Severe malaria - a case of fatal Plasmodium knowlesi infection with post-mortem findings: a case report

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.

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.

Primate malaria of human importance

Nonhuman primate (NHP) malaria poses a major threat to the malaria control programs. The last two decades have witnessed a paradigm shift in our understanding of the malaria caused by species other than the traditionally known human Plasmodium species - Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, and Plasmodium ovale. The emergence of the malaria parasite of long-tailed macaque monkeys, Plasmodium knowlesi, as the fifth malaria species of humans has made the scientific community consider the risk of other zoonotic malaria, such as Plasmodium cynomolgi, Plasmodium simium, Plasmodium inui, and others, to humans. The development of knowledge about P. knowlesi as a pathogen which was earlier only known to experimentally cause malaria in humans and rarely cause natural infection, toward its acknowledgment as a significant cause of human malaria and a threat of malaria control programs has been made possible by the use of advanced molecular techniques such as polymerase chain reaction and gene sequencing. This review explores the various aspects of NHP malaria, and the association of various factors with their emergence and potential to cause human malaria which are important to understand to be able to control these emerging infections.

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.

Rhino-orbital mucormycosis in a patient with no susceptibility following P.vivax malaria infection-a case report

Background

Mucormycosis is a potentially lethal, angioinvasive fungal infection caused by the Mucoracea family comprising Mucor, Rhizopus, and Absidia species. It is commonly associated with uncontrolled diabetes mellitus, the use of corticosteroids, immunosuppressive drugs, and Covid-19 infection. The occurrence of mucormycosis in an immunocompetent patient is rare. Also, only a few case reports have been published where patients developed mucormycosis with associated malarial infection.

Case presentation

A young female presented with a 3-weeks history of painful swelling and outward protrusion of the right eye with complete loss of vision. She had a history of P.vivax malaria two weeks before her ocular symptoms. On ocular examination, there was proptosis and total ophthalmoplegia with loss of corneal sensations in the right eye. Hematological examination revealed normocytic normochromic anemia and thrombocytopenia. MRI was suggestive of right-sided pansinusitis and orbital cellulitis with right superior ophthalmic vein thrombosis and bulky cavernous sinus.

Nasal biopsy was negative for fungal culture. An emergency surgical debridement of all the sinuses was done with right orbital exenteration. Histopathology confirmed the diagnosis of mucormycosis and the patient improved post-operatively on systemic antifungals.

Conclusion

Such an association of mucormycosis with malaria infection is rarely reported in the literature and is hypothesized to be a result of immunosuppression caused by malaria. Also, emphasis is laid upon having a high index of suspicion for fungal infection in the setting of pansinusitis even if the risk factors are absent.

We hereby report a case of rhino-orbital mucormycosis following P.vivax malaria in a 20-year-old female with anemia and thrombocytopenia.

IL-4, IL-10, CCL2 and TGF-β as potential biomarkers for severity in Plasmodium vivax malaria

Cytokines and chemokines are immune response molecules that display diverse functions, such as inflammation and immune regulation. In Plasmodium vivax infections, the uncontrolled production of these molecules is thought to contribute to pathogenesis and has been proposed as a possible predictor for disease complications. The objective of this study was to evaluate the cytokine profile of P. vivax malaria patients with different clinical outcomes to identify possible immune biomarkers for severe P. vivax malaria. The study included patients with non-severe (n = 56), or severe (n = 50) P. vivax malaria and healthy controls (n = 50). Patient plasma concentrations of IL-4, IL-2, CXCL10, IL-1β, TNF-α, CCL2, IL-17A, IL-6, IL-10, IFN-γ, IL-12p70, CXCL8 and active TGF-β1 were determined through flow cytometry. The levels of several cytokines and chemokines, CXCL10, IL-10, IL-6, IL-4, CCL2 and IFN-γ were found to be significantly higher in severe, compared to non-severe P. vivax malaria patients. Severe thrombocytopenia was positively correlated with IL-4, CXCL10, IL-6, IL-10 and IFN-γ levels, renal dysfunction was related to an increase in IL-2, IL-1β, IL-17A and IL-8, and hepatic impairment with CXCL10, MCP-1, IL-6 and IFN-γ. A Lasso regression model suggests that IL-4, IL-10, CCL2 and TGF-β might be developed as biomarkers for severity in P. vivax malaria. Severe P. vivax malaria patients present specific cytokine and chemokine profiles that are different from non-severe patients and that could potentially be developed as biomarkers for disease severity.

Plasmodium vivax is one of the main species responsible for malaria in the world. The pathogenic mechanisms leading to the development of severe P. vivax malaria are not yet fully understood. Immune system molecules such as cytokines and chemokines actively participate in the control of the infection, however, their uncontrolled production can influence alterations in organs such as the liver, kidneys, among others. In this study we show that there is a differential concentration of some cytokines and chemokines between patients with non-severe malaria and severe P. vivax malaria; and that there are associations between these molecules with manifestations that occur in severe malaria. Four molecules with potential to become biomarkers of severity were identified.

A member of the tryptophan-rich protein family is required for efficient sequestration of Plasmodium berghei schizonts

Protein export and host membrane remodeling are crucial for multiple Plasmodium species to establish a niche in infected hosts. To better understand the contribution of these processes to successful parasite infection in vivo, we sought to find and characterize protein components of the intraerythrocytic Plasmodium berghei-induced membrane structures (IBIS) that form in the cytoplasm of infected erythrocytes. We identified proteins that immunoprecipitate with IBIS1, a signature member of the IBIS in P. berghei-infected erythrocytes. In parallel, we also report our data describing proteins that co-precipitate with the PTEX (Plasmodium translocon of exported proteins) component EXP2. To validate our findings, we examined the location of three candidate IBIS1-interactors that are conserved across multiple Plasmodium species, and we found they localized to IBIS in infected red blood cells and two further colocalized with IBIS1 in the liver-stage parasitophorous vacuole membrane. Successful gene deletion revealed that these two tryptophan-rich domain-containing proteins, termed here IPIS2 and IPIS3 (for intraerythrocytic Plasmodium-induced membrane structures), are required for efficient blood-stage growth. Erythrocytes infected with IPIS2-deficient schizonts in particular fail to bind CD36 as efficiently as wild-type P. berghei-infected cells and therefore fail to effectively sequester out of the circulating blood. Our findings support the idea that intra-erythrocytic membrane compartments are required across species for alterations of the host erythrocyte that facilitate interactions of infected cells with host tissues.

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.

Plasmodium knowlesi Cytoadhesion Involves SICA Variant Proteins

Plasmodium knowlesi poses a health threat throughout Southeast Asian communities and currently causes most cases of malaria in Malaysia. This zoonotic parasite species has been studied in Macaca mulatta (rhesus monkeys) as a model for severe malarial infections, chronicity, and antigenic variation. The phenomenon of Plasmodium antigenic variation was first recognized during rhesus monkey infections. Plasmodium-encoded variant proteins were first discovered in this species and found to be expressed at the surface of infected erythrocytes, and then named the Schizont-Infected Cell Agglutination (SICA) antigens. SICA expression was shown to be spleen dependent, as SICA expression is lost after P. knowlesi is passaged in splenectomized rhesus. Here we present data from longitudinal P. knowlesi infections in rhesus with the most comprehensive analysis to date of clinical parameters and infected red blood cell sequestration in the vasculature of tissues from 22 organs. Based on the histopathological analysis of 22 tissue types from 11 rhesus monkeys, we show a comparative distribution of parasitized erythrocytes and the degree of margination of the infected erythrocytes with the endothelium. Interestingly, there was a significantly higher burden of parasites in the gastrointestinal tissues, and extensive margination of the parasites along the endothelium, which may help explain gastrointestinal symptoms frequently reported by patients with P. knowlesi malarial infections. Moreover, this margination was not observed in splenectomized rhesus that were infected with parasites not expressing the SICA proteins. This work provides data that directly supports the view that a subpopulation of P. knowlesi parasites cytoadheres and sequesters, likely via SICA variant antigens acting as ligands. This process is akin to the cytoadhesive function of the related variant antigen proteins, namely Erythrocyte Membrane Protein-1, expressed by Plasmodium falciparum.

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.

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.

De Novo Assembly of Plasmodium knowlesi Genomes From Clinical Samples Explains the Counterintuitive Intrachromosomal Organization of Variant SICAvar and kir Multiple Gene Family Members

Plasmodium knowlesi, a malaria parasite of Old World macaque monkeys, is used extensively to model Plasmodium biology. Recently, P. knowlesi was found in the human population of Southeast Asia, particularly Malaysia. P. knowlesi causes uncomplicated to severe and fatal malaria in the human host with features in common with the more prevalent and virulent malaria caused by Plasmodium falciparum. As such, P. knowlesi presents a unique opportunity to develop experimental translational model systems for malaria pathophysiology informed by clinical data from same-species human infections. Experimental lines of P. knowlesi represent well-characterized genetically stable parasites, and to maximize their utility as a backdrop for understanding malaria pathophysiology, genetically diverse contemporary clinical isolates, essentially wild-type, require comparable characterization. The Oxford Nanopore PCR-free long-read sequencing platform was used to sequence and de novo assemble P. knowlesi genomes from frozen clinical samples. The sequencing platform and assembly pipelines were designed to facilitate capturing data and describing, for the first time, P. knowlesi schizont-infected cell agglutination (SICA) var and Knowlesi-Interspersed Repeats (kir) multiple gene families in parasites acquired from nature. The SICAvar gene family members code for antigenically variant proteins analogous to the virulence-associated P. falciparum erythrocyte membrane protein (PfEMP1) multiple var gene family. Evidence presented here suggests that the SICAvar family members have arisen through a process of gene duplication, selection pressure, and variation. Highly evolving genes including PfEMP1family members tend to be restricted to relatively unstable sub-telomeric regions that drive change with core genes protected in genetically stable intrachromosomal locations. The comparable SICAvar and kir gene family members are counter-intuitively located across chromosomes. Here, we demonstrate that, in contrast to conserved core genes, SICAvar and kir genes occupy otherwise gene-sparse chromosomal locations that accommodate rapid evolution and change. The novel methods presented here offer the malaria research community not only new tools to generate comprehensive genome sequence data from small clinical samples but also new insight into the complexity of clinically important real-world parasites.

In-depth biological analysis of alteration in Plasmodium knowlesi-infected red blood cells using a noninvasive optical imaging technique

Background

Imaging techniques are commonly used to understand disease mechanisms and their biological features in the microenvironment of the cell. Many studies have added to our understanding of the biology of the malaria parasite Plasmodium knowlesi from functional in vitro and imaging analysis using serial block-face scanning electron microscopy (SEM). However, sample fixation and metal coating during SEM analysis can alter the parasite membrane.

Methods

In this study, we used noninvasive diffraction optical tomography (DOT), also known as holotomography, to explore the morphological, biochemical, and mechanical alterations of each stage of P. knowlesi-infected red blood cells (RBCs). Each stage of the parasite was synchronized using Nycodenz and magnetic-activated cell sorting (MACS) for P. knowlesi and P. falciparum, respectively. Holotomography was applied to measure individual three-dimensional refractive index tomograms without metal coating, fixation, or additional dye agent.

Results

Distinct profiles were found on the surface area and hemoglobin content of the two parasites. The surface area of P. knowlesi-infected RBCs showed significant expansion, while P. falciparum-infected RBCs did not show any changes compared to uninfected RBCs. In terms of hemoglobin consumption, P. falciparum tended to consume hemoglobin more than P. knowlesi. The observed profile of P. knowlesi-infected RBCs generally showed similar results to other studies, proving that this technique is unbiased.

Conclusions

The observed profile of the surface area and hemoglobin content of malaria infected-RBCs can potentially be used as a diagnostic parameter to distinguish P. knowlesi and P. falciparum infection. In addition, we showed that holotomography could be used to study each Plasmodium species in greater depth, supporting strategies for the development of diagnostic and treatment strategies for malaria.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05182-1.

Cytoadherence Properties of Plasmodium knowlesi-Infected Erythrocytes

Plasmodium knowlesi is responsible for zoonotic malaria infections that are potentially fatal. While the severe pathology of falciparum malaria is associated with cytoadherence phenomena by Plasmodium falciparum-infected erythrocytes (IRBC), information regarding cytoadherence properties of P. knowlesi-IRBC remained scarce. Here, we characterized the cytoadherence properties of RBC infected with the laboratory-adapted P. knowlesi A1-H.1 strain. We found that late-stage IRBC formed rosettes in a human serum-dependent manner, and rosettes hampered IRBC phagocytosis. IRBC did not adhere much to unexposed (unstimulated) human endothelial cell lines derived from the brain (hCMEC/D3), lungs (HPMEC), and kidneys (HRGEC). However, after being "primed" with P. knowlesi culture supernatant, the IRBC-endothelial cytoadherence rate increased in HPMEC and HRGEC, but not in hCMEC/D3 cells. Both endothelial cytoadherence and rosetting phenomena were abrogated by treatment of P. knowlesi-IRBC with trypsin. We also found that different receptors were involved in IRBC cytoadherence to different types of endothelial cells. Although some of the host receptors were shared by both P. falciparum- and P. knowlesi-IRBC, the availability of glycoconjugates on the receptors might influence the capacity of P. knowlesi-IRBC to cytoadhere to these receptors.

Clinical recovery of Macaca fascicularis infected with Plasmodium knowlesi

BACKGROUND

Kra monkeys (Macaca fascicularis), a natural host of Plasmodium knowlesi, control parasitaemia caused by this parasite species and escape death without treatment. Knowledge of the disease progression and resilience in kra monkeys will aid the effective use of this species to study mechanisms of resilience to malaria. This longitudinal study aimed to define clinical, physiological and pathological changes in kra monkeys infected with P. knowlesi, which could explain their resilient phenotype.

METHODS

Kra monkeys (n = 15, male, young adults) were infected intravenously with cryopreserved P. knowlesi sporozoites and the resulting parasitaemias were monitored daily. Complete blood counts, reticulocyte counts, blood chemistry and physiological telemetry data (n = 7) were acquired as described prior to infection to establish baseline values and then daily after inoculation for up to 50 days. Bone marrow aspirates, plasma samples, and 22 tissue samples were collected at specific time points to evaluate longitudinal clinical, physiological and pathological effects of P. knowlesi infections during acute and chronic infections.

RESULTS

As expected, the kra monkeys controlled acute infections and remained with low-level, persistent parasitaemias without anti-malarial intervention. Unexpectedly, early in the infection, fevers developed, which ultimately returned to baseline, as well as mild to moderate thrombocytopenia, and moderate to severe anaemia. Mathematical modelling and the reticulocyte production index indicated that the anaemia was largely due to the removal of uninfected erythrocytes and not impaired production of erythrocytes. Mild tissue damage was observed, and tissue parasite load was associated with tissue damage even though parasite accumulation in the tissues was generally low.

CONCLUSIONS

Kra monkeys experimentally infected with P. knowlesi sporozoites presented with multiple clinical signs of malaria that varied in severity among individuals. Overall, the animals shared common mechanisms of resilience characterized by controlling parasitaemia 3-5 days after patency, and controlling fever, coupled with physiological and bone marrow responses to compensate for anaemia. Together, these responses likely minimized tissue damage while supporting the establishment of chronic infections, which may be important for transmission in natural endemic settings. These results provide new foundational insights into malaria pathogenesis and resilience in kra monkeys, which may improve understanding of human infections.

Harnessing the Potential of miRNAs in Malaria Diagnostic and Prevention

Despite encouraging progress over the past decade, malaria remains a major global health challenge. Its severe form accounts for the majority of malaria-related deaths, and early diagnosis is key for a positive outcome. However, this is hindered by the non-specific symptoms caused by malaria, which often overlap with those of other viral, bacterial and parasitic infections. In addition, current tools are unable to detect the nature and degree of vital organ dysfunction associated with severe malaria, as complications develop silently until the effective treatment window is closed. It is therefore crucial to identify cheap and reliable early biomarkers of this wide-spectrum disease. microRNAs (miRNAs), a class of small non-coding RNAs, are rapidly released into the blood circulation upon physiological changes, including infection and organ damage. The present review details our current knowledge of miRNAs as biomarkers of specific organ dysfunction in patients with malaria, and both promising candidates identified by pre-clinical models and important knowledge gaps are highlighted for future evaluation in humans. miRNAs associated with infected vectors are also described, with a view to expandind this rapidly growing field of research to malaria transmission and surveillance.

Purifying and Characterizing Bacterially Expressed Soluble Lactate Dehydrogenase from Plasmodium knowlesi for the Development of Anti-Malarial Drugs

Plasmodium lactate dehydrogenase (pLH) is one of the enzymes in glycolysis with potential target for chemotherapy. This study aimed to clone, overexpress and characterize soluble recombinant lactate dehydrogenase from Plasmodium knowlesi in a bacterial system. Synthetic P. knowlesi lactate dehydrogenase (Pk-LDH) gene was cloned into pET21a expression vector, transformed into Escherichia coli strain BL21 (DE3) expression system and then incubated for 18 h, 20 °C with the presence of 0.5 mM isopropyl β-d-thiogalactoside in Terrific broth supplemented with Magnesium sulfate, followed by protein purifications using Immobilized Metal Ion Affinity Chromatography and size exclusion chromatography (SEC). Enzymatic assay was conducted to determine the activity of the enzyme. SDS-PAGE analysis revealed that protein of 34 kDa size was present in the soluble fraction. In SEC, a single peak corresponding to the size of Pk-LDH protein was observed, indicating that the protein has been successfully purified. From MALDI-TOF analysis findings, a peptide score of 282 was established, which is significant for lactate dehydrogenase from P. knowlesi revealed via MASCOT analysis. Secondary structure analysis of CD spectra indicated 79.4% α helix and 1.37% β strand structure. Specific activity of recombinant Pk-LDH was found to be 475.6 U/mg, confirming the presence of active protein. Soluble Pk-LDH that is biologically active was produced, which can be used further in other malaria studies.

Knowlesi malaria: Human risk factors, clinical spectrum, and pathophysiology

Plasmodium knowlesi is endemic across Southeast Asia, and is the commonest cause of zoonotic malaria. The spectrum of clinical disease from P. knowlesi infection ranges from asymptomatic infection, through to severe malaria and death. Over 90% of clinical disease occurs in adults, mostly living in forest edge areas undergoing intensive land use change. With a 24-h asexual life cycle in humans, high parasite counts are possible, but most clinical cases of knowlesi malaria are uncomplicated with low parasitaemia. In co-endemic areas, median parasitaemia in knowlesi malaria is lower than that seen in vivax and falciparum malaria, suggesting a lower fever threshold. Severe malaria occurs in 6-9% of symptomatic adults. Manifestations of severe malaria from P. knowlesi are similar to those seen with falciparum malaria, with the notable absence of coma. Age, parasitaemia, cardiovascular comorbidities and delayed diagnosis are risk factors for severe disease and death, which are only seen in adults. Thrombocytopenia is near-universal in adults, likely related to platelet-red cell binding and clearance. Mechanisms underlying the microvascular sludging seen in fatal disease in non-natural primate hosts and the microvascular accumulation of parasites in fatal human disease are not clear. Marked reductions in deformability of both infected and uninfected red blood cells are associated with disease severity in both humans and other non-natural primate hosts, likely contributing to impaired microvascular perfusion and organ dysfunction. Endothelial activation, endothelial dysfunction, glycocalyx degradation and haemolysis are also associated with, and likely contribute to, severe disease and organ dysfunction, particularly acute kidney injury.

Performance Evaluation of Biozentech Malaria Scanner in Plasmodium knowlesi and P. falciparum as a New Diagnostic Tool

The computer vision diagnostic approach currently generates several malaria diagnostic tools. It enhances the accessible and straightforward diagnostics that necessary for clinics and health centers in malaria-endemic areas. A new computer malaria diagnostics tool called the malaria scanner was used to investigate living malaria parasites with easy sample preparation, fast and user-friendly. The cultured Plasmodium parasites were used to confirm the sensitivity of this technique then compared to fluorescence-activated cell sorting (FACS) analysis and light microscopic examination. The measured percentage of parasitemia by the malaria scanner revealed higher precision than microscopy and was similar to FACS. The coefficients of variation of this technique were 1.2–6.7% for Plasmodium knowlesi and 0.3–4.8% for P. falciparum. It allowed determining parasitemia levels of 0.1% or higher, with coefficient of variation smaller than 10%. In terms of the precision range of parasitemia, both high and low ranges showed similar precision results. Pearson’s correlation test was used to evaluate the correlation data coming from all methods. A strong correlation of measured parasitemia (r²=0.99, P<0.05) was observed between each method. The parasitemia analysis using this new diagnostic tool needs technical improvement, particularly in the differentiation of malaria species.

Prevalence of Cardiovascular Complications in Malaria: A Systematic Review and Meta-Analysis

Recent studies have suggested that malaria may affect the cardiovascular system. The aim of this systematic review and meta-analysis was to determine the prevalence of cardiovascular complications in symptomatic malaria patients. We searched databases such as Pubmed, Embase, Cochrane, and Web of Science (January 1950-April 2020) for studies reporting on cardiovascular complications in adults and children with malaria. Cardiovascular complications were defined as abnormalities in electrocardiogram (ECG), cardiac biomarkers, and echocardiography on admission or during outpatient examination. Studies of patients with known heart disease or cardiovascular evaluation performed after the start of intravenous antimalarial medication were excluded. The study was registered in International Prospective Register of Systematic Reviews (PROSPERO) (No.: CRD42020167672). The literature search yielded 1,243 studies, and a total of 43 studies with symptomatic malaria patients were included. Clinical studies (n = 12 adults; n = 5 children) comprised 3,117 patients, of which a majority had Plasmodium falciparum (n = 15) and were diagnosed with severe malaria (n = 13). In random-effects models of adults, the pooled prevalence estimate for any cardiovascular complication was 7% (95% CI: 5-9). No meta-analysis was conducted in children, but the range of abnormal ECG was 0-8%, cardiac biomarkers 0-57%, and echocardiography 4-9%. We analyzed 33 cases (n = 10 postmortem), in which the most common cardiovascular pathologies were myocarditis and acute coronary syndrome. All histopathological studies found evidence of parasitized red blood cells in the myocardium. Cardiovascular complications are not uncommon in symptomatic adults and children with malaria. Additional studies investigating malaria and cardiovascular disease are encouraged.

Plasmodium knowlesi - Clinical Isolate Genome Sequencing to Inform Translational Same-Species Model System for Severe Malaria

Malaria is responsible for unacceptably high morbidity and mortality, especially in Sub-Saharan African Nations. Malaria is caused by member species' of the genus Plasmodium and despite concerted and at times valiant efforts, the underlying pathophysiological processes leading to severe disease are poorly understood. Here we describe zoonotic malaria caused by Plasmodium knowlesi and the utility of this parasite as a model system for severe malaria. We present a method to generate long-read third-generation Plasmodium genome sequence data from archived clinical samples using the MinION platform. The method and technology are accessible, affordable and data is generated in real-time. We propose that by widely adopting this methodology important information on clinically relevant parasite diversity, including multiple gene family members, from geographically distinct study sites will emerge. Our goal, over time, is to exploit the duality of P. knowlesi as a well-used laboratory model and human pathogen to develop a representative translational model system for severe malaria that is informed by clinically relevant parasite diversity.

Deaths From Plasmodium knowlesi Malaria: Case Series and Systematic Review

BACKGROUND

Plasmodium knowlesi causes severe and fatal malaria, and incidence in Southeast Asia is increasing. Factors associated with death are not clearly defined.

METHODS

All malaria deaths in Sabah, Malaysia, from 2015 to 2017 were identified from mandatory reporting to the Sabah Department of Health. Case notes were reviewed, and a systematic review of these and all previously reported fatal P. knowlesi cases was conducted. Case fatality rates (CFRs) during 2010-2017 were calculated using incidence data from the Sabah Department of Health.

RESULTS

Six malaria deaths occurred in Sabah during 2015-2017, all from P. knowlesi. Median age was 40 (range, 23-58) years; 4 cases (67%) were male. Three (50%) had significant cardiovascular comorbidities and 1 was pregnant. Delays in administering appropriate therapy contributed to 3 (50%) deaths. An additional 26 fatal cases were included in the systematic review. Among all 32 cases, 18 (56%) were male; median age was 56 (range, 23-84) years. Cardiovascular-metabolic disease, microscopic misdiagnosis, and delay in commencing intravenous treatment were identified in 11 of 32 (34%), 26 of 29 (90%), and 11 of 31 (36%) cases, respectively. The overall CFR during 2010-2017 was 2.5/1000: 6.0/1000 for women and 1.7/1000 for men (P = .01). Independent risk factors for death included female sex (odds ratio, 2.6; P = .04), and age ≥45 years (odds ratio, 4.7; P < .01).

CONCLUSIONS

Earlier presentation, more rapid diagnosis, and administration of intravenous artesunate may avoid fatal outcomes, particularly in females, older adults, and patients with cardiovascular comorbidities.

Identification of Plasmodium knowlesi Merozoite Surface Protein-119 (PkMSP-119 ) novel binding peptides from a phage display library

OBJECTIVE

Plasmodium knowlesi, the fifth human malaria parasite, has caused mortality in humans. We aimed to identify P. knowlesi novel binding peptides through a random linear dodecapeptide phage display targeting the 19-kDa fragment of Merozoite Surface Protein-1 protein.

METHODS

rPkMSP-1₁₉ protein was heterologously expressed using Expresso^® Solubility and Expression Screening System and competent E. cloni^® 10G cells according to protocol. Three rounds of biopanning were performed on purified rPkMSP-1₁₉ to identify binding peptides towards rPkMSP-1₁₉ using Ph.D.™-12 random phage display library. Binding sites of the identified peptides to PkMSP-1₁₉ were in silico predicted using the CABS-dock web server.

RESULTS

Four phage peptide variants that bound to PkMSP-1₁₉ were identified after three rounds of biopanning, namely Pkd1, Pkd2, Pkd3 and Pkd4. The sequences of both Pkd1 and Pkd2 consist of a large number of histidine residues. Pkd1 showed positive binding signal with 6.1× vs. BSA control. Docking results showed that Pkd1 and Pkd2 were ideal binding peptides for PkMSP-1₁₉ .

CONCLUSION

We identified two novel binding peptides of PkMSP-1₁₉ , Pkd1 (HFPFHHHKLRAH) and Pkd2 (HPMHMLHKRQHG), through phage display. They provide a valuable starting point for the development of novel therapeutics.

Case Report: Plasmodium knowlesi Infection with Rhabdomyolysis in a Japanese Traveler to Palawan, the Philippines

Skeletal muscle is known to be damaged by falciparum malaria via sequestration of infected erythrocytes. We present a case of rhabdomyolysis caused by Plasmodium knowlesi infection. The patient had fever, myalgia, and muscle weakness 5 days after returning to Japan from Palawan, the Philippines. Blood test revealed thrombocytopenia and an elevated creatine kinase level. Although rhabdomyolysis resolved with fluid therapy, fever of 24-hour cycle continued and thrombocytopenia intensified. On day 7 of illness, Giemsa-stained thin blood smear revealed malaria parasites, with a parasite count of 2,380/μL, which were morphologically indistinguishable between P. knowlesi and Plasmodium malariae. Rapid diagnostic test showed a negative result. The pathogen was later confirmed to be P. knowlesi by nested polymerase chain reaction (PCR). The patient was successfully treated with artemether/lumefantrine. This case suggests that knowlesi malaria might be able to cause skeletal muscle damage.

Genetic diversity of Plasmodium knowlesi among human and long-tailed macaque populations in Peninsular Malaysia: The utility of microsatellite markers

It has been discovered that Plasmodium knowlesi (P. knowlesi) is transmitted from macaque to man. Thus, the aim of the present study was to determine P. knowlesi genetic diversity in both human (n = 147) and long-tailed macaque (n = 26) samples from high- and low-endemicity localities. Genotyping was performed using seven neutral microsatellite loci markers. The size of the alleles, multiplicity of infection (MOI), mean number of alleles (Na), expected heterozygosity (H_E), linkage disequilibrium (LD), and genetic differentiation (F_ST) were determined. In highly endemic P. knowlesi localities, the MOI for human and long-tailed macaque isolates was 1.04 and 1.15, respectively, while the Na was 11.14 and 7.86, respectively. Based on the allele frequency distribution for all loci, and with F_ST < 0.1, no genetic differentiation was seen between human and long-tailed macaque. In localities characterised by lower P. knowlesi endemicity, the MOI for human and long-tailed macaque isolates was 1.05 and 1.11, respectively, while the Na was 6.14 and 2.71, respectively. Further molecular analysis of the allele frequencies indicated that there was a significant genetic differentiation in human P. knowlesi isolates as compared to long-tailed macaque isolates, with a very low fixation index (F_ST = 0.016, p < .05) based on multiple loci analysis. Our results further indicate that, in Peninsular Malaysia, humans are mostly affected by P. knowlesi of a single genotype, while long-tailed macaque tend to acquire polyclonal infections, which supports the assumption that there is a higher rate of transmission among long-tailed macaque. Understanding the genetic diversity of P. knowlesi isolates can provide invaluable information for characterising patterns of the population structure and the migration rate of P. knowlesi in peninsular Malaysia.

Reduced red blood cell deformability in Plasmodium knowlesi malaria

The simian parasite Plasmodium knowlesi can cause severe and fatal human malaria. However, little is known about the pathogenesis of this disease. In falciparum malaria, reduced red blood cell deformability (RBC-D) contributes to microvascular obstruction and impaired organ perfusion. In P knowlesi infection, impaired microcirculatory flow has been observed in Macaca mulatta (rhesus macaques), unnatural hosts who develop severe and fatal disease. However, RBC-D has not been measured in human infection or in the natural host M fascicularis (long-tailed macaques). Using ektacytometry, we measured RBC-D in adults with severe and non-severe knowlesi and falciparum malaria and in healthy controls. In addition, we used micropipette aspiration to determine the relative stiffness of infected RBCs (iRBCs) and uninfected RBCs (uRBCs) in P knowlesi-infected humans and M fascicularis Ektacytometry demonstrated that RBC-D overall was reduced in human knowlesi malaria in proportion to disease severity, and in severe knowlesi malaria, it was comparable to that of severe falciparum malaria. RBC-D correlated inversely with parasitemia and lactate in knowlesi malaria and HRP2 in falciparum malaria, and it correlated with hemoglobin nadir in knowlesi malaria. Micropipette aspiration confirmed that in humans, P knowlesi infection increased stiffness of both iRBCs and uRBCs, with the latter mostly the result of echinocytosis. In contrast, in the natural host M fascicularis, echinocyte formation was not observed, and the RBC-D of uRBCs was unaffected. In unnatural primate hosts of P knowlesi, including humans, reduced deformability of iRBCs and uRBCs may represent a key pathogenic mechanism leading to microvascular accumulation, impaired organ perfusion, and anemia.

Contribution of Plasmodium immunomics: potential impact for serological testing and surveillance of malaria

Introduction: Plasmodium vivax (Pv) and P. knowlesi account together for a considerable share of the global burden of malaria, along with P. falciparum (Pf). However, inaccurate diagnosis and undetectable asymptomatic/submicroscopic malaria infections remain very challenging. Blood-stage antigens involved in either invasion of red blood cells or sequestration/cytoadherence of parasitized erythrocytes have been immunomics-characterized, and are vital for the detection of malaria incidence. Areas covered: We review the recent advances in Plasmodium immunomics to discuss serological markers with potential for specific and sensitive diagnosis of malaria. Insights on alternative use of immunomics to assess malaria prevalence are also highlighted. Finally, we provide practical applications of serological markers as diagnostics, with an emphasis on dot immunogold filtration assay which holds promise for malaria diagnosis and epidemiological surveys. Expert commentary: The approach largely contributes to Pf and Pv research in identifying promising non-orthologous antigens able to detect malaria incidence and to differentiate between past and recent infections. However, further studies to profiling naturally acquired immune responses are expected in order to help discover/validate serological markers of no cross-seroreactivity and guide control interventions. More so, the application of immunomics to knowlesi infections would help validate the recently identified antigens and contribute to the discovery of additional biomarkers of exposure, immunity, or both.

Genetic polymorphism and natural selection in the C-terminal 42 kDa region of merozoite surface protein-1 (MSP-1) among Plasmodium knowlesi samples from Malaysia

Background

The merozoite surface protein-1 (MSP-1) gene encodes for a leading malaria vaccine candidate antigen. However, its extensive polymorphic nature represents a major obstacle to the development of a protective vaccine. Previously, a pilot study was carried out to explore the sequence variation of the C-terminal 42 kDa fragment within P. knowlesi MSP-1 gene (PkMSP-1₄₂) based on 12 clinical samples; however, further study on an adequate sample size is vital in estimating the genetic diversity of the parasite population.

Methods

In the present study, we included a larger sample size of P. knowlesi (83 samples) covering eight states of Malaysia to determine the genetic polymorphism, natural selection and haplotype groups of the gene fragment coding PkMSP-1₄₂. The region flanking PkMSP-1₄₂ was amplified by PCR and directly sequenced. Genetic diversity, haplotype diversity, population genetic differentiation and natural selection were determined in order to study the polymorphic characteristic of PkMSP-1₄₂.

Results

A high level of genetic diversity (Hd = 0.970 ± 0.007; л = 0.01079 ± 0.00033) was observed among the 83 P. knowlesi samples, confirming the extensive genetic polymorphism exhibited among the P. knowlesi population found in Malaysia. A total of 18 distinct haplotypes with 17 amino acid changes were identified, whereby 15 were new haplotypes. High population differentiation values were observed within samples from Peninsular Malaysia and Malaysian Borneo. The 42 kDa fragments of P. knowlesi from Malaysian Borneo were found to be acting on balancing selection whilst purifying selection was suggested to act on isolates from Peninsular Malaysia. The separation of PkMSP-1₄₂ haplotypes into two main groups based on geographical separation has further supported the existence of two distinct P. knowlesi lineages.

Conclusions

A high level of genetic diversity was observed among PkMSP-1₄₂ in Malaysia, whereby most of the polymorphisms were found within the 33 kDa region. Taken together, these data will be useful in order to understand the nature of P. knowlesi population in Malaysia as well as the design and development of a MSP-1₄₂ based knowlesi malaria vaccine.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3234-5) contains supplementary material, which is available to authorized users.

Laboratory challenges of Plasmodium species identification in Aceh Province, Indonesia, a malaria elimination setting with newly discovered P. knowlesi

The discovery of the life-threatening zoonotic infection Plasmodium knowlesi has added to the challenges of prompt and accurate malaria diagnosis and surveillance. In this study from Aceh Province, Indonesia, a malaria elimination setting where P. knowlesi endemicity was not previously known, we report the laboratory investigation and difficulties encountered when using molecular detection methods for quality assurance of microscopically identified clinical cases. From 2014 to 2015, 20 (49%) P. falciparum, 16 (39%) P. vivax, 3 (7%) P. malariae, and 2 (5%) indeterminate species were identified by microscopy from four sentinel health facilities. At a provincial-level reference laboratory, loop-mediated isothermal amplification (LAMP), a field-friendly molecular method, was performed and confirmed Plasmodium in all samples though further species-identification was limited by the unavailability of non-falciparum species-specific testing with the platform used. At a national reference laboratory, several molecular methods including nested PCR (nPCR) targeting the 18 small sub-unit (18S) ribosomal RNA, nPCR targeting the cytochrome-b (cytb) gene, a P. knowlesi-specific nPCR, and finally sequencing, were necessary to ultimately classify the samples as: 19 (46%) P. knowlesi, 8 (20%) P. falciparum, 14 (34%) P. vivax. Microscopy was unable to identify or mis-classified up to 56% of confirmed cases, including all cases of P. knowlesi. With the nPCR methods targeting the four human-only species, P. knowlesi was missed (18S rRNA method) or showed cross-reactivity for P. vivax (cytb method). To facilitate diagnosis and management of potentially fatal P. knowlesi infection and surveillance for elimination of human-only malaria in Indonesia and other affected settings, new detection methods are needed for testing at the point-of-care and in local reference laboratories.

In Southeast Asia, Plasmodium knowlesi, a malaria parasite of macaques, was recently discovered to infect humans. This emerging disease is important because it has potential for causing severe disease and death, and it is a threat to malaria elimination efforts in the region. In this report from Aceh Province, Indonesia, where P. knowlesi was only recently discovered, the authors report on the laboratory challenges of distinguishing this species from other human species. Using several different molecular methods, they investigated 41 malaria cases which by microscopy, were initially reported as: P. falciparum (49%), P. vivax (39%), P. malariae (7%), and indeterminate (5%). Only after using a P. knowlesi-specific nPCR method and sequencing, did they find that nearly half were P. knowlesi. Consistent with a sparse literature, a field-friendly molecular method (genus-specific LAMP) reliably detected P. knowlesi, while use of a more standard reference laboratory molecular method (18S rRNA nPCR targeting the four human-only species) missed the infections. Also another reference laboratory molecular method (cytb nPCR) mis-classified P. knowlesi infections as P. vivax due to cross-reactivity. To address the emerging threat of P. knowlesi, new detection methods are needed for point-of-care and reference testing.

Human infection with Plasmodium knowlesi on the Laos-Vietnam border

Background

Border malaria in the Greater Mekong region of Southeast Asia poses a serious threat to the health of the ethnic minority populations of the region. Traditionally thought to be caused primarily by the malaria parasites Plasmodium falciparum and Plasmodium vivax, recently a zoonotic parasite, Plasmodium knowlesi, has been identified in some countries of the region. The presence of this parasite poses a challenge to malaria control programmes, as it is maintained in a zoonotic reservoir of forest-dwelling macaque monkeys.

Methods

A cross-sectional malaria parasite species prevalence survey was conducted along the Laos-Vietnam border in the central part of the two countries. Human blood samples were collected from Savannakhet in Laos and Quang Tri in Vietnam between August and October 2010 and assayed for the presence of human malaria parasite species and P. knowlesi. A PCR targeting the 18S small subunit ribosomal RNA gene and circumsporozoite protein gene was used for Plasmodium species identification.

Results

Nine cases of P. knowlesi were detected by PCR in blood samples from the Laos side and three from the Vietnam side. All P. knowlesi infections were found in co-infection with P. vivax, with some triple infections of P. knowlesi, P. vivax and P. falciparum detected in Laos. Phylogenetic analysis of these parasites suggests that P. knowlesi is circulating in the Laos-Vietnam border region.

Conclusion

This report shows that P. knowlesi is transmited on both sides of the Vietnam-Laos border. Continued monitoring of the range and prevalence of P. knowlesi on both the sides of Laos-Vietnam border is of importance to the National Malaria Control Programmes of both countries.

Electronic supplementary material

The online version of this article (10.1186/s41182-018-0116-7) contains supplementary material, which is available to authorized users.

Age-Related Clinical Spectrum of Plasmodium knowlesi Malaria and Predictors of Severity

Background

Plasmodium knowlesi is increasingly reported in Southeast Asia, but prospective studies of its clinical spectrum in children and comparison with autochthonous human-only Plasmodium species are lacking.

Methods

Over 3.5 years, we prospectively assessed patients of any age with molecularly-confirmed Plasmodium monoinfection presenting to 3 district hospitals in Sabah, Malaysia.

Results

Of 481 knowlesi, 172 vivax, and 96 falciparum malaria cases enrolled, 44 (9%), 71 (41%), and 31 (32%) children aged ≤12 years. Median parasitemia was lower in knowlesi malaria (2480/μL [interquartile range, 538-8481/μL]) than in falciparum (9600/μL; P < .001) and vivax malaria. In P. knowlesi, World Health Organization-defined anemia was present in 82% (95% confidence interval [CI], 67%-92%) of children vs 36% (95% CI, 31%-41%) of adults. Severe knowlesi malaria occurred in 6.4% (95% CI, 3.9%-8.3%) of adults but not in children; the commenst severity criterion was acute kideny injury. No patient had coma. Age, parasitemia, schizont proportion, abdominal pain, and dyspnea were independently associated with severe knowlesi malaria, with parasitemia >15000/μL the best predictor (adjusted odds ratio, 16.1; negative predictive value, 98.5%; P < .001). Two knowlesi-related adult deaths occurred (fatality rate: 4.2/1000 adults).

Conclusions

Age distribution and parasitemia differed markedly in knowlesi malaria compared to human-only species, with both uncomplicated and severe disease occurring at low parasitemia. Severe knowlesi malaria occurred only in adults; however, anemia was more common in children despite lower parasitemia. Parasitemia independently predicted knowlesi disease severity: Intravenous artesunate is warranted initially for those with parasitemia >15000/μL.

Malaria Elimination: Time to Target All Species

Important strides have been made within the past decade toward malaria elimination in many regions, and with this progress, the feasibility of eradication is once again under discussion. If the ambitious goal of eradication is to be achieved by 2040, all species of Plasmodium infecting humans will need to be targeted with evidence-based and concerted interventions. In this perspective, the potential barriers to achieving global malaria elimination are discussed with respect to the related diversities in host, parasite, and vector populations. We argue that control strategies need to be reorientated from a sequential attack on each species, dominated by Plasmodium falciparum to one that targets all species in parallel. A set of research themes is proposed to mitigate the potential setbacks on the pathway to a malaria-free world.

Intravascular haemolysis in severe Plasmodium knowlesi malaria: association with endothelial activation, microvascular dysfunction, and acute kidney injury

Plasmodium knowlesi occurs throughout Southeast Asia, and is the most common cause of human malaria in Malaysia. Severe disease in humans is characterised by high parasite biomass, reduced red blood cell deformability, endothelial activation and microvascular dysfunction. However, the roles of intravascular haemolysis and nitric oxide (NO)-dependent endothelial dysfunction, important features of severe falciparum malaria, have not been evaluated, nor their role in acute kidney injury (AKI). In hospitalised Malaysian adults with severe (n = 48) and non-severe (n = 154) knowlesi malaria, and in healthy controls (n = 50), we measured cell-free haemoglobin (CFHb) and assessed associations with the endothelial Weibel–Palade body (WPB) constituents, angiopoietin-2 and osteoprotegerin, endothelial and microvascular function, and other markers of disease severity. CFHb was increased in knowlesi malaria in proportion to disease severity, and to a greater extent than previously reported in severe falciparum malaria patients from the same study cohort. In knowlesi malaria, CFHb was associated with parasitaemia, and independently associated with angiopoietin-2 and osteoprotegerin. As with angiopoietin-2, osteoprotegerin was increased in proportion to disease severity, and independently associated with severity markers including creatinine, lactate, interleukin-6, endothelial cell adhesion molecules ICAM-1 and E-selectin, and impaired microvascular reactivity. Osteoprotegerin was also independently associated with NO-dependent endothelial dysfunction. AKI was found in 88% of those with severe knowlesi malaria. Angiopoietin-2 and osteoprotegerin were both independent risk factors for acute kidney injury. Our findings suggest that haemolysis-mediated endothelial activation and release of WPB constituents is likely a key contributor to end-organ dysfunction, including AKI, in severe knowlesi malaria.

Bone Marrow Is a Major Parasite Reservoir in Plasmodium vivax Infection

Plasmodium vivax causes heavy burdens of disease across malarious regions worldwide. Mature P. vivax asexual and transmissive gametocyte stages occur in the blood circulation, and it is often assumed that accumulation/sequestration in tissues is not an important phase in their development. Here, we present a systematic study of P. vivax stage distributions in infected tissues of nonhuman primate (NHP) malaria models as well as in blood from human infections. In a comparative analysis of the transcriptomes of P. vivax and Plasmodium falciparum blood-stage parasites, we found a conserved cascade of stage-specific gene expression despite the greatly different gametocyte maturity times of these two species. Using this knowledge, we validated a set of conserved asexual- and gametocyte-stage markers both by quantitative real-time PCR and by antibody assays of peripheral blood samples from infected patients and NHP (Aotus sp.). Histological analyses of P. vivax parasites in organs of 13 infected NHP (Aotus and Saimiri species) demonstrated a major fraction of immature gametocytes in the parenchyma of the bone marrow, while asexual schizont forms were enriched to a somewhat lesser extent in this region of the bone marrow as well as in sinusoids of the liver. These findings suggest that the bone marrow is an important reservoir for gametocyte development and proliferation of malaria parasites.IMPORTANCEPlasmodium vivax malaria continues to cause major public health burdens worldwide. Yet, significant knowledge gaps in the basic biology and epidemiology of P. vivax malaria remain, largely due to limited available tools for research and diagnostics. Here, we present a systematic examination of tissue sequestration during P. vivax infection. Studies of nonhuman primates and malaria patients revealed enrichment of developing sexual stages (gametocytes) and mature replicative stages (schizonts) in the bone marrow and liver, relative to those present in peripheral blood. Identification of the bone marrow as a major P. vivax tissue reservoir has important implications for parasite diagnosis and treatment.

Genome-wide mosaicism in divergence between zoonotic malaria parasite subpopulations with separate sympatric transmission cycles

Plasmodium knowlesi is a significant cause of human malaria transmitted as a zoonosis from macaque reservoir hosts in South‐East Asia. Microsatellite genotyping has indicated that human infections in Malaysian Borneo are an admixture of two highly divergent sympatric parasite subpopulations that are, respectively, associated with long‐tailed macaques (Cluster 1) and pig‐tailed macaques (Cluster 2). Whole‐genome sequences of clinical isolates subsequently confirmed the separate clusters, although fewer of the less common Cluster 2 type were sequenced. Here, to analyse population structure and genomic divergence in subpopulation samples of comparable depth, genome sequences were generated from 21 new clinical infections identified as Cluster 2 by microsatellite analysis, yielding a cumulative sample size for this subpopulation similar to that for Cluster 1. Profound heterogeneity in the level of intercluster divergence was distributed across the genome, with long contiguous chromosomal blocks having high or low divergence. Different mitochondrial genome clades were associated with the two major subpopulations, but limited exchange of haplotypes from one to the other was evident, as was also the case for the maternally inherited apicoplast genome. These findings indicate deep divergence of the two sympatric P. knowlesi subpopulations, with introgression likely to have occurred recently. There is no evidence yet of specific adaptation at any introgressed locus, but the recombinant mosaic types offer enhanced diversity on which selection may operate in a currently changing landscape and human environment. Loci responsible for maintaining genetic isolation of the sympatric subpopulations need to be identified in the chromosomal regions showing fixed differences.

Malaria Pathogenesis

In the mosquito-human life cycle, the six species of malaria parasites infecting humans (Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale wallickeri, Plasmodium ovale curtisi, Plasmodium malariae, and Plasmodium knowlesi) undergo 10 or more morphological states, replicate from single to 10,000+ cells, and vary in total population from one to many more than 10⁶ organisms. In the human host, only a small number of these morphological stages lead to clinical disease and the vast majority of all malaria-infected patients in the world produce few (if any) symptoms in the human. Human clinical disease (e.g., fever, anemia, coma) is the result of the parasite preprogrammed biology in concert with the human pathophysiological response. Caveats and corollaries that add variation to this host-parasite interaction include parasite genetic diversity of key proteins, coinfections, comorbidities, delays in treatment, human polymorphisms, and environmental determinants.

Pathogenic CD8+ T Cells Cause Increased Levels of VEGF-A in Experimental Malaria-Associated Acute Respiratory Distress Syndrome, but Therapeutic VEGFR Inhibition Is Not Effective

Malaria is a severe disease and kills over 400,000 people each year. Malarial complications are the main cause of death and include cerebral malaria and malaria-associated acute respiratory distress syndrome (MA-ARDS). Despite antimalarial treatment, lethality rates of MA-ARDS are still between 20 and 80%. Patients develop pulmonary edema with hemorrhages and leukocyte extravasation in the lungs. The vascular endothelial growth factor-A (VEGF-A) and the placental growth factor (PlGF) are vascular permeability factors and may be involved in the disruption of the alveolar-capillary membrane, leading to alveolar edema. We demonstrated increased pulmonary VEGF-A and PlGF levels in lungs of mice with experimental MA-ARDS. Depletion of pathogenic CD8⁺ T cells blocked pulmonary edema and abolished the increase of VEGF-A and PlGF. However, neutralization of VEGF receptor-2 (VEGFR-2) with the monoclonal antibody clone DC101 did not decrease pulmonary pathology. The broader spectrum receptor tyrosine kinase inhibitor sunitinib even increased lung pathology. These data suggest that the increase in alveolar VEGF-A and PlGF is not a cause but rather a consequence of the pulmonary pathology in experimental MA-ARDS and that therapeutic inhibition of VEGF receptors is not effective and even contra-indicated.

Effects of Aging on Parasite Biomass, Inflammation, Endothelial Activation, Microvascular Dysfunction and Disease Severity in Plasmodium knowlesi and Plasmodium falciparum Malaria

Background

In populations pauci-immune to malaria, risk of severe malaria increases with age. This is particularly apparent in Plasmodium knowlesi malaria. However, pathophysiological mechanisms underlying knowlesi malaria, and of the age-related increase in risk of severe malaria in general, are poorly understood.

Methods

In Malaysian patients aged ≥12 years with severe (n = 47) and nonsevere (n = 99) knowlesi malaria, severe (n = 21) and nonsevere (n = 109) falciparum malaria, and healthy controls (n = 50), we measured parasite biomass, systemic inflammation (interleukin 6 [IL-6]), endothelial activation (angiopoietin-2), and microvascular function, and evaluated the effects of age.

Results

Plasmodium knowlesi parasitemia correlated with age (Spearman's correlation coefficient [rs] = 0.36; P < .0001). In knowlesi malaria, IL-6, angiopoietin-2, and microvascular dysfunction were increased in severe compared to nonsevere disease, and all correlated with age, independent of parasitemia. In falciparum malaria, angiopoietin-2 increased with age, independent of parasite biomass (histidine-rich protein 2 [HRP2]). Independent risk factors for severe malaria included parasitemia and angiopoietin-2 in knowlesi malaria, and HRP2, angiopoietin-2, and microvascular dysfunction in falciparum malaria.

Conclusions

Parasite biomass, endothelial activation, and microvascular dysfunction are associated with severe disease in knowlesi malaria and likely contribute to pathogenesis. The association of each of these processes with aging may account for the greater severity of malaria observed in older adults in low-endemic regions.

Respiratory Complications of Plasmodium vivax Malaria: Systematic Review and Meta-Analysis

Malaria, a major global public health problem, is mainly caused by Plasmodium falciparum and Plasmodium vivax, and is responsible for nearly half a million deaths annually. Although P. vivax malaria was not believed to cause severe disease, recent robust studies have proved otherwise. However, the clinical spectrum and pathogenesis of severe vivax malaria and, especially, its respiratory complications remain poorly understood. A systematic search for articles reporting respiratory complications associated with vivax malaria was performed in Lilacs, Cochrane, Scielo, Web of Science, and Medline databases irrespective of publication date. Prevalence of acute respiratory distress syndrome (ARDS) and associated mortality among vivax patients were calculated from cross-sectional and longitudinal studies, whereas factors associated with mortality were calculated from data pooled from case reports and series of cases. A total of 101 studies were included (49 cross-sectional or longitudinal and 52 case reports or series of cases). Prevalence of ARDS was 2.8% and 2.2% in children and adults, respectively, with nearly 50% mortality. Moreover, female sex (P = 0.013), having any comorbidity (P = 0.036), lower body temperature (P = 0.032), lower hemoglobin (P = 0.043), and oxygen saturation (P = 0.053) values were significantly associated with mortality. Plasmodium vivax malaria respiratory complications included ARDS and were associated with high mortality. Demographics and clinical characteristics upon presentation to hospital were associated with mortality among patients with respiratory complications in vivax malaria. This study reaffirms the evidence of severe and fatal complications of P. vivax malaria and its associated respiratory complications.

The Duffy binding protein (PkDBPαII) of Plasmodium knowlesi from Peninsular Malaysia and Malaysian Borneo show different binding activity level to human erythrocytes

Background

The zoonotic Plasmodium knowlesi is a major cause of human malaria in Malaysia. This parasite uses the Duffy binding protein (PkDBPαII) to interact with the Duffy antigen receptor for chemokines (DARC) receptor on human and macaque erythrocytes to initiate invasion. Previous studies on P. knowlesi have reported distinct Peninsular Malaysia and Malaysian Borneo PkDBPαII haplotypes. In the present study, the differential binding activity of these haplotypes with human and macaque (Macaca fascicularis) erythrocytes was investigated.

Methods

The PkDBPαII of Peninsular Malaysia and Malaysian Borneo were expressed on the surface of COS-7 cells and tested with human and monkey erythrocytes, with and without anti-Fy6 (anti-Duffy) monoclonal antibody treatment. Binding activity level was determined by counting the number of rosettes formed between the transfected COS-7 cells and the erythrocytes.

Results

Anti-Fy6 treatment was shown to completely block the binding of human erythrocytes with the transfected COS-7 cells, thus verifying the specific binding of human DARC with PkDBPαII. Interestingly, the PkDBPαII of Peninsular Malaysia displayed a higher binding activity with human erythrocytes when compared with the Malaysian Borneo PkDBPαII haplotype (mean number of rosettes formed = 156.89 ± 6.62 and 46.00 ± 3.57, respectively; P < 0.0001). However, no difference in binding activity level was seen in the binding assay using M. fascicularis erythrocytes.

Conclusion

This study is the first report of phenotypic difference between PkDBPαII haplotypes. The biological implication of this finding is yet to be determined. Therefore, further studies need to be carried out to determine whether this differential binding level can be associated with severity of knowlesi malaria in human.

Rupture and Release: A Role for Soluble Erythrocyte Content in the Pathology of Cerebral Malaria

Cerebral malaria (CM) is the most severe form of malaria and causes high associated mortality. We propose a multistep process for CM pathology that is initiated by cytoadhesion of infected erythrocytes to the brain vasculature, followed by rupture and release of contents that complete the disruption of the blood-brain barrier.

Genetic diversity in the C-terminus of merozoite surface protein 1 among Plasmodium knowlesi isolates from Selangor and Sabah Borneo, Malaysia

Plasmodium knowlesi, a malaria parasite of macaques, has emerged as an important parasite of humans. Despite the significance of P. knowlesi malaria in parts of Southeast Asia, very little is known about the genetic variation in this parasite. Our aim here was to explore sequence variation in a molecule called the 42kDa merozoite surface protein-1 (MSP-1), which is found on the surface of blood stages of Plasmodium spp. and plays a key role in erythrocyte invasion. Several studies of P. falciparum have reported that the C-terminus (a 42kDa fragment) of merozoite surface protein-1 (MSP-1₄₂; consisting of MSP-1₁₉ and MSP-1₃₃) is a potential candidate for a malaria vaccine. However, to date, no study has yet investigated the sequence diversity of the gene encoding P. knowlesi MSP-1₄₂ (comprising Pk-msp-1₁₉ and Pk-msp-1₃₃) among isolates in Malaysia. The present study explored this aspect. Twelve P. knowlesi isolates were collected from patients from hospitals in Selangor and Sabah Borneo, Malaysia, between 2012 and 2014. The Pk-msp-1₄₂ gene was amplified by PCR and directly sequenced. Haplotype diversity (Hd) and nucleotide diversity (л) were studied among the isolates. There was relatively high genetic variation among P. knowlesi isolates; overall Hd and л were 1±0.034 and 0.01132±0.00124, respectively. A total of nine different haplotypes related to amino acid alterations at 13 positions, and the Pk-MSP-1₁₉ sequence was found to be more conserved than Pk-msp-1₃₃. We have found evidence for negative selection in Pk-msp-₄₂ as well as the 33kDa and 19kDa fragments by comparing the rate of non-synonymous versus synonymous substitutions. Future investigations should study large numbers of samples from disparate geographical locations to critically assess whether this molecule might be a potential vaccine target for P. knowlesi.

Clinical features and management of Plasmodium knowlesi infections in humans

Plasmodium knowlesi is a simian malaria of primarily the macaque species of South East Asia. While it was known that human infections could be induced during the years of malariotherapy, naturally occurring P. knowlesi human infections were thought to be rare. However, in 2004, knowlesi infections became recognized as an important infection amongst human populations in Sarawak, Malaysian Borneo. Since then, it has become recognized as a disease affecting people living and visiting endemic areas across South East Asia. Over the last 12 years, clinical studies have improved our understanding of this potentially fatal disease. In this review article the current literature is reviewed to give a comprehensive description of the disease and treatment.

The dangers of accepting a single diagnosis: case report of concurrent Plasmodium knowlesi malaria and dengue infection

Background

Dengue and malaria are two common, mosquito-borne infections, which may lead to mortality if not managed properly. Concurrent infections of dengue and malaria are rare due to the different habitats of its vectors and activities of different carrier mosquitoes. The first case reported was in 2005. Since then, several concurrent infections have been reported between the dengue virus (DENV) and the malaria protozoans, Plasmodium falciparum and Plasmodium vivax. Symptoms of each infection may be masked by a simultaneous second infection, resulting in late treatment and severe complications. Plasmodium knowlesi is also a common cause of malaria in Malaysia with one of the highest rates of mortality. This report is one of the earliest in literature of concomitant infection between DENV and P. knowlesi in which a delay in diagnosis had placed a patient in a life-threatening situation.

Case presentation

A 59-year old man staying near the Belum-Temengor rainforest at the Malaysia–Thailand border was admitted with fever for 6 days, with respiratory distress. His non-structural protein 1 antigen and Anti-DENV Immunoglobulin M tests were positive. He was treated for severe dengue with compensated shock. Treating the dengue had so distracted the clinicians that a blood film for the malaria parasite was not done. Despite aggressive supportive treatment in the intensive care unit (ICU), the patient had unresolved acidosis as well as multi-organ failure involving respiratory, renal, liver, and haematological systems. It was due to the presentation of shivering in the ICU, that a blood film was done on the second day that revealed the presence of P. knowlesi with a parasite count of 520,000/μL. The patient was subsequently treated with artesunate-doxycycline and made a good recovery after nine days in ICU.

Conclusions

This case contributes to the body of literature on co-infection between DENV and P. knowlesi and highlights the clinical consequences, which can be severe. Awareness should be raised among health-care workers on the possibility of dengue-malaria co-infection in this region. Further research is required to determine the real incidence and risk of co-infection in order to improve the management of acute febrile illness.

Plasmodium knowlesi: a relevant, versatile experimental malaria model

The primate malaria Plasmodium knowlesi has a long-standing history as an experimental malaria model. Studies using this model parasite in combination with its various natural and experimental non-human primate hosts have led to important advances in vaccine development and in our understanding of malaria invasion, immunology and parasite-host interactions. The adaptation to long-term in vitro continuous blood stage culture in rhesus monkey, Macaca fascicularis and human red blood cells, as well as the development of various transfection methodologies has resulted in a highly versatile experimental malaria model, further increasing the potential of what was already a very powerful model. The growing evidence that P. knowlesi is an important human zoonosis in South-East Asia has added relevance to former and future studies of this parasite species.

Plasmodium knowlesi Skeleton-Binding Protein 1 Localizes to the 'Sinton and Mulligan' Stipplings in the Cytoplasm of Monkey and Human Erythrocytes

The malaria parasite, Plasmodium, exports protein products to the infected erythrocyte to introduce modifications necessary for the establishment of nutrient acquisition and surface display of host interaction ligands. Erythrocyte remodeling impacts parasite virulence and disease pathology and is well documented for the human malaria parasite Plasmodium falciparum, but has been less described for other Plasmodium species. For P. falciparum, the exported protein skeleton-binding protein 1 (PfSBP1) is involved in the trafficking of erythrocyte surface ligands and localized to membranous structures within the infected erythrocyte, termed Maurer's clefts. In this study, we analyzed SBP1 orthologs across the Plasmodium genus by BLAST analysis and conserved gene synteny, which were also recently described by de Niz et al. (2016). To evaluate the localization of an SBP1 ortholog, we utilized the zoonotic malaria parasite, Plasmodium knowlesi. Immunofluorescence assay of transgenic P. knowlesi parasites expressing epitope-tagged recombinant PkSBP1 revealed a punctate staining pattern reminiscent of Maurer's clefts, following infection of either monkey or human erythrocytes. The recombinant PkSBP1-positive puncta co-localized with Giemsa-stained structures, known as ‘Sinton and Mulligan’ stipplings. Immunoelectron microscopy also showed that recombinant PkSBP1 localizes within or on the membranous structures akin to the Maurer's clefts. The recombinant PkSBP1 expressed in P. falciparum-infected erythrocytes co-localized with PfSBP1 at the Maurer's clefts, indicating an analogous trafficking pattern. A member of the P. knowlesi 2TM protein family was also expressed and localized to membranous structures in infected monkey erythrocytes. These results suggest that the trafficking machinery and induced erythrocyte cellular structures of P. knowlesi are similar following infection of both monkey and human erythrocytes, and are conserved with P. falciparum.

Falling Plasmodium knowlesi Malaria Death Rate among Adults despite Rising Incidence, Sabah, Malaysia, 2010-2014

The decreased notification-fatality rate is likely associated with improved use of intravenous artesunate for severe malaria.

Deaths from Plasmodium knowlesi malaria have been linked to delayed parenteral treatment. In Malaysia, early intravenous artesunate is now recommended for all severe malaria cases. We describe P. knowlesi fatalities in Sabah, Malaysia, during 2012–2014 and report species-specific fatality rates based on 2010–2014 case notifications. Sixteen malaria-associated deaths (caused by PCR-confirmed P. knowlesi [7], P. falciparum [7], and P. vivax [1] and microscopy-diagnosed “P. malariae” [1]) were reported during 2012–2014. Six patients with severe P. knowlesi malaria received intravenous artesunate at hospital admission. For persons >15 years of age, overall fatality rates during 2010–2014 were 3.4, 4.2, and 1.0 deaths/1,000 P. knowlesi, P. falciparum, and P. vivax notifications, respectively; P. knowlesi–associated fatality rates fell from 9.2 to1.6 deaths/1,000 notifications. No P. knowlesi–associated deaths occurred among children, despite 373 notified cases. Although P. knowlesi malaria incidence is rising, the notification-fatality rate has decreased, likely due to improved use of intravenous artesunate.

Prevalence of malaria, prevention measures, and main clinical features in febrile children admitted to the Franceville Regional Hospital, Gabon

Recently, major progress has been made in controlling malaria in Africa. However, in Gabon, little information is available on the role of malaria in childhood febrile syndromes, the use and efficacy of preventive measures, and Plasmodium species distribution. Here, we characterized malaria in febrile children in Franceville, Gabon through a cross-sectional study at the pediatric unit of the Franceville Regional Hospital. We registered 940 febrile children. Their general condition was markedly altered in 11.7% of cases (n = 89/760); among them 19 (21.4%) had a severely altered condition. Malaria was the second most frequent etiology (22.0%; n = 162/738), after respiratory tract infections (37.3%; n = 275/738). Children with malaria (63 ± 39 months) were older than children without malaria (40 ± 37 months) (p = 0.0013). Hemoglobin, red blood cell, white blood cell, and platelet values were lower in children with malaria than in those without malaria (p < 0.0001). Anemia was the most common feature of severe malaria (70.6%; n = 12/17), followed by neurological involvement (23.5%; n = 4/17). The prevalence of malaria was significantly higher in children older than 60 months than in younger children (40% vs. 15.5%; p < 0.0001). Plasmodium falciparum accounted for 97.5% of cases (158/162), followed by Plasmodium malariae (2.5%; n = 4/162). Bed net use was high (74.4%; n = 697/936) and contributed to malaria prevention (p = 0.001). Good basic knowledge of malaria also had a preventive effect (p < 0.0001). The prevalence of malaria in children in Franceville did not decrease significantly from 2009 to 2012, remaining at about 20%, highlighting that preventive measures should be reinforced.