Advances in P. vivax Malaria Research

Resources

Websites

American Society of Parasitologists
A diverse group of over 800 scientists from industry, government, and academia who are interested in the study and teaching of parasitology.

Bill & Melinda Gates Foundation: Malaria
Provides details on the Gates Foundation's support for research and development leading to more effective treatments, diagnostics, and mosquito-control measures and a safe and effective malaria vaccine.

Brazilian Society of Tropical Medicine
Professional society that promotes and encourages studies and research on the numerous aspects of tropical medicine, including malaria.

Centers for Disease Control: Malaria
CDC resource providing information on malaria in the U.S. and around the world.

Irish Society for Parasitology
Professional society dedicated to advancing the study of parasitology by facilitating communication, promoting the dissemination of information, and developing resources for teaching and research in parasitology.

Fogarty International Center at the NIH: Malaria Information and Resources
Organization dedicated to supporting global health research conducted by U.S. and international investigators, building partnerships between U.S. and international health research institutions, and training the next generation of scientists to address global health needs.

Malaria Eradication Research Agenda (malERA), ISGlobal
Rigorous scientific consultative process intended to identify knowledge gaps and new tools that will be needed to eradicate malaria globally. Results of the process were published in a PLoS monograph in 2011.

Malaria Host–Pathogen Interaction Center (MaHPIC)
Team of interdisciplinary, global scientific thinkers interested in using systems biology approaches to understand malarial disease more thoroughly, specifically how recrudescence, relapse, host susceptibility, and coinfections affect the various clinical scenarios as they relate to human infection.

Medicines for Malaria Venture
Nonprofit public–private partnership established to reduce the burden of malaria in disease-endemic countries by discovering, developing, and facilitating delivery of new, effective, and affordable antimalarial drugs.

PATH Malaria Vaccine Initiative
Global program dedicated to accelerating the development of malaria vaccines and catalyzing timely access in endemic countries, established in 1999 by a grant from the Bill & Melinda Gates Foundation.

World Health Organization: World Malaria Report 2012
Report summarizing information received from 104 malaria-endemic countries and other sources, highlighting the progress toward global malaria targets for 2015 and describing current challenges for global malaria control and elimination.

Journal Articles

Alonso PL, Tanner M. Public health challenges and prospects for malaria control and elimination. Nat Med. 2013;19:150-5.

Anstey NM, Douglas NM, Poespoprodjo JR, et al. Plasmodium vivax: clinical spectrum, risk factors and pathogenesis. Adv Parasitol. 2012;80:151-201.

Baird JK. Elimination therapy for the endemic malarias. Curr Infect Dis Rep. 2012;14:227-37.

Baird JK. Primaquine toxicity forestalls effective therapeutic management of the endemic malarias. Int J Parasitol. 2012;42:1049-54.

Baird JK. Reinventing primaquine for endemic malaria. Expert Opin Emerg Drugs. 2012;17:439-44.

Baird JK. Evidence and implications of mortality associated with acute Plasmodium vivax malaria. Clin Microbiol Rev. 2013;26:36-57.

Baird KJ, Maguire JD, Price RN. Diagnosis and treatment of Plasmodium vivax malaria. Adv Parasitol. 2012;80:203-70.

Barber BE, William T, Grigg MJ, et al. Evaluation of the sensitivity of a pLDH-based and an aldolase-based rapid diagnostic test for diagnosis of uncomplicated and severe malaria caused by PCR-confirmed Plasmodium knowlesi, Plasmodium falciparum, and Plasmodium vivax. J Clin Microbiol. 2013;51:1118-23.

Bardají A, Bassat Q, Alonso PL, et al. Intermittent preventive treatment of malaria in pregnant women and infants: making best use of the available evidence. Expert Opin Pharmacother. 2012;13:1719-36.

Bardají A, Sigauque B, Sanz S, et al. Impact of malaria at the end of pregnancy on infant mortality and morbidity. J Infect Dis. 2011;203:691-9.

Battle KE, Gething PW, Elyazar IR, et al. The global public health significance of Plasmodium vivax. Adv Parasitol. 2012;80:1-111.

Betuela I, Bassat Q, Kiniboro B, et al. Tolerability and safety of primaquine in Papua New Guinean children 1 to 10 years of age. Antimicrob Agents Chemother. 2012;56:2146-9.

Betuela I, Rosanas-Urgell A, Kiniboro B, et al. Relapses contribute significantly to the risk of Plasmodium vivax infection and disease in Papua New Guinean children 1–5 years of age. J Infect Dis. 2012;206:1771-80.

Birkett AJ, Moorthy VS, Loucq C, et al. Malaria vaccine R&D in the Decade of Vaccines: breakthroughs, challenges and opportunities. Vaccine. 2013;31 Suppl 2:B233-43.

Brehm K, Carlton JM, Hoffmann KF. Parasite genomics and post-genomic activities: 21st century resources for the parasite immunologist. Parasite Immunol. 2012;34:47-9.

Carlton JM, Das A, Escalante AA. Genomics, population genetics and evolutionary history of Plasmodium vivax. Adv Parasitol. 2013;81:203-22.

Committee WHOMPA, Secretariat. Inaugural meeting of the malaria policy advisory committee to the WHO: conclusions and recommendations. Malar J. 2012;11:137.

Costa FT, Lopes SC, Albrecht L, et al. On the pathogenesis of Plasmodium vivax malaria: perspectives from the Brazilian field. Int J Parasitol. 2012;42:1099-105.

Culleton R, Coban C, Zeyrek FY, et al. The origins of African Plasmodium vivax; insights from mitochondrial genome sequencing. PLoS One. 2011;6:e29137.

D'Souza BJ, Newman RD. Strengthening the policy setting process for global malaria control and elimination. Malar J. 2012;11:28.

Del Portillo HA, Ferrer M, Brugat T, et al. The role of the spleen in malaria. Cell Microbiol. 2012;14:343-55.

Dembele L, Gego A, Zeeman AM, et al. Towards an in vitro model of Plasmodium hypnozoites suitable for drug discovery. PLoS One. 2011;6:e18162.

Douglas NM, John GK, von Seidlein L, et al. Chemotherapeutic strategies for reducing transmission of Plasmodium vivax malaria. Adv Parasitol. 2012;80:271-300.

Douglas NM, Simpson JA, Phyo AP, et al. Gametocyte dynamics and the role of drugs in reducing the transmission potential of Plasmodium vivax. J Infect Dis. 2013;208(5):801-12.

Dye C, Mertens T, Hirnschall G, et al. WHO and the future of disease control programmes. Lancet. 2013;381:413-8.

Galinski MR, Meyer EV, Barnwell JW. Plasmodium vivax: modern strategies to study a persistent parasite's life cycle. Adv Parasitol. 2013;81:1-26.

Garcia-Basteiro AL, Bassat Q, Alonso PL. Approaching the target: the path towards an effective malaria vaccine. Mediterr J Hematol Infect Dis. 2012;4:e2012015.

Gething PW, Elyazar IR, Moyes CL, et al. A long neglected world malaria map: Plasmodium vivax endemicity in 2010. PLoS Negl Trop Dis. 2012;6:e1814.

Jiang J, Barnwell JW, Meyer EV, et al. Plasmodium vivax merozoite surface protein-3 (PvMSP3): expression of an 11 member multigene family in blood-stage parasites. PLoS One. 2013;8:e63888.

Korenromp EL, Hosseini M, Newman RD, et al. Progress towards malaria control targets in relation to national malaria programme funding. Malar J. 2013;12:18.

Lacerda MV, Fragoso SC, Alecrim MG, et al. Postmortem characterization of patients with clinical diagnosis of Plasmodium vivax malaria: to what extent does this parasite kill? Clin Infect Dis. 2012;55:e67-74.

Lanca EF, Magalhaes BM, Vitor-Silva S, et al. Risk factors and characterization of Plasmodium vivax-associated admissions to pediatric intensive care units in the Brazilian Amazon. PLoS One. 2012;7:e35406.

Liu W, Li Y, Learn GH, et al. Origin of the human malaria parasite Plasmodium falciparum in gorillas. Nature. 2010;467: 420-5.

Lopez FJ, Bernabeu M, Fernandez-Becerra C, et al. A new computational approach redefines the subtelomeric vir superfamily of Plasmodium vivax. BMC Genomics. 2013;14:8.

Machado Siqueira A, Lopes Magalhaes BM, Cardoso Melo G, et al. Spleen rupture in a case of untreated Plasmodium vivax infection. PLoS Negl Trop Dis. 2012;6:e1934.

Mathias DK, Plieskatt JL, Armistead JS, et al. Expression, immunogenicity, histopathology, and potency of a mosquito-based malaria transmission-blocking recombinant vaccine. Infect Immun. 2012;80:1606-14.

Mayor A, Bardají A, Felger I, et al. Placental infection with Plasmodium vivax: a histopathological and molecular study. J Infect Dis. 2012;206:1904-10.

Meister S, Plouffe DM, Kuhen KL, et al. Imaging of Plasmodium liver stages to drive next-generation antimalarial drug discovery. Science. 2011;334:1372-7.

Mohien CU, Colquhoun DR, Mathias DK, et al. A bioinformatics approach for integrated transcriptomic and proteomic comparative analyses of model and non-sequenced anopheline vectors of human malaria parasites. Mol Cell Proteomics. 2013;12:120-31.

Moreno A, Cabrera-Mora M, Garcia A, et al. Plasmodium coatneyi in rhesus macaques replicates the multisystemic dysfunction of severe malaria in humans. Infect Immun. 2013;81:1889-904.

Mueller I, Galinski MR, Tsuboi T, et al. Natural acquisition of immunity to Plasmodium vivax: epidemiological observations and potential targets. Adv Parasitol. 2013;81:77-131.

Neafsey DE. Genome sequencing sheds light on emerging drug resistance in malaria parasites. Nat Genet. 2013;45:589-90.

Neafsey DE, Christophides GK, Collins FH, et al. The evolution of the anopheles 16 genomes project. G3 (Bethesda). 2013;3:1191-4.

Neafsey DE, Galinsky K, Jiang RH, et al. The malaria parasite Plasmodium vivax exhibits greater genetic diversity than Plasmodium falciparum. Nat Genet. 2012;44:1046-50.

Newman RD. Relegating malaria resurgences to history. Malar J. 2012;11:123.

Ntumngia FB, King CL, Adams JH. Finding the sweet spots of inhibition: understanding the targets of a functional antibody against Plasmodium vivax Duffy binding protein. Int J Parasitol. 2012;42:1055-62.

Parish LA, Colquhoun DR, Ubaida Mohien C, et al. Ookinete-interacting proteins on the microvillar surface are partitioned into detergent resistant membranes of Anopheles gambiae midguts. J Proteome Res. 2011;10:5150-62.

Price RN. Potential of artemisinin-based combination therapies to block malaria transmission. J Infect Dis. 2013;207:1627-9.

Price RN, Auburn S, Marfurt J, et al. Phenotypic and genotypic characterisation of drug-resistant Plasmodium vivax. Trends Parasitol. 2012;28:522-9.

Sarda V, Kaslow DC, Williamson KC. Approaches to malaria vaccine development using the retrospectroscope. Infect Immun. 2009;77:3130-40.

von Seidlein L, Auburn S, Espino F, et al. Review of key knowledge gaps in glucose-6-phosphate dehydrogenase deficiency detection with regard to the safe clinical deployment of 8-aminoquinoline treatment regimens: a workshop report. Malar J. 2013;12:112.

Shanks GD. Control and elimination of Plasmodium vivax. Adv Parasitol. 2012;80:301-41.

Shanks GD, White NJ. The activation of vivax malaria hypnozoites by infectious diseases. Lancet Infect Dis. 2013. [Epub ahead of print]

Sharp PM, Liu W, Learn GH, et al. Source of the human malaria parasite Plasmodium falciparum. Proc Natl Acad Sci U S A. 2011;108:E744-5.

Sharp PM, Rayner JC, Hahn BH. Evolution. Great apes and zoonoses. Science. 2013;340: 284-6.

Siddiqui AA, Xainli J, Schloegel J, et al. Fine specificity of Plasmodium vivax Duffy binding protein binding engagement of the Duffy antigen on human erythrocytes. Infect Immun. 2012;80:2920-8.

Sutanto I, Tjahjono B, Basri H, et al. Randomized, open-label trial of primaquine against vivax malaria relapse in Indonesia. Antimicrob Agents Chemother. 2013;57:1128-35.

Tachibana S, Sullivan SA, Kawai S, et al. Plasmodium cynomolgi genome sequences provide insight into Plasmodium vivax and the monkey malaria clade. Nat Genet. 2012;44:1051-5.

Volkman SK, Neafsey DE, Schaffner SF, et al. Harnessing genomics and genome biology to understand malaria biology. Nat Rev Genet. 2012;13:315-28.

Yeo TW, Lampah DA, Kenangalem E, et al. Increased carboxyhemoglobin in adult falciparum malaria is associated with disease severity and mortality. J Infect Dis. 2013;208(5):813-7.

Sponsors

Presented by

Bronze Sponsor

Fundación Ramón Areces

Grant Support

Funding for this conference was made possible [in part] by Award Number R13AI106238 from the National Institute of Allergy and Infectious Diseases of the National Institutes of Health. The views expressed in written conference materials or publications and by speakers and moderators do not necessarily reflect the official policies of the Department of Health and Human Services; nor does mention of trade names, commercial practices, or organizations imply endorsement by the U.S. Government.

The attendance of endemic country researchers was supported by Grant Number D43TW007884-06S2 from the Fogarty International Center of the U.S. National Institutes of Health and by grant #1012808 from the Burroughs Wellcome Fund.

The global burden of malaria

Malaria kills over 660 000 people every year, most of them children under the age of 5. Most cases are caused by the parasite Plasmodium falciparum, which is endemic to sub-Saharan Africa and causes the highest number of malaria deaths. However, another malaria parasite, Plasmodium vivax, is more widespread geographically and is a considerable source of morbidity and mortality. P. vivax caused an estimated 19.4 million malaria cases in 2010, concentrated in Asia and Latin America. As P. falciparum has come under control in some areas, there is a renewed focus on P. vivax. An estimated 2.6 billion people are at risk of contracting a P. vivax infection, but there are substantial gaps in our understanding of the pathophysiology of this virus. On May 28–29, 2013, a diverse group of P. vivax investigators gathered at the CosmoCaixa in Barcelona, Spain, to present their latest research on this challenging pathogen.

Malaria is a complex disease that involves the malaria parasite, a human or non-human primate host, and a vector, the Anopheles mosquito, which carries the parasites from one host to another. During its life cycle, the malaria parasite, in a form known as the sporozoite, is injected from mosquito salivary glands into the host's bloodstream. The sporozoite is carried to the liver, where it invades liver cells and differentiates into a schizont. The schizont ruptures and releases another form, the merozoite, back into the bloodstream. Merozoites invade red blood cells and form more schizonts, which again rupture to release merozoites. These blood-stage forms are responsible for the uncomfortable symptoms and negative clinical manifestations of malaria. Some merozoites differentiate into gametocytes, which are taken up when the infected host is bitten by another mosquito. In the mosquito, the gametocytes undergo further transformations and make their way from the mosquito's midgut back into the salivary glands, forming sporozoites that can be injected into another host.

Large-scale approaches

Malaria research has benefited from new research technologies, including genomics, proteomics, microfluidics, and structure-based drug design, which are accelerating the pace of new discoveries. The first day of the conference began with a series of presentations on large-scale approaches, featuring Jane M. Carlton, who is sequencing the genomes of different P. vivax strains; Daniel E. Neafsey, who is sequencing the genomes of 16 Anopheles mosquito species; and Rhoel R. Dinglasan, who is using multiple types of 'omic techniques to investigate the interactions of P. vivax with the mosquito midgut.

Mary R. Galinski described a systems biology approach to understanding P. vivax, its life cycle, and its host interactions. Later in the conference, Paul Sharp described complementary genetic research that has yielded evolutionary insights, based on sequencing of P. vivax mitochondrial DNA found in the feces of African great apes.

Drugs and vaccines

Vaccine development is essential to efforts to eliminate infectious diseases. Hernando del Portillo presented work to identify P. vivax antigens that might be included in new P. vivax-targeted vaccines. Christopher L. King described the characterization of natural immune responses to P. vivax infection. David C. Kaslow outlined broad strategies that will be needed to develop and test P. vivax vaccine candidates.

P. vivax differs from the more common P. falciparum in its ability to form a dormant liver stage known as the hypnozoite, which activates periodically to cause disease relapses. Several speakers presented work on the development of new treatments or the optimization of current treatments for P. vivax malaria. J. Kevin Baird discussed the challenges presented by the dormant hypnozoite stage, which makes it more difficult to use classical epidemiologic methods and to study the efficacy of new treatments. Dominique Mazier presented her work on novel in vitro systems for modeling hypnozoite formation and activation, which will be essential for testing new drugs against P. vivax.

Ric Price described the recent rise of chloroquine resistance in P. vivax and discussed strategies to circumvent it. George Dennis Shanks discussed how the use of primaquine, the only registered agent known to kill hypnozoites, could be improved through a better understanding of its pharmacology and by optimizing therapeutic regimens. Brice Campo provided an update on the Medicines for Malaria Venture drug-development portfolio, which includes multiple novel candidates against P. vivax in preclinical and clinical development.

Severe P. vivax disease

Although most previously healthy individuals recover well from P. vivax malaria, some patients develop severe disease which can lead to death. Nicholas Anstey provided an overview of severe P. vivax disease and the factors that lead to its development. Marcus V. G. Lacerda described findings from a recent series of autopsies on patients with confirmed P. vivax disease. Quique Bassat presented results from a series of nearly 800 hospitalized P. vivax cases, with detailed characterization of clinical factors, comorbidities, and coinfections for the severe cases. Pregnancy is a risk factor for developing severe disease, and later in the conference Azucena Bardají presented preliminary results from a multicenter collaborative cohort study of P. vivax in pregnant women.

Three speakers, Robert D. Newman, Pedro Alonso, and Alan J. Magill, provided broad overviews of the worldwide impact of P. vivax and described progress toward eliminating the infection. Much of the research presented at the conference was strategically designed to bridge knowledge gaps that must be filled before P. vivax malaria can successfully be eradicated.

Highlights

The WHO has made P. vivax prevention and control a priority in anticipation of the planned Global Technical Strategy for Malaria Control and Elimination, 2016–2025.

Malaria has been eliminated in nearly 20 countries, but 2.6 billion people are currently at risk of contracting P. vivax malaria, with the largest burdens of P. vivax disease found in India, Indonesia, Brazil, and Ethiopia.

Strategic Plan for P. vivax Control and Elimination

The keynote speaker, Robert D. Newman of the World Health Organization (WHO), provided insights on the current epidemiology of P. vivax malaria. Malaria has been eliminated in close to 20 countries, and cases of malaria are decreasing in almost all areas of the world. Of the 219 million cases in 2010, 45 million occurred outside Africa, and 39% of those were due to P. vivax infection. According to WHO estimates, 2.6 billion people are currently at risk of contracting P. vivax malaria. The largest burdens of P. vivax disease occur in India, Indonesia, Brazil, and Ethiopia.

P. falciparum and P. vivax have several biological differences, the most important of which is that P. vivax forms a dormant liver stage, the hypnozoite, which requires 14 days of primaquine therapy to eradicate. This dormant liver stage leads to multiple relapses if left untreated. Because it does not have a dormant stage, P. falciparum transmission is somewhat easier to control; therefore, large-scale prevention and control measures lead to a higher proportion of malaria cases that are caused by P. vivax infection. Preventing malaria depends on understanding mosquito resting and biting behaviors in specific geographic areas, but there is a lack of information about the mosquito species that carry P. vivax, further complicating control efforts.

The status of malaria elimination in 2012. (Image courtesy of Robert D. Newman)

WHO treatment guidelines for P. vivax malaria recommend chloroquine combined with primaquine, including at least a 14-day course of primaquine to eliminate hypnozoites. Both drugs have been in use for about six decades, and P. vivax has become increasingly resistant to chloroquine in some areas. Newer combination therapies are much more expensive, raising questions about optimal therapeutic choices. In addition, primaquine use has not been adopted in all transmission areas. Primaquine treatment is associated with a risk of hemolytic anemia in individuals who carry genotypes that lead to deficiency in the enzyme glucose-6-phosphate dehydrogenase (G6PD). Health workers may hesitate to administer primaquine because of this risk, particularly since reliable field tests to detect G6PD genetic polymorphisms are not currently available.

The WHO Malaria Policy Advisory Committee has called for the development of a global P. vivax prevention and control strategy to respond to the high burden of disease and the technical challenges involved in eradicating P. vivax malaria. This strategy will form part of the Global Technical Strategy for Malaria Control and Elimination, which will be implemented from 2016 to 2025. Accomplishing the goals outlined in these strategies will depend on continued funding for research, control, and elimination activities, as well as the political will to keep programs running. In addition, key research questions need to be answered before public health officials can design long-term plans to eradicate P. vivax malaria.

Highlights

Recent genomic studies of P. vivax and the mosquito vectors that carry it are poised to add considerably to our knowledge of how malaria is transmitted, particularly outside sub-Saharan Africa.

Proteomic and glycoproteomic studies have identified potential targets for drugs or vaccines that would block transmission of P. vivax from mosquito to host.

The P. vivax genome

The first major session of the conference was devoted to some of the newest approaches to understanding P. vivax epidemiology and pathophysiology: genomic and proteomic studies. The Malaria Eradication Research Agenda project (malERA) identified a lack of genome sequences as an important knowledge gap, and multiple research groups have stepped in to supply this information.

Jane M. Carlton of New York University described her work sequencing the genomes of four geographically distinct P. vivax isolates from patients in Brazil, India, Mauritania, and North Korea. These four genomes show a much higher genetic diversity than equivalent isolates of P. falciparum, suggesting that it may be difficult to develop vaccines that work against multiple P. vivax strains. There are now five published complete genome sequences for P. vivax, and another seven sequences that provide information at the level of single nucleotide polymorphisms. Carlton and her group are also sampling the genetic diversity of about 160 P. vivax isolates from around the world in collaboration with the Broad Institute.

Carlton has pioneered techniques for reducing human DNA contamination, one of the major obstacles to sequencing P. vivax genomes. Her group has also sequenced the first reference genome for Plasmodium cynomolgi, the closest living relative of P. vivax. The parasite P. cynomolgi is a one of a number of Plasmodium species that infects Old World monkeys, and it can infect humans in experimental situations. Two other research groups are preparing draft sequences for this organism. Comparisons of these genomes to each other and to other Plasmodium species that infect monkeys and humans will provide insights into common pathways that are involved in transmission, infection, and the parasite's life cycle, identifying targets for new therapeutic approaches. The high degree of homology between the P. vivax and P. cynomolgi genomes also reinforces the idea that the latter is a useful experimental system that will produce results that are applicable to human malarial infections.

Summary of P. vivax genomes published to date. (Image courtesy of Jane M. Carlton)

Vector genomes

To understand the transmission and spread of malaria, it is necessary to understand the biology of the vector, which can be any one of more than 60 different species of Anopheles mosquito. Until recently, the only publicly available, high-quality malaria vector genome sequence was that of Anopheles gambiae, which is largely responsible for transmitting P. falciparum. This dearth of sequence information limited the possibilities for using genomics to study transmission of P. vivax outside sub-Saharan Africa, where A. gambiae is endemic. Daniel E. Neafsey and his colleagues at the Broad Institute are leading an effort to sequence the genomes of 16 other Anopheles species, including several important vectors of P. vivax. Nine gene assemblies were released in the spring of 2013 and seven more are to follow in the near future.

Comparisons of Anopheles genomes may answer some important questions about the mosquito's role in malaria. Researchers will better understand the evolutionary relationships between Anopheles species, and they hope to find out why certain mosquitoes transmit Plasmodium parasites and others do not. A better understanding of physiological functions, such as chemoreception, reproduction, and blood digestion, should provide leads for new methods to control mosquitoes and parasites. Genome sequences may eventually lead to systems that can rapidly identify mosquito species in the field, which will aid epidemiologic studies as well as prevention and control efforts.

Evolutionary relationships among Anopheles mosquito species. (Image courtesy of Daniel E. Neafsey)

Anopheles species genomes carry a high level of genetic diversity but are also highly repetitive, to a much greater degree than either humans or P. falciparum. Genomic studies require large quantities of DNA, which can be difficult to obtain, particularly as not all mosquito species are amenable to laboratory cultivation. Neafsey and his colleagues have developed new protocols for obtaining DNA from mosquitoes and new methods for assembling large-scale sequences with a high level of repetition, which are likely to benefit other vector-sequencing projects.

A hybrid approach

Rhoel R. Dinglasan of Johns Hopkins University described a hybrid approach that uses multiple types of 'omic techniques to investigate the interactions between P. vivax and its mosquito vectors. He and his group are interested in developing agents that block P. vivax transmission by preventing the parasite from establishing itself within the mosquito. Such an approach would block a single, early stage of transmission, potentially alleviating the need to block polymorphic forms as they develop later in the parasite's life cycle and catching the parasites at a stage in which their total population is low. Dinglasan noted that blocking transmission is a particularly attractive approach to malaria prevention because it would reduce the spread of resistant parasites and allow existing drugs to be used for longer.

Transmission-blocking approaches to eradicating malaria. (Image courtesy of Rhoel R. Dinglasan)

The first place where malaria parasite and vector interact is in the mosquito midgut, after the mosquito has taken up a blood meal. Multiple midgut proteins have been identified as potential sites of parasite interaction, and vaccines targeting two of these proteins are in development. One potential target, known as P25, is currently under development by an Asian research group, whose results suggest that this protein will make an effective vaccine component. Dinglasan and his group are working on another potential target, aminopeptidase 1, which was identified using glycoproteomics. They have developed a vaccine targeting a conserved domain of this protein, which has been shown to block malaria transmission in macaques using an experimental system including A. gambiae and P. falciparum.

Dinglasan has used glycomics to identify a glycosaminoglycan compound (similar to chondroitin sulfate) that appears to mediate Plasmodium invasion of the mosquito midgut. He is working on creating molecular mimics that would block transmission through this avenue. Another approach uses combined transcriptomics and proteomics to identify membrane-associated proteins from the midguts of A. gambiae and A. albimanus that might serve as vaccine or drug targets. The research group has developed many new techniques that could be applied to other types of malaria mosquitoes, particularly those that are hard to colonize in the laboratory.

Two hot topics presentations concluded this session: one on the proteomics of P. vivax sporozoites isolated from the salivary glands of A. dirus, presented by Rapatbhorn Patrapuvich of Mahidol University in Thailand; the other on using microsatellite markers as a means of tracking the genetic diversity of P. vivax strains around the world, presented by Patrick L. Sutton of New York University. A short poster presentation turbo talk highlighted research by Tanasee Panichakul of Suan Dusit Rajabhat University in Thailand on the mechanisms by which P. vivax inhibits erythropoiesis, causing anemia.

Highlights

Systems biology approaches can provide information about the complex interactions between the malaria parasite and host blood and liver cells, leading to new therapeutic and vaccine approaches.

P. vivax proteins that are expressed in hosts with intact spleens show promise as potential antigens for use in new vaccines.

Systems biology approaches

Host–parasite and parasite–vector interactions are complex phenomena that are difficult to model in vitro, particularly for blood- and liver-stage forms that depend on complex physiological environments. Mary R. Galinski of Emory University described the use of systems biology to collect comprehensive data on infections caused by Plasmodium species in primate model systems in an attempt to better understand these interactions. Galinski is a program director for the Malaria Host–Pathogen Interaction Center (MaHPIC), an international, interdisciplinary team that is collecting large 'omic datasets in combination with clinical parameters to investigate important features of malarial disease. Such information may allow researchers to identify new biomarkers for disease stages and should also provide missing information on the basic processes of malaria transmission and pathogenesis.

In her own laboratory, Galinski is studying the molecular mechanisms involved in P. vivax invasion of red blood cells via reticulocyte-binding proteins. Once inside the red blood cell, the parasites form complex vesicular structures, which Galinski's group has recently characterized extensively using electron microscopy. She also described the work of a colleague at Emory University, Dean Jones, who is using high-resolution metabolomics approaches to investigate host–parasite interactions and the effects of host exposures, such as diet and environmental factors, on malarial infections.

Other members of the MaHPIC are developing new primate models of malaria. Using systems biology approaches, these new models will be used to investigate the characteristics of P. vivax relapses, the effects of coinfection on the dynamics of host–parasite interactions, and questions such as why some hosts are resistant to specific types of malarial infections. These researchers are collecting blood and tissue samples from infected non-human primates at each stage of the disease and isolating DNA, proteins, and lipids to compare stages of infection, infections with different Plasmodium species, and other parameters. In response to the large datasets generated by these projects, they have developed an information-technology infrastructure, novel computational approaches to analyze the data and simulate clinical scenarios, and standardized methods to develop and test hypotheses.

Systems biology approaches are intended to elucidate all stages of the P. vivax life cycle, using people and monkeys as experimental systems. (Image courtesy of Mary R. Galinski)

Working toward a P. vivax vaccine

Out of 30 or so current vaccine trials, only one candidate targets P. vivax. It is still in a very early stage of development, and other candidates are needed to increase the likelihood of success. Hernando del Portillo of the Barcelona Institute for Global Health and the Barcelona Centre for International Health Research presented his group's work to identify P. vivax proteins that could act as antigens for a vaccine. They are examining two types of host tissue: the spleen, where important aspects of the immune response to P. vivax occur; and exosomes, which are vesicles, derived from red blood cells, that present antigens to the immune system. As the natural targets of immune responses to P. vivax, proteins in these tissues should represent effective antigens for vaccine development.

In order to identify P. vivax proteins involved in interactions with the spleen, del Portillo and his group looked for differences in P. vivax gene expression when monkeys with and without spleens were infected. Among the genes identified as spleen-specific were several known to be involved in P. vivax virulence. In vitro studies of the encoded proteins have yielded new information about the potential roles of these proteins in P. vivax infection. To facilitate further investigation of the interactions between these proteins and the intact spleen, the group has developed a microfluidics-based spleen model (a spleen-on-a-chip) that uses very small amounts of experimental reagents. In addition, they have produced some of these proteins in cell-free systems and are testing them against blood sera from children in Papua New Guinea, to determine whether human sera contains naturally produced antibodies against the proteins. Their studies challenge the previously held idea that human Plasmodium-infected cells do not pass through the spleen and suggest that spleen-derived antigens will be useful targets for P. vivax vaccines.

The second session ended with a hot topic presentation by Letusa Albrecht of the University of Campinas in Brazil (UNICAMP), who discussed work on host and parasite factors involved in "rosetting," or the clumping of red blood cells, which is a hallmark of severe malaria. Turbo talks were given by poster presenters Celine Borlon of the Institute of Tropical Medicine in Belgium, who discussed cryopreservation of P. vivax isolates; Stefanie C. P. Lopes of UNICAMP, who described work on the P. vivax schizont stage; and Paulo F. P. Pimenta of the Oswaldo Cruz Foundation in Brazil, who described the Anopheles vector's immune response to malaria parasite invasion.

Highlights

Proteins involved in P. vivax invasion of and release from red blood cells are attractive targets for new vaccines.

Other potential vaccine opportunities exist at the sexual/sporogonic stage, while the parasite is still in the mosquito, and at other stages, when the parasite is free in the host bloodstream.

Targeting blood stages

Speakers in the next session focused on research designed to improve our understanding of the natural mechanisms by which humans become immune to P. vivax. Knowledge of these mechanisms is essential for vaccine development.

Christopher L. King of Case Western Reserve University described work from his laboratory and others comparing immune responses in P. vivax- and P. falciparum-infected individuals. Evidence suggests that P. vivax immunity is acquired more quickly than P. falciparum immunity, reaching a peak among children of younger ages. Several factors that are unique to P. vivax pathophysiology may be responsible for this more rapid acquisition of immunity, such as the repeated relapses associated with P. vivax malaria and a lower but more persistent parasite burden.

To identify the mechanisms that lead to these immunological differences, King and his group are studying humoral and cellular immune responses that target P. vivax while it is in the bloodstream. They have identified potential antigens and are working to characterize their functions, as well as the types of immune responses that they provoke. Proteins that are involved in parasite invasion of and release from red blood cells are particularly attractive as candidate vaccine targets, but these mechanisms are not well understood. This avenue of research supports the idea that it may be possible to develop an effective and long-lasting P. vivax vaccine that prevents the parasite from interacting with red blood cells, thus stopping the infection at a very early stage.

Strategic thinking on vaccine development

David C. Kaslow of the PATH Malaria Vaccine Initiative described general strategic approaches toward developing a P. vivax vaccine. Kaslow reviewed several previous clinical trials of potential vaccines. Most did not continue because they showed only modest immunogenic responses or, in one case, caused severe adverse reactions. As mentioned above, one new P. vivax vaccine candidate is currently in very early stages of development and preliminary results are expected in 2014.

Kaslow noted that the 2006 Malaria Vaccine Technology Roadmap emphasized the development of vaccines against P. falciparum that were intended to prevent severe disease and death, by improving immunological response in those infected, and to be used in children under the age of five in sub-Saharan Africa and other highly endemic areas. More recently, this vision has been expanded to include vaccines against P. vivax; vaccines that interrupt transmission, in addition to improving immunological responses; other age groups; and all endemic areas. The latest strategies advocate using vaccines in combination with other interventions to control and eliminate malaria infection while also reducing morbidity and mortality.

Strategic targets for vaccines that will interrupt malaria transmission and prevent new cases from developing. (Image courtesy of David C. Kaslow)

Kaslow described vaccine targets derived from basic research into the P. vivax life cycle. He noted that potential antigenic targets have been identified at many stages and suggested that a multi-agent, multi-stage P. vivax vaccine is feasible. Vaccine opportunities exist at the sexual/sporogonic stage, while the parasite is still in the mosquito; at the pre-erythrocytic stage, when the parasite has entered the blood but has not yet invaded red blood cells; and during the asexual stage, when the parasite has been released from red blood cells. The first two stages represent opportunities to interrupt parasite transmission. They also target low points in the parasite population, and dovetail well with drugs that target the same stages and with anti-vector interventions. Controlled human malaria infection models in which human volunteers are inoculated with specified amounts of blood-stage parasites, rather than infected by a mosquito bite, are in development and are likely to play a key role in P. vivax vaccine development.

The session ended with hot topic presentations. Jessica Hostetler of the National Institute of Allergy and Infectious Diseases is using a high-throughput protein-interaction screening assay to identify proteins involved in P. vivax invasion of red blood cells. Francis B. Ntumngia of the University of South Florida is developing a synthetic antigen based on the P. vivax Duffy binding protein, which has been shown to be essential for red blood cell invasion. Ruth Payne of the University of Oxford described a current phase Ia clinical trial assessing the safety and immunogenicity of P. vivax candidate vaccines.

Turbo talks were presented by Chetan E. Chitnis of the International Centre for Genetic Engineering and Biotechnology, India, who described the development of a vaccine based on the Duffy protein; Sonal Gupta of the International Centre for Genetic Engineering and Biotechnology, India, who presented work on the P. vivax reticulocyte binding protein; and Flora Kano of Centro de Pesquisas Ren&eacute Rachou, Brazil, who described work on Duffy binding protein polymorphisms and antibodies among people living in the rural Amazon region.

Highlights

The P. vivax hypnozoite form greatly complicates epidemiologic research on transmission and disease burden, as well as the interpretation of interventional studies.

New in vitro and ex vivo models of hypnozoites will benefit research on the mechanisms of quiescence, activation, and development of the parasite into mature liver forms.

Tackling the hypnozoite

J. Kevin Baird of the Eijkman-Oxford Clinical Research Unit provided an overview of P. vivax epidemiology, including biological and pathological characteristics of P. vivax that make it more difficult to study than P. falciparum. The P. vivax life cycle closely resembles that of P. falciparum except for the presence of the hypnozoite, which lies dormant in the human liver. A single infectious mosquito bite transmitting P. vivax results in a primary attack of malaria within two weeks, followed by as many as 20 or more secondary attacks, known as relapses, over the next two years. These secondary attacks are caused by the activation of dormant hypnozoites.

The presence of hypnozoites greatly complicates epidemiologic study of P. vivax infections, with implications for research aimed at malaria prevention and treatment. In P. falciparum malaria, the presence of parasites in the blood (known as parasitemia) indicates that an individual has recently been infected by a mosquito, while in P. vivax malaria, parasitemia can arise from a mosquito bite or from activation of hypnozoites from a past infection. The triggers for hypnozoite activation are poorly understood, and the risk, frequency, and timing of relapses vary greatly between P. vivax strains in different geographic areas, further complicating research.

P. vivax epidemiology is complicated by relapses. (Image courtesy of J. Kevin Baird)

These factors make it difficult to use classic epidemiologic parameters to study the transmission and spread of P. vivax infections. For example, the true prevalence of P. vivax infection in a population includes a large proportion of infections that are not readily measurable as a result of low-level undetectable parasitemia, the presence of unknown levels of parasites in the marrow and spleen, and dormant hypnozoites in the liver. Relapses also confound the use of genotyping as a tool for distinguishing between re-infection and an increase in infection severity that occurs after a period of remission, known as recrudescence, which can occur with any strain of malaria. These challenges also affect efforts to model interventions and to test the efficacy of drugs and vaccines. Baird said that treatment and control strategies that do not include an attack on the hypnozoite are unlikely to reduce endemic P. vivax infection rates.

Modeling the hypnozoite

Dominique Mazier of the University of Pierre & Marie Curie, INSERM, gave a detailed history of early findings on hypnozoites. Although the occurrence of a dormant form of malaria parasite had been suggested in the 1940s, it was only discovered in the early 1980s and methods to study it at the molecular, biochemical, and cellular levels have only recently been developed.

Mazier and her group have developed methods for culturing hypnozoites in human or non-human primate liver cells. The hypnozoites grown in these cultures can be maintained for extended periods, during which some become activated, allowing Mazier and her colleagues to study the mechanisms that allow hypnozoites to remain quiescent, to become activated, and to develop into mature liver stages. Thus far, they have identified a possible epigenetic mechanism for the control of quiescence.

Mazier's in vitro system can be used to test drug susceptibility as well, which may facilitate the development of drugs to replace primaquine for targeting the hypnozoite. Mazier has also developed an ex vivo model that uses humanized mice implanted with human red blood cells and hepatocytes. This model recapitulates many of the stages of the malaria life cycle in humans, including the transition from liver to blood stages, and thus may be useful for studying the basic biology and drug susceptibility of hypnozoites.

In vitro culture of hypnozoites will help elucidate the molecular mechanisms of quiescence and activation. (Image courtesy of Dominique Mazier and Georges Snounou)

Stefan H. I. Kappe of the Seattle Biomedical Research Institute presented a hot topic talk describing his work developing a human liver-chimeric mouse model that supports P. vivax liver-stage development. His studies have uncovered unique hypnozoite properties as well as evidence for divergent hypnozoite frequencies in P. vivax patient isolates from Thailand.

Highlights

P. vivax strains around the world have begun to develop resistance to chloroquine, the first line therapy for eradicating blood-stage infections.

Multiple factors limit the use of primaquine to eradicate hypnozoites, including the risk of severe hemolysis in individuals with genetic variants that lead to G6PD deficiency.

Several new therapies are in preclinical and clinical development for P. vivax malaria, including tafenoquine, which has the ability to kill both blood-stage and hypnozoite forms.

Rising chloroquine resistance

P. vivax infections are usually treated with chloroquine to target blood-stage infections and with primaquine to eradicate hypnozoites; both therapies have been in use for about six decades. Chloroquine-resistant strains of P. vivax have developed over this time but have been difficult to recognize and study because it is hard to distinguish resistance from relapse. Ric Price of the University of Oxford, Menzies School of Health Research described findings from published case reports and clinical trials performed between 1960 and 2012 which suggest that chloroquine efficacy has declined at least somewhat in most locations with endemic P. vivax malaria. He cautioned that there are considerable difficulties involved in interpreting these studies, which together with challenges in diagnosing P. vivax infection and recognizing resistance, make it likely that chloroquine resistance is more widespread than the evidence currently suggests. He also said that clinical efficacy trials for P. vivax drugs should be re-designed to detect drug resistance and to better inform malaria drug-use policies.

Rising levels of chloroquine resistance in P. vivax isolates around the world. (Image courtesy of Ric Price)

The rise of chloroquine resistance in P. vivax creates a need to develop new therapies, as well as to take advantage of available alternative therapies that have shown activity against resistant strains. Many of these alternative therapies are artemisinin combination therapies (ACTs), the current standard of care for P. falciparum malaria. Price explored the pros and cons of replacing chloroquine therapy with ACTs for P. vivax malaria. It could be more effective to treat all strains of malaria with ACTs in areas with high P. vivax chloroquine resistance, because this obviates the need to identify mosquito species and reduces the risk that resistant P. falciparum infections will be treated with chloroquine. Ongoing ACT treatment also reduces relapses, which are associated with an increased risk of severe malaria, anemia, and associated comorbidities that are largely responsible for P. vivax mortality. The downside of this approach is that reducing the need for species identification might circumvent surveillance efforts that are needed to inform choices of control measures. ACTs are also considerably more expensive than chloroquine.

Optimizing primaquine therapy

Primaquine is the only registered therapy that is known to kill hypnozoites, with activity against hypnozoites formed by both P. vivax and P. ovale. However, there are many unanswered questions about its clinical use. George Dennis Shanks of the Australian Army Malaria Institute discussed how primaquine therapy could be improved by optimizing its ability to eliminate P. vivax hypnozoites, by developing simplified regimens to promote patient adherence, and by developing better methods to identify individuals who should not receive primaquine because of a G6PD deficiency.

As is often the case with drugs that have been in clinical use since the early 20th century, the pharmacodynamics of primaquine are incompletely understood, but evidence suggests that the molecule that kills the hypnozoite is an active metabolite rather than primaquine itself. It is also unclear which treatment regimen is optimal for eliminating hypnozoites while minimizing adverse effects. Better knowledge of primaquine metabolism might explain why the drug fails in some individuals and would lead to a better understanding of dosing properties that could be used to optimize regimens.

Primaquine-induced severe hemolysis is also poorly understood. Many individuals with G6PD deficiencies do not have a severe reaction, and better methods to identify those who are at risk would help to optimize regimens and would improve attempts to eradicate malaria with mass drug administration. Better drug formulations are needed, such as liposomal formulations, which could help to minimize the risk of hemolysis by increasing the time the drug spends in the liver rather than in the bloodstream. In addition, because individuals with dormant hypnozoite infections do not feel sick, it can be difficult to convince patients to adhere to a full primaquine regimen. Patients may also be deterred by the risk of hemolysis. These problems could be addressed by designing simplified, reduced-risk drug regimens, which could also include direct observation of drug-taking to ensure patient adherence. More research into these and other questions is needed before primaquine can gain more widespread use as a tool for the eradication of P. vivax malaria.

Strategies for improving primaquine eradication of P. vivax using mass drug administration. (Image courtesy of George Dennis Shanks)

Emerging new therapies

The Medicines for Malaria Venture (MMV) of Geneva, Switzerland, is a nonprofit public–private collaboration that is working to develop new malaria drugs that are effective as well as affordable for malaria-endemic areas. Brice Campo provided an update on the MMV drug-development portfolio and discussed some of the reasons why P. vivax-specific medications present challenges. Their goal is to produce cheap, safe, child-friendly drugs that prevent relapses and/or block transmission and that require only one to three doses to be effective.

The P. vivax drug-development pipeline includes nine drug candidates in preclinical through phase IIa testing, including several molecules with novel mechanisms, and six candidates in phase IIb testing. One of the most promising candidates is tafenoquine, which is active against all P. vivax life stages, including hypnozoites. Tafenoquine has a long half-life that allows single-dose treatment, but it also carries a risk of hemolysis for G6PD-deficient patients. This drug awaits evaluation in phase III trials. Its usefulness may ultimately depend on the development of a robust G6PD point-of-care diagnostic test that would reduce the risk of accidentally dosing G6PD-deficient patients.

Most new therapies in the MMV portfolio target stages other than the cryptic hypnozoite stage that is responsible for P. vivax relapse. This is partly because current drug-development pathways favor compounds that are active against blood-stage forms. There is a need for in vitro assays that can be used to test large compound libraries and to optimize potential lead compounds. Campo said that MMV investigators are working to improve their abilities to develop drugs against hypnozoites.

Three turbo talks concluded this session, given by Katherine E. Battle of the University of Oxford, who described geographical variations in P. vivax relapse rates; Larry Walker of the University of Mississippi, who presented work on a new, single enantiomer drug, NPC1161B; and Anne-Marie Zeeman of the Biomedical Primate Research Center in the Netherlands, who described a chemical biology approach to developing new drugs against P. vivax hypnozoites. Juliana M. Sá of the National Institute of Allergy and Infectious Diseases also gave a hot topic presentation on her work using genetic approaches to uncover the molecular basis of chloroquine resistance in P. vivax.

Highlights

Severe P. vivax malaria is less well understood than severe P. falciparum malaria but it is associated with both acute and chronic comorbidities and is nearly always accompanied by coinfection.

The primary complications that lead to death from severe P. vivax malaria are acute respiratory distress syndrome and pulmonary edema, spleen rupture, and multiple organ dysfunction syndrome.

Severe P. vivax malaria shows a high degree of diversity that should be accounted for in clinical recommendations for treatment.

Severe P. vivax malaria

Most otherwise healthy individuals who contract P. vivax malaria do not progress to severe illness or death, but severe disease does occur in about one in 270 patients, and about one in 2000 to 4000 die. Nicholas Anstey of the Menzies School of Health Research gave an overview of the characteristics of severe P. vivax disease and the factors that lead to its development.

Severe manifestations of P. vivax infection often include severe anemia and acute lung injury, as well as kidney injury, shock, multiorgan failure, coma (rarely), and death. Severe anemia tends to occur in younger children who contract P. vivax malaria, compared with other types of malaria, and the risk of severe anemia rises steeply with the number of relapses. Acute lung injury is well characterized in adults. Other complications, such as coma, occur at lower rates, and their contributions to morbidity and mortality are less well understood.

The risk of developing severe P. falciparum malaria is increased by comorbidities such as malnutrition, coinfection with HIV, and bacterial infection of the bloodstream, but less is known about the role of these factors in severe P. vivax malaria. Additional comorbidities that have been observed in patients who develop severe P. vivax malaria include acute gastroenteritis, pneumonia, stroke, and chronic conditions such as diabetes and chronic obstructive pulmonary disease. Concurrent infections, such as Dengue fever or coinfection with another malaria parasite, are also associated with more severe disease. Further research is needed to investigate the contribution of these comorbidities to severe P. vivax malaria and to develop better means of treating these patients.

Autopsy studies of P. vivax malaria

Marcus V. G. Lacerda of the Tropical Medicine Foundation Dr. Heitor Vieira Dourado also discussed fatal P. vivax malaria, describing current and historical findings from autopsies. Because P. vivax malaria does not kill as often as the more common P. falciparum malaria, less data on its pathophysiology are available from postmortem tissue examinations. However, Lacerda described historical autopsy findings as well as isolated case studies from the literature that reveal some of the pathological changes that occurred in liver, lung, kidney, and brain tissue of patients who died with known P. vivax infections. Nearly all of these studies show the presence of coinfecting pathogens, reaffirming the importance of coinfection in P. vivax severe disease and death but at the same time complicating analysis because pathological changes and cause of death cannot be assigned to a specific pathogen.

To bridge this knowledge gap, Lacerda and his colleagues performed a series of 17 autopsies on patients who died from severe P. vivax mono-infection in the Brazilian Amazon between 1996 and 2010. The group used highly sensitive PCR analyses of tissues to determine whether these patients were infected with P. vivax alone. Thirteen of the patients definitely or probably died as a result of P. vivax infection alone, while the other four were found to have died of acute infectious diseases other than malaria. Autopsies on the patients who died of P. vivax infection alone showed that the primary complications that led to death were acute respiratory distress syndrome and pulmonary edema, spleen rupture, and multiple organ dysfunction syndrome. Three patients died exclusively of spleen rupture, which is unusual in P. falciparum malaria but appears to be a more frequent occurrence with P. vivax disease.

Lacerda noted that although studies such as this of mono-infected patients are important for understanding the pathophysiology of P. vivax infection, these patients are the exception to the rule. Coinfection is much more common, particularly in the low socioeconomic areas where malaria is endemic. He argued that clinical trials of potential P. vivax medications should enroll more patients with coinfections, in order to study severe P. vivax disease in its most common and clinically relevant setting.

A severe P. vivax malaria case series

Cases of severe P. vivax malaria have increased over the past decade but case descriptions can vary greatly in different geographic areas. Quique Bassat and his colleagues at the Barcelona Institute for Global Health performed parallel studies of nearly 800 P. vivax malaria admitted cases seen in two hospitals, one in Brazil and one in India, using a commonly agreed upon clinical protocol. Both hospitals are in areas where P. vivax predominates over P. falciparum and where low-to-moderate transmission rates lead to a patient population that is generally adult rather than pediatric. Cases of P. vivax mono-infection were confirmed by PCR analysis and standardized clinical definitions were used to identify clinical and severe cases of malaria. Many clinical parameters were collected, as well as microbiological data for all patients.

Comorbid infections associated with severe malaria cases. (Image courtesy of Quique Bassat)

About 13% of the Brazilian cases and 34% of the Indian cases included one or more of the WHO's criteria for severe malaria. The major causes of hospitalization were hyperbilirubinemia, severe anemia, and acute renal failure. Patients at both sites showed a high prevalence of multiple organ dysfunction syndrome and acute respiratory distress syndrome, as has previously been described for severe P. vivax malaria. Brazilian patients had a higher prevalence of both acute and chronic comorbidities and coinfections, including high rates of hemolysis due to primaquine use among G6PD-deficient patients and a high rate of dengue fever. Although there were common features, results show a high degree of heterogeneity both within and between the two sites, as well as differences from P. falciparum disease that should be accounted for in future studies and in the development of treatment guidelines.

Eline Kattenberg of the Papua New Guinea Institute of Medical Research gave a hot topic presentation on red blood cell polymorphisms that are associated with protection against P. vivax malaria in children from Papua New Guinea. James McCarthy of the Queensland Institute for Medical Research described his work to establish a cryobank of clinical-grade P. vivax collected from human volunteers. His work opens the possibility of using the more controlled conditions of induced blood stage infection rather than infection by mosquito bite in clinical research.

Turbo talks were given by Karina P. Leiva of the U.S. Naval Medical Research Unit No. Six in Peru, who described work that is intended to identify biomarkers of severe malaria; and Leanne J. Robinson from the Papua New Guinea Institute of Medical Research, who described the contribution of hypnozoites to the burden of P. vivax malaria among children in Papua New Guinea.

Highlights

Mitochondrial DNA sequence comparisons suggest that human P. vivax malaria originated in Africa and that African great apes represent a previously unrecognized zoonotic reservoir for malaria parasites.

Pregnant women carry a low burden of P. vivax infection in areas with low malaria transmission rates.

P. vivax and the great apes

In sub-Saharan Africa, a majority of the population carries a mutation that prevents expression of the P. vivax receptor, also known as the Duffy antigen, on red blood cells. This mutation reduces the prevalence of P. vivax malaria to nearly zero in some African regions. However, Africa is also home to several large non-human primate species, the African apes, which can also contract malaria. Previous research into the types of African malarial species and the evolutionary relationships among them was incomplete, so Paul Sharp of the University of Edinburgh and colleagues set out to study this topic. They collected more than 5000 fecal samples from chimpanzees and wild gorillas in West and Central Africa and used DNA amplification techniques to isolate and sequence DNA from Plasmodium species, as well as to analyze the samples for other pathogens and for variations in host DNA.

Noninvasive sampling to detect malaria parasites in wild African great apes. (Image courtesy of Paul Sharp)

Despite the almost complete absence of P. vivax malaria among humans in West and Central Africa, Sharp and his group were able to isolate of P. vivax DNA from fecal samples of both chimpanzees and gorillas. Comparison of parasite mitochondrial DNA sequences from these samples with those of malaria parasites around the world suggests that human P. vivax originated in Africa and that P. vivax strains have been passed frequently back and forth between humans and gorillas. Their results also suggest that P. falciparum may have been transmitted to humans from gorillas, perhaps quite recently. Based on these results and those from other laboratories, Sharp suggested that great apes represent a previously unrecognized zoonotic reservoir for malaria parasites, which probably impacts the distribution of malaria around the world.

P. vivax and pregnancy

Pregnant women are known to be at increased risk of P. falciparum infection, but little is known about the impact of P. vivax on pregnant women or on maternal and child health. Azucena Bardají of the Barcelona Centre for International Health Research described her work as part of a multicenter collaborative cohort study, PregVax, which is designed to estimate the burden of P. vivax infection during pregnancy in Brazil, Colombia, Guatemala, Papua New Guinea, and India. The study recruited over 9000 pregnant women and estimated P. vivax prevalence at routine prenatal visits and at delivery and assessed the clinical impact of P. vivax infection on maternal anemia, low birth weight, premature birth, and congenital malaria.

Preliminary results from the study suggest that the prevalence of P. vivax infection during pregnancy was low at enrollment and at delivery at most sites, which are in low-transmission endemic areas. More infections were discovered by PCR than by microscopic evaluation of blood samples, suggesting a low parasite burden. Analysis of this study, which is not yet complete, will provide insights into the epidemiology of malaria in areas of low endemicity, which should be applied to the design of future epidemiologic and clinical studies.

Two hot topic presentations ended this session. Muzamil Mahdi Abdel Hamid of the University of Khartoum presented work on the epidemiology and genetics of P. vivax in the Sudan. Rosalind Howes of the University of Oxford discussed her studies into the worldwide distribution and epidemiology of human genetic variants that cause G6PD deficiency.

Highlights

P. vivax has already been eradicated in some areas of the world but is likely to remain a challenge in highly endemic areas.

The hypnozoite is the key target for drugs and vaccines against P. vivax, and there are many knowledge gaps regarding this dormant form and other aspects of P. vivax pathophysiology.

Strategic overview of research targets

Since the 1950s, researchers have been working to eradicate malaria. They have succeeded in some parts of the world. However, inherent challenges in eliminating P. vivax infection have led to a renewed emphasis on prevention, control, and treatment strategies specifically designed for this parasite. Pedro Alonso of the Barcelona Institute for Global Health discussed the status of P. vivax eradication efforts around the world and provided a summary of clinical tools that will be needed before this infection can be completely or even largely eradicated.

P. vivax elimination is not impossible. Of four countries that were recently certified as malaria-free, two are in areas with only P. vivax infection and two are in areas where P. vivax predominates. Many of the other countries that are moving toward eliminating malaria are also in areas where P. vivax predominates. Turkmenistan, for example, saw major outbreaks of P. vivax malaria in the 1990s and early 2000s, yet in 2010 it was declared malaria-free.

However, many areas of P. vivax malaria endemicity remain. Alonso summarized the challenges associated with the effort to eliminate P. vivax malaria, listing the tools that will be needed to surmount the obstacles described by previous speakers. These tools include drugs that can clear the parasite completely from humans, vaccines that prevent transmission or promote clearance of infection by inducing cellular immune responses, and diagnostic tests that will allow detection of hypnozoites and identify individuals who are G6PD-mutation carriers. Answering outstanding basic research questions will aid in the development of these tools.

Alonso’s presentation was followed by a panel discussion on practical aspects of P. vivax eradication, moderated by Barbara Sina of the Fogarty International Center at the National Institutes of Health. Panelists from different countries, including China, India, Thailand, and Brazil, described P. vivax endemicity and transmission rates, the prevalence of asymptomatic cases, and surveillance and control efforts in their parts of the world. Panelists mentioned more active surveillance strategies and mass administration of anti-malarial drugs as interventions that would be likely to promote the success of malaria eradication efforts.

After the panel discussion, Alan J. Magill of the Bill & Melinda Gates Foundation delivered closing remarks for the conference. After summarizing basic and clinical research needs, Magill reiterated the key aspect of P. vivax biology that requires attention: the hypnozoite. Elimination of P. vivax malaria will require the ability to detect and kill hypnozoites in humans, as these forms are the most important reservoir of this infection. Although at least one old medication, primaquine, and at least one potential new medication, tafenoquine, are available for this purpose, their uses should be optimized and the risk of severe hemolysis should be mitigated in individuals with G6PD deficiencies. Better methods to detect hypnozoites and a better understanding of their biology will also be needed to completely eradicate this disease. P. vivax cases tend to increase as P. falciparum comes under control in a given population, lending urgency to the acquisition of knowledge that is needed to tackle this challenging pathogen.