Human infections and detection of Plasmodium knowlesi

Babesia duncani, a Model Organism for Investigating Intraerythrocytic Parasitism and Novel Antiparasitic Therapeutic Strategies

Pathogens such as Plasmodium, Babesia, and Theileria invade and multiply within host red blood cells, leading to the pathological consequences of malaria, babesiosis, and theileriosis. Establishing continuous in vitro culture systems and suitable animal models is crucial for studying these pathogens. This review spotlights the Babesia duncani in culture-in mouse (ICIM) model as a promising resource for advancing research on the biology, pathogenicity, and virulence of intraerythrocytic parasites. The model offers practical benefits, encompassing well-defined culture conditions, ease of manipulation, and a well-annotated genome. Moreover, B. duncani serves as a surrogate system for drug discovery, facilitating the evaluation of new antiparasitic drugs in vitro and in animals, elucidating their modes of action, and uncovering potential resistance mechanisms. The B. duncani ICIM model thus emerges as a multifaceted tool with profound implications, promising advancements in our understanding of parasitic biology and shaping the development of future therapies.

Real-time fluorescence loop-mediated isothermal amplification assays for detection of zoonotic malaria Plasmodium parasites

BACKGROUND

Natural human infections by Plasmodium cynomolgi and P. inui have been reported recently and gain the substantial attention from Southeast Asian countries. Zoonotic transmission of non-human malaria parasites to humans from macaque monkeys occurred through the bites of the infected mosquitoes. The objective of this study is to establish real-time fluorescence loop-mediated isothermal amplification (LAMP) assays for the detection of zoonotic malaria parasites by combining real-time fluorescent technology with the isothermal amplification technique.

METHODS

By using 18S rRNA as the target gene, the primers for P. cynomolgi, P. coatneyi and P. inui were newly designed in the present study. Four novel real-time fluorescence LAMP assays were developed for the detection of P. cynomolgi, P. coatneyi, P. inui and P. knowlesi. The entire amplification process was completed in 60 min, with the assays performed at 65 °C. By using SYTO-9 as the nucleic acid intercalating dye, the reaction was monitored via real-time fluorescence signal.

RESULTS

There was no observed cross-reactivity among the primers from different species. All 70 field-collected monkey samples were successfully amplified by real-time fluorescence LAMP assays. The detection limit for P. cynomolgi, P. coatneyi and P. knowlesi was 5 × 109 copies/µL. Meanwhile, the detection limit of P. inui was 5 × 1010 copies/µL.

CONCLUSION

This is the first report of the detection of four zoonotic malaria parasites by real-time fluorescence LAMP approaches. It is an effective, rapid and simple-to-use technique. This presented platform exhibits considerable potential as an alternative detection for zoonotic malaria parasites.

Proteomes of plasmodium knowlesi early and late ring-stage parasites and infected host erythrocytes

The emerging malaria parasite Plasmodium knowlesi threatens the goal of worldwide malaria elimination due to its zoonotic spread in Southeast Asia. After brief ex-vivo culture we used 2D LC/MS/MS to examine the early and late ring stages of infected Macaca mulatta red blood cells harboring P. knowlesi. The M. mulatta clathrin heavy chain and T-cell and macrophage inhibitor ERMAP were overexpressed in the early ring stage; glutaredoxin 3 was overexpressed in the late ring stage; GO term differential enrichments included response to oxidative stress and the cortical cytoskeleton in the early ring stage. P. knowlesi clathrin heavy chain and 60S acidic ribosomal protein P2 were overexpressed in the late ring stage; GO term differential enrichments included vacuoles in the early ring stage, ribosomes and translation in the late ring stage, and Golgi- and COPI-coated vesicles, proteasomes, nucleosomes, vacuoles, ion-, peptide-, protein-, nucleocytoplasmic- and RNA-transport, antioxidant activity and glycolysis in both stages. SIGNIFICANCE: Due to its zoonotic spread, cases of the emerging human pathogen Plasmodium knowlesi in southeast Asia, and particularly in Malaysia, threaten regional and worldwide goals for malaria elimination. Infection by this parasite can be fatal to humans, and can be associated with significant morbidity. Due to zoonotic transmission from large macaque reservoirs that are untreatable by drugs, and outdoor biting mosquito vectors that negate use of preventive measures such as bed nets, its containment remains a challenge. Its biology remains incompletely understood. Thus we examine the expressed proteome of the early and late ex-vivo cultured ring stages, the first intraerythrocyte developmental stages after infection of host rhesus macaque erythrocytes. We used GO term enrichment strategies and differential protein expression to compare early and late ring stages. The early ring stage is characterized by the enrichment of P. knowlesi vacuoles, and overexpression of the M. mulatta clathrin heavy chain, important for clathrin-coated pits and vesicles, and clathrin-mediated endocytosis. The M. mulatta protein ERMAP was also overexpressed in the early ring stage, suggesting a potential role in early ring stage inhibition of T-cells and macrophages responding to P. knowlesi infection of reticulocytes. This could allow expansion of the host P. knowlesi cellular niche, allowing parasite adaptation to invasion of a wider age range of RBCs than the preferred young RBCs or reticulocytes, resulting in proliferation and increased pathogenesis in infected humans. Other GO terms differentially enriched in the early ring stage include the M. mulatta cortical cytoskeleton and response to oxidative stress. The late ring stage is characterized by overexpression of the P. knowlesi clathrin heavy chain. Combined with late ring stage GO term enrichment of Golgi-associated and coated vesicles, and enrichment of COPI-coated vesicles in both stages, this suggests the importance to P. knowlesi biology of clathrin-mediated endocytosis. P. knowlesi ribosomes and translation were also differentially enriched in the late ring stage. With expression of a variety of heat shock proteins, these results suggest production of folded parasite proteins is increasing by the late ring stage. M. mulatta endocytosis was differentially enriched in the late ring stage, as were clathrin-coated vesicles and endocytic vesicles. This suggests that M. mulatta clathrin-based endocytosis, perhaps in infected reticulocytes rather than mature RBC, may be an important process in the late ring stage. Additional ring stage biology from enriched GO terms includes M. mulatta proteasomes, protein folding and the chaperonin-containing T complex, actin and cortical actin cytoskeletons. P knowlesi biology also includes proteasomes, as well as nucleosomes, antioxidant activity, a variety of transport processes, glycolysis, vacuoles and protein folding. Mature RBCs have lost internal organelles, suggesting infection here may involve immature reticulocytes still retaining organelles. P. knowlesi parasite proteasomes and translational machinery may be ring stage drug targets for known selective inhibitors of these processes in other Plasmodium species. To our knowledge this is the first examination of more than one timepoint within the ring stage. Our results expand knowledge of both host and parasite proteins, pathways and organelles underlying P. knowlesi ring stage biology.

Prevalence and genetic diversity of simian malaria in wild macaque populations across Thailand: Implications for human health

Over the past year, P. falciparum infections have declined in Thailand, yet nonhuman primate malaria infections have correspondingly increased, including Plasmodium knowlesi and P. cynomolgi. Nevertheless, little is known about simian malaria in its natural macaque hosts, Macaca mulatta and Macaca fascicularis. This study aims to address several research questions, including the prevalence and distribution of simian malaria in these two Thai wild macaque species, variations in infection between different macaque species and between M. fascicularis subspecies, and the genetic composition of these pathogens. Blood samples were collected from 82 M. mulatta and 690 M. fascicularis across 15 locations in Thailand, as well as two locations in Vietnam and Myanmar. We employed quantitative real-time PCR targeting the Plasmodium genus-specific 18S ribosomal RNA (rRNA) gene to detect malaria infection, with a limit of detection set at 1,215.98 parasites per mL. We genotyped eight microsatellite markers, and the P. cynomolgi dihydrofolate reductase gene (DHFR) was sequenced (N = 29). In total, 100 of 772 samples (13 %) tested positive for malaria, including 45 (13 %) for P. cynomolgi, 37 (13 %) for P. inui, 16 (5 %) for P. coatneyi, and 2 (0.25 %) for Hepatocystis sp. in Saraburi, central and Ranong, southern Thailand. Notably, simian malaria infection was observed exclusively in M. fascicularis and not in M. mulatta (P = 0.0002). Particularly, P. cynomolgi was detected in 21.7 % (45/207) of M. f. fascicularis living in Wat Tham Phrapothisat, Saraburi Province. The infection with simian malaria was statistically different between M. fascicularis and M. mulatta (P = 0.0002) but not within M. fascicularis subspecies (P = 0.78). A haplotype network analysis revealed that P. cynomolgi shares a lineage with reference strains obtained from macaques. No mutation in the predicted binding pocket of PcyDHFR to pyrimethamine was observed. This study reveals a significant prevalence of simian malaria infection in M. fascicularis. The clonal genotypes of P. cynomolgi suggest in-reservoir breeding. These findings raise concerns about the potential spread of nonhuman primate malaria to humans and underscore the need for preventive measures.

Analysis of the interaction of antimalarial agents with Plasmodium falciparum glutathione reductase through molecular mechanical calculations

CONTEXT

Malaria remains a significant global health challenge with emerging resistance to current treatments. Plasmodium falciparum glutathione reductase (PfGR) plays a critical role in the defense mechanisms of malaria parasites against oxidative stress. In this study, we investigate the potential of targeting PfGR with conventional antimalarials and dual drugs combining aminoquinoline derivatives with GR inhibitors, which reveal promising interactions between PfGR and studied drugs. The naphthoquinone Atovaquone demonstrated particularly high affinity and potential dual-mode binding with the enzyme active site and cavity. Furthermore, dual drugs exhibit enhanced binding affinity, suggesting their efficacy in inhibiting PfGR, where the aliphatic ester bond (linker) is essential for effective binding with the enzyme's active site. Overall, this research provides important insights into the interactions between antimalarial agents and PfGR and encourages further exploration of its role in the mechanisms of action of antimalarials, including dual drugs, to enhance antiparasitic efficacy.

METHODS

The drugs were tested as PfGR potential inhibitors via molecular docking on AutoDock 4, which was performed based on the preoptimized structures in HF/3-21G-PCM level of theory on ORCA 5. Drug-receptor systems with the most promising binding affinities were then studied with a molecular dynamic's simulation on AMBER 16. The molecular dynamics simulations were performed with a 100 ns NPT ensemble employing GAFF2 forcefield in the temperature of 310 K, integration time step of 2 fs, and non-bond cutoff distance of 6.0 Å.

Cloning, Expression, Purification, and Characterization of Lactate Dehydrogenase from Plasmodium knowlesi: A Zoonotic Malaria Parasite

Plasmodium knowlesi is the only Plasmodium that causes zoonotic disease among the Plasmodium that cause infection in humans. It is fatal due to its short asexual growth cycle within 24 h. Lactate dehydrogenase (LDH), an enzyme that catalyzes the final step of glycolysis, is a biomarker for diagnosing infection by Plasmodium spp. parasite. Therefore, this study aimed to efficiently produce the soluble form of P. knowlesi LDH (PkLDH) using a bacterial expression system for studying malaria caused by P. knowlesi. Recombinant pET-21a(+)-PkLDH plasmid was constructed by inserting the PkLDH gene into a pET-21a(+) expression vector. Subsequently, the recombinant plasmid was inserted into the protein-expressing Escherichia coli Rosetta(DE3) strain, and the optimal conditions for overexpression of the PkLDH protein were established using this strain. We obtained a yield of 52.0 mg/L PkLDH from the Rosetta(DE3) strain and confirmed an activity of 483.9 U/mg through experiments. This methodology for high-efficiency PkLDH production can be utilized for the development of diagnostic methods and drug candidates for distinguishing malaria caused by P. knowlesi.

Exploring the Recent Pioneering Developments of Small Molecules in Antimalarial Drug Armamentarium: A Chemistry Prospective Appraisal

Malaria is a very destructive and lethal parasitic disease that causes significant mortality worldwide, resulting in the loss of millions of lives annually. It is an infectious disease transmitted by mosquitoes, which is caused by different species of the parasite protozoan belonging to the genus Plasmodium. The uncontrolled intake of antimalarial drugs often employed in clinical settings has resulted in the emergence of numerous strains of plasmodium that are resistant to these drugs, including multidrug-resistant strains. This resistance significantly diminishes the effectiveness of many primary drugs used in the treatment of malaria. Hence, there is an urgent need for developing unique classes of antimalarial drugs that function with distinct mechanisms of action. In this context, the design and development of hybrid compounds that combine pharmacophoric properties from different lead molecules into a single unit gives a unique perspective towards further development of malaria drugs in the next generation. In recent years, the field of medicinal chemistry has made significant efforts resulting in the discovery and synthesis of numerous small novel compounds that exhibit potent antimalarial properties, while also demonstrating reduced toxicity and desirable efficacy. In light of this, we have reviewed the progress of hybrid antimalarial agents from 2021 up to the present. This manuscript presents a comprehensive overview of the latest advancements in the medicinal chemistry pertaining to small molecules, with a specific focus on their potential as antimalarial agents. As possible antimalarial drugs that might target both the dual stage and multi-stage stages of the parasite life cycle, these small hybrid molecules have been studied. This review explores a variety of physiologically active compounds that have been described in the literature in order to lay a strong foundation for the logical design and eventual identification of antimalarial drugs based on lead frameworks.

Cross-reactivity of rPvs48/45, a recombinant Plasmodium vivax protein, with sera from Plasmodium falciparum endemic areas of Africa

Background

Ps48/45, a Plasmodium gametocyte surface protein, is a promising candidate for malaria transmission-blocking (TB) vaccine. Due to its relevance for a multispecies vaccine, we explored the cross-reactivity and TB activity of a recombinant P. vivax Ps48/45 protein (rPvs48/45) with sera from P. falciparum-exposed African donors.

Methods

rPvs48/45 was produced in Chinese hamster ovary cell lines and tested by ELISA for its cross-reactivity with sera from Burkina Faso, Tanzania, Mali, and Nigeria - In addition, BALB/c mice were immunized with the rPvs48/45 protein formulated in Montanide ISA-51 and inoculated with a crude extract of P. falciparum NF-54 gametocytes to evaluate the parasite-boosting effect on rPvs48/45 antibody titers. Specific anti-rPvs48/45 IgG purified from African sera was used to evaluate the ex vivo TB activity on P. falciparum, using standard mosquito membrane feeding assays (SMFA).

Results

rPvs48/45 protein showed cross-reactivity with sera of individuals from all four African countries, in proportions ranging from 94% (Tanzania) to 40% (Nigeria). Also, the level of cross-reactive antibodies varied significantly between countries (p<0.0001), with a higher antibody level in Mali and the lowest in Nigeria. In addition, antibody levels were higher in adults (≥ 17 years) than young children (≤ 5 years) in both Mali and Tanzania, with a higher proportion of responders in adults (90%) than in children (61%) (p<0.0001) in Mali, where male (75%) and female (80%) displayed similar antibody responses. Furthermore, immunization of mice with P. falciparum gametocytes boosted anti-Pvs48/45 antibody responses, recognizing P. falciparum gametocytes in indirect immunofluorescence antibody test. Notably, rPvs48/45 affinity-purified African IgG exhibited a TB activity of 61% against P. falciparum in SMFA.

Conclusion

African sera (exposed only to P. falciparum) cross-recognized the rPvs48/45 protein. This, together with the functional activity of IgG, warrants further studies for the potential development of a P. vivax and P. falciparum cross-protective TB vaccine.

Preliminary insight on diarylpentanoids as potential antimalarials: In silico, in vitro pLDH and in vivo zebrafish toxicity assessment

Malaria remains a major public health problem worldwide, including in Southeast Asia. Chemotherapeutic agents such as chloroquine (CQ) are effective, but problems with drug resistance and toxicity have necessitated a continuous search for new effective antimalarial agents. Here we report on a virtual screening of ∼300 diarylpentanoids and derivatives, in search of potential Plasmodium falciparum lactate dehydrogenase (PfLDH) inhibitors with acceptable drug-like properties. Several molecules with binding affinities comparable to CQ were chosen for in vitro validation of antimalarial efficacy. Among them, MS33A, MS33C and MS34C are the most promising against CQ-sensitive (3D7) with EC50 values of 1.6, 2.5 and 3.1 μM, respectively. Meanwhile, MS87 (EC50 of 1.85 μM) shown the most active against the CQ-resistant Gombak A strain, and MS33A and MS33C the most effective P. knowlesi inhibitors (EC50 of 3.6 and 5.1 μM, respectively). The in vitro cytotoxicity of selected diarylpentanoids (MS33A, MS33C, MS34C and MS87) was tested on Vero mammalian cells to evaluate parasite selectivity (SI), showing moderate to low cytotoxicity (CC50 > 82 μM). In addition, MS87 exhibited a high SI and the lowest resistance index (RI), suggesting that MS87 may exert effective parasite inhibition with low resistance potential in the CQ-resistant P. falciparum strain. Furthermore, the in vivo toxicity of the molecules on early embryonic development, the cardiovascular system, heart rate, motor activity and apoptosis were assessed in a zebrafish animal model. The overall results indicate the preliminary potential of diarylpentanoids, which need further investigation for their development as new antimalarial agents.

The One Health Concept

The concept of One Health has been developed as the appreciation that human health is intricately connected to those of other animals and the environment that they inhabit. In recent years, the COVID-19 pandemic and noticeable effects of climate change have encouraged national and international cooperation to apply One Health strategies to address key issues of health and welfare. The United Nations (UN) Sustainable Development Goals have established targets for health and wellbeing, clean water and sanitation, climate action, as well as sustainability in marine and terrestrial ecosystems. The One Health Quadripartite comprises the World Health Organization (WHO), the World Organization for Animal Health (WOAH-formerly OIE), the United Nations Food and Agriculture Organization (FAO) and the United Nations Environment Programme (UNEP). There are six areas of focus which are Laboratory services, Control of zoonotic diseases, Neglected tropical diseases, Antimicrobial resistance, Food safety and Environmental health. This article discusses the concept of One Health by considering examples of infectious diseases and environmental issues under each of those six headings. Biomedical Scientists, Clinical Scientists and their colleagues working in diagnostic and research laboratories have a key role to play in applying the One Health approach to key areas of healthcare in the 21st Century.

Updating estimates of Plasmodium knowlesi malaria risk in response to changing land use patterns across Southeast Asia

BACKGROUND

Plasmodium knowlesi is a zoonotic parasite that causes malaria in humans. The pathogen has a natural host reservoir in certain macaque species and is transmitted to humans via mosquitoes of the Anopheles Leucosphyrus Group. The risk of human P. knowlesi infection varies across Southeast Asia and is dependent upon environmental factors. Understanding this geographic variation in risk is important both for enabling appropriate diagnosis and treatment of the disease and for improving the planning and evaluation of malaria elimination. However, the data available on P. knowlesi occurrence are biased towards regions with greater surveillance and sampling effort. Predicting the spatial variation in risk of P. knowlesi malaria requires methods that can both incorporate environmental risk factors and account for spatial bias in detection.

METHODS & RESULTS

We extend and apply an environmental niche modelling framework as implemented by a previous mapping study of P. knowlesi transmission risk which included data up to 2015. We reviewed the literature from October 2015 through to March 2020 and identified 264 new records of P. knowlesi, with a total of 524 occurrences included in the current study following consolidation with the 2015 study. The modelling framework used in the 2015 study was extended, with changes including the addition of new covariates to capture the effect of deforestation and urbanisation on P. knowlesi transmission.

DISCUSSION

Our map of P. knowlesi relative transmission suitability estimates that the risk posed by the pathogen is highest in Malaysia and Indonesia, with localised areas of high risk also predicted in the Greater Mekong Subregion, The Philippines and Northeast India. These results highlight areas of priority for P. knowlesi surveillance and prospective sampling to address the challenge the disease poses to malaria elimination planning.

Merozoite surface protein 1 paralog is involved in the human erythrocyte invasion of a zoonotic malaria, Plasmodium knowlesi

The zoonotic malaria parasite Plasmodium knowlesi is an important public health concern in Southeast Asia. Invasion of host erythrocytes is essential for parasite growth, and thus, understanding the repertoire of parasite proteins that enable this process is vital for identifying vaccine candidates and how some species are able to cause zoonotic infection. Merozoite surface protein 1 (MSP1) is found in all malaria parasite species and is perhaps the most well-studied as a potential vaccine candidate. While MSP1 is encoded by a single gene in P. falciparum, all other human infective species (P. vivax, P. knowlesi, P. ovale, and P. malariae) additionally encode a divergent paralogue known as MSP1P, and little is known about its role or potential functional redundancy with MSP1. We, therefore, studied the function of P. knowlesi merozoite surface protein 1 paralog (PkMSP1P), using both recombinant protein and CRISPR-Cas9 genome editing. The recombinant 19-kDa C-terminus of PkMSP1P (PkMSP1P-19) was shown to bind specifically to human reticulocytes. However, immunoblotting data suggested that PkMSP1P-19-induced antibodies can recognize PkMSP1-19 and vice versa, confounding our ability to separate the properties of these two proteins. Targeted disruption of the pkmsp1p gene profoundly impacts parasite growth, demonstrating for the first time that PkMSP1P is important in in vitro growth of P. knowlesi and likely plays a distinct role from PkMSP1. Importantly, the MSP1P KO also enabled functional characterization of the PkMSP1P-19 antibodies, revealing clear immune cross-reactivity between the two paralogues, highlighting the vital importance of genetic studies in contextualizing recombinant protein studies.

Ultrastructural alterations due to sodium metavanadate treatment in the blood stages of Plasmodium yoelii yoelii

Malaria is a potentially mortal disease caused by parasites of the genus Plasmodium spp. It has a wide distribution in the world and unfortunately there are several factors that make its control difficult; among which the development of pharmacological resistance to the different drugs used to treat this disease stands out, which makes it necessary to design new compounds that have an antimalarial effect. Previous studies have shown that vanadium has a broad antiparasitic spectrum and is also safe for the host, so the objective of this research was to evaluate the antimalarial potential of sodium metavanadate (SM) and to analyze the ultrastructural changes in parasites exposed. The method consisted of inoculating CD-1 male mice with Plasmodium yoelii yoelii and administering a 10 mg/kg/day dose of SM orally for 4 days. On the fifth day, whole blood samples were obtained, processed for ultrastructural analysis, and the changes in the different parasite stages were compared against the control. Our results showed that SM decreased parasitemia compared to the group that did not receive treatment and modified the ultrastructure in all parasitic stages because it damaged the membranes, causing alterations mainly in the nucleus and in the mitochondria as well as the loss of cellular organization, which could affect the integrity of these parasites and decrease its viability.

Current development of 1,2,3-triazole derived potential antimalarial scaffolds: Structure- activity relationship (SAR) and bioactive compounds

Malaria is among one of the most devastating and deadliest parasitic disease in the world claiming millions of lives every year around the globe. It is a mosquito-borne infectious disease caused by various species of the parasitic protozoan of the genus Plasmodium. The indiscriminate exploitation of the clinically used antimalarial drugs led to the development of various drug-resistant and multidrug-resistant strains of plasmodium which severely reduces the therapeutic effectiveness of most frontline medicines. Therefore, there is urgent need to develop novel structural classes of antimalarial agents acting with unique mechanism of action(s). In this context, design and development of hybrid molecules containing pharmacophoric features of different lead molecules in a single entity represents a unique strategy for the development of next-generation antimalarial drugs. Research efforts by the scientific community over the past few years has led to the identification and development of several heterocyclic small molecules as antimalarial agents with high potency, less toxicity and desired efficacy. Triazole derivatives have become indispensable units in the medicinal chemistry due to their diverse spectrum of biological profiles and many triazole based hybrids and conjugates have demonstrated potential in vitro and in vivo antimalarial activities. The manuscript compiled recent developments in the medicinal chemistry of triazole based small heterocyclic molecules as antimalarial agents and discusses various reported biologically active compounds to lay the groundwork for the rationale design and discovery of triazole based antimalarial compounds. The article emphasised on biological activities, structure activity relationships, and molecular docking studies of various triazole based hybrids with heterocycles such as quinoline, artemisinins, naphthyl, naphthoquinone, etc. as potential antimalarial agents which could act on the dual stage and multi stage of the parasitic life cycle.

Plasmodium knowlesi (Pk) Malaria: A Review & Proposal of Therapeutically Rational Exchange (T-REX) of Pk-Resistant Red Blood Cells

Plasmodium knowlesi (Pk) causes zoonotic malaria and is known as the "fifth human malaria parasite". Pk malaria is an emerging threat because infections are increasing and can be fatal. While most infections are in Southeast Asia (SEA), especially Malaysia, travelers frequently visit this region and can present with Pk malaria around the world. So, clinicians need to know (1) patients who present with fever after recent travel to SEA might be infected with Pk and (2) Pk is often misdiagnosed as P. malariae (which typically causes less severe malaria). Here we review the history, pathophysiology, clinical features, diagnosis, and treatment of Pk malaria. Severe disease is most common in adults. Signs and symptoms can include fever, abdominal pain, jaundice, acute kidney injury, acute respiratory distress syndrome, hyponatremia, hyperparasitemia, and thrombocytopenia. Dengue is one of the diseases to be considered in the differential. Regarding pathophysiologic mechanisms, when Pk parasites invade mature red blood cells (RBCs, i.e., normocytes) and reticulocytes, changes in the red blood cell (RBC) surface can result in life-threatening cytoadherence, sequestration, and reduced RBC deformability. Since molecular mechanisms involving the erythrocytic stage are responsible for onset of severe disease and lethal outcomes, it is biologically plausible that manual exchange transfusion (ET) or automated RBC exchange (RBCX) could be highly beneficial by replacing "sticky" parasitized RBCs with uninfected, deformable, healthy donor RBCs. Here we suggest use of special Pk-resistant donor RBCs to optimize adjunctive manual ET/RBCX for malaria. "Therapeutically-rational exchange transfusion" (T-REX) is proposed in which Pk-resistant RBCs are transfused (instead of disease-promoting RBCs). Because expression of the Duffy antigen on the surface of human RBCs is essential for parasite invasion, T-REX of Duffy-negative RBCs-also known as Fy(a-b-) RBCs-could replace the majority of the patient's circulating normocytes with Pk invasion-resistant RBCs (in a single procedure lasting about 2 h). When sequestered or non-sequestered iRBCs rupture-in a 24 h Pk asexual life cycle-the released merozoites cannot invade Fy(a-b-) RBCs. When Fy(a-b-) RBC units are scarce (e.g., in Malaysia), clinicians can consider the risks and benefits of transfusing plausibly Pk-resistant RBCs, such as glucose-6-phosphate dehydrogenase deficient (G6PDd) RBCs and Southeast Asian ovalocytes (SAO). Patients typically require a very short recovery time (<1 h) after the procedure. Fy(a-b-) RBCs should have a normal lifespan, while SAO and G6PDd RBCs may have mildly reduced half-lives. Because SAO and G6PDd RBCs come from screened blood donors who are healthy and not anemic, these RBCs have a low-risk for hemolysis and do not need to be removed after the patient recovers from malaria. T-REX could be especially useful if (1) antimalarial medications are not readily available, (2) patients are likely to progress to severe disease, or (3) drug-resistant strains emerge. In conclusion, T-REX is a proposed optimization of manual ET/RBCX that has not yet been utilized but can be considered by physicians to treat Pk malaria patients.

In Silico and In Vitro Potential of FDA-Approved Drugs for Antimalarial Drug Repurposing against Plasmodium Serine Hydroxymethyltransferases

Malaria has spread in many countries, with a 12% increase in deaths after the coronavirus disease 2019 pandemic. Malaria is one of the most concerning diseases in the Greater Mekong subregion, showing increased drug-resistant rates. Serine hydroxymethyltransferase (SHMT), a key enzyme in the deoxythymidylate synthesis pathway, has been identified as a promising antimalarial drug target due to its conserved folate binding pocket. This study used a molecular docking approach to screen 2509 US Food and Drug Administration (FDA)-approved drugs against seven Plasmodium SHMT structures. Eight compounds had significantly lower binding energies than the known SHMT inhibitors pyrazolopyran(+)-86, tetrahydrofolate, and antimalarial drugs, ranging from 4 to 10 kcal/mol. Inhibition assays testing the eight compounds against Plasmodium falciparum SHMT (PfSHMT) showed that amphotericin B was a competitive inhibitor of PfSHMT with a half-maximal inhibitory concentration (IC50) of 106 ± 1 μM. Therefore, a 500 ns molecular dynamics simulation of PfSHMT/PLS/amphotericin B was performed. The backbone root-mean-square deviation of the protein-ligand complex indicated the high complex stability during simulations, supported by its radius of gyration, hydrogen-bond interactions, and number of atom contacts. The appreciable binding affinity of amphotericin B for PfSHMT was indicated by their solvated interaction energy (-11.15 ± 0.09 kcal/mol) and supported by strong ligand-protein interactions (≥80% occurrences) with its essential residues (i.e., Y78, K151, N262, F266, and V365) predicted by pharmacophore modeling and per-residue decomposition free energy methods. Therefore, our findings identify a promising new PfSHMT inhibitor, albeit with less inhibitory activity, and suggest a core structure that differs from that of previous SHMT inhibitors, thus being a rational approach for novel antimalarial drug design.

Nanocarrier based active targeting strategies against erythrocytic stage of malaria

The Global Technical Strategy for Malaria 2016-2030 aims to achieve a 90% reduction in malaria cases, and strategic planning and execution are crucial for accomplishing this target. This review aims to understand the complex interaction between erythrocytic receptors and parasites and to use this knowledge to actively target the erythrocytic stage of malaria. The review provides insight into the malaria life cycle, which involves various receptors such as glycophorin A, B, C, and D (GPA/B/C/D), complement receptor 1, basigin, semaphorin 7a, Band 3/ GPA, Kx, and heparan sulfate proteoglycan for parasite cellular binding and ingress in the erythrocytic and exo-erythrocytic stages. Synthetic peptides mimicking P. falciparum receptor binding ligands, human serum albumin, chondroitin sulfate, synthetic polymers, and lipids have been utilized as ligands and decorated onto nanocarriers for specific targeting to parasite-infected erythrocytes. The need of the hour for treatment and prophylaxis against malaria is a broadened horizon that includes multiple targeting strategies against the entry, proliferation, and transmission stages of the parasite. Platform technologies with established pre-clinical safety and efficacy should be translated into clinical evaluation and formulation scale-up. Future development should be directed towards nanovaccines as proactive tools against malaria infection.

Plasmodium knowlesi Infection in Traveler Returning to Canada from the Philippines, 2023

A 55-year-old man sought treatment for an uncomplicated febrile illness after returning to Canada from the Philippines. A suspected diagnosis of Plasmodium knowlesi infection was confirmed by PCR, and treatment with atovaquone/proguanil brought successful recovery. We review the evolving epidemiology of P. knowlesi malaria in the Philippines, specifically within Palawan Island.

Genetic diversity and in silico analysis of Plasmodium knowlesi Serine Repeat Antigen (SERA) 3 antigen 2 in Malaysia

Plasmodium knowlesi is the leading cause of malaria in Malaysia. Serine Repeat Antigens (SERAs) have an essential role in the parasite life cycle. However, genetic characterization on P. knowlesi SERA3 Ag2 (PkSERA3 Ag2) is lacking. In the present study, nucleotide diversity, natural selection, and haplotypes of PkSERA3 Ag2 in clinical samples from Peninsular Malaysia and Malaysian Borneo were investigated. A total of 50 P. knowlesi clinical samples were collected from Peninsular Malaysia and Malaysian Borneo. The PkSERA3 Ag2 gene was amplified using PCR, and subsequently cloned and sequenced. Genetic diversity, haplotype, natural selection as well as genetic structure and differentiation of PkSERA3 Ag2 were analysed. In addition, in silico analyses were performed to identify repeat motifs, B-cell epitopes, and antigenicity indices of the protein. Analysis of 114 PkSERA3 Ag2 sequences revealed high nucleotide diversity of the gene in Malaysia. A codon-based Z-test indicated that the gene underwent purifying selection. Haplotype and population structure analyses identified two distinct PkSERA3 Ag2 clusters (K = 2, ΔK = 721.14) but no clear genetic distinction between PkSERA3 Ag2 from Peninsular Malaysia and Malaysian Borneo. FST index indicated moderate differentiation of the gene. In silico analyses revealed unique repeat motifs among PkSERA3 Ag2 isolates. Moreover, the amino acid sequence of PkSERA3 Ag2 exhibited potential B-cell epitopes and possessed high antigenicity indices. These findings enhance the understanding of PkSERA3 Ag2 gene as well as its antigenic properties. Further validation is necessary to ascertain the utility of PkSERA3 Ag2 as a serological marker for P. knowlesi infection.

Updating estimates of Plasmodium knowlesi malaria risk in response to changing land use patterns across Southeast Asia

Background

Plasmodium knowlesi is a zoonotic parasite that causes malaria in humans. The pathogen has a natural host reservoir in certain macaque species and is transmitted to humans via mosquitoes of the Anopheles Leucosphyrus Group. The risk of human P. knowlesi infection varies across Southeast Asia and is dependent upon environmental factors. Understanding this geographic variation in risk is important both for enabling appropriate diagnosis and treatment of the disease and for improving the planning and evaluation of malaria elimination. However, the data available on P. knowlesi occurrence are biased towards regions with greater surveillance and sampling effort. Predicting the spatial variation in risk of P. knowlesi malaria requires methods that can both incorporate environmental risk factors and account for spatial bias in detection.

Methods & Results

We extend and apply an environmental niche modelling framework as implemented by a previous mapping study of P. knowlesi transmission risk which included data up to 2015. We reviewed the literature from October 2015 through to March 2020 and identified 264 new records of P. knowlesi, with a total of 524 occurrences included in the current study following consolidation with the 2015 study. The modelling framework used in the 2015 study was extended, with changes including the addition of new covariates to capture the effect of deforestation and urbanisation on P. knowlesi transmission.

Discussion

Our map of P. knowlesi relative transmission suitability estimates that the risk posed by the pathogen is highest in Malaysia and Indonesia, with localised areas of high risk also predicted in the Greater Mekong Subregion, The Philippines and Northeast India. These results highlight areas of priority for P. knowlesi surveillance and prospective sampling to address the challenge the disease poses to malaria elimination planning.

In Silico Analysis Reveals High Levels of Genetic Diversity of Plasmodium knowlesi Cell Traversal Protein for Ookinetes and Sporozoites (PkCelTOS) in Clinical Samples

The cell-traversal protein for ookinetes and sporozoites (CelTOS), expressed on the surface of ookinetes and sporozoitesin Plasmodium species, is a promising malaria vaccine candidate. CelTOS is essential for parasite invasion into mosquito midgut and human hepatocytes, thereby contributing to malaria transmission and disease pathogenesis. This study explores the genetic diversity, polymorphisms, haplotypes, natural selection, phylogenetic analysis, and epitope prediction in the full-length Plasmodium knowlesi CelTOS gene in clinical samples from Sarawak, Malaysian Borneo, and long-term laboratory strains from Peninsular Malaysia and the Philippines. Our analysis revealed a high level of genetic variation in the PkCelTOS gene, with a nucleotide diversity of π ~ 0.021, which was skewed towards the 3' end of the gene. This level of diversity is double that observed in PfCelTOS and 20 times that observed in PvCelTOS from worldwide clinical samples. Tests of natural selection revealed evidence for positive selection within clinical samples. Phylogenetic analysis of the amino acid sequence of PkCelTOS revealed the presence of two distinct groups, although no geographical clustering was observed. Epitope prediction analysis identified two potential epitopes (96AQLKATA102 and 124TIKPPRIKED133) using the IEDB server and one epitope (125IKPPRIKED133) by Bcepred server on the C' terminal region of PkCelTOS protein. Both the servers predicted a common epitope region of nine amino acid length (IKPPRIKED) peptide, which can be studied in the future as a potential candidate for vaccine development. These findings shed light on the genetic diversity, polymorphism, haplotypes, and natural selection within PkCelTOS in clinical samples and provide insights about its future prospects as a potential candidate for P. knowlesi malaria vaccine development.

Sensitive Detection of Asymptomatic and Symptomatic Malaria with Seven Novel Parasite-Specific LAMP Assays and Translation for Use at Point-of-Care

Human malaria is a life-threatening parasitic disease with high impact in the sub-Saharan Africa region, where 95% of global cases occurred in 2021. While most malaria diagnostic tools are focused on Plasmodium falciparum, there is a current lack of testing non-P. falciparum cases, which may be underreported and, if undiagnosed or untreated, may lead to severe consequences. In this work, seven species-specific loop-mediated isothermal amplification (LAMP) assays were designed and evaluated against TaqMan quantitative PCR (qPCR), microscopy, and enzyme-linked immunosorbent assays (ELISAs). Their clinical performance was assessed with a cohort of 164 samples of symptomatic and asymptomatic patients from Ghana. All asymptomatic samples with a parasite load above 80 genomic DNA (gDNA) copies per μL of extracted sample were detected with the Plasmodium falciparum LAMP assay, reporting 95.6% (95% confidence interval [95% CI] of 89.9 to 98.5) sensitivity and 100% (95% CI of 87.2 to 100) specificity. This assay showed higher sensitivity than microscopy and ELISA, which were 52.7% (95% CI of 39.7 to 67%) and 67.3% (95% CI of 53.3 to 79.3%), respectively. Nine samples were positive for P. malariae, indicating coinfections with P. falciparum, which represented 5.5% of the tested population. No samples were detected as positive for P. vivax, P. ovale, P. knowlesi, or P. cynomolgi by any method. Furthermore, translation to the point-of-care was demonstrated with a subcohort of 18 samples tested locally in Ghana using our handheld lab-on-chip platform, Lacewing, showing comparable results to a conventional fluorescence-based instrument. The developed molecular diagnostic test could detect asymptomatic malaria cases, including submicroscopic parasitemia, and it has the potential to be used for point-of-care applications. IMPORTANCE The spread of Plasmodium falciparum parasites with Pfhrp2/3 gene deletions presents a major threat to reliable point-of-care diagnosis with current rapid diagnostic tests (RDTs). Novel molecular diagnostics based on nucleic acid amplification are needed to address this liability. In this work, we overcome this challenge by developing sensitive tools for the detection of Plasmodium falciparum and non-P. falciparum species. Furthermore, we evaluate these tools with a cohort of symptomatic and asymptomatic malaria patients and test a subcohort locally in Ghana. The findings of this work could lead to the implementation of DNA-based diagnostics to fight against the spread of malaria and provide reliable, sensitive, and specific diagnostics at the point of care.

Detection of Plasmodium simium gametocytes in non-human primates from the Brazilian Atlantic Forest

BACKGROUND

Plasmodium species of non-human primates (NHP) are of great interest because they can naturally infect humans. Plasmodium simium, a parasite restricted to the Brazilian Atlantic Forest, was recently shown to cause a zoonotic outbreak in the state of Rio de Janeiro. The potential of NHP to act as reservoirs of Plasmodium infection presents a challenge for malaria elimination, as NHP will contribute to the persistence of the parasite. The aim of the current study was to identify and quantify gametocytes in NHP naturally-infected by P. simium.

METHODS

Whole blood samples from 35 NHP were used in quantitative reverse transcription PCR (RT-qPCR) assays targeting 18S rRNA, Pss25 and Pss48/45 malaria parasite transcripts. Absolute quantification was performed in positive samples for 18S rRNA and Pss25 targets. Linear regression was used to compare the quantification cycle (Cq) and the Spearman's rank correlation coefficient was used to assess the correlation between the copy numbers of 18S rRNA and Pss25 transcripts. The number of gametocytes/µL was calculated by applying a conversion factor of 4.17 Pss25 transcript copies per gametocyte.

RESULTS

Overall, 87.5% of the 26 samples, previously diagnosed as P. simium, were positive for 18S rRNA transcript amplification, of which 13 samples (62%) were positive for Pss25 transcript amplification and 7 samples (54%) were also positive for Pss48/45 transcript. A strong positive correlation was identified between the Cq of the 18S rRNA and Pss25 and between the Pss25 and Pss48/45 transcripts. The 18S rRNA and Pss25 transcripts had an average of 1665.88 and 3.07 copies/µL, respectively. A positive correlation was observed between the copy number of Pss25 and 18S rRNA transcripts. Almost all gametocyte carriers exhibited low numbers of gametocytes (< 1/µL), with only one howler monkey having 5.8 gametocytes/µL.

CONCLUSIONS

For the first time, a molecular detection of P. simium gametocytes in the blood of naturally-infected brown howler monkeys (Alouatta guariba clamitans) was reported here, providing evidence that they are likely to be infectious and transmit P. simium infection, and, therefore, may act as a reservoir of malaria infection for humans in the Brazilian Atlantic Forest.

Natural vectors of Plasmodium knowlesi and other primate, avian and ungulate malaria parasites in Narathiwat Province, Southern Thailand

To date, four species of simian malaria parasites including Plasmodium knowlesi, P. cynomolgi, P. inui and P. fieldi have been incriminated in human infections in Thailand. Although the prevalence of malaria in macaque natural hosts has been investigated, their vectors remain unknown in this country. Herein, we performed a survey of Anopheles mosquitoes during rainy and dry seasons in Narathiwat Province, Southern Thailand. Altogether 367 Anopheles mosquitoes were captured for 40 nights during 18:00 to 06:00 h by using human-landing catches. Based on morphological and molecular identification, species composition comprised An. maculatus (37.06%), An. barbirostris s.l. (31.34%), An. latens (17.71%), An. introlatus (10.08%) and others (3.81%) including An. umbrosus s.l., An. minimus, An. hyrcanus s.l., An. aconitus, An. macarthuri and An. kochi. Analyses of individual mosquitoes by PCR, sequencing and phylogenetic inference of the mitochondrial cytochrome genes of both malaria parasites and mosquitoes have revealed that the salivary gland samples of An. latens harbored P. knowlesi (n = 1), P. inui (n = 2), P. fieldi (n = 1), P. coatneyi (n = 1), P. hylobati (n = 1) and an unnamed Plasmodium species known to infect both long-tailed and pig-tailed macaques (n = 2). The salivary glands of An. introlatus possessed P. cynomolgi (n = 1), P. inui (n = 1), P. hylobati (n = 1) and coexistence of P. knowlesi and P. inui (n = 1). An avian malaria parasite P. juxtanucleare has been identified in the salivary gland sample of An. latens. Three other distinct lineages of Plasmodium with phylogenetic affinity to avian malaria species were detected in An. latens, An. introlatus and An. macarthuri. Interestingly, the salivary gland sample of An. maculatus contained P. caprae, an ungulate malaria parasite known to infect domestic goats. Most infected mosquitoes harbored multiclonal Plasmodium infections. All Plasmodium-infected mosquitoes were captured during the first quarter of the night and predominantly occurred during rainy season. Since simian malaria in humans has a wide geographic distribution in Thailand, further studies in other endemic areas of the country are mandatory for understanding transmission and prevention of zoonotic malaria.

Malaria: An Overview

Malaria is a global public health burden with an estimated 229 million cases reported worldwide in 2019. About 94% of the reported cases were recorded in the African region. About 200 different species of protozoa have been identified so far and among them, at least 13 species are known to be pathogenic to humans. The life cycle of the malaria parasite is a complex process comprising an Anopheles mosquito and a vertebrate host. Its pathophysiology is characterized by fever secondary to the rupture of erythrocytes, macrophage ingestion of merozoites, and/or the presence of antigen-presenting trophozoites in the circulation or spleen which mediates the release of tumor necrosis factor α (TNF-α). Malaria can be diagnosed through clinical observation of the signs and symptoms of the disease. Other diagnostic techniques used to diagnose malaria are the microscopic detection of parasites from blood smears and antigen-based rapid diagnostic tests. The management of malaria involves preventive and/or curative approaches. Since untreated uncomplicated malaria can progress to severe malaria. To prevent or delay the spread of antimalarial drug resistance, WHO recommends the use of combination therapy for all episodes of malaria with at least two effective antimalarial agents having a different mechanism of action. The Centers for Disease Control (CDC) emphasizes that there is no prophylactic agent that can prevent malaria 100%. Therefore, prophylaxis shall be augmented with the use of personal protective measures.

Current advances in nanodrug delivery systems for malaria prevention and treatment

Malaria is a life-threatening, blood-borne disease with over two hundred million cases throughout the world and is more prevalent in Sub-Saharan Africa than anywhere else in the world. Over the years, several treatment agents have been developed for malaria; however, most of these active pharmaceutical ingredients exhibit poor aqueous solubility and low bioavailability and may result in drug-resistant parasites, thus increasing malaria cases and eventually, deaths. Factors such as these in therapeutics have led to a better appreciation of nanomaterials. The ability of nanomaterials to function as drug carriers with a high loading capacity and targeted drug delivery, good biocompatibility, and low toxicity renders them an appealing alternative to conventional therapy. Nanomaterials such as dendrimers and liposomes have been demonstrated to be capable of enhancing the efficacy of antimalarial drugs. This review discusses the recent development of nanomaterials and their benefits in drug delivery for the potential treatment of malaria.

Biological activity of 1,2,3-triazole-2-amino-1,4-naphthoquinone derivatives and their evaluation as therapeutic strategy for malaria control

Malaria can be caused by several Plasmodium species and the development of an effective vaccine is challenging. Currently, the most effective tool to control the disease is the administration of specific chemotherapy; however, resistance to the frontline antimalarials is one of the major problems in malaria control and thus the development of new drugs becomes urgent. The study presented here sought to evaluate the antimalarial activities of compounds derived from 2-amino-1,4-naphthoquinones containing 1,2,3-triazole using in vivo and in vitro models. 1H-1,2,3-Triazole 2-amino-1,4-naphthoquinone derivatives were synthesized and evaluated for antimalarial activity in vitro, using P. falciparum W2 chloroquine (CQ) resistant strain and in vivo using the murine-P. berghei ANKA strain. Acute toxicity was determined as established by the OECD (2001). Cytotoxicity was evaluated against HepG2 and Vero mammalian cell lines. Transmission electron microscopy of the Plasmodium falciparum trophozoite (early and late stages) was used to evaluate the action of compounds derived at ultra-structural level. The compounds displayed low cytotoxicity CC₅₀ > 100 μM, neither did they cause hemolysis at the tested doses and nor the signs of toxicity in the in vivo acute toxicity test. Among the five compounds tested, one showed IC₅₀ values in submicromolar range of 0.8 μM. Compounds 7, 8 and 11 showed IC₅₀ values < 5 μM, and selectivity index (SI) ranging from 6.8 to 343 for HepG2, and from 13.7 to 494.8 for Vero cells. Compounds 8 and 11 were partially active against P. berghei induced parasitemia in vivo. Analysis of the ultrastructural changes associated with the treatment of these two compounds, showed trophozoites with completely degraded cytoplasm, loss of membrane integrity, organelles in the decomposition stage and possible food vacuole deterioration. Our results indicated that compounds 8 and 11 may be considered hit molecules for antimalarial drug discovery platform and deserve further optimization studies.

Universal two-dimensional labelled probe-mediated melting curve analysis based on multiplex PCR for rapid typing of Plasmodium in a single closed tube

Currently, malaria is still one of the major public health problems commonly caused by the four Plasmodium species. The similar symptoms of malaria and the COVID-19 epidemic of fever or fatigue lead to frequent misdiagnosis. The disadvantages of existing detection methods, such as time-consuming, costly, complicated operation, need for experienced technicians, and indistinguishable typing, lead to difficulties in meeting the clinical requirements of rapid, easy, and accurate typing of common Plasmodium species. In this study, we developed and optimized a universal two-dimensional labelled probe-mediated melting curve analysis (UP-MCA) assay based on multiplex and asymmetric PCR for rapid and accurate typing of five Plasmodium species, including novel human Plasmodium, Plasmodium knowlesi (Pk), in a single closed tube following genome extraction. The assay showed a limit of detection (LOD) of 10 copies per reaction and could accurately distinguish Plasmodium species from intra-plasmodium and other pathogens. Additionally, we proposed and validated different methods of fluorescence quenching and tag design for probes that are suitable for UP-MCA assays. Moreover, the clinical performance of the Plasmodium UP-MCA assay using a base-quenched universal probe was evaluated using 226 samples and showed a sensitivity of 100% (164/164) and specificity of 100% (62/62) at a 99% confidence interval, with the microscopy method as the gold standard. In summary, the UP-MCA assay showed excellent sensitivity, specificity, and accuracy for genotyping Plasmodium species spp. Additionally, it facilitates convenient and rapid Plasmodium detection in routine clinical practice and has great potential for clinical translation.

Point-of-Care Diagnosis of Malaria Using a Simple, Purification-Free DNA Extraction Method Coupled with Loop-Mediated Isothermal Amplification-Lateral Flow

We propose a protocol suitable for point-of-care diagnosis of malaria utilizing a simple and purification-free DNA extraction method with the combination of loop-mediated isothermal amplification assay and lateral flow (LAMP-LF). The multiplex LAMP-LF platform developed here can simultaneously detect Plasmodium knowlesi, P. vivax, P. falciparum, and Plasmodium genus (for P. malariae and P. ovale). Through the capillary effect, the results can be observed by the red band signal on the test and control lines within 5 min. The developed multiplex LAMP-LF was tested with 86 clinical blood samples on-site at Hospital Kapit, Sarawak, Malaysia. By using microscopy as the reference method, the multiplex LAMP-LF showed 100% sensitivity (95% confidence interval (CI): 91.4 to 100.00%) and 97.8% specificity (95% CI: 88.2% to 99.9%). The high sensitivity and specificity of multiplex LAMP-LF make it ideal for use as a point-of-care diagnostic tool. The simple and purification-free DNA extraction protocol can be employed as an alternative DNA extraction method for malaria diagnosis in resource-limited settings. By combining the simple DNA extraction protocol and multiplex LAMP-LF approach, we aim to develop a simple-to-handle and easy-to-read molecular diagnostic tool for malaria in both laboratory and on-site settings.

International modified Delphi study on malaria preventive behaviour: new themes, contexts and framework for future research on Plasmodium knowlesi malaria

INTRODUCTION

Plasmodium knowlesi malaria is a non-human simian malaria that threatens Southeast Asian rural communities. Studies indicate that non-compliant bednet usage, travelling into the forest and working as farmers and rubber tappers put communities at risk for infection. Despite guidelines, malaria incidence increases yearly and has become a public health concern. In addition to research gaps addressing factors that influence malaria prevention behaviour in these communities, there are no specific guidelines to facilitate strategies against the threat of P. knowlesi malaria.

METHOD

To examine potential factors that influence malaria-prevention behaviour in communities exposed to P. knowlesi malaria, 12 malaria experts participated in a modified Delphi study; every participant maintained their anonymity throughout the study. Three Delphi rounds were conducted via different online platforms between 15 November 2021 and 26 February 2022, and consensus was achieved when 70% of the participants agreed on a particular point with a 4-5 median. The results from the open-ended questions were then subjected to thematic analysis, and the dataset generated by this study was analysed using a deductive and inductive approach.

RESULTS

After a systematic, iterative process, knowledge and belief, social support, cognitive and environmental factors, past experience as a malaria patient, and the affordability and feasibility of a given intervention were critical contributors to malaria-prevention behaviour.

CONCLUSION

Future research on P. knowlesi malaria could adapt this study's findings for a more nuanced understanding of factors that influence malaria-prevention behaviour and improve P. knowlesi malaria programmes based on the expert consensus.

The Impact of Geographical Variation in Plasmodium knowlesi Apical Membrane Protein 1 (PkAMA-1) on Invasion Dynamics of P. knowlesi

Plasmodium knowlesi has emerged as an important zoonotic parasite that causes persistent symptomatic malaria in humans. The signs and symptoms of malaria are attributed to the blood stages of the parasites, which start from the invasion of erythrocytes by the blood stage merozoites. The apical membrane protein 1 (AMA-1) plays an important role in the invasion. In this study, we constructed and expressed recombinant PkAMA-1 domain II (PkAMA-1-DII) representing the predominant haplotypes from Peninsular Malaysia and Malaysian Borneo and raised specific antibodies against the recombinant proteins in rabbits. Despite the minor amino acid sequence variation, antibodies raised against haplotypes from Peninsular Malaysia and Malaysian Borneo demonstrated different invasion inhibition (46.81% and 39.45%, respectively) to P. knowlesi A1-H.1, a reference strain derived from Peninsular Malaysia. Here, we demonstrated how a minor variation in a conserved parasite protein could cast a significant impact on parasite invasion biology, suggesting a complex host-switching of P. knowlesi from different locations. This may challenge the implementation of a standardized One Health approach against the transmission of knowlesi malaria.

Perspectives of vector management in the control and elimination of vector-borne zoonoses

The complex transmission profiles of vector-borne zoonoses (VZB) and vector-borne infections with animal reservoirs (VBIAR) complicate efforts to break the transmission circuit of these infections. To control and eliminate VZB and VBIAR, insecticide application may not be conducted easily in all circumstances, particularly for infections with sylvatic transmission cycle. As a result, alternative approaches have been considered in the vector management against these infections. In this review, we highlighted differences among the environmental, chemical, and biological control approaches in vector management, from the perspectives of VZB and VBIAR. Concerns and knowledge gaps pertaining to the available control approaches were discussed to better understand the prospects of integrating these vector control approaches to synergistically break the transmission of VZB and VBIAR in humans, in line with the integrated vector management (IVM) developed by the World Health Organization (WHO) since 2004.

A systematic review of asymptomatic Plasmodium knowlesi infection: an emerging challenge involving an emerging infectious disease

BACKGROUND

In the last decade Plasmodium knowlesi has been detected in humans throughout South East Asia. The highest risk groups for this infection are males, adults and those performing forest-related work. Furthermore, asymptomatic cases of P. knowlesi malaria have been reported including among women and children.

METHODS

Pubmed, Scopus and the Web of Science databases for literature describing asymptomatic P. knowlesi malaria published between 2010 and 2020 were searched. A systematic literature review was conducted to identify studies reporting the prevalence and incidence of laboratory confirmed asymptomatic P. knowlesi cases in humans, their clinical and demographic characteristics, and methods used to diagnose these cases.

RESULTS

By analysing over 102 papers, thirteen were eligible for this review. Asymptomatic P. knowlesi infections have been detected in 0.03%-4.0% of the population depending on region, and infections have been described in children as young as 2 years old. Various different diagnostic methods were used to detect P. knowlesi cases and there were differing definitions of asymptomatic cases in these studies. The literature indicates that regionally-differing immune-related mechanisms may play a part on the prevalence of asymptomatic P. knowlesi.

CONCLUSION

Differing epidemiological characteristics of asymptomatic P. knowlesi malaria in different regions reinforces the need to further investigate disease transmission mechanics. Effective public health responses to changes in P. knowlesi epidemiology require proactive intervention and multisectoral collaboration.

Genetic Diversity and Clustering of the Rhoptry Associated Protein-1 of Plasmodium knowlesi from Peninsular Malaysia and Malaysian Borneo

Human infection with simian malaria Plasmodium knowlesi is a cause for concern in Southeast Asian countries, especially in Malaysia. A previous study on Peninsular Malaysia P. knowlesi rhoptry associated protein-1 (PkRAP1) gene has discovered the existence of dimorphism. In this study, genetic analysis of PkRAP1 in a larger number of P. knowlesi samples from Malaysian Borneo was conducted. The PkRAP1 of these P. knowlesi isolates was PCR-amplified and sequenced. The newly obtained PkRAP1 gene sequences (n = 34) were combined with those from the previous study (n = 26) and analysed for polymorphism and natural selection. Sequence analysis revealed a higher genetic diversity of PkRAP1 compared to the previous study. Exon II of the gene had higher diversity (π = 0.0172) than exon I (π = 0.0128). The diversity of the total coding region (π = 0.0167) was much higher than those of RAP1 orthologues such as PfRAP-1 (π = 0.0041) and PvRAP1 (π = 0.00088). Z-test results indicated that the gene was under purifying selection. Phylogenetic tree and haplotype network showed distinct clustering of Peninsular Malaysia and Malaysian Borneo PkRAP1 haplotypes. This geographical-based clustering of PkRAP1 haplotypes provides further evidence of the dimorphism of the gene and possible existence of 2 distinct P. knowlesi lineages in Malaysia.

Kaempferol: Antimicrobial Properties, Sources, Clinical, and Traditional Applications

Flavonoids are a category of plant-derived compounds which exhibit a large number of health-related effects. One of the most well-known and studied flavonoids is kaempferol, which can be found in a wide variety of herbs and plant families. Apart from their anticarcinogenic and anti-inflammatory effects, kaempferol and its associated compounds also exhibit antibacterial, antifungal, and antiprotozoal activities. The development of drugs and treatment schemes based on these compounds is becoming increasingly important in the face of emerging resistance of numerous pathogens as well as complex molecular interactions between various drug therapies. In addition, many of the kaempferol-containing plants are used in traditional systems all over the world for centuries to treat numerous conditions. Due to its variety of sources and associated compounds, some molecular mechanisms of kaempferol antimicrobial activity are well known while others are still under analysis. This paper thoroughly documents the vegetal and food sources of kaempferol as well as the most recent and significant studies regarding its antimicrobial applications.

Elevated plasma α1-antichymotrypsin is a biomarker candidate for malaria patients

Advancements in the field of proteomics have provided opportunities to develop diagnostic and therapeutic strategies against various diseases. About half of the world's population remains at risk of malaria. Caused by protozoan parasites of the genus Plasmodium, malaria is one of the oldest and largest risk factors responsible for the global burden of infectious diseases with an estimated 3.2 billion persons at risk of infection. For epidemiological surveillance and appropriate treatment of individuals infected with Plasmodium spp., timely detection is critical. In this study, we used combinations of depletion of abundant plasma proteins, 2-dimensional gel electrophoresis (2-DE), image analysis, LC-MS/MS and western blot analysis on the plasma of healthy donors (100 individuals) and vivax and falciparum malaria patients (100 vivax malaria patients and 8 falciparum malaria patients). These analyses revealed that α1-antichymotrypsin (AACT) protein levels were elevated in vivax malaria patient plasma samples (mean fold-change ± standard error: 2.83 ± 0.11, based on band intensities), but not in plasma from patients with other mosquito-borne infectious diseases. The results of AACT immunoblot analyses showed that AACT protein was significantly elevated in vivax and falciparum malaria patient plasma samples (≥ 2-fold) compared to healthy control donor plasma samples, which has not been previously reported. [BMB Reports 2022; 55(11): 571-576].

Analogues of Oxamate, Pyruvate, and Lactate as Potential Inhibitors of Plasmodium knowlesi Lactate Dehydrogenase Identified Using Virtual Screening and Verified via Inhibition Assays

Malaria management remains a challenge, due to the resistance of malaria parasites to current antimalarial agents. This resistance consequently delays the global elimination of malaria throughout the world. Hence, the demand is increasing for new and effective antimalarial drugs. The identification of potential drugs that target Pk-LDH can be obtained through virtual screening analyses, as this has been previously applied to discover Pf-LDH inhibitors. In this study, the selected candidates from our virtual screening analyses were subsequently tested against purified Pk-LDH, and verified through an inhibition of Pk-LDH via enzymatic activity assays. Virtual screening analysis from this study showed that 3,3-Difluoropyrrolidine hydrochloride and 3-hydroxytetrahydrofuran exhibited binding affinity values of −3.25 kcal/mol and −3.74, respectively. These compounds were selected for evaluation towards inhibitory activity against Pk-LDH assays, including two compounds from a previous study which are oxalic acid and glycolamide. The earlier compounds were structurally similar to lactate and pyruvate, and the latter two compounds were structurally similar to a known LDH inhibitor, oxamate. Among all of the compounds tested, oxalic acid showed the highest inhibition activity at 54.12%; interestingly, this correlated well with the virtual screening analyses, which showed that this compound was the best among the oxamate analogues, with a binding affinity value of −2.59 kcal/mol. Hence, further exploration and development of this compound may result in a promising antimalarial drug for malaria treatment, especially for infection involving P. knowlesi.

The ecology and epidemiology of malaria parasitism in wild chimpanzee reservoirs

Chimpanzees (Pan troglodytes) harbor rich assemblages of malaria parasites, including three species closely related to P. falciparum (sub-genus Laverania), the most malignant human malaria parasite. Here, we characterize the ecology and epidemiology of malaria infection in wild chimpanzee reservoirs. We used molecular assays to screen chimpanzee fecal samples, collected longitudinally and cross-sectionally from wild populations, for malaria parasite mitochondrial DNA. We found that chimpanzee malaria parasitism has an early age of onset and varies seasonally in prevalence. A subset of samples revealed Hepatocystis mitochondrial DNA, with phylogenetic analyses suggesting that Hepatocystis appears to cross species barriers more easily than Laverania. Longitudinal and cross-sectional sampling independently support the hypothesis that mean ambient temperature drives spatiotemporal variation in chimpanzee Laverania infection. Infection probability peaked at ~24.5 °C, consistent with the empirical transmission optimum of P. falciparum in humans. Forest cover was also positively correlated with spatial variation in Laverania prevalence, consistent with the observation that forest-dwelling Anophelines are the primary vectors. Extrapolating these relationships across equatorial Africa, we map spatiotemporal variation in the suitability of chimpanzee habitat for Laverania transmission, offering a hypothetical baseline indicator of human exposure risk.

SICA-mediated cytoadhesion of Plasmodium knowlesi-infected red blood cells to human umbilical vein endothelial cells

Zoonotic malaria due to Plasmodium knowlesi infection in Southeast Asia is sometimes life-threatening. Post-mortem examination of human knowlesi malaria cases showed sequestration of P. knowlesi-infected red blood cells (iRBCs) in blood vessels, which has been proposed to be linked to disease severity. This sequestration is likely mediated by the cytoadhesion of parasite-iRBCs to vascular endothelial cells; however, the responsible parasite ligands remain undetermined. This study selected P. knowlesi lines with increased iRBC cytoadhesion activity by repeated panning against human umbilical vein endothelial cells (HUVECs). Transcriptome analysis revealed that the transcript level of one gene, encoding a Schizont Infected Cell Agglutination (SICA) protein, herein termed SICA-HUVEC, was more than 100-fold increased after the panning. Transcripts of other P. knowlesi proteins were also significantly increased, such as PIR proteins exported to the iRBC cytosol, suggesting their potential role in increasing cytoadhesion activity. Transgenic P. knowlesi parasites expressing Myc-fused SICA-HUVEC increased cytoadhesion activity following infection of monkey as well as human RBCs, confirming that SICA-HUVEC conveys activity to bind to HUVECs.

Narrative Review of the Control and Prevention of Knowlesi Malaria

Despite the reduction in the number of cases of human malaria throughout the world, the incidence rate of knowlesi malaria is continuing to rise, especially in Southeast Asia. The conventional strategies for the prevention and control of human malaria can provide some protection against knowlesi malaria. Despite the numerous studies on the risk factors and the innovative methods that may be used to prevent and control the vectors of Plasmodium knowlesi, the incidence rate remains high. An integrated approach that includes environmental intervention should be adopted in order to ensure the successful control of zoonotic malaria. A combination of personal-level protection, vector control and environmental control may mitigate the risk of Plasmodium knowlesi transmission from macaques to humans and, ultimately, reduce the incidence rate of knowlesi malaria.

Clinical performance testing of the automated haematology analyzer XN-31 prototype using whole blood samples from patients with imported malaria in Japan

Background

The automated haematology analyzer XN-31 prototype (XN-31p) is a new flow cytometry-based device developed to measure the number and the ratio of malaria-infected red blood cells (MI-RBC) with a complete blood count (CBC). The XN-31p can provide results in about one minute and also can simultaneously provide information on the malaria parasite (Plasmodium) species. In this study, clinical testing of the XN-31p was performed using blood samples from patients with imported malaria in Japan.

Methods

Blood samples were collected from 80 patients who visited the hospital of the National Center for Global Health and Medicine, Tokyo, Japan, for malaria diagnosis from January 2017 to January 2019. The test results by the XN-31p were compared with those by other standard methods, such as microscopic observation, rapid diagnostic tests and the nested PCR.

Results

Thirty-three patients were diagnosed by the nested PCR as being malaria positive (28 Plasmodium falciparum, 2 Plasmodium vivax, 1 Plasmodium knowlesi, 1 mixed infection of P. falciparum and Plasmodium malariae, and 1 mixed infection of P. falciparum and Plasmodium ovale), and the other 47 were negative. The XN-31p detected 32 patients as “MI-RBC positive”, which almost matched the results by the nested PCR and, in fact, completely matched with the microscopic observations. The ratio of RBCs infected with malaria parasites as determined by the XN-31p showed a high correlation coefficient of more than 0.99 with the parasitaemia counted under microscopic observation. The XN-31p can analyse the size and nucleic acid contents of each cell, and the results were visualized on a two-dimensional cytogram termed the “M scattergram”. Information on species and developmental stages of the parasites could also be predicted from the patterns visualized in the M scattergrams. The XN-31p showed a positive coincidence rate of 0.848 with the nested PCR in discriminating P. falciparum from the other species.

Conclusions

The XN-31p could rapidly provide instructive information on the ratio of MI-RBC and the infecting Plasmodium species. It was regarded to be of great help for the clinical diagnosis of malaria.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-022-04247-x.

Prevalence of simian malaria among macaques in Malaysia (2000–2021): A systematic review

Background

The aim of Malaysia to eliminate malaria nationwide by 2020 seems need to be prolonged. Whilst Malaysia has successfully eliminated human malaria transmission, simian malaria parasites such as Plasmodium knowlesi, P. cynomolgi, P. inui and P. cynomolgi are the emerging cause of malaria in humans. The epidemiological study of simian malaria in primates provides useful information in identifying the risk of human-macaques Plasmodium infection.

Methodology/Principal findings

This study was performed to gather all available data in terms of simian malaria epidemiology study among macaques in Malaysia over the last two decades. This systematic review was conducted according to the PRISMA guidelines to select appropriate articles as references. Data searches were performed through international databases such as Google Scholar, PubMed, CrossRef, Scopus, Web of Science and Science Direct for original articles published from 2000 until 2021. The review identified seven simian malaria epidemiology studies in Malaysia over the 20-year study period. Most studies were conducted in Peninsular Malaysia (5/7; 71%) followed by East Malaysia (2/7; 29%). All studies showed positive detection of Plasmodium parasites in macaques. The most prevalent Plasmodium species in macaques was P. inui (49.27%) and the least prevalent was P. fieldi (4.76%). The prevalence of simian malaria was higher in East Malaysia compared to Peninsular Malaysia. The mono, dual and triple infection types were the most common among macaques.

Conclusion/Significance

The non-human primates like macaques are the reservoir of simian plasmodium in Malaysia. Hence, the study of host epidemiology is an important insight to public health management as there is a high occurrence of simian malaria in Malaysia. The right measurement can be taken as well to prevent the transmission of simian malaria from macaques to humans.

Macaques are the most abundant primates in south east Asia including Malaysia. Due to deforestation, macaques came closer to human settlements searching for food. Macaques like the long-tailed and pig-tailed harbouring several Plasmodium species that can cause zoonotic malaria in humans. Close contact of human and macaques cause zoonotic transmission of simian malaria. The simian plasmodium such as P. knowlesi, P. inui and P. cynomolgi have been found infecting humans in Malaysia; mainly in East Malaysia (Borneo). Zoonotic malaria poses great risk to public health as prolonged in treatment often lead to fatal outcomes. Hence the knowledge of prevalence and diversity is important to access, this can therefore enlighten the authorities to plan a control strategy that will minimize the zoonotic transmission between non-human primate host to human. This systematic review has summarised all publish data of macaques-plasmodium infection from the year 2000–2021 by using PRISMA guidelines. Our result showed that P. inui (49.27%) is the most prevalent Plasmodium species found in macaques, followed by P. cynomolgi (33.05%) and P. knowlesi (26.86%). Simian plasmodium prevalent was also found higher in East Malaysia (97.0%) compared to Peninsular Malaysia (45.18%). The significant increase of simian malaria incidences in human have jeopardized the national malaria elimination programme. Hence, this study provides a compact insight into the plasmodium epidemiology of macaques in Malaysia.

Case Report: Cambodian National Malaria Surveillance Program Detection of Plasmodium knowlesi

Despite recent success in reducing the regional incidence of Plasmodium falciparum malaria, cases of zoonotic malaria are on the rise in Southeast Asia. The Cambodian National Malaria Surveillance Program has previously relied on rapid diagnostic tests and blood smear microscopy with confirmatory polymerase chain reaction (PCR) testing in a subset of cases to further distinguish P. falciparum, P. malariae, P. ovale, and P. vivax species. Here, metagenomic next-generation sequencing identified P. knowlesi mono-infection in six Cambodian patients initially diagnosed with P. malariae by blood smear microscopy in February–May 2020. These findings of recent human infections with P. knowlesi in Cambodia led to the incorporation of P. knowlesi–specific PCR diagnostics to national malaria surveillance efforts.

Non-Human Primate Malaria Infections: A Review on the Epidemiology in Malaysia

Malaria remains a public health problem in many parts of the world, including Malaysia. Although Malaysia has been recognized as one of the countries free from indigenous human malaria since 2018, the rising trend of zoonotic malaria, particularly Plasmodium knowlesi cases, poses a threat to public health and is of great concern to the country’s healthcare system. We reviewed previously scattered information on zoonotic malaria infections in both Peninsular Malaysia and Malaysian Borneo to determine the epidemiology and distribution of emerging zoonotic malaria infections. Given the high prevalence of zoonotic malaria in Malaysia, efforts should be made to detect zoonotic malaria in humans, mosquito vectors, and natural hosts to ensure the success of the National Malaria Elimination Strategic Plan.

Macaca fascicularis and Macaca nemestrina infected with zoonotic malaria parasites are widely distributed in Sarawak, Malaysian Borneo

Human infections with Plasmodium knowlesi, a malaria parasite of Macaca fascicularis and Macaca nemestrina (long-tailed and pig-tailed macaques respectively), occur throughout Southeast Asia, especially Malaysian Borneo. Other naturally-acquired human infections with malaria parasites from macaques in Southeast Asia are P. cynomolgi, P. inui-like, P. coatneyi and P. simiovale. In Sarawak, Malaysian Borneo, M. fascicularis and M. nemestrina from only the Kapit Division have been examined previously for malaria parasites. In order to determine the distribution of P. knowlesi and other zoonotic malaria parasites, 73 macaque blood samples derived from 7 other administrative divisions in Sarawak were studied. Of 45 blood samples from M. fascicularis and 28 from M. nemestrina tested by nested PCR assays, 23 (51.1%) M. fascicularis and 15 (53.6%) M. nemestrina samples were positive for Plasmodium DNA. Thirty-two of these macaques from 7 divisions sampled, harboured either single (n = 12), double (n = 9), triple (n = 7) or quadruple (n = 4) infections of P. knowlesi, P. inui, P. cynomolgi and P. coatneyi, while the infecting species of Plasmodium could not be identified for 6 samples. P. knowlesi was detected in 15.5% (7/45) M. fascicularis and in 7.1% (2/28) M. nemestrina sampled. Despite the small number of samples analysed from each administrative division, the current study indicates that macaques infected with the zoonotic malaria parasites P. knowlesi, P. cynomolgi, P. inui and P. coatneyi are widely distributed throughout Sarawak, Malaysian Borneo. Travelers to forested areas in Sarawak should be made aware of the potential risk of acquiring zoonotic malaria.

Plasmodium vivax malaria serological exposure markers: Assessing the degree and implications of cross-reactivity with P. knowlesi

Serological markers are a promising tool for surveillance and targeted interventions for Plasmodium vivax malaria. P. vivax is closely related to the zoonotic parasite P. knowlesi, which also infects humans. P. vivax and P. knowlesi are co-endemic across much of South East Asia, making it important to design serological markers that minimize cross-reactivity in this region. To determine the degree of IgG cross-reactivity against a panel of P. vivax serological markers, we assayed samples from human patients with P. knowlesi malaria. IgG antibody reactivity is high against P. vivax proteins with high sequence identity with their P. knowlesi ortholog. IgG reactivity peaks at 7 days post-P. knowlesi infection and is short-lived, with minimal responses 1 year post-infection. We designed a panel of eight P. vivax proteins with low levels of cross-reactivity with P. knowlesi. This panel can accurately classify recent P. vivax infections while reducing misclassification of recent P. knowlesi infections.

Epidemiology of indigenous Plasmodium knowlesi infection in Sarawak, 2011-2019

OBJECTIVES

To characterize the state-wide epidemiology of indigenous knowlesi malaria in Sarawak from 2011-2019.

METHODS

Longitudinal retrospective study based on Sarawak knowlesi malaria surveillance data recorded 2011-2019. Only indigenous cases were included and information extracted for analysis comprised age, sex, occupation, ethnicity, case severity, hospital admission, and parasite density.

RESULTS

Over the 9 years 8473 indigenous knowlesi malaria cases were recorded. Age group 40 to 49 years, males, plantation workers, and Iban communities recorded the highest percentage of cases in each demographic variable. Most of the cases were uncomplicated (85.9%) and 89.5% of the total cases were reported with ≤20,000 parasites/μL of blood. Age group and ethnic group are associated with severity of knowlesi malaria in Sarawak. Multivariable logistic regression indicated that the age group 60+ years had the highest odds of developing severe knowlesi malaria compared to other age groups (AOR 2.48; 95% CI 1.22, 5.02; p=0.012). Bidayuh patients were more likely to develop severe knowlesi malaria than Ibans, the largest ethnic group among knowlesi malaria patients (AOR 1.97; 95% CI 1.31, 2.97; p=0.001).

CONCLUSIONS

Identification of risk groups is important for the implementation of prevention programs and treatments targeting at specific group to combat knowlesi malaria effectively.