate for P. vivax, and to estimate the mean age of the last common ancestor between European and American strains to the 15th century. This date points to an introduction of the parasite during the European colonization of the Americas. In addition, we found that some known variants for resistance to antimalarial drugs, including Chloroquine and Sulfadoxine, were already present in this European strain, predating their use. Our results shed light on the evolution of an important human pathogen and illustrate the value of antique medical collections as a resource for retrieving genomic information on pathogens from the past.

Microbiota, a Third Player in the Host-Plasmodium Affair

Plasmodium, the causative agent of malaria, is responsible for more than 200 million new infections and 400 000 deaths yearly. While in recent years the influence of the microbiota in homeostasis and a wide variety of disorders has taken center stage, its contribution during malaria infections has only now started to emerge. The few published studies suggest two distinct but complementary directions. Plasmodium infections can cause significant alterations in host (at least gut) microbiota, and host gut microbiota can influence the clinical outcome of malaria infections. In this opinion article, we highlight the most fundamental unanswered questions in the field that will, hopefully, point future research directions towards unveiling key mechanistic insights of the Plasmodium-host-microbiota axis.

Cost-utility of tafenoquine vs. primaquine for the radical cure (prevention of relapse) of Plasmodium vivax malaria

The aim of this study was to compare cost-utility of tafenoquine (TQ) and primaquine (PQ) for a radical cure (prevention of relapse) of Plasmodium vivax (PV) malaria in Serbia using A five-state, 1-month cycle Markov model. The perspective of Republic Health Insurance Fund was chosen, and the time horizon was 10 years. The model results were obtained after Monte Carlo microsimulation of a sample with 1000 virtual patients. After base case analysis PQ was dominated by TQ, as the net monetary benefit was positive (20,713.84 ± 7,167.46 RSD (99% CI) (174.95 ± 60.54 €)) and incremental cost-effectiveness ratio was below the willingness-to-pay line of 1 Serbian gross national product per capita per quality-adjusted life year gained. Multiple one-way sensitivity analysis and probabilistic sensitivity analysis confirmed the results of the base case simulation. In conclusion, TQ was cost-effective in comparison to PQ for radical cure of PV malaria in socio-economic settings of a South-Eastern European country.

Short-course primaquine for the radical cure of Plasmodium vivax malaria: a multicentre, randomised, placebo-controlled non-inferiority trial

BACKGROUND

Primaquine is the only widely used drug that prevents Plasmodium vivax malaria relapses, but adherence to the standard 14-day regimen is poor. We aimed to assess the efficacy of a shorter course (7 days) of primaquine for radical cure of vivax malaria.

METHODS

We did a randomised, double-blind, placebo-controlled, non-inferiority trial in eight health-care clinics (two each in Afghanistan, Ethiopia, Indonesia, and Vietnam). Patients (aged ≥6 months) with normal glucose-6-phosphate dehydrogenase (G6PD) and presenting with uncomplicated vivax malaria were enrolled. Patients were given standard blood schizontocidal treatment and randomly assigned (2:2:1) to receive 7 days of supervised primaquine (1·0 mg/kg per day), 14 days of supervised primaquine (0·5 mg/kg per day), or placebo. The primary endpoint was the incidence rate of symptomatic P vivax parasitaemia during the 12-month follow-up period, assessed in the intention-to-treat population. A margin of 0·07 recurrences per person-year was used to establish non-inferiority of the 7-day regimen compared with the 14-day regimen. This trial is registered at ClinicalTrials.gov (NCT01814683).

FINDINGS

Between July 20, 2014, and Nov 25, 2017, 2336 patients were enrolled. The incidence rate of symptomatic recurrent P vivax malaria was 0·18 (95% CI 0·15 to 0·21) recurrences per person-year for 935 patients in the 7-day primaquine group and 0·16 (0·13 to 0·18) for 937 patients in the 14-day primaquine group, a difference of 0·02 (-0·02 to 0·05, p=0·3405). The incidence rate for 464 patients in the placebo group was 0·96 (95% CI 0·83 to 1·08) recurrences per person-year. Potentially drug-related serious adverse events within 42 days of starting treatment were reported in nine (1·0%) of 935 patients in the 7-day group, one (0·1%) of 937 in the 14-day group and none of 464 in the control arm. Four of the serious adverse events were significant haemolysis (three in the 7-day group and one in the 14-day group).

INTERPRETATION

In patients with normal G6PD, 7-day primaquine was well tolerated and non-inferior to 14-day primaquine. The short-course regimen might improve adherence and therefore the effectiveness of primaquine for radical cure of P vivax malaria.

FUNDING

UK Department for International Development, UK Medical Research Council, UK National Institute for Health Research, and the Wellcome Trust through the Joint Global Health Trials Scheme (MR/K007424/1) and the Bill & Melinda Gates Foundation (OPP1054404).

Plasmodium vivax Infections in Duffy-Negative Individuals in the Democratic Republic of the Congo

Although Plasmodium vivax has been assumed to be absent from sub-Saharan Africa because of the protective mutation conferring the Duffy-negative phenotype, recent evidence has suggested that P. vivax cases are prevalent in these regions. We selected 292 dried blood spots from children who participated in the 2013-2014 Demographic and Health Survey of the Democratic Republic of the Congo (DRC), to assess for P. vivax infection. Four P. vivax infections were identified by polymerase chain reaction, each in a geographically different survey cluster. Using these as index cases, we tested the remaining 73 samples from the four clusters. With this approach, 10 confirmed cases, three probable cases, and one possible case of P. vivax were identified. Among the 14 P. vivax cases, nine were coinfected with Plasmodium falciparum. All 14 individuals were confirmed to be Duffy-negative by sequencing for the single point mutation in the GATA motif that represses the expression of the Duffy antigen. This finding is consistent with a growing body of literature that suggests that P. vivax can infect Duffy-negative individuals in Africa. Future molecular and sequencing work is needed to understand the relationship of these isolates with other P. vivax samples from Asia and South America and discover variants linked to P. vivax virulence and erythrocyte invasion.

Efficacy of Primaquine in Preventing Short- and Long-Latency Plasmodium vivax Relapses in Nepal

BACKGROUND

Plasmodium vivax is the main cause of malaria in Nepal. Relapse patterns have not been characterized previously.

METHODS

Patients with P. vivax malaria were randomized to receive chloroquine (CQ; 25 mg base/kg given over 3 days) alone or together with primaquine (PQ; 0.25 mg base/kg/day for 14 days) and followed intensively for 1 month, then at 1- to 2-month intervals for 1 year. Parasite isolates were genotyped.

RESULTS

One hundred and one (49%) patients received CQ and 105 (51%) received CQ + PQ. In the CQ + PQ arm, there were 3 (4.1%) recurrences in the 73 patients who completed 1 year of follow-up compared with 22 of 78 (28.2%) in the CQ-only arm (risk ratio, 0.146 [95% confidence interval, .046-.467]; P < .0001). Microsatellite genotyping showed relatively high P. vivax genetic diversity (mean heterozygosity, 0.843 [range 0.570-0.989] with low multiplicity of infection (mean, 1.05) reflecting a low transmission preelimination setting. Of the 12 genetically homologous relapses, 5 (42%) occurred in a cluster after 9 months, indicating long latency.

CONCLUSIONS

Although there may be emerging CQ resistance, the combination of CQ and the standard-dose 14-day PQ regimen is highly efficacious in providing radical cure of short- and long-latency P. vivax malaria in Nepal.

Malaria situation in Iran: 2002-2017

Background

Malaria is considered as a major threat to health systems. It is still considered as one of the most important infectious diseases in Iran, but with an elimination goal in 2025. This study aimed to review the malaria situation in Iran over the 16 years.

Methods

The data was collected from epidemiological registration forms that had been completed by physicians and malaria focal points in the National Centers for Disease Control and Prevention.

Results

During the study period, 134,273 malaria cases were reported. The malaria incidence decreased from 0.24/1000 cases in 2002 to 0.01/1000 in 2017. From 2009 onward, the number of imported cases increased in comparison with the autochthonous and indigenous cases. Most cases were seen in males and people over 15 years of age. Moreover, the dominant registered reports were from rural areas. Most malaria cases were reported from the south and southeastern of Iran. Plasmodium vivax was the dominant species.

Conclusion

The dramatic drop in the incidence of autochthonous cases can hopefully support malaria elimination as a major goal in the near future.

Plasmodium knowlesi as a model system for characterising Plasmodium vivax drug resistance candidate genes

Plasmodium vivax causes the majority of malaria outside Africa, but is poorly understood at a cellular level partly due to technical difficulties in maintaining it in in vitro culture conditions. In the past decades, drug resistant P. vivax parasites have emerged, mainly in Southeast Asia, but while some molecular markers of resistance have been identified, none have so far been confirmed experimentally, which limits interpretation of the markers, and hence our ability to monitor and control the spread of resistance. Some of these potential markers have been identified through P. vivax genome-wide population genetic analyses, which highlighted genes under recent evolutionary selection in Southeast Asia, where chloroquine resistance is most prevalent. These genes could be involved in drug resistance, but no experimental proof currently exists to support this hypothesis. In this study, we used Plasmodium knowlesi, the most closely related species to P. vivax that can be cultured in human erythrocytes, as a model system to express P. vivax genes and test for their role in drug resistance. We adopted a strategy of episomal expression, and were able to express fourteen P. vivax genes, including two allelic variants of several hypothetical resistance genes. Their expression level and localisation were assessed, confirming cellular locations conjectured from orthologous species, and suggesting locations for several previously unlocalised proteins, including an apical location for PVX_101445. These findings establish P. knowlesi as a suitable model for P. vivax protein expression. We performed chloroquine and mefloquine drug assays, finding no significant differences in drug sensitivity: these results could be due to technical issues, or could indicate that these genes are not actually involved in drug resistance, despite being under positive selection pressure in Southeast Asia. These data confirm that in vitro P. knowlesi is a useful tool for studying P. vivax biology. Its close evolutionary relationship to P. vivax, high transfection efficiency, and the availability of markers for colocalisation, all make it a powerful model system. Our study is the first of its kind using P. knowlesi to study unknown P. vivax proteins and investigate drug resistance mechanisms.

A Multi-Stage Plasmodium vivax Malaria Vaccine Candidate Able to Induce Long-Lived Antibody Responses Against Blood Stage Parasites and Robust Transmission-Blocking Activity

Malaria control and interventions including long-lasting insecticide-treated nets, indoor residual spraying, and intermittent preventative treatment in pregnancy have resulted in a significant reduction in the number of Plasmodium falciparum cases. Considerable efforts have been devoted to P. falciparum vaccines development with much less to P. vivax. Transmission-blocking vaccines, which can elicit antibodies targeting Plasmodium antigens expressed during sexual stage development and interrupt transmission, offer an alternative strategy to achieve malaria control. The post-fertilization antigen P25 mediates several functions essential to ookinete survival but is poorly immunogenic in humans. Previous clinical trials targeting this antigen have suggested that conjugation to a carrier protein could improve the immunogenicity of P25. Here we report the production, and characterization of a vaccine candidate composed of a chimeric P. vivax Merozoite Surface Protein 1 (cPvMSP1) genetically fused to P. vivax P25 (Pvs25) designed to enhance CD4+ T cell responses and its assessment in a murine model. We demonstrate that antibodies elicited by immunization with this chimeric protein recognize both the erythrocytic and sexual stages and are able to block the transmission of P. vivax field isolates in direct membrane-feeding assays. These findings provide support for the continued development of multi-stage transmission blocking vaccines targeting the life-cycle stage responsible for clinical disease and the sexual-stage development accountable for disease transmission simultaneously.

Chloroquine Versus Dihydroartemisinin-Piperaquine With Standard High-dose Primaquine Given Either for 7 Days or 14 Days in Plasmodium vivax Malaria

BACKGROUND

Primaquine is necessary for the radical cure of Plasmodium vivax malaria, but the optimum duration of treatment and best partner drug are uncertain. A randomized controlled trial was performed to compare the tolerability and radical curative efficacy of 7-day versus 14-day high-dose primaquine regimens (total dose 7mg/kg) with either chloroquine or dihydroartemisinin-piperaquine.

METHODS

Patients with uncomplicated P. vivax malaria on the Thailand-Myanmar border were randomized to either chloroquine (25mg base/kg) or dihydroartemisinin-piperaquine (dihydroartemisinin 7mg/kg and piperaquine 55mg/kg) plus primaquine, either 0.5 mg/kg/day for 14 days or 1 mg/kg/day for 7 days. Adverse events within 42 days and 1-year recurrence rates were compared and their relationship with day 6 drug concentrations assessed.

RESULTS

Between February 2012 and July 2014, 680 patients were enrolled. P. vivax recurrences (all after day 35) occurred in 80/654 (12%) patients; there was no difference between treatments. Compared to the 7-day primaquine groups the pooled relative risk of recurrence in the 14-day groups was 1.15 (95% confidence interval 0.7 to 1.8). Hematocrit reductions were clinically insignificant except in G6PD female heterozygotes, 2 of whom had hematocrit reductions to <23% requiring blood transfusion.

CONCLUSION

Radical cure should be deployed more widely. The radical curative efficacy in vivax malaria of 7-day high-dose primaquine is similar to the standard 14-day high-dose regimen. Chloroquine and dihydroartemisinin-piperaquine are both highly effective treatments of the blood stage infection. Quantitative point of care G6PD testing would ensure safe use of the 7-day high-dose primaquine regimen in G6PD heterozygous females.

CLINICAL TRIALS REGISTRATION

NCT01640574.

Discovery and Structural Optimization of Acridones as Broad-Spectrum Antimalarials

Malaria remains one of the deadliest diseases in the world today. Novel chemoprophylactic and chemotherapeutic antimalarials are needed to support the renewed eradication agenda. We have discovered a novel antimalarial acridone chemotype with dual-stage activity against both liver-stage and blood-stage malaria. Several lead compounds generated from structural optimization of a large library of novel acridones exhibit efficacy in the following systems: (1) picomolar inhibition of in vitro Plasmodium falciparum blood-stage growth against multidrug-resistant parasites; (2) curative efficacy after oral administration in an erythrocytic Plasmodium yoelii murine malaria model; (3) prevention of in vitro Plasmodium berghei sporozoite-induced development in human hepatocytes; and (4) protection of in vivo P. berghei sporozoite-induced infection in mice. This study offers the first account of liver-stage antimalarial activity in an acridone chemotype. Details of the design, chemistry, structure-activity relationships, safety, metabolic/pharmacokinetic studies, and mechanistic investigation are presented herein.

Are malaria elimination efforts on right track? An analysis of gains achieved and challenges ahead

Background

Malaria causes significant morbidity and mortality each year. In the past few years, the global malaria cases have been declining and many endemic countries are heading towards malaria elimination. Nevertheless, reducing the number of cases seems to be easy than sustained elimination. Therefore to achieve the objective of complete elimination and maintaining the elimination status, it is necessary to assess the gains made during the recent years.

Main text

With inclining global support and World Health Organisation (WHO) efforts, the control programmes have been implemented effectively in many endemic countries. Given the aroused interest and investments into malaria elimination programmes at global level, the ambitious goal of elimination appears feasible. Sustainable interventions have played a pivotal role in malaria contraction, however drug and insecticide resistance, social, demographic, cultural and behavioural beliefs and practices, and unreformed health infrastructure could drift back the progress attained so far. Ignoring such impeding factors coupled with certain region specific factors may jeopardise our ability to abide righteous track to achieve global elimination of malaria parasite. Although support beyond the territories is important, but well managed integrated vector management approach at regional and country level using scrupulously selected area specific interventions targeting both vector and parasite along with the community involvement is necessary. A brief incline in malaria during 2016 has raised fresh perturbation on whether elimination could be achieved on time or not.

Conclusions

The intervention tools available currently can most likely reduce transmission but clearing of malaria epicentres from where the disease can flare up any time, is not possible without involving local population. Nevertheless maintaining zero malaria transmission and checks on malaria import in declared malaria free countries, and further speeding up of interventions to stop transmission in elimination countries is most desirable. Strong collaboration backed by adequate political and financial support among the countries with a common objective to eliminate malaria must be on top priority. The present review attempts to assess the progress gained in malaria elimination during the past few years and highlights some issues that could be important in successful malaria elimination.

Electronic supplementary material

The online version of this article (10.1186/s40249-019-0524-x) contains supplementary material, which is available to authorized users.

Plasmodium vivax readiness to transmit: implication for malaria eradication

Background

The lack of a continuous long-term in vitro culture system for Plasmodium vivax severely limits our knowledge of pathophysiology of the most widespread malaria parasite. To gain direct understanding of P. vivax human infections, we used Next Generation Sequencing data mining to unravel parasite in vivo expression profiles for P. vivax, and P. falciparum as comparison.

Results

We performed cloud and local computing to extract parasite transcriptomes from publicly available raw data of human blood samples. We developed a Poisson Modelling (PM) method to confidently identify parasite derived transcripts in mixed RNAseq signals of infected host tissues. We successfully retrieved and reconstructed parasite transcriptomes from infected patient blood as early as the first blood stage cycle; and the same methodology did not recover any significant signal from controls. Surprisingly, these first generation blood parasites already show strong signature of transmission, which indicates the commitment from asexual-to-sexual stages. Further, we place the results within the context of P. vivax’s complex life cycle, by developing mathematical models for P. vivax and P. falciparum and using sensitivity analysis assess the relative epidemiological impact of possible early stage transmission.

Conclusion

The study uncovers the earliest onset of P. vivax blood pathogenesis and highlights the challenges of P. vivax eradication programs.

Electronic supplementary material

The online version of this article (10.1186/s12918-018-0669-4) contains supplementary material, which is available to authorized users.

Epidemiology of Plasmodium vivax Malaria Infection in Nepal

Malaria is endemic in the southern plain of Nepal which shares a porous border with India. More than 80% cases of malaria in Nepal are caused by Plasmodium vivax. The main objective of this study was to review the epidemiology of P. vivax malaria infections as recorded by the national malaria control program of Nepal between 1963 and 2016. National malaria data were retrieved from the National Malaria program in the Ministry of Health, Government of Nepal. The epidemiological trends and malariometric indicators were analyzed. Vivax malaria has predominated over falciparum malaria in the past 53 years, with P. vivax malaria comprising 70-95% of the annual malaria infections. In 1985, a malaria epidemic occurred with 42,321 cases (82% P. vivax and 17% Plasmodium falciparum). Nepal had experienced further outbreaks of malaria in 1991 and 2002. Plasmodium falciparum cases increased from 2005 to 2010 but since then declined. Analyzing the overall trend between 2002 (12,786 cases) until 2016 (1,009 cases) shows a case reduction by 92%. The proportion of imported malaria cases has increased from 18% of cases in 2001 to 50% in 2016. The current trends of malariometric indices indicate that Nepal is making a significant progress toward achieving the goal of malaria elimination by 2025. Most of the cases are caused by P. vivax with imported malaria comprising an increasing proportion of cases. The malaria control program in Nepal needs to counter importation of malaria at high risk areas with collaborative cross border malaria control activities.

Therapeutic and Transmission-Blocking Efficacy of Dihydroartemisinin/Piperaquine and Chloroquine against Plasmodium vivax Malaria, Cambodia

We assessed the efficacy of standard 3-day courses of chloroquine and dihydroartemisinin/piperaquine against Plasmodium vivax malaria. Compared with chloroquine, dihydroartemisinin/piperaquine was faster in clearing asexual P. vivax parasites and blocking human-to-mosquito transmission. This drug combination was also more effective in preventing potential recurrences for >2 months.

Unraveling the Plasmodium vivax sporozoite transcriptional journey from mosquito vector to human host

Malaria parasites transmitted by mosquito bite are remarkably efficient in establishing human infections. The infection process requires roughly 30 minutes and is highly complex as quiescent sporozoites injected with mosquito saliva must be rapidly activated in the skin, migrate through the body, and infect the liver. This process is poorly understood for Plasmodium vivax due to low infectivity in the in vitro models. To study this skin-to-liver-stage of malaria, we used quantitative bioassays coupled with transcriptomics to evaluate parasite changes linked with mammalian microenvironmental factors. Our in vitro phenotyping and RNA-seq analyses revealed key microenvironmental relationships with distinct biological functions. Most notable, preservation of sporozoite quiescence by exposure to insect-like factors coupled with strategic activation limits untimely activation of invasion-associated genes to dramatically increase hepatocyte invasion rates. We also report the first transcriptomic analysis of the P. vivax sporozoite interaction in salivary glands identifying 118 infection-related differentially-regulated Anopheles dirus genes. These results provide important new insights in malaria parasite biology and identify priority targets for antimalarial therapeutic interventions to block P. vivax infection.

A comprehensive model for assessment of liver stage therapies targeting Plasmodium vivax and Plasmodium falciparum

Malaria liver stages represent an ideal therapeutic target with a bottleneck in parasite load and reduced clinical symptoms; however, current in vitro pre-erythrocytic (PE) models for Plasmodium vivax and P. falciparum lack the efficiency necessary for rapid identification and effective evaluation of new vaccines and drugs, especially targeting late liver-stage development and hypnozoites. Herein we report the development of a 384-well plate culture system using commercially available materials, including cryopreserved primary human hepatocytes. Hepatocyte physiology is maintained for at least 30 days and supports development of P. vivax hypnozoites and complete maturation of P. vivax and P. falciparum schizonts. Our multimodal analysis in antimalarial therapeutic research identifies important PE inhibition mechanisms: immune antibodies against sporozoite surface proteins functionally inhibit liver stage development and ion homeostasis is essential for schizont and hypnozoite viability. This model can be implemented in laboratories in disease-endemic areas to accelerate vaccine and drug discovery research.

Plasmodium vivax Biology: Insights Provided by Genomics, Transcriptomics and Proteomics

During the last decade, the vast omics field has revolutionized biological research, especially the genomics, transcriptomics and proteomics branches, as technological tools become available to the field researcher and allow difficult question-driven studies to be addressed. Parasitology has greatly benefited from next generation sequencing (NGS) projects, which have resulted in a broadened comprehension of basic parasite molecular biology, ecology and epidemiology. Malariology is one example where application of this technology has greatly contributed to a better understanding of Plasmodium spp. biology and host-parasite interactions. Among the several parasite species that cause human malaria, the neglected Plasmodium vivax presents great research challenges, as in vitro culturing is not yet feasible and functional assays are heavily limited. Therefore, there are gaps in our P. vivax biology knowledge that affect decisions for control policies aiming to eradicate vivax malaria in the near future. In this review, we provide a snapshot of key discoveries already achieved in P. vivax sequencing projects, focusing on developments, hurdles, and limitations currently faced by the research community, as well as perspectives on future vivax malaria research.

Unexpected high circulation of Plasmodium vivax in asymptomatic children from Kédougou, southeastern Senegal

Background

Malaria in Senegal is due essentially to infections by Plasmodium falciparum and, to a lesser extent to Plasmodium malariae and Plasmodium ovale. By the use of molecular methods, detection of Plasmodium vivax has been recently reported in the region of Kedougou, raising the question of appraisal of its potential prevalence in this setting.

Methods

A retrospective serological study was carried out using 188 samples taken from 2010 to 2011 in a longitudinal school survey during which 48 asymptomatic children (9–11 years) were recruited. Four collections of samples collected during two successive dry and rainy seasons were analysed for antibody responses to P. vivax and P. falciparum. Recombinant P. falciparum and P. vivax MSP1 antigens and total P. falciparum schizont lysate from African 07/03 strain (adapted to culture) were used for ELISA. Nested PCR amplification was used for molecular detection of P. vivax.

Results

A surprising high prevalence of IgG responses against P. vivax MSP1 was evidenced with 53% of positive samples and 58% of the individuals that were found positive to this antigen. There was 77% of responders to P. falciparum outlined by 63% of positive samples. Prevalence of responders did not differ as function of seasons. Levels of antibodies to P. falciparum fluctuated with significant increasing between dry and rainy season (P < 0.05), contrary to responses to P. vivax. There was a significant reciprocal relationship (P < 10⁻³) between antibody responses to the different antigens, but with weak coefficient of correlation (Rho around 0.3) underlining a variable profile at the individual level. Clear molecular signature was found in positive IgG to P. vivax msp1 samples by PCR.

Conclusion

This cross-sectional longitudinal study highlights the unexpected high circulation of P. vivax in this endemic area. Sero-immunology and molecular methods are powerful additive tools to identify endemic sites where relevant control measures have to be settled and monitored.

Determination of multiple-clone infection at allelic dimorphism site of Plasmodium vivax merozoite surface protein-1 in the Republic of Korea by pyrosequencing assay

Allelic diversity leading to multiple gene polymorphisms of vivax malaria parasites has been shown to greatly contribute to antigenic variation and drug resistance, increasing the potential for multiple-clone infections within the host. Therefore, to identify multiple-clone infections and the predominant haplotype of Plasmodium vivax in a South Korean population, P. vivax merozoite surface protein-1 (PvMSP-1) was analyzed by pyrosequencing. Pyrosequencing of 156 vivax malaria-infected samples yielded 97 (62.18%) output pyrograms showing two main types of peak patterns of the dimorphic allele for threonine and alanine (T1476A). Most of the samples evaluated (88.66%) carried multiple-clone infections (wild- and mutant-types), whereas 11.34% of the same population carried only the mutant-type (1476A). In addition, each allele showed a high frequency of guanine (G) base substitution at both the first and third positions (86.07% and 81.13%, respectively) of the nucleotide combinations. Pyrosequencing of the PvMSP-1 42-kDa fragment revealed a heterogeneous parasite population, with the mutant-type dominant compared to the wild-type. Understanding the genetic diversity and multiple-clone infection rates may lead to improvements in vivax malaria prevention and strategic control plans. Further studies are needed to improve the efficacy of the pyrosequencing assay with large sample sizes and additional nucleotide positions.

Proximity to vector breeding site and risk of Plasmodium vivax infection: a prospective cohort study in rural Ethiopia

Background

Despite falling incidence and mortality since the turn of the century, malaria remains an important global health challenge. In the future fight against malaria, greater emphasis will have to be placed on understanding and addressing malaria caused by the Plasmodium vivax parasite. Unfortunately, due to years of neglect and underfunding, there are currently many gaps in knowledge of P. vivax malaria. The aims of the present study were to explore the association between distance to vector breeding site and P. vivax infection in rural Ethiopia, and, secondarily, to test whether this association varies with age.

Methods

A prospective, cohort study of all residents in the Chano Mille Kebele in southern Ethiopia from April 2009 to March 2011 (n = 8121). Weekly household follow up visits included screening for febrile cases (active surveillance). Participants were also asked to contact the local health centre if they experienced subjective fever between visits (passive surveillance). Plasmodium vivax infection was confirmed using microscopy by two independent readers. Information was collected on demographics and household characteristics including GPS-determined distance to vector breeding site. Data was analysed using Cox regression modelling.

Results

Overall the P. vivax infection rate was 12.3/1000 person-years (95% CI 10.5–14.5). Mean household distance to breeding site was 2449 m (range 1646–3717 m). Fully adjusted results showed very strong evidence of an association between proximity to breeding site and P. vivax infection: rate ratio = 3.47 (95% CI 2.15–5.60; P < 0.001) comparing the group closest to the breeding site (distance < 2100 m; n = 1383) to the group furthest away (distance > 2700 m; n = 2460). There was no evidence that age was an effect modifier in the association.

Conclusion

Results showed strong evidence that household proximity to vector breeding site is positively associated with P. vivax infection in rural Ethiopia, and that this association is constant across age groups. The findings might influence how net-distribution and indoor residual spraying campaigns are planned, help guide strategies on water resource development by highlighting potential health effects of man-made dams near human habitats, and add to current educational information given to people living close to breeding sites.

Hemolytic Potential of Tafenoquine in Female Volunteers Heterozygous for Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency (G6PD Mahidol Variant) versus G6PD-Normal Volunteers

Tafenoquine is an 8-aminoquinoline under investigation for the prevention of relapse in Plasmodium vivax malaria. This open-label, dose-escalation study assessed quantitatively the hemolytic risk with tafenoquine in female healthy volunteers heterozygous for the Mahidol^487A glucose-6-phosphate dehydrogenase (G6PD)-deficient variant versus G6PD-normal females, and with reference to primaquine. Six G6PD-heterozygous subjects (G6PD enzyme activity 40-60% of normal) and six G6PD-normal subjects per treatment group received single-dose tafenoquine (100, 200, or 300 mg) or primaquine (15 mg × 14 days). All participants had pretreatment hemoglobin levels ≥ 12.0 g/dL. Tafenoquine dose escalation stopped when hemoglobin decreased by ≥ 2.5 g/dL (or hematocrit decline ≥ 7.5%) versus pretreatment values in ≥ 3/6 subjects. A dose-response was evident in G6PD-heterozygous subjects (N = 15) receiving tafenoquine for the maximum decrease in hemoglobin versus pretreatment values. Hemoglobin declines were similar for tafenoquine 300 mg (-2.65 to -2.95 g/dL [N = 3]) and primaquine (-1.25 to -3.0 g/dL [N = 5]). Two further cohorts of G6PD-heterozygous subjects with G6PD enzyme levels 61-80% (N = 2) and > 80% (N = 5) of the site median normal received tafenoquine 200 mg; hemolysis was less pronounced at higher G6PD enzyme activities. Tafenoquine hemolytic potential was dose dependent, and hemolysis was greater in G6PD-heterozygous females with lower G6PD enzyme activity levels. Single-dose tafenoquine 300 mg did not appear to increase the severity of hemolysis versus primaquine 15 mg × 14 days.

Further evaluation of the NWF filter for the purification of Plasmodium vivax-infected erythrocytes

BACKGROUND

Isolation of Plasmodium-infected red blood cells (iRBCs) from clinical blood samples is often required for experiments, such as ex vivo drug assays, in vitro invasion assays and genome sequencing. Current methods for removing white blood cells (WBCs) from malaria-infected blood are time-consuming or costly. A prototype non-woven fabric (NWF) filter was developed for the purification of iRBCs, which showed great efficiency for removing WBCs in a pilot study. Previous work was performed with prototype filters optimized for processing 5-10 mL of blood. With the commercialization of the filters, this study aims to evaluate the efficiency and suitability of the commercial NWF filter for the purification of Plasmodium vivax-infected RBCs in smaller volumes of blood and to compare its performance with that of Plasmodipur^® filters.

METHODS

Forty-three clinical P. vivax blood samples taken from symptomatic patients attending malaria clinics at the China-Myanmar border were processed using the NWF filters in a nearby field laboratory. The numbers of WBCs and iRBCs and morphology of P. vivax parasites in the blood samples before and after NWF filtration were compared. The viability of P. vivax parasites after filtration from 27 blood samples was examined by in vitro short-term culture. In addition, the effectiveness of the NWF filter for removing WBCs was compared with that of the Plasmodipur^® filter in six P. vivax blood samples.

RESULTS

Filtration of 1-2 mL of P. vivax-infected blood with the NWF filter removed 99.68% WBCs. The densities of total iRBCs, ring and trophozoite stages before and after filtration were not significantly different (P > 0.05). However, the recovery rates of schizont- and gametocyte-infected RBCs, which were minor parasite stages in the clinical samples, were relatively low. After filtration, the P. vivax parasites did not show apparent morphological changes. Culture of 27 P. vivax-infected blood samples after filtration showed that parasites successfully matured into the schizont stage. The WBC removal rates and iRBC recovery rates were not significantly different between the NWF and Plasmodipur^® filters (P > 0.05).

CONCLUSIONS

When tested with 1-2 mL of P. vivax-infected blood, the NWF filter could effectively remove WBCs and the recovery rates for ring- and trophozoite-iRBCs were high. P. vivax parasites after filtration could be successfully cultured in vitro to reach maturity. The performance of the NWF and Plasmodipur^® filters for removing WBCs and recovering iRBCs was comparable.

Higher Complexity of Infection and Genetic Diversity of Plasmodium vivax Than Plasmodium falciparum Across All Malaria Transmission Zones of Papua New Guinea

Plasmodium falciparum and Plasmodium vivax have varying transmission dynamics that are informed by molecular epidemiology. This study aimed to determine the complexity of infection and genetic diversity of P. vivax and P. falciparum throughout Papua New Guinea (PNG) to evaluate transmission dynamics across the country. In 2008-2009, a nationwide malaria indicator survey collected 8,936 samples from all 16 endemic provinces of PNG. Of these, 892 positive P. vivax samples were genotyped at PvMS16 and PvmspF3, and 758 positive P. falciparum samples were genotyped at Pfmsp2. The data were analyzed for multiplicity of infection (MOI) and genetic diversity. Overall, P. vivax had higher polyclonality (71%) and mean MOI (2.32) than P. falciparum (20%, 1.39). These measures were significantly associated with prevalence for P. falciparum but not for P. vivax. The genetic diversity of P. vivax (PvMS16: expected heterozygosity = 0.95, 0.85-0.98; PvMsp1F3: 0.78, 0.66-0.89) was higher and less variable than that of P. falciparum (Pfmsp2: 0.89, 0.65-0.97). Significant associations of MOI with allelic richness (rho = 0.69, P = 0.009) and expected heterozygosity (rho = 0.87, P < 0.001) were observed for P. falciparum. Conversely, genetic diversity was not correlated with polyclonality nor mean MOI for P. vivax. The results demonstrate higher complexity of infection and genetic diversity of P. vivax across the country. Although P. falciparum shows a strong association of these parameters with prevalence, a lack of association was observed for P. vivax and is consistent with higher potential for outcrossing of this species.

Geographical distributions of African malaria vector sibling species and evidence for insecticide resistance

Background

Many of the mosquito species responsible for malaria transmission belong to a sibling complex; a taxonomic group of morphologically identical, closely related species. Sibling species often differ in several important factors that have the potential to impact malaria control, including their geographical distribution, resistance to insecticides, biting and resting locations, and host preference. The aim of this study was to define the geographical distributions of dominant malaria vector sibling species in Africa so these distributions can be coupled with data on key factors such as insecticide resistance to aid more focussed, species-selective vector control.

Results

Within the Anopheles gambiae species complex and the Anopheles funestus subgroup, predicted geographical distributions for Anopheles coluzzii, An. gambiae (as now defined) and An. funestus (distinct from the subgroup) have been produced for the first time. Improved predicted geographical distributions for Anopheles arabiensis, Anopheles melas and Anopheles merus have been generated based on records that were confirmed using molecular identification methods and a model that addresses issues of sampling bias and past changes to the environment. The data available for insecticide resistance has been evaluated and differences between sibling species are apparent although further analysis is required to elucidate trends in resistance.

Conclusions

Sibling species display important variability in their geographical distributions and the most important malaria vector sibling species in Africa have been mapped here for the first time. This will allow geographical occurrence data to be coupled with species-specific data on important factors for vector control including insecticide resistance. Species-specific data on insecticide resistance is available for the most important malaria vectors in Africa, namely An. arabiensis, An. coluzzii, An. gambiae and An. funestus. Future work to combine these data with the geographical distributions mapped here will allow more focussed and resource-efficient vector control and provide information to greatly improve and inform existing malaria transmission models.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-017-1734-y) contains supplementary material, which is available to authorized users.

Global Epidemiology of Plasmodium vivax

Plasmodium vivax is the most widespread human malaria, putting 2.5 billion people at risk of infection. Its unique biological and epidemiological characteristics pose challenges to control strategies that have been principally targeted against Plasmodium falciparum Unlike P. falciparum, P. vivax infections have typically low blood-stage parasitemia with gametocytes emerging before illness manifests, and dormant liver stages causing relapses. These traits affect both its geographic distribution and transmission patterns. Asymptomatic infections, high-risk groups, and resulting case burdens are described in this review. Despite relatively low prevalence measurements and parasitemia levels, along with high proportions of asymptomatic cases, this parasite is not benign. Plasmodium vivax can be associated with severe and even fatal illness. Spreading resistance to chloroquine against the acute attack, and the operational inadequacy of primaquine against the multiple attacks of relapse, exacerbates the risk of poor outcomes among the tens of millions suffering from infection each year. Without strategies accounting for these P. vivax-specific characteristics, progress toward elimination of endemic malaria transmission will be substantially impeded.

Costs and Cost-Effectiveness of Plasmodium vivax Control

The continued success of efforts to reduce the global malaria burden will require sustained funding for interventions specifically targeting Plasmodium vivax The optimal use of limited financial resources necessitates cost and cost-effectiveness analyses of strategies for diagnosing and treating P. vivax and vector control tools. Herein, we review the existing published evidence on the costs and cost-effectiveness of interventions for controlling P. vivax, identifying nine studies focused on diagnosis and treatment and seven studies focused on vector control. Although many of the results from the much more extensive P. falciparum literature can be applied to P. vivax, it is not always possible to extrapolate results from P. falciparum-specific cost-effectiveness analyses. Notably, there is a need for additional studies to evaluate the potential cost-effectiveness of radical cure with primaquine for the prevention of P. vivax relapses with glucose-6-phosphate dehydrogenase testing.

Management of relapsing Plasmodium vivax malaria

Introduction: Relapses are important contributors to illness and morbidity in Plasmodium vivax and P. ovale infections. Relapse prevention (radical cure) with primaquine is required for optimal management, control and ultimately elimination of Plasmodium vivax malaria. A review was conducted with publications in English, French, Portuguese and Spanish using the search terms ‘P. vivax’ and ‘relapse’.

Areas covered: Hypnozoites causing relapses may be activated weeks or months after initial infection. Incidence and temporal patterns of relapse varies geographically. Relapses derive from parasites either genetically similar or different from the primary infection indicating that some derive from previous infections. Malaria illness itself may activate relapse. Primaquine is the only widely available treatment for radical cure. However, it is often not given because of uncertainty over the risks of primaquine induced haemolysis when G6PD deficiency testing is unavailable. Recommended dosing of primaquine for radical cure in East Asia and Oceania is 0.5 mg base/kg/day and elsewhere is 0.25 mg base/kg/day. Alternative treatments are under investigation.

Expert commentary: Geographic heterogeneity in relapse patterns and chloroquine susceptibility of P. vivax, and G6PD deficiency epidemiology mean that radical treatment should be given much more than it is today. G6PD testing should be made widely available so primaquine can be given more safely.

Genomic analysis of local variation and recent evolution in Plasmodium vivax

The widespread distribution and relapsing nature of Plasmodium vivax infection present major challenges for the elimination of malaria. To characterize the genetic diversity of this parasite in individual infections and across the population, we performed deep genome sequencing of >200 clinical samples collected across the Asia-Pacific region and analyzed data on >300,000 SNPs and nine regions of the genome with large copy number variations. Individual infections showed complex patterns of genetic structure, with variation not only in the number of dominant clones but also in their level of relatedness and inbreeding. At the population level, we observed strong signals of recent evolutionary selection both in known drug resistance genes and at new loci, and these varied markedly between geographical locations. These findings demonstrate a dynamic landscape of local evolutionary adaptation in the parasite population and provide a foundation for genomic surveillance to guide effective strategies for control and elimination of P. vivax.