Parasitemia characteristics of Plasmodium vivax malaria patients in the Republic of Korea

Morphological atypia and molecular profile of Plasmodium vivax: Findings from an outbreak in the Brazilian Amazon

This study aimed to perform morphological and molecular analyses of parasites isolated from the blood of malaria-infected individuals during an outbreak in the Microregion of Cametá, State of Pará, Brazilian Amazon. A total of 260 positive samples were identified by microscopy as Plasmodium vivax; however, in three samples, forms considered unusual for the species were found and defined as morphological atypia of P. vivax. Single P. vivax infection was confirmed by qPCR in all samples. Among 256 genotyped samples, the VK247 genotype alone was identified in 255 samples, and the VK210 genotype was found in only one. The study showed that this malaria outbreak was caused by the etiological agent P. vivax, and for the first time, morphological atypia was described in isolates circulating in Brazil. Likewise, for the first time, the VK247 genotype was detected predominantly in single infections in an area of the State of Pará, which may suggest a greater circulation of the genotype in the region.

Alternative Invasion Mechanisms and Host Immune Response to Plasmodium vivax Malaria: Trends and Future Directions

Plasmodium vivax malaria is a neglected tropical disease, despite being more geographically widespread than any other form of malaria. The documentation of P. vivax infections in different parts of Africa where Duffy-negative individuals are predominant suggested that there are alternative pathways for P. vivax to invade human erythrocytes. Duffy-negative individuals may be just as fit as Duffy-positive individuals and are no longer resistant to P.vivax malaria. In this review, we describe the complexity of P. vivax malaria, characterize pathogenesis and candidate invasion genes of P. vivax, and host immune responses to P. vivax infections. We provide a comprehensive review on parasite ligands in several Plasmodium species that further justify candidate genes in P. vivax. We also summarize previous genomic and transcriptomic studies related to the identification of ligand and receptor proteins in P. vivax erythrocyte invasion. Finally, we identify topics that remain unclear and propose future studies that will greatly contribute to our knowledge of P. vivax.

Genetic Diversity of Plasmodium vivax Causing Epidemic Malaria in the Republic of Korea

Plasmodium vivax is more challenging to control and eliminate than P. falciparum due to its more asymptomatic infections with low parasite densities making diagnosis more difficult, in addition to its unique biological characteristics. The potential re-introduction of incidence cases, either through borders or via human migrations, is another major hurdle to sustained control and elimination. The Republic of Korea has experienced re-emergence of vivax malaria in 1993 but is one of the 32 malaria-eliminating countries to-date. Despite achieving successful nationwide control and elimination of vivax malaria, the evolutionary characteristics of vivax malaria isolates in the Republic of Korea have not been fully understood. In this review, we present an overview of the genetic variability of such isolates to increase understanding of the epidemiology, diversity, and dynamics of vivax populations in the Republic of Korea.

Blood-Stage Parasitaemia and Age Determine Plasmodium falciparum and P. vivax Gametocytaemia in Papua New Guinea

A better understanding of human-to-mosquito transmission is crucial to control malaria. In order to assess factors associated with gametocyte carriage, 2083 samples were collected in a cross-sectional survey in Papua New Guinea. Plasmodium species were detected by light microscopy and qPCR and gametocytes by detection of pfs25 and pvs25 mRNA transcripts by reverse-transcriptase PCR (qRT-PCR). The parasite prevalence by PCR was 18.5% for Plasmodium falciparum and 13.0% for P. vivax. 52.5% of all infections were submicroscopic. Gametocytes were detected in 60% of P. falciparum-positive and 51% of P. vivax-positive samples. Each 10-fold increase in parasite density led to a 1.8-fold and 3.3-fold increase in the odds of carrying P. falciparum and P. vivax gametocytes. Thus the proportion of gametocyte positive and gametocyte densities was highest in young children carrying high asexual parasite densities and in symptomatic individuals. Dilution series of gametocytes allowed absolute quantification of gametocyte densities by qRT-PCR and showed that pvs25 expression is 10-20 fold lower than pfs25 expression. Between 2006 and 2010 parasite prevalence in the study site has decreased by half. 90% of the remaining infections were asymptomatic and likely constitute an important reservoir of transmission. However, mean gametocyte densities were low (approx. 1-2 gametocyte/μL) and it remains to be determined to what extent low-density gametocyte positive individuals are infective to mosquitos.

Chemotherapeutic strategies for reducing transmission of Plasmodium vivax malaria

Effective use of anti-malarial drugs is key to reducing the transmission potential of Plasmodium vivax. In patients presenting with symptomatic disease, treatment with potent and relatively slowly eliminated blood schizontocidal regimens administered concurrently with a supervised course of 7 mg/kg primaquine over 7-14 days has potential to exert the greatest transmission-blocking benefit. Given the spread of chloroquine-resistant P. vivax strains, the artemisinin combination therapies dihydroartemisinin + piperaquine and artesunate + mefloquine are currently the most assured means of preventing P. vivax recrudescence. Preliminary evidence suggests that, like chloroquine, these combinations potentiate the hypnozoitocidal effect of primaquine, but further supportive evidence is required. In view of the high rate of P. vivax relapse following falciparum infections in co-endemic regions, there is a strong argument for broadening current radical cure policy to include the administration of hypnozoitocidal doses of primaquine to patients with Plasmodium falciparum malaria. The most important reservoir for P. vivax transmission is likely to be very low-density, asymptomatic infections, the majority of which will arise from liver-stage relapses. Therefore, judicious mass administration of hypnozoitocidal therapy will reduce transmission of P. vivax to a greater extent than strategies focused on treatment of symptomatic patients. An efficacious hypnozoitocidal agent with a short curative treatment course would be particularly useful in mass drug administration campaigns.

Quantitative determination of Plasmodium parasitemia by flow cytometry and microscopy

The traditional light microscopy has limitations for precise growth assays of malaria parasites in culture or for assessment of new compounds for antimalarial activity; the speed and high reproducibility of flow cytometry can overcome these limitations. A flow cytometric method using PicoGreen, a DNA-binding fluorochrome, was developed with optimal precision suitable for performing growth assays of low-parasitemia field isolates. In addition, intra- and inter-person reproducibility of the flow cytometric and the microscopic method were compared in order to quantitatively demonstrate the improved precision. RNase treatment contributed to the precision of the flow cytometric measurements by enhancing the signal-to-noise ratios. Coefficients of variation of the method were smaller than 10% for 0.1% or higher parasitemia samples. The intra- and inter-person coefficients of variation of the flow cytometric method were three to six times smaller than those of the microscopic method. The flow cytometric method developed in this study yielded substantially more precise results than the microscopic method, allowing determination of parasitemia levels of 0.1% or higher, with coefficients of variation smaller than 10%. Thus, the PicoGreen method could be a reliable high sensitivity assay for analysis of low parasitemia samples and might be applied to a high throughput system testing antimalarial drug activity.

Clinical proteomics of the neglected human malarial parasite Plasmodium vivax

Recent reports highlight the severity and the morbidity of disease caused by the long neglected malaria parasite Plasmodium vivax. Due to inherent difficulties in the laboratory-propagation of P. vivax, the biology of this parasite has not been adequately explored. While the proteome of P. falciparum, the causative agent of cerebral malaria, has been extensively explored from several sources, there is limited information on the proteome of P. vivax. We have, for the first time, examined the proteome of P. vivax isolated directly from patients without adaptation to laboratory conditions. We have identified 153 proteins from clinical P. vivax, majority of which do not show homology to any previously known gene products. We also report 29 new proteins that were found to be expressed in P. vivax for the first time. In addition, several proteins previously implicated as anti-malarial targets, were also found in our analysis. Most importantly, we found several unique proteins expressed by P. vivax.This study is an important step in providing insight into physiology of the parasite under clinical settings.

Lack of molecular correlates of Plasmodium vivax ookinete development

Previous studies of Plasmodium vivax transmission to Anopheles spp. mosquitoes have not been able to predict mosquito infectivity on the basis of microscopic or molecular quantification of parasites (total parasites in the sample or total number of gametocytes) in infected blood. Two methods for production of P. vivax ookinete cultures in vitro, with yields of 10(6) macrogametocytes, 10(4) zygotes, and 10(3) ookinetes, respectively, per 10 mL of P. vivax-infected patient blood with approximately 0.01% parasitemia, were used to study P. vivax sexual stage development. The quantity of gametocytes, determined by counting Giemsa-stained blood smears, and quantity and type of gametocyte as determined by quantitative reverse transcriptase-polymerase chain reaction for Pvalpha tubulin II and macrogametocyte-specific pvg377 did not predict ookinete yield. Factors that affect the efficiency of in vitro P. vivax ookinete transformation remain poorly understood.

Epidemiology and infectivity of Plasmodium falciparum and Plasmodium vivax gametocytes in relation to malaria control and elimination

Malaria remains a major cause of morbidity and mortality in the tropics, with Plasmodium falciparum responsible for the majority of the disease burden and P. vivax being the geographically most widely distributed cause of malaria. Gametocytes are the sexual-stage parasites that infect Anopheles mosquitoes and mediate the onward transmission of the disease. Gametocytes are poorly studied despite this crucial role, but with a recent resurgence of interest in malaria elimination, the study of gametocytes is in vogue. This review highlights the current state of knowledge with regard to the development and longevity of P. falciparum and P. vivax gametocytes in the human host and the factors influencing their distribution within endemic populations. The evidence for immune responses, antimalarial drugs, and drug resistance influencing infectiousness to mosquitoes is reviewed. We discuss how the application of molecular techniques has led to the identification of submicroscopic gametocyte carriage and to a reassessment of the human infectious reservoir. These components are drawn together to show how control measures that aim to reduce malaria transmission, such as mass drug administration and a transmission-blocking vaccine, might better be deployed.