Evolutionary and historical aspects of the burden of malaria

Bioactive Alkaloids from South American Psychotria and Related Species

Many important molecules have been discovered from tropical and sub-tropical plant biodiversity. However, the largest part of the chemical profile of such biodiversity remains unknown. Combining ethnopharmacological and chemotaxonomical investigation can be a good strategy in bioactive compound discovery. South American Psychotria species studied by this approach proved to be a rich source of new bioactive alkaloids, some of which bear unique chemical skeletons.

Defining the relationship between Plasmodium falciparum parasite rate and clinical disease: statistical models for disease burden estimation

BACKGROUND

Clinical malaria has proven an elusive burden to enumerate. Many cases go undetected by routine disease recording systems. Epidemiologists have, therefore, frequently defaulted to actively measuring malaria in population cohorts through time. Measuring the clinical incidence of malaria longitudinally is labour-intensive and impossible to undertake universally. There is a need, therefore, to define a relationship between clinical incidence and the easier and more commonly measured index of infection prevalence: the "parasite rate". This relationship can help provide an informed basis to define malaria burdens in areas where health statistics are inadequate.

METHODS

Formal literature searches were conducted for Plasmodium falciparum malaria incidence surveys undertaken prospectively through active case detection at least every 14 days. The data were abstracted, standardized and geo-referenced. Incidence surveys were time-space matched with modelled estimates of infection prevalence derived from a larger database of parasite prevalence surveys and modelling procedures developed for a global malaria endemicity map. Several potential relationships between clinical incidence and infection prevalence were then specified in a non-parametric Gaussian process model with minimal, biologically informed, prior constraints. Bayesian inference was then used to choose between the candidate models.

RESULTS

The suggested relationships with credible intervals are shown for the Africa and a combined America and Central and South East Asia regions. In both regions clinical incidence increased slowly and smoothly as a function of infection prevalence. In Africa, when infection prevalence exceeded 40%, clinical incidence reached a plateau of 500 cases per thousand of the population per annum. In the combined America and Central and South East Asia regions, this plateau was reached at 250 cases per thousand of the population per annum. A temporal volatility model was also incorporated to facilitate a closer description of the variance in the observed data.

CONCLUSION

It was possible to model a relationship between clinical incidence and P. falciparum infection prevalence but the best-fit models were very noisy reflecting the large variance within the observed opportunistic data sample. This continuous quantification allows for estimates of the clinical burden of P. falciparum of known confidence from wherever an estimate of P. falciparum prevalence is available.

Development of a Polymerase Chain Reaction (PCR) method based on amplification of mitochondrial DNA to detect Plasmodium falciparum and Plasmodium vivax

In this study we standardized a new technical approach in which the target mitochondrial DNA sequence (mtDNA) is amplified using a simple but sensitive PCR method as a tool to detect Plasmodium falciparum and Plasmodium vivax. Specific primers were designed to hybridize with cytochrome c oxidase genes of P. falciparum (cox III) and P. vivax (cox I). Amplification products were obtained for all positive samples, presenting homology only for species-specific mtDNA. Sensitivity and specificity were 100%. The applicability of the method was tested in a cross-sectional study, in which 88 blood samples from individuals naturally exposed to malaria in the Brazilian Amazon region were analyzed. Based on the results, the sensitivity and specificity were 100% and 88.3%, respectively. This simple and sensitive PCR method can be useful in specific situations and in different settings of malaria management, in endemic as well as non-endemic areas (travelers), and in clinical or epidemiological studies, with applications in malaria control programs.

Antimalarial activity of natural product extracts from Papua New Guinean and Australian plants against Plasmodium falciparum

In the search for new antimalarial compounds, a subset of a natural product extract library prepared from plant samples collected from Papua New Guinea and Australia was screened for in vitro activity against the chloroquine-sensitive 3D7 and chloroquine-resistant Dd2 strains of Plasmodium falciparum. Using the incorporation of ((3)H)-hypoxanthine into parasite nucleic acid as a marker of growth, 93 of the 794 extracts screened displayed >40% inhibition against 3D7 infected erythrocytes at 312 microge/mL. Antimalarial activity was confirmed in 48 of these extracts against both 3D7 and Dd2 infected erythrocytes at concentrations between 78 and 390 microge/mL, 14 of which caused >90% growth inhibition of 3D7 at the lowest concentration screened. Extracts were also tested for mammalian cell cytotoxicity to evaluate selectivity of action.

Artemisia annua as a self-reliant treatment for malaria in developing countries

Malaria is a vector-borne infectious disease caused by the protozoan Plasmodium parasites. Each year, it causes disease in approximately 515 million people and kills between one and three million people, the majority of whom are young children in sub-Saharan Africa. It is widespread in tropical and subtropical regions, including parts of the Americas, Asia, and Africa. Due to climate change and the gradual warming of the temperate regions the future distribution of the malaria disease might include regions which are today seen as safe. Currently, malaria control requires an integrated approach comprising of mainly prevention, including vector control and the use of effective prophylactic medicines, and treatment of infected patients with antimalarials. The antimalarial chloroquine, which was in the past a mainstay of malaria control, is now ineffective in most malaria areas and resistance to other antimalarials is also increasing rapidly. The discovery and development of artemisinins from Artemisia annua have provided a new class of highly effective antimalarials. ACTs are now generally considered as the best current treatment for uncomplicated Plasmodium falciparum malaria. This review gives a short history of the malaria disease, the people forming a high risk group and the botanical aspects of A. annua. Furthermore the review provides an insight in the use of ART and its derivatives for the treatment of malaria. Its mechanism of action and kinetics will be described as well as the possibilities for a self-reliant treatment will be revealed. This self-reliant treatment includes the local production practices of A. annua followed by the possibilities for using traditional prepared teas from A. annua as an effective treatment for malaria. Finally, HMM will be described and the advantages and disadvantages discussed.

An overview of the Babesia, Plasmodium and Theileria genomes: a comparative perspective

Babesia, Plasmodium and Theileria form a triad of apicomplexan hemoparasites and are accountable for significant mortality and morbidity to humans and animals globally. Understanding the pathobiology of these three genera is crucial as multiple drug resistant strains continue to arise in endemic areas along with pesticide and acaricide resistant vector hosts. Vastly improved sequencing technology has produced whole genome sequences of several apicomplexan species and subsequent comparative analyses of these genomes have identified unique as well as common features among the different species, information that will help in the pursuit of alternative therapies, management and perhaps elimination of the disease. This review, therefore, summarizes comparisons of genome structure, protein families, metabolic pathways and organelle biology in these three apicomplexans and how such knowledge has and will continue to enhance the field.

Failure to detect Plasmodium vivax in West and Central Africa by PCR species typing

Background

Plasmodium vivax is estimated to affect 75 million people annually. It is reportedly absent, however, from west and central Africa due to the high prevalence of the Duffy negative phenotype in the indigenous populations. Despite this, non-African travellers consistently return to their own countries with P. vivax malaria after visiting this region. An attempt was made, therefore, to detect the presence of P. vivax parasites in blood samples collected from the indigenous populations of west and central Africa.

Methods

Parasite species typing (for all four human malaria parasites) was carried out by PCR on 2,588 blood samples collected from individuals from nine African malaria-endemic countries.

Results

Most infections (98.5%) were Plasmodium falciparum, Plasmodium malariae was identified in 8.5% of all infections, and Plasmodium ovale in 3.9%. The prevalence of both parasites varied greatly by country. Only one case of P. vivax was detected from Sao Tome, an island off the west coast of Africa, confirming the scarcity of this parasite in Africa.

Conclusion

The prevalence of P. vivax in local populations in sub-Saharan Africa is very low, despite the frequent identification of this parasite in non-African travellers.

Consanguineous marriages and endemic malaria: can inbreeding increase population fitness?

Background

The practice of consanguineous marriages is widespread in countries with endemic malaria. In these regions, consanguinity increases the prevalence of α⁺-thalassemia, which is protective against malaria. However, it also causes an excessive mortality amongst the offspring due to an increase in homozygosis of recessive lethal alleles. The aim of this study was to explore the overall effects of inbreeding on the fitness of a population infested with malaria.

Methods

In a stochastic computer model of population growth, the sizes of inbred and outbred populations were compared. The model has been previously validated producing results for inbred populations that have agreed with analytical predictions. Survival likelihoods for different α⁺-thalassemia genotypes were obtained from the odds of severe forms of disease from a field study. Survivals were further estimated for different values of mortality from malaria.

Results

Inbreeding increases the frequency of α⁺-thalassemia allele and the loss of life due to homozygosis of recessive lethal alleles; both are proportional to the coefficient of inbreeding and the frequency of alleles in population. Inbreeding-mediated decrease in mortality from malaria (produced via enhanced α⁺-thalassemia frequency) mitigates inbreeding-related increases in fatality (produced via increased homozygosity of recessive lethals). When the death rate due to malaria is high, the net effect of inbreeding is a reduction in the overall mortality of the population.

Conclusion

Consanguineous marriages may increase the overall fitness of populations with endemic malaria.

Genetic studies of African populations: an overview on disease susceptibility and response to vaccines and therapeutics

Africa is the ultimate source of modern humans and as such harbors more genetic variation than any other continent. For this reason, studies of the patterns of genetic variation in African populations are crucial to understanding how genes affect phenotypic variation, including disease predisposition. In addition, the patterns of extant genetic variation in Africa are important for understanding how genetic variation affects infectious diseases that are a major problem in Africa, such as malaria, tuberculosis, schistosomiasis, and HIV/AIDS. Therefore, elucidating the role that genetic susceptibility to infectious diseases plays is critical to improving the health of people in Africa. It is also of note that recent and ongoing social and cultural changes in sub-Saharan Africa have increased the prevalence of non-communicable diseases that will also require genetic analyses to improve disease prevention and treatment. In this review we give special attention to many of the past and ongoing studies, emphasizing those in Sub-Saharan Africans that address the role of genetic variation in human disease.

Consanguinity affects selection of alpha-thalassemia genotypes and the size of populations under selection pressure from malaria

BACKGROUND

In populations with alpha(+)-thalassemia gene deletion, the practice of consanguineous marriages is common.

AIM

The study explored the impact of consanguinity (inbreeding) on the selection of alpha(+)-thalassemia genotypes in a computer model.

METHOD

In a population under selection pressure from malaria, a single protective mutation (-alpha/alphaalpha genotype) was introduced among normal genotypes (alphaalpha/alphaalpha), and mating allowed to proceed. Heterozygote (-alpha/alphaalpha) and homozygote (-alpha/-alpha) children were 1.5 and 2.5 times more likely to survive malaria than those with normal genotypes. Using different coefficients of inbreeding (F, range 0-0.12), we examined the effect of population size, and the mean number of generations required for the homozygote frequency to reach 0.5.

RESULTS

On average, consanguineous populations were larger than randomly mating populations and the size was directly proportional to F. In more inbred populations,-alpha/-alpha homozygotes reached a frequency of 0.5 faster than in less inbred populations. As the frequency of the alpha(+)-thalassemia allele in a population increases, however, the positive effect of inbreeding on the population growth decreases.

CONCLUSION

Under selection pressure from malaria, consanguinity may increase the speed of selection of-alpha/-alpha homozygotes and provide an advantage regarding population growth over non-consanguineous populations.

Duffy blood group gene polymorphisms among malaria vivax patients in four areas of the Brazilian Amazon region

Background

Duffy blood group polymorphisms are important in areas where Plasmodium vivax predominates, because this molecule acts as a receptor for this protozoan. In the present study, Duffy blood group genotyping in P. vivax malaria patients from four different Brazilian endemic areas is reported, exploring significant associations between blood group variants and susceptibility or resistance to malaria.

Methods

The P. vivax identification was determined by non-genotypic and genotypic screening tests. The Duffy blood group was genotyped by PCR/RFLP in 330 blood donors and 312 malaria patients from four Brazilian Amazon areas. In order to assess the variables significance and to obtain independence among the proportions, the Fisher's exact test was used.

Results

The data show a high frequency of the FYA/FYB genotype, followed by FYB/FYB, FYA/FYA, FYA/FYB-33 and FYB/FYB-33. Low frequencies were detected for the FYA/FY^X, FYB/FY^X, FYX/FY^X and FYB-33/FYB-33 genotypes. Negative Duffy genotype (FYB-33/FYB-33) was found in both groups: individuals infected and non-infected (blood donors). No individual carried the FY^X/FYB-33 genotype. Some of the Duffy genotypes frequencies showed significant differences between donors and malaria patients.

Conclusion

The obtained data suggest that individuals with the FYA/FYB genotype have higher susceptibility to malaria. The presence of the FYB-33 allele may be a selective advantage in the population, reducing the rate of infection by P. vivax in this region. Additional efforts may contribute to better elucidate the physiopathologic differences in this parasite/host relationship in regions endemic for P. vivax malaria, in particular the Brazilian Amazon region.

Coadaptation and malaria control

Malaria emerges from a disequilibrium of the system 'human-plasmodium-mosquito' (HPM). If the equilibrium is maintained, malaria does not ensue and the result is asymptomatic plasmodium infection. The relationships among the components of the system involve coadaptive linkages that lead to equilibrium. A vast body of evidence supports this assumption, including the strategies involved in the relationships between plasmodium and human and mosquito immune systems, and the emergence of resistance of plasmodia to antimalarial drugs and of mosquitoes to insecticides. Coadaptive strategies for malaria control are based on the following principles: (1) the system HPM is composed of three highly complex and dynamic components, whose interplay involves coadaptive linkages that tend to maintain the equilibrium of the system; (2) human and mosquito immune systems play a central role in the coadaptive interplay with plasmodium, and hence, in the maintenance of the system's equilibrium; the under- or overfunction of human immune system may result in malaria and influence its severity; (3) coadaptation depends on genetic and epigenetic phenomena occurring at the interfaces of the components of the system, and may involve exchange of infectrons (genes or gene fragments) between the partners; (4) plasmodia and mosquitoes have been submitted to selective pressures, leading to adaptation, for an extremely long while and are, therefore, endowed with the capacity to circumvent both natural (immunity) and artificial (drugs, insecticides, vaccines) measures aiming at destroying them; (5) since malaria represents disequilibrium of the system HPM, its control should aim at maintaining or restoring this equilibrium; (6) the disequilibrium of integrated systems involves the disequilibrium of their components, therefore the maintenance or restoration of the system's equilibrium depend on the adoption of integrated and coordinated measures acting on all components, that means, panadaptive strategies. Coadaptive strategies for malaria control should consider that: (1) host immune response has to be induced, since without it, no coadaptation is attained; (2) the immune response has to be sustained and efficient enough to avoid plasmodium overgrowth; (3) the immune response should not destroy all parasites; (4) the immune response has to be well controlled in order to not harm the host. These conditions are mostly influenced by antimalarial drugs, and should also be taken into account for the development of coadaptive malaria vaccines.

Genetic linkage and association analyses for trait mapping in Plasmodium falciparum

Genetic studies of Plasmodium falciparum laboratory crosses and field isolates have produced valuable insights into determinants of drug responses, antigenic variation, disease virulence, cellular development and population structures of these virulent human malaria parasites. Full-genome sequences and high-resolution haplotype maps of SNPs and microsatellites are now available for all 14 parasite chromosomes. Rapidly increasing genetic and genomic information on Plasmodium parasites, mosquitoes and humans will combine as a rich resource for new advances in our understanding of malaria, its transmission and its manifestations of disease.

The genetic diversity of Plasmodium vivax: a review

The genetic diversity of Plasmodium vivax has been investigated in several malaria-endemic areas, including the Brazilian Amazon region, where this is currently the most prevalent species causing malaria in humans. This review summarizes current views on the use of molecular markers to examine P. vivax populations, with a focus on studies performed in Brazilian research laboratories. We emphasize the importance of phylogenetic studies on this parasite and discuss the perspectives created by our increasing understanding of genetic diversity and population structure of this parasite for the development of new control strategies, including vaccines, and more effective drugs for the treatment of P. vivax malaria.

Immunogenicity of Plasmodium vivax combination subunit vaccine formulated with human compatible adjuvants in mice

An effective malaria vaccine will probably require the delivery of multiple antigens that induce several layers of immunity. Malaria antigens expressed on the surface and in apical organelles of blood-stage merozoites are potential vaccine candidates given their importance in the invasion of erythrocytes. The present study examined the kinetics of humoral response in BALB/c mice following immunization with combination of two blood-stage Plasmodium vivax invasion related molecules, the N-terminal, cysteine-rich region II of P. vivax Duffy binding protein (PvRII) and the 19kDa C-terminal region of merozoite surface protein 1 (PvMSP1(19)) formulated with Montanide ISA 720 and alhydrogel. Immunization with combination of recombinant PvRII and PvMSP1(19) formulated with the Montanide ISA 720 elicited higher antibody titer compared to the alhydrogel formulation. In case of both the adjuvants tested, combination of PvRII and PvMSP1(19) did not result in suppression of antibody response against either antigen when compared to immunization with individual antigens alone. Analysis of IgG subclasses showed that combination of both the recombinant proteins induced a mixed Th1/Th2-type response with almost all IgG subtypes being expressed in equivalent amount. Antibodies elicited against PvRII showed significant inhibitory effect on the binding of PvRII to recombinant Duffy antigen receptor for chemokines (DARC) in an in vitro binding assay. The results of the present study provide a rationale for a combination vaccine against P. vivax malaria based on PvMSP1(19) and PvRII.

The ABO blood group system and Plasmodium falciparum malaria

In the century since the discovery of the ABO blood groups, numerous associations between ABO groups and disease have been noted. However, the selection pressures defining the ABO distributions remain uncertain. We review published information on Plasmodium falciparum infection and ABO blood groups. DNA sequence information dates the emergence and development of the group O allele to a period of evolution before human migration out of Africa, concomitant with P falciparum's activity. The current geographic distribution of group O is also consistent with a selection pressure by P falciparum in favor of group O individuals in malaria-endemic regions. We critically review clinical reports of ABO and P falciparum infection, documenting a correlation between disease severity and ABO group. Finally, we review published data on the pathogenesis of P falciparum infection, and propose a biologic model to summarize the role of ABO blood groups in cytoadherence biology. Such ABO-related mechanisms also point to a new hypothesis to account for selection of the Le(a-b-) phenotype. Taken together, a broad range of available evidence suggests that the origin, distribution, and relative proportion of ABO blood groups in humans may have been directly influenced by selective genetic pressure from P falciparum infection.

Systemic lupus erythematosus-associated defects in the inhibitory receptor FcgammaRIIb reduce susceptibility to malaria

Polygenic autoimmune diseases, such as systemic lupus erythematosus (SLE), are a significant cause of morbidity and mortality worldwide. In recent years, functionally important genetic polymorphisms conferring susceptibility to SLE have been identified, but the evolutionary pressures driving their retention in the gene pool remain elusive. A defunctioning, SLE-associated polymorphism of the inhibitory receptor FcgammaRIIb is found at an increased frequency in African and Asian populations, broadly corresponding to areas where malaria is endemic. Here, we show that FcgammaRIIb-deficient mice have increased clearance of malarial parasites (Plasmodium chabaudi chabaudi) and develop less severe disease. In vitro, the human lupus associated FcgammaRIIb polymorphism enhances phagocytosis of Plasmodium falciparum-infected erythrocytes. These results demonstrate that FcgammaRIIb is important in controlling the immune response to malarial parasites and suggests that the higher frequency of human FcgammaRIIb polymorphisms predisposing to SLE in Asians and Africans may be maintained because these variants reduce susceptibility to malaria.

Haemoglobinopathies and glucose-6-phosphate dehydrogenase deficiency in a Scandinavian perspective

Haemoglobinopathies (mainly thalassaemia and sickle-cell anaemia syndromes) and glucose-6-phosphate dehydrogenase deficiency (G6PD) are globally among the most prevalent single-genomic diseases. About 3% of the world's population are heterozygotic for beta-thalassaemia and about 1-2% for sickle-cell anaemia, and it is estimated that more than 400 million people are affected by G6PD deficiency worldwide. The disorders are most prevalent in the Mediterranean area, in Asia and Africa. The Scandinavian countries, among others, have seen a boom in immigration during the past 20 years, and therefore migration makes haemoglobinopathies as well as G6PD deficiency increasingly more important from a differential diagnostic perspective in most countries. The purpose of the present special issue of the Journal is to summarize current epidemiological data and elucidate trends and practices in the laboratory diagnosis of these disorders.

Mudanças no padrão epidemiológico da malária em área rural do médio Rio Negro, Amazônia brasileira: análise retrospectiva

Foi realizado um estudo retrospectivo dos casos de malária ocorridos entre 1992 e 2004 no Município de Barcelos, Amazonas, Brasil, aprofundando as informações existentes na área altamente endêmica do Rio Padauri, afluente do Rio Negro. Foram identificados 16.795 casos de malária, sendo 61,4% (10.318) da área rural e 38,6% (6.477) da área urbana, com uma incidência parasitária anual média de 136,7 por mil habitantes na área urbana e 613,6 na área rural. A incidência parasitária anual média no Rio Padauri foi de 708,9/1.000. Nesse rio, foram diferenciados dois períodos epidemiológicos, sendo um denominado epidêmico (1992-1998) e outro pós-epidêmico (1999-2004). Comparando os dois períodos, a proporção homem/mulher mudou de 1,8 para 1,14, a média de idade dos pacientes passou de 17,9 para 14,8, o percentual de casos por Plasmodium falciparum reduziu-se de 51,9% para 23,7, e o percentual de lâminas com baixa densidade parasitária de P. falciparum passou de 35,3% para 44,9%, e as de P. vivax de 24% para 35%. Todas essas diferenças foram estatisticamente significativas (p < 0,05). Tais mudanças no padrão epidemiológico da malária no rio Padauiri devem ser esclarecidas mediante estudos prospectivos.

Differences in trans-stimulated chloroquine efflux kinetics are linked to PfCRT in Plasmodium falciparum

The mechanism underpinning chloroquine drug resistance in the human malarial parasite Plasmodium falciparum has remained controversial. Currently discussed models include a carrier or a channel for chloroquine, the former actively expelling the drug, the latter facilitating its passive diffusion, out of the parasite's food vacuole, where chloroquine accumulates and inhibits haem detoxification. Here we have challenged both models using an established trans-stimulation efflux protocol. While carriers may demonstrate trans-stimulation, channels do not. Our data reveal that extracellular chloroquine stimulates chloroquine efflux in the presence and absence of metabolic energy in both chloroquine-sensitive and -resistant parasites, resulting in a hyperbolic increase in the apparent initial efflux rates as the concentration of external chloroquine increases. In the absence of metabolic energy, the apparent initial efflux rates were comparable in both parasites. Significant differences were only observed in the presence of metabolic energy, where consistently higher apparent initial efflux rates were found in chloroquine-resistant parasites. As trans-stimulation is characteristic of a carrier, and not a channel, we interpret our data in favour of a carrier for chloroquine being present in both chloroquine-sensitive and -resistant parasites, however, with different transport modalities.

Bayesian analysis of new and old malaria parasite DNA sequence data demonstrates the need for more phylogenetic signal to clarify the descent of Plasmodium falciparum

Molecular systematic studies published during the last 15 years to clarify the phylogenetic relationships among the malaria parasites have led to two major hypotheses on the descent of Plasmodium falciparum: One supports an avian origin as a result of a relatively recent host switch, and another one favours the evolutionary development of P. falciparum together with its human host from primate ancestors. In this paper, we present phylogenetic analyses of three different Plasmodium genes, the nuclear 18 small sub-unit (SSU) ribosomal ribonucleic acid (rRNA), the mitochondrial cytochrome b (cyt b) and the plastid caseinolytic protease C (ClpC) gene, using numerous haemosporidian parasite DNA sequences obtained from the GenBank as well as several new sequences for major malaria parasites including the avian one Plasmodium cathemerium, which has never been considered in molecular phylogenetic analyses before. Most modern and sophisticated DNA substitution models based on Bayesian inference analysis were applied to estimate the cyt b and ClpC phylogenetic trees, whereas the 18 SSU rRNA gene was examined with regards to its secondary structure using PHASE software. Our results indicate that the data presently available are generally neither sufficient in number nor in information to solve the problem of the phylogenetic origin of P. falciparum.

Plasmodium vivax in Africa: hidden in plain sight?

People who live in tropical Africa, south of the Sahara, are predominantly negative for the Duffy blood-group antigen, which mediates invasion of reticulocytes by Plasmodium vivax. Recent reports of a parasite that was molecularly diagnosed as P. vivax from populations who are suspected, or known, to be Duffy negative confound a large body of evidence that states that invasion of P. vivax requires the Duffy antigen. If confirmed, one of several possible explanations is that P. vivax, which originated in Asia, is now evolving to exploit alternate invasion receptors in Africa.

Point-of-Care Diagnostics Enter the Mouth

In this succinct review, I delineate a case supporting point-of-care (POC) diagnostics to provide a brief outline of why oral fluid/saliva-based POC offer several advantages over more traditional blood-based tests and conclude with a focused overview of the ethical, legal, and social implications of more widespread access to oral fluid/saliva-based POC diagnostics.

Urbanization and tropical health--then and now

Since the launch of the Annals of Tropical Medicine and Parasitology 100 years ago, the percentage of the world's population living in urban settings has more than tripled and is now approaching 50%. Urbanization will continue at a high pace, particularly in the less developed regions of Africa and Asia. The profound demographic, ecological and socio-economic transformations that accompany the process of urbanization have important impacts on health and well-being. In industrialized countries, urbanization led to the so-called 'epidemiological transition', from acute infectious and deficiency diseases to chronic non-communicable diseases, many decades ago. In the developing world, surprisingly little research has been carried out on the health-related aspects of urbanization. In a temporal analysis of publications in the Annals of Tropical Medicine and Parasitology, for example, in which the first volume in every decade from 1907 was examined, only 16 (2.6%) of the 604 articles investigated focused on epidemiological and/or public-health issues in urban tropical settings. This review begins with the question 'what is urban?' and then provides a summary of the trends seen in urbanization, and its impacts on human health, over the past century, on both a global and regional scale. For the main tropical diseases, estimates of the at-risk populations and the numbers of cases are updated and then split into urban and non-urban categories. The inhabitants of urban slums are particularly vulnerable to many of these diseases and require special attention if internationally-set targets for development are to be met. Heterogeneity, a major feature of urban settings in the tropics that complicates all efforts at health improvement, is demonstrated in an exploration of a densely populated municipality of a large West African town. Urban planners, public-health experts and other relevant stakeholders clearly need to make much more progress in alleviating poverty and enhancing the health and well-being of urban residents, in an equity-effective and sustainable manner.

Targeting purine and pyrimidine metabolism in human apicomplexan parasites

Synthesis de novo, acquisition by salvage and interconversion of purines and pyrimidines represent the fundamental requirements for their eventual assembly into nucleic acids as nucleotides and the deployment of their derivatives in other biochemical pathways. A small number of drugs targeted to nucleotide metabolism, by virtue of their effect on folate biosynthesis and recycling, have been successfully used against apicomplexan parasites such as Plasmodium and Toxoplasma for many years, although resistance is now a major problem in the prevention and treatment of malaria. Many targets not involving folate metabolism have also been explored at the experimental level. However, the unravelling of the genome sequences of these eukaryotic unicellular organisms, together with increasingly sophisticated molecular analyses, opens up possibilities of introducing new drugs that could interfere with these processes. This review examines the status of established drugs of this type and the potential for further exploiting the vulnerability of apicomplexan human pathogens to inhibition of this key area of metabolism.

Uric acid nephrolithiasis: proton titration of an essential molecule?

PURPOSE OF REVIEW

The majority of uric acid nephrolithiasis in humans occurs in the absence of frank hyperuricosuria and is primarily a disease of excessively low urinary pH. Uric acid is substantially less soluble than urate salts so in low urine pH urate is protonated, thus favoring precipitation even under what is considered physiologic concentrations of total urinary uric acid/urate. This commentary examines the rationales behind the existence of uric acid in urine and body fluids in vertebrate evolution.

RECENT FINDINGS

The purpose of uric acid in arthropod, avian and reptilian species is to enable nitrogen excretion in solid state without loss of water. The re-emergence of uric acid in higher primates as an end product of metabolism is intriguing since urea functions perfectly well as a nitrogenous waste. Uric acid must purvey important physiologic functions in primate biology. Numerous roles of uric acid as an antioxidant, immune signaling molecule, and a defender of circulatory integrity have recently been proposed.

SUMMARY

There is little doubt that uric acid serves multiple important functions in higher primates. It is also conceivable, however, that this important molecule when present in the wrong concentration or context can lead to undesirable phenotypes.

Comparison of naturally acquired antibody responses against the C-terminal processing products of Plasmodium vivax Merozoite Surface Protein-1 under low transmission and unstable malaria conditions in Sri Lanka

We report here, for the first time, a comparison of naturally acquired antibody responses to the 42 and 19 kDa C-terminal processing products of Plasmodium vivax Merozoite Surface Protein-1 assayed by ELISA using p42 and p19 baculovirus-derived recombinant proteins, respectively. Test populations comprised patients with microscopy confirmed acute P. vivax infections from two regions endemic for vivax malaria where low transmission and unstable malaria conditions prevail, and a non-endemic urban area, in Sri Lanka. The antibody prevalence to the two proteins, both at the individual and population levels, tend to respond more to p42 than to p19 in all test areas, where >14% of individuals preferentially recognized p42, compared with <2% for p19. In patients with no previous exposure to malaria, 21% preferentially recognized p42, whereas none exclusively recognized p19. A significantly lower prevalence of anti-p19 IgM, but not anti-p42 IgM, was observed among residents from endemic areas compared with their non-endemic counterparts. Individuals from both endemic areas produced significantly less anti-p19 IgM compared with anti-p42 IgM. IgG1 was the predominant IgG isotype for both antigens in all individuals. With increasing exposure to malaria in both endemic areas, anti-p19 antibody responses were dominated by the functionally important IgG1 and IgG3 isotypes, with a concurrent reduction in IgM that was lacking in the non-endemic residents. This antibody switch was also reflected for PvAMA-1 as we previously reported with the identical battery of sera. In contrast, the antibody switch for p42 was restricted to endemic residents with more extensive exposure. These results suggest that an IgM-dominated antibody response against the p42 polymorphic region in endemic residents may interfere with the development of an IgG-dominated "protective" isotype shift to p19, that may complicate vaccine development.

Design, synthesis and activity against Toxoplasma gondii, Plasmodium spp., and Mycobacterium tuberculosis of new 6-fluoroquinolones

This paper reports on the rational design of a series of new 6-fluoroquinolones by QSAR analysis against Toxoplasma (T.) gondii, their synthesis, their biological evaluation against T. gondii and Plasmodium (P.) spp., and their effect on Mycobacterium (M.) tuberculosis DNA gyrase and growth inhibition. Of the 12 computer-designed 8-ethyl(or methoxy)- and 5-ethyl-8-methoxy-6-fluoroquinolones predicted to be active against T. gondii, we succeeded in the synthesis of four 6-fluoro-8-methoxy-quinolones. The four 6-fluoro-8-methoxy-quinolones are active on T. gondii but only one is as active as predicted. One of these four compounds appears to be an antiparasitical drug of great potential with inhibitory activities comparable to or higher than that of trovafloxacin, gatifloxacin, and moxifloxacin. They also inhibit DNA supercoiling by M. tuberculosis gyrase with an efficiency comparable to that of the most active quinolones but are poor inhibitors of M. tuberculosis growth.

The distribution and intensity of parasite sequestration in comatose Malawian children

BACKGROUND

The sequestration of Plasmodium falciparum-infected erythrocytes in capillary beds is a characteristic feature of severe malaria and is believed to be central to disease pathogenesis. Sequestration occurs in all P. falciparum infections, including those in asymptomatic individuals. Therefore, sequestration cannot be the sole determinant of severe disease; the intensity or distribution of infected erythrocytes may also contribute. Discerning the relationship between sequestration and well-defined clinical syndromes may enhance understanding of disease mechanisms.

METHODS

We measured the concentration of parasite-derived lactate dehydrogenase (pLDH) in tissue samples obtained at autopsy from patients with clinically defined cerebral malaria. On the basis of the autopsy findings, patients were divided into 2 groups: those with an identifiable, nonmalarial cause of death and those without, who were presumed to have died of cerebral malaria. The concentration of pLDH, as determined by enzyme-linked immunosorbent assay, was used to estimate parasite load in different organs.

RESULTS

When pLDH could be detected, the parasite load was higher in patients with presumed cerebral malaria than in parasitemic patients with assumed cerebral malaria with a nonmalaria cause of death identified at autopsy (P<.05 for brain, intestine, and skin).

CONCLUSIONS

These findings suggest that sequestration in patients with fatal cerebral malaria occurs in multiple organs and does not reflect a predilection in the parasite for the cerebral vasculature.

A new challenge for malaria control in Brazil: asymptomatic Plasmodium infection - a review

The evolution of malaria in Brazil, its morbidity, the malaria control programs, and the new challenges for these programs in the light of the emergence of asymptomatic infection in the Amazon region of Brazil were reviewed. At least six Brazilian research groups have demonstrated that asymptomatic infection by Plasmodium is an important impediment to malaria control, among mineral prospectors in Mato Grosso and riverside communities in Rondônia and, in our group, in the middle and upper reaches of the Negro river, in the state of Amazonas. Likewise, other researchers have studied the problem among indigenous communities in the Colombian, Peruvian, and Venezuelan parts of the Amazon basin, adjacent to Brazil. The frequency of positive results from the polymerase chain reaction (PCR) among asymptomatic individuals has ranged from 20.4 to 49.5%, and the presence of Plasmodium in the thick blood smears, from 4.2 to 38.5%. Infection with Anopheles darlingi has also been demonstrated by xenodiagnosis among asymptomatic patients with positive PCR results. If a mean of 25% is taken for the asymptomatic infection caused by Plasmodium sp. in the Amazon region of Brazil, malaria control will be difficult to achieve in that region with the measures currently utilized for such control.

Genetic diversity and genetic burden in humans

We discuss categories of genetic diversity in humans. Neutral diversity, population differences in frequencies of genetic markers that we think are invisible to natural selection, provides a passive record of population history but is otherwise of little interest in human biology. Genetic variation related to disease can be separated into mutational noise and variation due to selection, either ongoing selection else effects of a past environment. We distinguish consequences of genetic diversity for fitness, relevant to evolution, and consequences for well-being, relevant to medicine and public health. We call genetic variation that causes impairment of health or well-being of individual humans "apparent genetic burden" and variation that has effects on fitness but not well-being "unapparent genetic burden". We use "burden" to distinguish these notions from the classical concept of "genetic load" that refers to effects on population fitness, a concept formulated by Morton et al. [Morton, N.E., Crow, J.F., Muller, H.J., 1956. An estimate of the mutational damage in man from data on consanguineous marriages. Proc. Natl. Acad. Sci. U.S.A. 42, 855-863]. We distinguish adapted genes and adapted genotypes: an adapted gene is a gene that increases fitness of its bearer either in heterozygous or homozygous state or both, while an adapted genotype is a genotype that increases fitness of its bearer but is not transmitted intact to future generations. Balanced polymorphisms in which the heterozygote is superior in fitness may generate most adapted genotypes. In the face of major rapid environmental change adapted genotypes appear first but over time they are replaced by adapted genes. The presence of adapted genotypes is a good indication of recent environmental change: for example, there are apparently many polymorphisms in domestic animals of this nature, responses to domestication, and many fewer in wild animals (and in humans).

Signal-mediated export of proteins from the malaria parasite to the host erythrocyte

Intracellular parasites from the genus Plasmodium reside and multiply in a variety of cells during their development. After invasion of human erythrocytes, asexual stages from the most virulent malaria parasite, P. falciparum, drastically change their host cell and export remodelling and virulence proteins. Recent data demonstrate that a specific NH₂-terminal signal conserved across the genus Plasmodium plays a central role in this export process.

Incestuous gene in consanguinophilia and incest: Toward a consilience theory of incest taboo

Westermarck's theory of incest taboo states that inhibition of sexual attraction between biologically close relatives is situational and develops during co-residence in early childhood. By contrast, the biological (genetic) basis of incest taboo is presumed from its universality in all human societies and animals and teleologically, from the need to prevent the detrimental effects of inbreeding. As incest taboo violation is infrequent, the frequency of the presumed gene in the population is believed to be near 100%. We present arguments which suggest that the incestuous gene may exist in all populations and could play an important role in evolution. When malaria emerged 10,000 years ago, human adaptation proceeded by the selection of protective genotypes. Among them, homozygotes for alpha-thalassemia, hemoglobin C, and Duffy antigen negative blood group, have better survival odds in malarious regions than heterozygotes and those with normal genotypes. Since consanguinity increases homozygosity, it increases the number of persons who are resistant to malaria. To pro-create, however, biologically close individuals must not feel sexual aversion that normally develops between those who spend their early childhood together (Westermarck effect). It is reasonable to assume, therefore, that mutation of the gene that discourages inbreeding may have appeared at an early time in evolution, and produced a weak Westermarck effect. This gene (we will call it anti-w) failed to inhibit mating between kins. Inbred offspring of anti-w carriers, would statistically, more likely carry both anti-w and homozygote genotypes which increase fitness in the presence of malaria. Today, alpha-thalassemia is the single most common monogenetic disorders in man with over 500 millions carriers concentrated in malarious regions of the world. The world's consanguineous population is some 500-800 millions and is also concentrated in malarious regions. Population migration has spread the gene outside areas of high malaria endemicity. However, endemicity of malaria provides a worldwide gradient of genotype frequencies which makes the incestuous gene hypothesis testable. We propose that the incestuous anti-w allele was co-selected with some of the genes protective against malaria because anti-w facilitates mating between genetically close individuals whose offspring better survive malaria.

Defining the global spatial limits of malaria transmission in 2005

There is no accurate contemporary global map of the distribution of malaria. We show how guidelines formulated to advise travellers on appropriate chemoprophylaxis for areas of reported Plasmodium falciparum and Plasmodium vivax malaria risk can be used to generate crude spatial limits. We first review and amalgamate information on these guidelines to define malaria risk at national and sub-national administrative boundary levels globally. We then adopt an iterative approach to reduce these extents by applying a series of biological limits imposed by altitude, climate and population density to malaria transmission, specific to the local dominant vector species. Global areas of, and population at risk from, P. falciparum and often-neglected P. vivax malaria are presented for 2005 for all malaria endemic countries. These results reveal that more than 3 billion people were at risk of malaria in 2005.

Plasmodium vivax: recent world expansion and genetic identity to Plasmodium simium

Plasmodium vivax causes the most geographically widespread human malaria, accounting annually for 70-80 million clinical cases throughout the tropical and subtropical regions of the world's continents. We have analyzed the DNA sequences of the Csp (circumsporozoite protein) gene in 24 geographically representative strains of P. vivax and 2 of P. simium, which parasitizes several species of New World monkeys. The Csp sequences are of two types, VK210 and VK247, which differ by three diagnostic amino acid replacements, one in each of the 5' and 3' terminal regions [5' nonrepeat (NR) and 3' NR] of the gene and in an insertion sequence that precedes the 3' NR region. The central region of the gene consists of approximately 38 repetitive "motifs," which are alternatively four and five amino acids long, which also are diagnostically different between the VK210 and VK247 types. There are very few synonymous substitutions within and between the two types of strains, which we hypothesize reflects that the worldwide spread of P. vivax is very recent. The two P. simium Csp sequences belong one to each of the two VK types and are genetically indistinguishable from the corresponding P. vivax strains, suggesting that at least two host transfers have occurred between humans and New World monkeys. We exclude as unlikely the possibility that the two types of sequences could have independently arisen in humans and platyrrhines by natural selection. There are reasons favoring each of the two possible directions of host transfer between humans and monkeys.

Positive selection on a high-sensitivity allele of the human bitter-taste receptor TAS2R16

BACKGROUND

During periods of human expansion into new environments, recognition of bitter natural toxins through taste may have conferred an important selective advantage. The G protein-coupled receptor encoded by TAS2R16 mediates response to salicin, amygdalin, and many bitter beta-glucopyranosides. beta-glucopyranosides are ubiquitous in nature, with many having a highly toxic cyanogenic activity.

RESULTS

We examined evidence for natural selection on the human receptor TAS2R16 by sequencing the entire coding region, as well as part of the 5' and 3' UTRs, in 997 individuals from 60 human populations. We detected signatures of positive selection, indicated by an excess of evolutionarily derived alleles at the nonsynonymous site K172N and two linked sites and significant values of Fay and Wu's H statistics in 19 populations. The estimated age range for the common ancestor of the derived N172 variant is 78,700-791,000 years, placing it in the Middle Pleistocene and before the expansion of early humans out of Africa. Using calcium imaging in cells expressing different receptor variants, we showed that N172 is associated with an increased sensitivity to salicin, arbutin, and five different cyanogenic glycosides.

CONCLUSION

We have detected a clear signal of positive selection at the bitter-taste receptor gene TAS2R16. We speculate that the increased sensitivity that is shown toward harmful cyanogenic glycosides and conferred by the N172 allele may have driven the signal of selection at an early stage of human evolution.

Identification of the optimal third generation antifolate against P. falciparum and P. vivax

Inhibitors of dihydrofolate reductase (DHFR) have been mainstays in the treatment of falciparum malaria. Resistance to one of these antifolates, pyrimethamine, is now common in Plasmodium falciparum populations. Antifolates have not traditionally been recommended for treatment of vivax malaria. However, recent studies have suggested that a third-generation antifolate, WR99210, is remarkably effective even against highly pyrimethamine-resistant parasites from both species. Two methods were used to identify a compound that is effective against quadruple mutant alleles from P. falciparum (N51I/C59R/S108N/I164L) and from Plasmodium vivax (57L/111L/117T/173F). The first was simple yeast system used to screen a panel of WR99210 analogs. The biguanide prodrug, JPC-2056, of the 2-chloro-4-trifluoromethoxy analog of WR99210 was effective against both the P. falciparum and P. vivax enzymes, and has been selected for further development. The second method compared the analogs in silico by docking them in the known structure of the P. falciparum DHFR-thymidylate synthase. The program reproduced well the position of the triazine ring, but the calculated energies of ligand binding were very similar for different compounds and therefore did not reproduce the observed trends in biological activity. The WR99210 family of molecules is flexible due to a long bridge between the triazine ring and the substituted benzene. During docking, multiple conformations were observed for the benzene ring part of the molecules in the DHFR active site, making computer-based predictions of binding energy less informative than for more rigid ligands. This flexibility is a key factor in their effectiveness against the highly mutant forms of DHFR.

Epidemiology and surveillance: changing the global picture of malaria--myth or reality?

The burden of malaria remains unacceptably high, and the greatest burden is borne by the African continent. There is now wide agreement on the control strategies and tools, and a considerable increase in global resources invested in malaria control efforts. However, challenges remain, especially with achieving the necessary population coverage with these interventions in order to change the epidemiology of the disease. Existing tools may not be optimal, and are liable to fail with time, just as previous ones did. Current global antimalarial efforts need to be underpinned by a strong research and development agenda. If all these factors are taken into consideration, reducing the burden of malaria would seem more like a reality than a myth in the foreseeable future.

ABO-blood-group types and protection against severe, Plasmodium falciparum malaria

Although the ABO blood group of the human host has been reported to influence malarial infection, there have been few clinical observations on this effect. A hospital-based, comparative study was therefore performed to investigate the relationship between blood-group type and severe disease i nPlasmodium falciparum malaria. Overall, 243 cases of malaria (163 uncomplicated and 80 severe) and 65 patients with severe, non-malarial infections were studied. In terms of ABO-blood-group composition, the patients with severe malaria were significantly different from the patients with the uncomplicated disease (P<0.001) and also from a population control described previously (P<0.0001). The patients with uncomplicated malaria or severe but non-malarial disease were, however, similar to the population control. The cases of severe malaria were significantly less likely to be of blood group O (P=0.0003), and significantly more likely to be of group AB (P<0.0001), than the patients with nonsevere malaria. It appears that individuals who are of blood-group O are relatively resistant to the severe disease caused by P. falciparum infection.

The hydration state of human red blood cells and their susceptibility to invasion by Plasmodium falciparum

In most inherited red blood cell (RBC) disorders with high gene frequencies in malaria-endemic regions, the distribution of RBC hydration states is much wider than normal. The relationship between the hydration state of circulating RBCs and protection against severe falciparum malaria remains unexplored. The present investigation was prompted by a casual observation suggesting that falciparum merozoites were unable to invade isotonically dehydrated normal RBCs. We designed an experimental model to induce uniform and stable isotonic volume changes in RBC populations from healthy donors by increasing or decreasing their KCl contents through a reversible K(+) permeabilization pulse. Swollen and mildly dehydrated RBCs were able to sustain Plasmodium falciparum cultures with similar efficiency to untreated RBCs. However, parasite invasion and growth were progressively reduced in dehydrated RBCs. In a parallel study, P falciparum invasion was investigated in density-fractionated RBCs from healthy subjects and from individuals with inherited RBC abnormalities affecting primarily hemoglobin (Hb) or the RBC membrane (thalassemias, hereditary ovalocytosis, xerocytosis, Hb CC, and Hb CS). Invasion was invariably reduced in the dense cell fractions in all conditions. These results suggest that the presence of dense RBCs is a protective factor, additional to any other protection mechanism prevailing in each of the different pathologies.

Pediatric malaria in the developing world

Hundreds of millions of people suffer from malaria, and more than a million children die of malaria each year. Malaria typically presents with fever and headache, but the presentation often is nonspecific. The diagnosis should be based on blood tests, and thick and thin smears are the standard means of identifying parasites. In some areas, chloroquine still is effective as treatment, but other medications are needed in most parts of the world. Patients with severe disease (altered consciousness, marked anemia, and/or respiratory distress) should begin therapy parenterally. Control measures depend on the use of insecticide-treated bednets, early identification and treatment of symptomatic individuals, and intermittent preventive therapy. Progress continues toward the development of a useful vaccine.

Mitochondrial genome sequences support ancient population expansion in Plasmodium vivax

Examination of nucleotide diversity in 106 mitochondrial genomes of the most geographically widespread human malaria parasite, Plasmodium vivax, revealed a level of diversity similar to, but slightly higher than, that seen in the virulent human malaria parasite Plasmodium falciparum. The pairwise distribution of nucleotide differences among mitochondrial genome sequences supported the hypothesis that both these parasites underwent ancient population expansions. We estimated the age of the most recent common ancestor (MRCA) of the mitochondrial genomes of both P. vivax and P. falciparum at around 200,000-300,000 years ago. This is close to the previous estimates of the time of the human mitochondrial MRCA and the origin of modern Homo sapiens, consistent with the hypothesis that both these Plasmodium species were parasites of the hominid lineage before the origin of modern H. sapiens and that their population expansion coincided with the population expansion of their host.

Dictyostelium discoideum expresses a malaria chloroquine resistance mechanism upon transfection with mutant, but not wild-type, Plasmodium falciparum transporter PfCRT

Chloroquine resistance in Plasmodium falciparum malaria results from mutations in PfCRT, a member of a unique family of transporters present in apicomplexan parasites and Dictyostelium discoideum. Mechanisms that have been proposed to explain chloroquine resistance are difficult to evaluate within malaria parasites. Here we report on the targeted expression of wild-type and mutant forms of PfCRT to acidic vesicles in D. discoideum. We show that wild-type PfCRT has minimal effect on the accumulation of chloroquine by D. discoideum, whereas forms of PfCRT carrying a key charge-loss mutation of lysine 76 (e.g. K76T) enable D. discoideum to expel chloroquine. As in P. falciparum, the chloroquine resistance phenotype conferred on transformed D. discoideum can be reversed by the channel-blocking agent verapamil. Although intravesicular pH levels in D. discoideum show small acidic changes with the expression of different forms of PfCRT, these changes would tend to promote intravesicular trapping of chloroquine (a weak base) and do not account for reduced drug accumulation in transformed D. discoideum. Our results instead support outward-directed chloroquine efflux for the mechanism of chloroquine resistance by mutant PfCRT. This mechanism shows structural specificity as D. discoideum transformants that expel chloroquine do not expel piperaquine, a bisquinoline analog of chloroquine used frequently against chloroquine-resistant parasites in Southeast Asia. PfCRT, nevertheless, may have some ability to act on quinine and quinidine. Transformed D. discoideum will be useful for further studies of the chloroquine resistance mechanism and may assist in the development and evaluation of new antimalarial drugs.