Avian malaria: a new lease of life for an old experimental model to study the evolutionary ecology of Plasmodium

Avian malaria has historically played an important role as a model in the study of human malaria, being a stimulus for the development of medical parasitology. Avian malaria has recently come back to the research scene as a unique animal model to understand the ecology and evolution of the disease, both in the field and in the laboratory. Avian malaria is highly prevalent in birds and mosquitoes around the world and is amenable to laboratory experimentation at each stage of the parasite''s life cycle. Here, we take stock of 5 years of experimental laboratory research carried out using Plasmodium relictum SGS1, the most prevalent avian malaria lineage in Europe, and its natural vector, the mosquito Culex pipiens. For this purpose, we compile and analyse data obtained in our laboratory in 14 different experiments. We provide statistical relationships between different infection-related parameters, including parasitaemia, gametocytaemia, host morbidity (anaemia) and transmission rates to mosquitoes. This analysis provides a wide-ranging picture of the within-host and between-host parameters that may bear on malaria transmission and epidemiology.     

The evolution of Brazilian studies on helminths and the Escola tropicalista bahiana (Bahia Tropical School) (1866-1892)

In the mid nineteenth century, laboratory research in embryology, development, ecology and pathogeny of helminths unexpectedly resulted in a change in the explication of the etiology of some human pathology that was traditionally attributed to environmental factors. The new postulates of the parasitology of helminths put in question the traditional explications as well as the recognized authorities in the fields of clinical medicine, hygiene, and of medical geography, affecting not only the traditional circles where the innovations were discussed, but also the institutions that controlled and validated medical knowledge. From the beginning of the decade of the 1860s, a group of Brazilian medical doctors, initially led by Otto Wucherer, took part in this scientific movement that had an important impact on the area of the domestic and international academic medicine. The causality of known diseases, such as intertropical hypoemia, hematochyluria and elephantiasis, became the focus of intense debate between 1866 and 1892. By the end of this period, the field of helminthic parasitology had available well-established methodological and conceptual criteria and the role of Ankylostomum duodenale and of Wuchereria bancrofti in the production of those diseases was fully accepted.     

The birth of the anti-malaria campaign in Spain during the first 30 years of the 20th century: scientific and social aspects

The relations between the scientific and the social domains in the particular case of malaria had developed at three levels. The first one operates around the transformation of malaria into an escapable disease and the combined efforts of parasitology and entomology are mixed with the growing development of the concept of social medicine. The second one deals with the rhythm and the content of the measures taken to fight the disease in each concerned country; in that case, very peculiar site-time coordinates ask precise questions. At the same level can be placed the differences in the campaigns led against malaria in colonies and in the mainland, differences that it would be misleading to approach on the sole side of social determinism. Finally the third level corresponds to the complexity of the relationships between the international and the local domains, as present from the birth of "Office International d'Hygiène de la Société des Nations" and the involvement of the Rockefeller Foundation where two strategic positions can be detected opposing people minoring problems and those seeking for eradication, who indeed were opponents at the level of the scientific direction, but also originate within the socio-professional boundary of the members of each group, respectively.     

Spreading the seeds of million-murdering death: metamorphoses of malaria in the mosquito

Plasmodium spp. undergo a complex obligate developmental cycle within their invertebrate vectors that enables transmission between vertebrate hosts. This developmental cycle involves sexual reproduction and then asexual multiplication, separated by phases of invasion and colonization of distinct vector tissues. As with other stages in the Plasmodium life cycle, there is exquisite adaptation of the malaria parasite to its changing environment as it transforms within the blood of its vertebrate host, through the different tissues of its mosquito vector and onwards to infect a new vertebrate host. Despite the intricacies inherent in these successive transformations, malaria parasites remain staggeringly successful at disseminating through their vertebrate host populations.     

Academician E. N. Pavlovskiĭ's parasitology school in the Zoological institute RAS

The first parasitological division in the Zoological Museum was created in 1924 by the initiative of E. N. Pavlovsky and A. A. Schtakelberg and originally had a named "The permanent commission on the study of malaria mosquitoes". In the process of reorganisation of the Zoological Museum into Zoological Institute in 1930, it was modified into the Department of Parasitology with the E. N. Pavlovsky as a head. In 1934-1935, two laboratories were formed within this department: the Laboratory arachno-entomology and Laboratory of parasitic worms. In subsequent history of ZIN, these parasitological laboratories existed at first as subdivisions of the Department of Parasitology and finally the they were reorganised into independent administrative divisions. The study of parasitic and blood-sucking arthropodes is concentrated in the Laboratory of Parasitology (the head Yu. S. Balashov). A creation of the most important concepts of ecological parasitology was taking place in the Zoological Institute in the middle of 30th. E. N. Pavlovsky for the first time had formulated the principle of an organism as an environment for parasites, the concept of communities of parasitic organisms (concept of parasitocoenosis), and the theory of natural focuses of transmissive diseases. In the process of development of these scientific generalisations, a scientific direction named "Academician E. N. Pavlovsky's school of thought in parasitology" was formed in the USSR in 40-50th. In the frame of this school of thought, the main tusks of the Laboratory of Parasitology ZIN are to work out fundamental problems in ecology, systematics and morphology of parasitic and blood-sucking ticks, mites and insects. Within the ecological parasitology, different aspects of host-parasite relationships are studied at organism and population levels. The main basis of systematics studies of parasitic arthropodes is a scientific collection including over 250,000 samples. Based on this material, 40 key books and monographs on the USSR's fauna were created. Over 20 doctors of science and 50 candidates of science have been prepared within the laboratory or under the promotion of its stuff during 70 years of the existence of the Laboratory of Parasitology.     

The impact of whole genome sequence data on drug discovery--a malaria case study.

BACKGROUND: Identification and validation of a drug discovery target is a prominent step in drug development. In the post-genomic era it is possible to reevaluate the association of a gene with a specific biological function to see if a homologous gene can subsume this role. This concept has special relevance to drug discovery in human infectious diseases, like malaria. A trophozoite cysteine protease (falcipain-1) from the papain family, thought to be responsible for the degradation of erythrocyte hemoglobin, has been considered a promising target for drug discovery efforts owing to the antimalarial activity of peptide based covalent cysteine protease inhibitors. This led to the development of non-peptidic non-covalent inhibitors of falcipain-1 and their characterization as antimalarials. It is now clear from sequencing efforts that the malaria genome contains more than one cysteine protease and that falcipain-1 is not the most important contributor to hemoglobin degradation. Rather, falcipain-2 and falcipain-3 appear to account for the majority of cysteine hemoglobinase activity in the plasmodium trophozoite. MATERIALS AND METHODS: We have modeled the falcipain-2 cysteine protease from one of the major human malaria species, Plasmodium falciparum and compared it to our original work on falcipain-1. As with falcipain-1, computa-tional screening of the falcipain-2 active site was conducted using DOCK. Using structural superpositions within the protease family and evolutionary analysis of substrate specificity sites, we focused on the commonalities and the protein specific features to direct our drug discovery effort. RESULTS: Since 1993, the size of the Available Chemicals Directory had increased from 55313 to 195419 unique chemical structures. For falcipain-2, eight inhibitors were identified with IC50's against the enzyme between 1 and 7 microM. Application of three of these inhibitors to infected erythrocytes cured malaria in culture, but parasite death did not correlate with food vacuole abnormalities associated with the activity of mechanistic inhibitors of cysteine proteases like the epoxide E64. CONCLUSIONS: Using plasmodial falcipain proteases, we show how a protein family perspective can influence target discovery and inhibitor design. We suspect that parallel drug discovery programs where a family of targets is considered, rather than serial programs built on a single therapeutic focus, will become the dominant industrial paradigm. Economies of scale in assay development and in compound synthesis are expected owing to the functional and structural features of individual family members. One of the remaining challenges in post-genomic drug discovery is that inhibitors of one target are likely to show some activity against other family members. This lack of specificity may lead to difficulties in functional assignments and target validation as well as a complex side effect profile.     

Unraveling the ubiquitome of the human malaria parasite

Malaria is one of the deadliest infectious diseases worldwide. The most severe form is caused by the eukaryotic protozoan parasite Plasmodium falciparum. Recent studies have highlighted the importance of post-translational regulations for the parasite's progression throughout its life cycle, protein ubiquitylation being certainly one of the most abundant. The specificity of its components and the wide range of biological processes in which it is involved make the ubiquitylation pathway a promising source of suitable targets for anti-malarial drug development. Here, we combined immunofluorescent microscopy, biochemical assays, in silico prediction, and mass spectrometry analysis using the multidimensional protein identification technology, or MudPIT, to describe the P. falciparum ubiquitome. We found that ubiquitin conjugates are detected at every morphological stage of the parasite erythrocytic cycle. Furthermore, we detected that more than half of the parasite's proteome represents possible targets for ubiquitylation, especially proteins found to be present at the most replicative stage of the asexual cycle, the trophozoite stage. A large proportion of ubiquitin conjugates were also detected at the schizont stage, consistent with a cell activity slowdown to prepare for merozoite differentiation and invasion. Finally, for the first time in the human malaria parasite, our results strongly indicate the presence of heterologous mixed conjugations, SUMO/UB. This discovery suggests that sumoylated proteins may be regulated by ubiquitylation in P. falciparum. Altogether, our results present the first stepping stone toward a better understanding of ubiquitylation and its role(s) in the biology of the human malaria parasite.     

Parasites, parasitology and parasitologists

The science of parasitology is one of the many new disciplines of the twentieth century, as such it is a dynamic and rapidly evolving science which encompasses an increasing number of sub-disciplines and technologies. However, will the fragmentation involved in current methods of scientific enquiry and the competition for funding mean the decline of certain areas of parasitology or perhaps the complete loss of the discipline. This paper attempts to address these questions by considering the development of the discipline of parasitology especially within Australia and by considering the mechanisms of attaining funding for science. Technological change and its impact on parasitology is also considered, and requirements for maintenance of the discipline and its practitioners are suggested.     

Criticism,commitment, and the growth of human sociobiology

The fundamental unit of assessment in the sociobiology debate is neither a field nor a theory, but a framework of group commitments. Recourse to the framework concept is motivated, in general, by post-Kuhnian philosophy of scientific change and, in particular, by the dispute between E. O. Wilson and R. C. Lewontin. The framework concept is explicated in terms of commitments about problems, domain, disciplinary relations, exemplars, and performance evaluations. One upshot is that debate over such charges as genetic determinism, reductionism, adaptationism, and the biologization of human nature has been vexed. It has lost sight of human sociobiology's central problem, namely to help show that the modern synthesis is complete.     

Environmental, pharmacological and genetic influences on the spread of drug-resistant malaria

Plasmodium falciparum malaria is subject to artificial selection from antimalarial drugs that select for drug-resistant parasites. We describe and apply a flexible new approach to investigate how epistasis, inbreeding, selection heterogeneity and multiple simultaneous drug deployments interact to influence the spread of drug-resistant malaria. This framework recognizes that different human 'environments' within which treatment may occur (such as semi- and non-immune humans taking full or partial drug courses) influence the genetic interactions between parasite loci involved in resistance. Our model provides an explanation for how the rate of spread varies according to different malaria transmission intensities, why resistance might stabilize at intermediate frequencies and also identifies several factors that influence the decline of resistance after a drug is removed. Results suggest that studies based on clinical outcomes might overestimate the spread of resistant parasites, especially in high-transmission areas. We show that when transmission decreases, prevalence might decrease without a corresponding change in frequency of resistance and that this relationship is heavily influenced by the extent of linkage disequilibrium between loci. This has important consequences on the interpretation of data from areas where control is being successful and suggests that reducing transmission might have less impact on the spread of resistance than previously expected.     

Recent advances in malaria drug discovery

This digest covers some of the most relevant progress in malaria drug discovery published between 2010 and 2012. There is an urgent need to develop new antimalarial drugs. Such drugs can target the blood stage of the disease to alleviate the symptoms, the liver stage to prevent relapses, and the transmission stage to protect other humans. The pipeline for the blood stage is becoming robust, but this should not be a source of complacency, as the current therapies set a high standard. Drug discovery efforts directed towards the liver and transmission stages are in their infancy but are receiving increasing attention as targeting these stages could be instrumental in eradicating malaria.     

PbGEST mediates malaria transmission to both mosquito and vertebrate host

The malaria life cycle relies on the successful transfer of the parasite between its human and mosquito hosts. We identified a Plasmodium berghei secreted protein (PBANKA_131270) that plays distinct roles in both the mammal-to-mosquito and the mosquito-to-mammal transitions. This protein, here named gamete egress and sporozoite traversal (GEST), plays an important role in the egress of male and female gametes from the vertebrate red blood cell. Interestingly, GEST is also required following the bite of the infected mosquito, for sporozoite progression through the skin. We found PbGEST to be secreted shortly after activation of the intraerythrocytic gametocyte, and during sporozoite migration. These findings indicate that a single malaria protein may have pleiotropic roles in different parasites stages mediating transmission between its insect and mammalian hosts.     

A mitogen-activated protein kinase regulates male gametogenesis and transmission of the malaria parasite Plasmodium berghei

Differentiation of malaria parasites into sexual forms (gametocytes) in the vertebrate host and their subsequent development into gametes in the mosquito vector are crucial steps in the completion of the parasite's life cycle and transmission of the disease. The molecular mechanisms that regulate the sexual cycle are poorly understood. Although several signal transduction pathways have been implicated, a clear understanding of the pathways involved has yet to emerge. Here, we show that a Plasmodium berghei homologue of Plasmodium falciparum mitogen-activated kinase-2 (Pfmap-2), a gametocyte-specific mitogen-activated protein kinase (MAPK), is required for male gamete formation. Parasites lacking Pbmap-2 are competent for gametocytogenesis, but exflagellation of male gametocytes, the process that leads to male gamete formation, is almost entirely abolished in mutant parasites. Consistent with this result, transmission of mutant parasites to mosquitoes is grossly impaired. This finding identifies a crucial role for a MAPK pathway in malaria transmission.     

The relevance of non-human primate and rodent malaria models for humans

At the 2010 Keystone Symposium on "Malaria: new approaches to understanding Host-Parasite interactions", an extra scientific session to discuss animal models in malaria research was convened at the request of participants. This was prompted by the concern of investigators that skepticism in the malaria community about the use and relevance of animal models, particularly rodent models of severe malaria, has impacted on funding decisions and publication of research using animal models. Several speakers took the opportunity to demonstrate the similarities between findings in rodent models and human severe disease, as well as points of difference. The variety of malaria presentations in the different experimental models parallels the wide diversity of human malaria disease and, therefore, might be viewed as a strength. Many of the key features of human malaria can be replicated in a variety of nonhuman primate models, which are very under-utilized. The importance of animal models in the discovery of new anti-malarial drugs was emphasized. The major conclusions of the session were that experimental and human studies should be more closely linked so that they inform each other, and that there should be wider access to relevant clinical material.     

Data Do Not Speak for Themselves: The Role of Data in Scientific Controversies

This article presents a learning cycle with the aim of helping students understand the evidentiary basis of scientific claims. Students consider data and interpretations as used to support contradictory views in the debate surrounding the causal relationship between human immunodeficiency virus (HIV) and acquired immunodeficiency syndrome (AIDS). Students apply their understandings of the role of data to explorations of other scientific controversies.     

Genetic diversity of Plasmodium falciparum and its implications in the epidemiology of malaria

Genetic diversity provides Plasmodium falciparum with the potential capacity of avoiding the immune response, and possibly supporting the selection of drug or vaccine resistant parasites. These genetic characters play key roles in the selection of appropriate malaria control measures. Diverse clones of Plasmodium falciparum, often denoted as strains, has been documented, and the degree of genetic diversity supported by several kinds of PCR (polymerase chain reaction) assays. Many studies in different endemic regions with differences in their level of disease transmission have clarified the interactions between the parasite populations and malaria epidemiology. This paper describes recombination events of the malaria parasite life cycle that originate such genetic diversity in P. falciparum, reviewing different studies on this aspect and its implications in the immunity and development of control measures in regions with different degrees of endemicity.     

Exploring the Seasonality of Reported Treated Malaria Cases in Mpumalanga,South Africa

South Africa, having met the World Health Organisation''s pre-elimination criteria, has set a goal to achieve malaria elimination by 2018. Mpumalanga, one of three provinces where malaria transmission still occurs, has a malaria season subject to unstable transmission that is prone to sporadic outbreaks. As South Africa prepares to intensify efforts towards malaria elimination, there is a need to understand patterns in malaria transmission so that efforts may be targeted appropriately. This paper describes the seasonality of transmission by exploring the relationship between malaria cases and three potential drivers: rainfall, geography (physical location) and the source of infection (local/imported). Seasonal decomposition of the time series by Locally estimated scatterplot smoothing is applied to the case data for the geographical and source of infection sub-groups. The relationship between cases and rainfall is assessed using a cross-correlation analysis. The malaria season was found to have a short period of no/low level of reported cases and a triple peak in reported cases between September and May; the three peaks occurring in October, January and May. The seasonal pattern of locally-sourced infection mimics the triple-peak characteristic of the total series while imported infections contribute mostly to the second and third peak of the season (Christmas and Easter respectively). Geographically, Bushbuckridge municipality, which exhibits a different pattern of cases, contributed mostly to the first and second peaks in cases while Maputo province (Mozambique) experienced a similar pattern in transmission to the imported cases. Though rainfall lagged at 4 weeks was significantly correlated with malaria cases, this effect was dampened due to the growing proportion of imported cases since 2006. These findings may be useful as they enhance the understanding of the current incidence pattern and may inform mathematical models that enable one to predict the impact changes in these drivers will have on malaria transmission.     

Parasites of the superorganism: are they indicators of ecosystem health?

The concept of ecosystem health is derived from analogies with human health, which subsequently leads to the implication that the ecosystem has organismal properties, a 'superorganism' in the Clementsian sense. Its application and usefulness has been the subject of a contentious debate; yet, the term 'ecosystem health' has captured the public's imagination and woven its way into the current lexicon, even incorporated into public policy. However, the application of parasites as bioindicators of ecosystem health poses a curious conundrum. Perceptions of parasites range from mild distaste to sheer disgust among the general public, the media, environmental managers and non-parasitologists in the scientific community. Nevertheless, the biological nature of parasitism incorporates natural characteristics that are informative and useful for environmental management. The helminths in particular have evolved elegant means to ensure their transmission, often relying on complex life cycle interactions that include a variety of invertebrate and vertebrate hosts. The assemblage of these diverse parasites within a host organism potentially reflect that host's trophic position within the food web as well as the presence in the ecosystem of any other organisms that participate in the various parasite life cycles. Perturbations in ecosystem structure and function that affect food web topology will also impact upon parasite transmission, thus affecting parasite species abundance and composition. As such, parasite populations and communities are useful indicators of environmental stress, food web structure and biodiversity. In addition, there may be useful other means to utilise parasitic organisms based on their biology and life histories such as suites or guilds that may be effective bioindicators of particular forms of environmental degradation. The challenge for parasitology is to convince resource managers and fellow scientists that parasites are a natural part of all ecosystems, each species being a potentially useful information unit, and that healthy ecosystems have healthy parasites.     

Placental Malaria: Decreased Transfer of Maternal Antibodies Directed to Plasmodium falciparum and Impact on the Incidence of Febrile Infections in Infants

The efficacy of mother-to-child placental transfer of antibodies specific to malaria blood stage antigens was investigated in the context of placental malaria infection, taking into account IgG specificity and maternal hypergammaglobulinemia. The impact of the resulting maternal antibody transfer on infections in infants up to the age of 6 months was also explored. This study showed that i) placental malaria was associated with a reduced placental transfer of total and specific IgG, ii) antibody placental transfer varied according to IgG specificity and iii) cord blood malaria IgG levels were similar in infants born to mothers with or without placental malaria. The number of malaria infections was negatively associated with maternal age, whereas it was not associated with the transfer of any malaria-specific IgG from the mother to the fetus. These results suggest that i) malaria-specific IgG may serve as a marker of maternal exposure but not as a useful marker of infant protection from malaria and ii) increasing maternal age contributes to diminishing febrile infections diagnosed in infants, perhaps by means of the transmission of an effective antibody response.