LOOKING BACK

The current malaria research community surely recognizes that we are in a land of fabulous riches. Never have we had more comprehensive resources at our disposal. When looking at the parasites, experimental technologies, reagents and databases, it is difficult to comprehend the barriers to research that preceded the current environment. Despite these riches, current well-meaning legislation has not enhanced our capacity to work on the malaria parasites either in man or animal models. Of the mammalian malarias the rodent parasites, and Plasmodium berghei in particular, offer systems in which to explore plausible hypotheses, hypotheses that nonetheless require validation in the human parasites, not because P. berghei in the laboratory mouse, or its laboratory vectors Anopheles stephensi/A. gambiae is atypical of the majority of Plasmodium spp., but because the biology of most important human pathogen, P. falciparum is, in key aspects of its development, distinct from most other Plasmodium species.

How has our understanding of malaria biology changed in the lifetime of this journal? By the middle of the last century detailed light microscopic studies had described and classified the majority of malaria-like species known today, in hosts ranging from living fossils e.g. the Tuatara (Sphenodon) and crocodilians, to the most recent mammals (man). These researches are comprehensively recorded in a series of seminal publications by Maegraith (Reference Maegraith1948), Boyd (Reference Boyd1949), Garnham (Reference Garnham1966), Coatney et al. (Reference Coatney, Collins, Warren and Contacos1971), Bruce-Chwatt (Reference Bruce-Chwatt1985), Wernsdorfer and McGregor (Reference Wernsdsorfer and McGregor1988) and Valkiunas (Reference Valkiunas2005) – texts that should be compulsory reading for any malaria researcher today.

Studies on the parasite life cycle continued well into the 1980s when P.C.C. Garnham, the joint discoverer of the liver stages, promoted a successful international effort to search for the elusive relapse form of P. vivax, now termed the hypnozoite (see Krotoski, Reference Krotoski1985). Likewise, the enigmatic journey of the sporozoite from the skin to liver provoked widespread microscopic and physiological studies in the 1970s. The disparate observations reported were only ‘recently’ resolved with the advent of fluorescent parasites and in situ multiphoton confocal microscopy (Pradel and Frevert, Reference Pradel and Frevert2001; Baer et al. Reference Baer, Roosevelt, Clarkson, van Rooijen, Schneider and Frevert2007). The major question was whether the Kupffer cell was a protagonist or an antagonist of sporozoite sequestration in the liver. It turns out to be the former – prompting interesting studies as to how the parasite avoids destruction (Brown and Kreier, Reference Brown and Kreier1986). Important though remaining questions may be (e.g. what is the role of lymphatic system in infection biology), the key pieces of the morphological jigsaw puzzle are clearly in place ……… or so we believe (Landau et al. Reference Landau, Chabaud, Mora-Silvera, Coquelin, Boulard, Rénia and Snounou1999; Amino et al. Reference Amino, Giovannini, Thiberge, Gueirard, Boisson, Dubremetz, Prévost, Ishino, Yuda and Ménard2008).

A deeper appreciation of malaria organization was provided by the application of electron microscopy (scanning, transmission and high voltage), approaches initiated by Garnham, Trager and Rudzinska and subsequently applied widely by many others of whom Masamichi Aikawa set a technical standard that few could match. These studies, while still classically descriptive, nonetheless revealed the amazing, indeed unique, subcellular structures mediating the parasites' complex life cycle, structures that led to the revision and re-naming of the phylum (Apicomplexa). In the author's opinion perhaps the most important accomplishment of this phase of malaria research was the recognition that the parasite alternates 2 key ‘conserved’ strategies, invasion (merozoite, sporozoite and ookinete); and growth (erythrocytic schizont, liver schizont, oocyst), with a single excursion into sexual development (gametocytes). This unique phase of development, as a sheer spectacle of what a eukaryotic cell can do is, in the eyes of the writer, one of the wonders of biology. The male gametocyte undergoes 3 mitotic divisions and the assembly of 8 axonemes in 5–20 min. These spectacular capacities should have forewarned investigators that novel processes were afoot, making their analysis not so much an examination of the parasite, but more of the investigators' imaginations. But, as a deeply committed zoologist, it is with a sense of sadness that I have to recognize the profound ‘challenge’ Plasmodium inflicted, when the multi-laminate organelle (an ill-defined series of concentric cytoplasmic membranes) was finally revealed to be a degenerate chloroplast! – respectfully renamed the apicoplast (Sullivan et al. Reference Sullivan, Li, Kumar, Rogers and McCutchan2000). Plasmodium…. a plant! No wonder the work on the yeasts had been useful in guiding analysis of the parasite's mitotic divisions.

The current molecular revolution in malaria research was made possible by the development of methods for the culture of the parasite. The substantive early efforts of Bass and Johns (Reference Bass and Johns1912) to culture blood stages and of Ball, Chao and Schneider to culture the sporogonic forms (Chao and Ball, Reference Chao and Ball1964; Schneider and Vanderberg, Reference Schneider, Vanderberg and Kreier1980) were rewarded when Trager and Jensen (Reference Trager and Jensen1976) successfully cultured the asexual blood stages of P. falciparum. This was rapidly complemented by methods for the culture of gametocytes (Smalley, Reference Smalley1976), pre-erythrocytic schizonts (Strome et al. Reference Strome, DeSantis and Beaudoin1979) and ookinetes (Yoeli and Upmanis, Reference Yoeli and Upmanis1968; Janse et al. Reference Janse, Mons, Rouwenhorst, Klooster van der, Overdulve and Kaay van der1985), but only much later by the culture of the oocyst (Al-Olayan et al. Reference Al-Olayan, Beetsma, Butcher, Sinden and Hurd2002) – the last method remains difficult to reproduce even today.

Without doubt, it is the genomic revolution, by now founded in a largely secure understanding of parasite ultrastructure, that has transformed our capacity to analyse the molecular cell biology of the parasite. Previously, molecules of interest were often identified because they were significantly immunogenic. First, a monoclonal or mono-specific antibody had to be generated and bacterial colonies screened and parasite genes cloned and sequenced by individual researchers. Each identified gene represented many months of work. The hard-won monoclonal antibodies, however, had significant benefit, permitting not only identification of the encoding gene, but also the subcellular location of the immunogen; its expression profiling, and where relevant its evaluation as a target for immune intervention. These diverse opportunities have prompted some to suggest that a new ‘omics’ approach could be the generation of monoclonal antibodies to every protein expressed by the parasite. The benefits that have accrued from the sequencing of even one malarial gene are many, but the projected ability to sequence in their entirety, genomes of many thousands of malarial parasites will permit a deep understanding of the evolution of these organisms, and additionally promises powerful new insights into potential drug resistance mechanisms.

Perhaps the greatest contribution of genomic era to the understanding of malaria cell biology, is the new-found ability to detect coordinated patterns of gene regulation (the whole question of how does a genome make an organism?). The low-hanging fruit of this analysis has been transcriptomics, which spawned the elegant ‘just-in-time’ hypothesis of gene product synthesis (Hayward et al. Reference Hayward, DeRisi, Alfadhli, Kaslow, Brown and Rathod2000; Bozdech et al. Reference Bozdech, Zhu, Joachimiak, Cohen, Pulliam and DeRisi2003; Le Roch et al. Reference Le Roch, Johnson, Florens, Zhou, Santrosyan, Grainger, Yan, Williamson, Holder, Carucci, Yates and Winzeler2004; Silvestrini et al. Reference Silvestrini, Bozdech, Lanfrancotti, Di Guilio, Bultrini, Picci, deRisi, Pizzi and Alano2005). The limitations of this conclusion are immediately apparent when considering how far removed the mRNA transcript is from the final/active gene product. An early indication of this distance was forecast by the observation that expression of the ookinete protein P28 was subject to translational repression in the mature female gametocyte (Paton et al. Reference Paton, Barker, Matsuoka, Ramesar, Janse, Waters and Sinden1993). With the advent of high-throughput proteomics (only possible following genome sequencing), it was possible to ask more directly ‘What proteins are expressed at each accessible stage of development?’ This has resulted in a deeper, but still far from complete, understanding of the parasite's biochemical pathways, and molecular machines (Florens et al. Reference Florens, Washburn, Raine, Anthony, Grainger, Haynes, Moch, Muster, Sacci, Tabb, Witney, Wolters, Wu, Gardner, Holder, Sinden, Yates and Carucci2002; Lasonder et al. Reference Lasonder, Ishihama, Andersen, Vermunt, Paln, Sauervein, Eling, Hall, Waters, Stunnenberg and Mann2002, Reference Lasonder, Janse, van Gemert, Mair, Vermunt, Douradinha, van Noort, Huynen, Luty, Kroeze, Khan, Sauerwein, Waters, Mann and Stunnenberg2008; Hall et al. Reference Hall, Karras, Raine, Carlton, Kooij, Berriman, Florens, Janssen, Pain, Christophides, James, Rutherford, Harris, Harris, Churcher, Quail, Ormond, Doggett, Trueman, Mendoza, Bidwell, Rajandream, Carucci, Yates, Kafatos, Janse, Barrell, Turner, Waters and Sinden2005; Khan et al. Reference Khan, Franke-Fayard, Mair, Lasonder, Janse, Mann and Waters2005; Lal et al. Reference Lal, Delves, Bromley, Wastling, Tomley and Sinden2009a, Reference Lal, Bromley, Prieto, Sanderson, Yates, Wastling, Tomley and Sindenb; Talman, personal communication).

Where does HTP analysis go from here? Existing datasets are incomplete and incompletely analysed. If we are to understand the parasite's regulatory networks, not just within the parasite, but between the parasites and their hosts we must apply Systems Analysis to openly shared/integrated datasets. Malaria cell biology has made amazing progress, and it might be argued that some Plasmodium species (notably the rodent malarias) are close to being model systems, but to analyse their biology requires diverse and unique resources simply to permit experimentation, not least amongst these resources are biologists who understand the organisms themselves.

BIOLOGY OF SEXUAL DEVELOPMENT

Whereas in Haemoproteids all bloodstage parasites are ‘by default’ gametocytes, in Plasmodium a small percentage of asexual bloodstages are, it is believed, induced by ill-defined ‘stress’ factors at each round of schizogony to form gametocytes in the next generation. Not only are all merozoites from a single schizont committed to the sexual pathway (Bruce et al. Reference Bruce, Alano, Duthie and Carter1990), but all become either male or females (Silvestrini et al. Reference Silvestrini, Alano and Williams2000; Smith et al. Reference Smith, Lourenço, Carter, Walliker and Ranford-Cartwright2000). In most species gametocytes mature only fractionally slower than that of the asexual parasite; however, in 2 species Plasmodium falciparum and Plasmodium reichenovi maturation is dramatically extended to 8–12 days, compared to the asexual maturation period of just 48 h. Early studies demonstrated that the immature gametocyte during its first 3 days of development was susceptible to bloodstage schizonticides (reviewed by Butcher, Reference Butcher1997). However, from stage III and beyond these gametocytes became progressively insensitive to many drugs, with the exception of inhibitors of energy metabolism e.g. the 8-aminoquinolines and artemisinin which therefore have the critical potential not only to provide direct protection to the patient, but, recognizing the local patterns of transmission (Carter et al. Reference Carter, Mendis, Miller, Molineaux and Saul2000) also protect the immediate community from transmission.

Mature gametocytes are morphologically distinct from the asexual parasite and must therefore be expected to contain a significantly different repertoire of proteins. Transcriptomics (Bozdech et al. Reference Bozdech, Zhu, Joachimiak, Cohen, Pulliam and DeRisi2003; Le Roch Reference Le Roch, Zhou, Blair, Grainger, Moch, Haynes, de la Vega, Holder, Batalov, Carucci and Winzeler2003, Reference Le Roch, Johnson, Florens, Zhou, Santrosyan, Grainger, Yan, Williamson, Holder, Carucci, Yates and Winzeler2004; Young et al. Reference Young, Fivelman, Blair, de la Vega, Le Roch, Zhou, Carucci, Baker and Winzeler2005; Silvestrini et al. Reference Silvestrini, Bozdech, Lanfrancotti, Di Guilio, Bultrini, Picci, deRisi, Pizzi and Alano2005) and proteomics (Florens et al. Reference Florens, Washburn, Raine, Anthony, Grainger, Haynes, Moch, Muster, Sacci, Tabb, Witney, Wolters, Wu, Gardner, Holder, Sinden, Yates and Carucci2002; Lasonder et al. Reference Lasonder, Ishihama, Andersen, Vermunt, Paln, Sauervein, Eling, Hall, Waters, Stunnenberg and Mann2002, Reference Lasonder, Janse, van Gemert, Mair, Vermunt, Douradinha, van Noort, Huynen, Luty, Kroeze, Khan, Sauerwein, Waters, Mann and Stunnenberg2008; Hall et al. Reference Hall, Karras, Raine, Carlton, Kooij, Berriman, Florens, Janssen, Pain, Christophides, James, Rutherford, Harris, Harris, Churcher, Quail, Ormond, Doggett, Trueman, Mendoza, Bidwell, Rajandream, Carucci, Yates, Kafatos, Janse, Barrell, Turner, Waters and Sinden2005; Khan et al. Reference Khan, Franke-Fayard, Mair, Lasonder, Janse, Mann and Waters2005; Patra et al. Reference Patra, Johnson, Cantin, Yates and Vinetz2008; Tarun et al. Reference Tarun, Peng, Dumpit, Ogata, Silva-Rivera, Camargo, Bergman and Kappe2008; Talman, personal communication) have confirmed this obvious conclusion. Notable amongst the proteins found in the mature female gametocyte are proteins Pfs230 and Pfs48/45, in the male gametocyte tubulin is amongst the most abundant of the cytoplasmic proteins. These and other proteins are required for sexual development, not only for the gametocyte itself but in the ensuing gametes which develop in the mosquito midgut.

The development of the gametocytes in the mosquito bloodmeal is without doubt one of the most dramatic developmental events in the parasite life cycle. In just a matter of minutes, both male and female parasites emerge from the red-cell, a process mediated by the exocytosis of vesicles (osmiophilic bodies) containing Pfg377 (Alano et al. Reference Alano, Read, Bruce, Aikawa, Kaido, Tegoshi, Bhatti, Smith, Luo, Hansra, Carter and Elliott1995) and Mdv-1 (Lal et al. Reference Lal, Delves, Bromley, Wastling, Tomley and Sinden2009), which may mediate dissolution and disruption of the enveloping erythrocyte. The now-naked female cell is fertilization competent; males by contrast, additionally undergo three rounds of DNA replication, each round followed by endomitotic genome segregation. Prior to activation of the microgametocyte 1 amorphous MTOC lies in the cytoplasm adjacent to a nuclear pore, it is connected to the genome through the pore. It rapidly transforms into 2 orthogonal tetrads of basal bodies, which are then segregated at each mitotic division such that at the third division 1 basal body is bound to each of the resultant 8 haploid genomes. The basal bodies act as the nucleation centres for the formation of the axonemes that will drive the flagella of the male gamete when released from the parental cell. All these events occur with incredible rapidity, microgametes are released typically within 15 min of bloodfeeding. The signalling processes regulating this dramatic development are now extensively characterized (see Billker et al. Reference Billker, Dechamps, Tewari, Wenig, Franke-Fayard and Brinkmann2004). It is known that the primary inducers of the process are a fall in temperature of 5°C in concert with presence of raised concentrations of the mosquito excretory product xanthurenic acid (Billker et al. Reference Billker, Shaw, Margos and Sinden1997, Reference Billker, Lindo, Panico, Etienne, Paxton, Dell, Rogers, Sinden and Morris1998). Together, these events activate the phosphoinositol pathway resulting in the release of calcium from cytoplasmic stores. The downstream activation of CDPK and thereafter MAPkinase, organize all the events of DNA replication, axoneme assembly, nuclear division and expulsion of the gametes.

It is now recognized that the most abundant proteins (e.g. tubulin, P230 etc) required for gamete formation are pre-synthesized in the terminally arrested mature gametocytes. Enigmatically many of these early proteins (e.g. the LAP/CCP family) could be knocked out with no immediate phenotype. The first detectable morphological change is the failure of the oocyst to undergo cytokinesis when sporozoites are normally formed some 10 days later (Raine et al. Reference Raine, Ecker, Mendoza, Tewari, Stanway and Sinden2007). An elegant analysis (Mair et al. Reference Mair, Braks, Garver, Wiegant, Hall, Dirks, Khan, Dimopoulos, Janse and Waters2006) identified in the female gametocyte a large number of mRNA species, that are under translational control (>370 species to date, including the ookinete surface proteins P25 and P28) that are translated only following activation of the gametocyte in the mosquito gut. Key motifs were identified either the 5′ or 3′ UTR's of these inhibited messages. A critical component of the regulatory machinery was the DDX6-class RNA helicase DOZI. Subsequently protein expression in the developing zygote/ookinete (e.g. CTRP) is controlled by transcription control factors including AP2-O (Yuda et al. Reference Yuda, Iwanaga, Shigenobu, Mair, Janse, Waters, Kato and Kaneko2009). Both gamete and ookinete surface proteins have been shown to be extremely immunogenic, if antibodies to these molecules are ingested in the bloodmeal they have been shown to inhibit fertilization (P230, P48/45, HAP2) or ookinete development (P25, P28) most effectively (Carter et al. Reference Carter, Mendis, Miller, Molineaux and Saul2000).

The properly formed male gamete is one of the simplest eukaryotic cells. It is composed solely of a nucleus, an axoneme (attached to its basal body) and a plasmalemma (Sinden et al. Reference Sinden, Canning and Spain1976). Proteomic analysis has confirmed the simplicity of this organization (Talman, personal communication). Unsurprisingly mitochondrial and apicoplast genomes were subsequently shown to be maternally inherited (Creasey et al. Reference Creasey, Ranford-Cartwright, Moore, Williamson, Wilson, Walliker and Carter1993; Vaidya, Reference Vaidya, Morrisey, Plowe, Kaslow and Wellems1993). The characteristic stop-go motility of the male gamete is exclusively energized by glycolysis, a process that can be readily inhibited with appropriate compounds in the surrounding medium – thus offering a very accessible and novel target for intervention (Talman, personal communication).

Fertilization of the female by the male gamete is now known to be mediated by a number of surface molecules. Pfs230 and Pfs48/45 are believed to act in a multi-molecular complex and are perhaps fertilization receptors, P47 is female specific (van Schaijk et al. Reference van Schaijk, van Dijk, van de Vegte-Bolmer, van Gemert, van Dooren, Eksi, Roeffen, Janse, Waters and Sauerwein2006); Pfs48/45, although present on both male and female gametes, is essential only to the fertility of the male (van Dijk et al. Reference van Dijk, Janse, Thompson, Waters, Braks, Dodemont, Stunnenberg, van Gemert, Sauerwein and Eling2001). Following molecular recognition of male and female, fusion of the gamete membranes is mediated by HAP2/GCS (Liu et al. Reference Liu, Tewari, Ning, Blagborough, Pei, Grishin, Steele, Sinden, Snell and Billker2008). This is followed by the movement of the entire male gamete into the cytoplasm of the female. Nuclear fusion follows within a few hours, forming the now diploid zygote. The parasite immediately returns to a haploid organization, meiosis being the first 2 divisions of the zygote genome (Sinden et al. Reference Sinden, Hartley and Winger1985). Development of the ookinete from the zygote is analogous to the formation of either the merozoite or sporozoite with the exception that there is only 1 daughter cell. Proteomic analysis of the ookinete, and of its isolated secretory organelles suggests, very strongly, that ookinetes unlike the sporozoite and merozoite, contain micronemes but not rhoptries (Lal et al. Reference Lal, Delves, Bromley, Wastling, Tomley and Sinden2009b). The presence of this single class of secretory organelle is reflected in a dramatically different invasive biology. The ookinete, on reaching the midgut epithelial cell, ruptures the plasma membrane and migrates directly into the cytoplasm of the now-dying host cell (Han et al. Reference Han, Thompson, Kafatos and Barillas-Mury2000). Invasion provokes a series of well-characterized molecular responses in susceptible mosquitoes, including synthesis of reactive nitrogen intermediates (Luckhart et al. Reference Luckhart, Vodovotz, Cui and Rosenberg1998) and peroxidase (Kumar and Barillas-Mury, Reference Kumar and Barillas-Mury2005). Should the ookinete survive this assault, it will, on leaving the epithelial cell, meet the mosquito haemolymph within which lie many immune effector molecules including the lethal complement-like molecule TEP1 (Levashina et al. Reference Levashina, Moita, Blandin, Vriend, Lagueux and Kafatos2001), the activity of which is regulated by molecules including LRIM 1 and 2 (Povolones et al. Reference Povolones, Waterhouse, Kafatos and Christophides2009). In refractory mosquito genotypes all ookinetes may be killed, either by lysis (Vernick et al. Reference Vernick, Fujioka, Seeley, Tandler, Aikawa and Miller1995) or by melanization (Collins et al. Reference Collins, Sakai, Vernick, Paskewitz, Seeley, Miller, Collins, Campbell and Gwadz1986).

The few ookinetes that survive this assault will, on meeting the basal lamina of the mosquito midgut, transform into oocysts, which over a period of 9–20 days undergo approximately 11 synchronous endomitotic divisions to form 2000–8000 sporozoites. We still know surprisingly little of these events at the molecular level, but we should anticipate that future transcriptomic and proteomic analyses will provide fascinating insights into these processes which, at the molecular level, may be expected to reiterate, many of the events observed in both blood- and liver-stage schizonts.

POPULATION DYNAMICS OF DEVELOPMENT IN THE MOSQUITO

Whereas the explosive expansion of knowledge on parasite molecular development reflects the rational application of the new and diverse technologies, other, less glamorous approaches have revealed fascinating insights into the dynamics of parasite development in the mosquito vector. Substantive studies in the rodent malaria parasites revealed massive sequential density-dependent losses as the parasite transformed from the gametocyte to oocyst, such that the ingestion of even many thousands of pre-committed sexual cells in the blood often yields, as few as 5 surviving oocysts, revealing the most critical population bottleneck in the parasite's life cycle (Alavi et al. Reference Alavi, Arai, Mendoza, Tufet-Bayona, Sinha, Fowler, Billker, Franke-Fayard, Janse, Waters and Sinden2003; Vaughan, Reference Vaughan2006; Poudel et al. Reference Poudel, Newman and Vaughan2008). Not only will this significantly influence the genetic structure of endemic parasite populations, it provides an attractive focus for effective intervention. The ensuing increase in parasite number, as sporozoites are formed within the oocyst, is temporary respite, as few as 10% of the sporozoites successfully enter the mosquito salivary glands, and of these only the sporozoites residing in the duct of the gland are likely to be injected by the mosquito into the skin of the next host. These two population bottlenecks resulting from the obligatory jump of the parasite from host to host offer the most attractive targets for attacking the parasite as part of any strategy looking to control or eliminate malaria endemic populations.

Further observations on the population structure of Plasmodium berghei in Anopheles stephensi have emphasized the non-linearity of the relationship between successive developmental stages (Sinden et al. Reference Sinden, Dawes, Alavi, Waldock, Finney, Mendoza, Butcher, Andrews, Hill, Gilbert and Basanez2008). All developmental transitions of the parasite in the mosquito exhibit a hyperbolic saturation curve i.e., developmental transitions become less efficient at progressively higher parasite densities. Uniquely, at very low ookinete numbers, increasing parasite density initially results in an increase in transmission efficiency. This is followed by a short, approximately linear relationship, and then at high intensities the transition saturates commonly at ~100 oocysts. The mechanisms by which these relationships are achieved at present remain unknown, but clearly a major focus of interest is on innate immune system of the mosquito (Dimopoulos et al. Reference Dimopoulos, Muller, Levashina and Kafatos2001; Blandin and Levashina, Reference Blandin and Levashina2004; Osta et al. Reference Osta, Christophides and Kafatos2004). A further dynamic that has been examined for many years, and is now the subject of renewed interest, asks whether the parasite places an evolutionary burden on the mosquito. Past studies suggested that flight performance and fecundity are adversely affected (Schiefer et al. Reference Schiefer, Ward and Eldridge1977; Rowland and Boersma, Reference Rowland and Boersma1988; Ahmed et al. Reference Ahmed, Maingon, Taylor and Hurd1999). Recent studies (Dawes et al. Reference Dawes, Zhuang, Sinden and Basáñez2009), suggest that the survival of the mosquito vector (and hence its reproductive potential) is both age- and parasite density-dependent. It is obvious that we have a great deal to learn about the parasite/vector relationship and how this will impact on the transmission of parasite. Mathematical models, securely based in this new understanding of parasite biology, will have an important role to play in attempting to predict the impact of any transmission blocking measures.

THE FUTURE

It is right that the malaria research community has recently been challenged as to whether it is feasible to reconsider malaria eradication (Greenwood, Reference Greenwood2009). Re-assessment of global research objectives naturally recognizes the need to treat the infected individual – a focus, which has dominated malarial research for the past 40 years, but it also appreciates that we have failed to address adequately the need to suppress the numbers of new cases of malaria. This latter objective brings into acute focus the need, in endemic populations, to suppress transmission. Objective evaluation of past eradication campaigns, of current interventions based on bed nets or the use of drugs with transmission-blocking activity (e.g. artemisinin; 8-aminoquinolines), and the rational analysis of parasite biology, strongly suggests that attacking transmission in the mosquito vector is likely to be one of the more effective control strategies in the future. Additional new concepts to achieve this objective through reduction or modification of mosquito populations are being entertained, and include the development of new fungal (Thomas and Read, Reference Thomas and Read2007; Read et al. Reference Read, Lynch and Thomas2009), bacterial (Geissbühler et al. Reference Geissbühler, Kannady, Chaki, Emidi, Govella, Mayagaya, Kiama, Mtasiwa, Mshinda, Lindsay, Tanner, Fillinger, de Castro and Killeen2009) and microsporidan (Hulls, Reference Hulls1971) biocides, or the use of genetic manipulation technology (Terenius et al. Reference Terenius, Marinotti, Sieglaff and James2008) to modulate mosquito breeding habits, olfaction or susceptibility to the parasite. Perhaps one of the most powerful mechanisms, previously under-appreciated, remains the application of transmission blocking vaccines (Carter et al. Reference Carter, Mendis, Miller, Molineaux and Saul2000).

Without doubt, research over the past century has generated numerous and exciting concepts to attack Plasmodium – a parasite of truly horrifying global impact. The careful re-evaluation of well-established concepts, together with the rational prioritization and careful implementation of additional new interventions in the field has the potential to reduce malaria burden within the foreseeable future. Without doubt, to engage in new campaigns to achieve local or national elimination or global eradication will require that the scientific community works together more effectively than ever before to understand the optimal implementation of appropriate interventions in diverse endemic situations. Then ‘all’ we will require is the political and moral integrity to see the campaigns to completion.