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Keywords:

  • Adaptation;
  • life-history;
  • malaria;
  • strategies

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Invasion of the host vertebrate, exoerythrocytic expansion and latency
  5. Expansion and transmission – the erythrocytic parasite development
  6. Why bother with asexual proliferation in the blood?
  7. Red blood cells: survival vs. self-restraint
  8. Anaemia and transmission
  9. Parasite responses to the host
  10. Host responses to the parasite – extending its phenotype?
  11. Fertilization and transmission – the development within the insect vector
  12. Transmission and restraint
  13. Developmental time within the insect vector: why so long?
  14. Infections in context
  15. Sex allocation and conflict between conspecific clones
  16. Cooperation between conspecific clones
  17. Concluding remarks
  18. Box 1. Malaria parasite life cycle
  19. Acknowledgements
  20. References

Plasmodium, the aetiological agent of malaria, imposes a substantial public health burden on human society and one that is likely to deteriorate. Hitherto, the recent Darwinian medicine movement has promoted the important role evolutionary biology can play in issues of public health. Recasting the malaria parasite two-host life cycle within an evolutionary framework has generated considerable insight into how the parasite has adapted to life within both vertebrate and insect hosts. Coupled with the rapid advances in the molecular basis to host–parasite interactions, exploration of the evolutionary ecology of Plasmodium will enable identification of key steps in the life cycle and highlight fruitful avenues of research for developing malaria control strategies. In addition, elucidating the extent to which Plasmodium can respond to short- and long-term changes in selection pressures, i.e. its adaptive capacity, is even more crucial in predicting how the burden of malaria will alter with our rapidly evolving ecology.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Invasion of the host vertebrate, exoerythrocytic expansion and latency
  5. Expansion and transmission – the erythrocytic parasite development
  6. Why bother with asexual proliferation in the blood?
  7. Red blood cells: survival vs. self-restraint
  8. Anaemia and transmission
  9. Parasite responses to the host
  10. Host responses to the parasite – extending its phenotype?
  11. Fertilization and transmission – the development within the insect vector
  12. Transmission and restraint
  13. Developmental time within the insect vector: why so long?
  14. Infections in context
  15. Sex allocation and conflict between conspecific clones
  16. Cooperation between conspecific clones
  17. Concluding remarks
  18. Box 1. Malaria parasite life cycle
  19. Acknowledgements
  20. References

Plasmodium spp. are insect vector-borne protozoan parasites infecting reptiles, birds and mammals, several species of which have received considerable attention for their medical (e.g. P. falciparum and P. vivax in man) or veterinary (e.g. P. gallinaceum in chickens), or ecological (e.g. P. relictum in birds) importance (Atkinson et al. 1995). Despite the research emphasis on P. falciparum, the agent of lethal malaria in man, the genus is estimated to include at least 172 spp., of which 89 occur in reptiles (Telford 1994), 32 in birds (van Riper et al. 1994) and 51 in mammals; this last group includes 25 spp. occurring in non-human primates (Collins & Aikawa 1993), the four spp. infecting man and several well-known rodent laboratory models (Cox 1993). The evolutionary history of Plasmodium, as revealed by increasingly thorough phylogenetic analyses, is proving both complex and surprising. Plasmodium appears to be a paraphyletic group with respect to two other related apicomplexan genera, Haemoproteus (parasites of birds and lizards) and Hepatocystis (parasites of mammals) (Escalante et al. 1998; Perkins & Schall 2002). The major parasite clades cluster within their vertebrate host classes, suggesting that parasite switching between classes is rare and that the establishment of parasite lineages within particular vertebrate host classes has an ancient origin (Perkins & Schall 2002). Importantly, this recent phylogeny, based on cytochrome b, tends to refute the hypothesis that P. falciparum and its sister species P. reichenowi (infecting chimpanzees) had an avian origin (Waters et al. 1991; Escalante et al. 1998; Perkins & Schall 2002). At least some of this earlier confusion stems from the use of ribosomal DNA, which has since been shown to occur in multiple copies that are expressed differentially during the life cycle in vertebrate and vector hosts (Mercereau-Puijalon et al. 2002). Within a vertebrate class, however, there may be considerable host switching. Plasmodium spp. infecting mammalian and lizard hosts are believed to be generally more host-specific (Garnham 1966; Telford 1994) than those infecting birds (Bennett et al. 1982). Haemoproteus spp., by contrast, were previously considered to be highly bird host-specific (Atkinson & van Riper 1991). However, detailed phylogenetic analyses have shown that cross-species transmission of Haemoproteus spp. most likely occurs frequently and has led to repeated host shifts even across host bird families (Bensch et al. 2000; Waldenstrom et al. 2002).

In contrast to our knowledge concerning the parasite–vertebrate host evolutionary histories, no thorough assessment of the evolutionary relationship between Plasmodium and insect vector spp. has been carried out to date. Although anopheline mosquito spp. are well-known vectors of human and several other mammalian malaria parasites, both anopheline and culicine mosquitoes vector bird malarias; other haematophagous insects, notably phlebotomine flies, are included among the vector spp. of lizard malaria parasites (Telford 1994; Fialho & Schall 1995). Haemoproteus and Hepatocystis are vectored by ceratopogonid midges, and the more distantly related Leucocytozoa by simulid black flies. Although vector choice and specificity may simply be a consequence of local vector species availability, the refractory nature of specific parasite–vector combinations suggests a more detailed and complex co-evolutionary history. Sadly, the natural vectors of the majority of Plasmodium spp. remain unknown. Indeed there is a general lack of basic knowledge concerning all aspects of the life cycle of less ‘famous’Plasmodium spp.

The last decade has generated unprecedented advances in our understanding of fundamental molecular mechanisms employed by parasites throughout their life cycle. With the decoding of all genomes of one triad (human, Anopheles gambiae and P. falciparum) and advances in molecular technology, our knowledge of this system will become increasingly precise. This, however, will place an ever increasing emphasis on a single host –Plasmodium system at the neglect of others. This bias is already evident in this review, where the majority of studies are based on a handful of laboratory strains of few model species and field data is largely restricted to human malaria parasites, notably P. falciparum. One notable exception is the long-term field study of lizard Plasmodium spp. that continues to yield invaluable insights into the behaviour of host–Plasmodium interactions under natural conditions (Schall 1996; also see Schall 2002 and references therein for an overview); unfortunately detailed immunological and molecular study of such host–parasite systems remains marginal and poorly understood.

This review addresses our current knowledge of how observed parasite traits reflect parasite evolutionary responses to selective pressures imposed by their two hosts and the complexities of transmission from man to mosquito and vice versa. Crucial to the success of Plasmodium is its ability to evade the host immune responses, whether those of the vertebrate or the insect vector. Successful immune evasion engenders the risk of over-exploitation and a central theme to host–parasite interactions is the trade-off between virulence and transmission (Frank 1996). The two-host life cycle of Plasmodium introduces a further level of complexity where the strategies used to optimize interaction with one host may also be subject to selection for optimization in the other. Thus considering the parasite life cycle simply with respect to each host type independently ignores the connectivity of the hosts through transmission. Therefore, in our progression through the parasite life cycle (Box 1) we place emphasis on interpreting parasite life histories within the evolutionary framework of fitness maximization: i.e. what strategies does the parasite employ when in the vertebrate host in order to maximize the number of insect vectors infected and vice versa. Where Plasmodium spp. differ, we consider what pressures may have selected for such alternative strategies. Despite an increasingly precise parasite phylogeny, we do not attempt a rigorous comparative analysis of life history traits, largely because the available information is incomplete or derived from studies on model hosts. The first part of the review deals with the adaptive nature of parasite interactions with its two hosts on a one-to-one basis. The subsequent section considers parasites in the context of natural conditions and particularly the importance of intraspecific co-infections and the parasite population structure. Finally, we address the important contribution evolutionary ecology can make to the major public health burden of human malaria.

Invasion of the host vertebrate, exoerythrocytic expansion and latency

  1. Top of page
  2. Abstract
  3. Introduction
  4. Invasion of the host vertebrate, exoerythrocytic expansion and latency
  5. Expansion and transmission – the erythrocytic parasite development
  6. Why bother with asexual proliferation in the blood?
  7. Red blood cells: survival vs. self-restraint
  8. Anaemia and transmission
  9. Parasite responses to the host
  10. Host responses to the parasite – extending its phenotype?
  11. Fertilization and transmission – the development within the insect vector
  12. Transmission and restraint
  13. Developmental time within the insect vector: why so long?
  14. Infections in context
  15. Sex allocation and conflict between conspecific clones
  16. Cooperation between conspecific clones
  17. Concluding remarks
  18. Box 1. Malaria parasite life cycle
  19. Acknowledgements
  20. References

An infected female mosquito injects sporozoite stage parasites into the vertebrate host during her bloodmeal. The sporozoite must evade the vertebrate immune system and invade host cells. The initial target cell varies but is never the red blood cell (RBC). The liver cells are the targets in mammals, whereas in birds and reptiles this pre-erythrocytic cycle is more complex, involving several rounds of invasion and asexual multiplication initially in skin macrophages before spreading throughout the body. In mammals, the movement of the sporozoite from the site of injection to its target cell remains enigmatic but it is capable of passing through cells in the skin prior to entering the bloodstream (Mota et al. 2001). Do sporozoites of mammalian malaria parasites, like their avian and saurian relatives, have in fact a more complex life cycle than hitherto thought? Although the infection process is very efficient, there is room for improvement: 50% of bites resulted in an infection in human-induced studies (Rickman et al. 1990), but a minimum of four to five infectious bites are required to generate an infection in animal models (Vaughan et al. 1999). Several questions immediately spring to mind during this initial phase: Why are so few sporozoites within the insect salivary glands injected during a bloodmeal? There can be many thousands within an insect vector salivary gland and yet only 10–20 are actually injected, although the observed range is several fold higher. What governs inoculum size? Are there any counter-selective forces placing an upper limit on inoculum size affecting either vector longevity or vertebrate susceptibility? What is the relationship between the number of sporozoites injected and the subsequent disease severity? As discussed later, parasites do appear to manipulate the insect vector to augment its transmission, for example by altering saliva apyrase activity to increase time spent probing on the vertebrate host (Rossignol et al. 1984) and hence increase transmission (Rossignol et al. 1986). Does the parasite alter the insect saliva to manipulate the vertebrate immune response? The saliva of sandflies, the vector of Leishmania in mammals, has been shown to affect immune activity in the skin resulting in exacerbation of Leishmania disease (Belkaid et al. 1998). Does sandfly saliva have a similar effect with regard to Plasmodium in lizards? And what of mosquito saliva and human Plasmodium?

During the pre-erythrocytic phase, each sporozoite undergoes extensive mitosis in its target cell. In mammalian parasites, each sporozoite generates a consistent 10 000–30 000 merozoite stages (responsible for RBC invasion), although requiring a variable length of time from 48 h to several weeks (Table 1). At maturation, these merozoites are released into the blood system, initiating the erythrocytic cycle, which is necessary for eventual transmission to the blood-feeding insect vectors. Why is there such invariance in this number in mammalian parasite spp.? Is this the maximum number sustainable in one host cell or the optimal number to ensure successful infection of the blood system? How many merozoites are eventually generated from the multiple pre-erythrocytic cycles characteristic of avian and saurian parasite spp.? In several Plasmodium spp. exo-erythrocytic forms (EEF) play an additional major role in maintaining a parasite reservoir of infection from which blood infections are intermittently seeded. Such latent EEF originate through either re-infection from the blood stages (some avian and lizard Plasmodium spp.) or persistence of EEF from the initial sporozoite inoculation (some mammalian spp., e.g. P. vivax). The long-term persistence of such a reservoir may be of particular importance to avoid elimination by the host's immune response whilst waiting for the seasonal return of the insect vector. However, not all spp. have EEF but rather have evolved alternative mechanisms for prolonging an infection. What are the selective forces determining this life history trait? What costs do the alternative strategies incur? What are the consequences for infection and transmission success? This interface, between the exo-erythrocytic and eryrthocytic environments, remains one of the least explored and most enigmatic parts of the Plasmodium life cycle, but whose study would certainly be fructuous.

Table 1.  Biological characteristics of Plasmodium spp. within their vertebrate hosts
Host groupPlasmodium spp.Pre-erythrocytic cell (no. of merozoites per cell)Pre-erythrocytic mean duration (days)Red blood cell preference (no. of merozoites per cell)Erythrocytic cycle duration (h)Latent or relapse stages
  1. n.k., not known. Where range given, too much variability for informative mean value (Garnham 1966; Collins & Aikawa 1993; Cox 1993; van Riper et al. 1994; Telford 1994; Schall 1996, 2002).

HumanP. falciparumHepatocyte (30 000)6All (16–24)48No
P. vivaxHepatocyte (10 000)7Retics (14–20)48Liver hypnozoites
P. malariaeHepatocyte (15 000)15Mature (8–10)72No
P. ovaleHepatocyte (15 000)9Retics (6–12)50Liver hypnozoites
PrimateP. knowlesiHepatocyte7Mature (8–16)24No
P. cynomolgiHepatocyte (5000–10 000) All/retics (16)48Yes
RodentP. yoeliiHepatocyte (10 000)2Retics (8)18No
P. chabaudiHepatocyte (10 000)2–3All (8)24No
AvianP. gallinaceumMacrophages; several cycles (100–200)7Mature (16–20)36Epithelial phanerozoites (500 per cell)
P. juxtanucleareMacrophages; several cycles? (28–50)Range 11–21Mature (2–6)24Epithelial phanerozoites
LizardP. agamaeMacrophages? Monocytes? (15–35)Range 2–45Mature (8)n.k.Suspected
P. giganteumMacrophages? (16–48)n.k.Reticulocytes (>100)n.k.n.k.

Why bother with asexual proliferation in the blood?

  1. Top of page
  2. Abstract
  3. Introduction
  4. Invasion of the host vertebrate, exoerythrocytic expansion and latency
  5. Expansion and transmission – the erythrocytic parasite development
  6. Why bother with asexual proliferation in the blood?
  7. Red blood cells: survival vs. self-restraint
  8. Anaemia and transmission
  9. Parasite responses to the host
  10. Host responses to the parasite – extending its phenotype?
  11. Fertilization and transmission – the development within the insect vector
  12. Transmission and restraint
  13. Developmental time within the insect vector: why so long?
  14. Infections in context
  15. Sex allocation and conflict between conspecific clones
  16. Cooperation between conspecific clones
  17. Concluding remarks
  18. Box 1. Malaria parasite life cycle
  19. Acknowledgements
  20. References

Invasion of the blood system is of obvious importance to enable successful transmission to haematophagous insects. However, it is less clear why the parasite should undertake further rounds of asexual multiplication, subjecting itself to the immunologically hostile blood environment, prior to producing specialized transmission stages. In stark contrast to Plasmodium, the related Apicomplexa genera, Leucocytozoa and Haemoproteus (as well as its mammalian parasite counterpart, Hepatocystis), liberate only sexual stages into the blood system (in leucocytes and erythrocytes, respectively). Indeed, several Plasmodium spp. can produce gametocytes directly from EEF (e.g. P. gallinaceum and P. yoelii). In man, parasite proliferation within the blood system results in severe symptoms associated with malaria, including death, and invokes a strong immune reaction. Why, therefore, does Plasmodium undertake this additional asexual expansion in erythrocytes rather than simply seeding the blood with benign transmission stages? What are the advantages of incorporating this asexual blood phase? The first consideration is that advantages to asexual expansion might be expected to relate to increasing transmission through the production of more transmission stages (gametocytes). Certainly the asexual expansion phase increases the overall biomass of parasite cells and thus generates a greater source for gametocytes. Indeed, transmission success to mosquitoes does generally increase with gametocyte density (Schall 2000; Robert & Boudin 2003). However, paradoxically, in mammalian Plasmodium spp. in particular, only a small proportion of asexually dividing cells convert to becoming sexual gametocytes (Taylor & Read 1997). Why, therefore, should the parasite increase potential gametocyte production via increased asexual replication on the one hand, and yet on the other exhibit such self-restraint in gametocyte production? The ‘classical’ explanation is that the immunodominance of the asexual stages distracts and thus shields the gametocytes from the immune system. If so, then how do Haemoproteus spp., which release only gametocytes into the blood system, overcome this problem? Perhaps the often exceedingly high gametocyte densities of Haemoproteus spp. overwhelm the host immune response and provide an alternative strategy to ensure transmission. Have Haemoproteus and Hepatocystis spp. abandoned blood stage replication or rather have Plasmodium spp. acquired this character? The most recent constructed phylogeny would suggest the former (Perkins & Schall 2002).

An alternative approach to address the apparent paradoxical relationship between asexual replication and transmission potential is to recast the question in terms of evolutionary virulence theory (Frank 1996). Considerable theoretical work has emphasized the adaptive nature of host–pathogen interactions. The extent of pathogen virulence depends on the trade-off between the exploitation of the host (virulence) and the benefits accrued by increasing R0, the reproductive rate, either via increasing transmission directly or by reducing host recovery rate (Anderson & May 1991). Pathogens are expected to evolve a schedule of host exploitation that maximizes their transmission. Thus the virulent nature of the infection is simply the by-product of the parasite's transmission strategy. The level of virulence depends critically on the relationship between transmission and virulence. Even if transmission increases positively with virulence, excessively virulent parasites may result in premature host death reducing lifetime transmission success. Host exploitation strategies, and thus virulence, are therefore expected to evolve to some intermediate value. Despite the high profile mortality burden on human populations, parasite-induced mortality is not actually the predominant outcome of infection. By contrast, malaria parasites have been shown to have numerous sub-lethal negative fitness effects on their hosts (Schall 1990; Schall 2002). The extent to which there may be an adaptive basis to virulence in malaria parasites has been the subject of a series of recent laboratory studies. As predicted from virulence theory (van Baalen & Sabelis 1995), mixed clone infections not only resulted in increased virulence but also higher overall transmission (Taylor et al. 1997, 1998). Moreover, examination of genetic variation in trade-offs among virulence and life history traits revealed positive genetic correlations between virulence, asexual replication rate and transmission (Mackinnon & Read 1999a). Importantly, sub-lethal effects, rather than lethal infection outcome, seem to impose an upper threshold to virulence (Mackinnon & Read 1999b) and transmission was maximized at intermediate levels of host morbidity (Mackinnon et al. 2002). Thus, there does seem to be a transmission advantage to asexual replication within the blood, although this poses the question as to why it is absent in Haemoproteus spp.?

An additional intriguing adaptive possibility has emerged from the recent finding that blood stage infection results in the suppression of the anti-EEF immune response (Ocana-Morgner et al. 2003). In its original formulation, as a general mechanism to enable novel infections, group selection was strongly implied. Although parasite population viscosity could favour such a mechanism under kin selection (Read et al. 2002), we suggest that such immune-suppression concerns persistence and hence re-infection from the parasite reservoir EEF population rather than new infections. Indeed, it is well recognized in animal models that current infection actually blocks invasion by a second clone until the original infection has been cleared (McGhee 1988). Thus positive selection for asexual blood stage multiplication could result from the benefits accrued through long-term survival of the parasite as EEF (Fig. 1). Once again Haemoproteus remains an anomaly and importantly not all Plasmodium spp. have latent EEF forms.

image

Figure 1. Infection initiation and persistence in the vertebrate host. Following inoculation of sporozoites during a bloodmeal by an infected insect vector, the parasite undergoes mitotic asexual reproduction within liver, macrophage or epithelial cells. At maturity merozoites are released into the blood system and the erythrocytic cycle begins, where further asexual replication occurs every 24–72 h according to the Plasmodium spp. (Table 1). Re-infection of exo-erythrocytic cells from blood stages occurs in avian and saurian Plasmodium spp. Latent exo-erythrocytic stages (EEF) from the initial sporozoite inoculation exist in several mammalian Plasmodium spp. (1). There is suppression of the host anti-EEF immune response by the blood stage infection.

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Red blood cells: survival vs. self-restraint

  1. Top of page
  2. Abstract
  3. Introduction
  4. Invasion of the host vertebrate, exoerythrocytic expansion and latency
  5. Expansion and transmission – the erythrocytic parasite development
  6. Why bother with asexual proliferation in the blood?
  7. Red blood cells: survival vs. self-restraint
  8. Anaemia and transmission
  9. Parasite responses to the host
  10. Host responses to the parasite – extending its phenotype?
  11. Fertilization and transmission – the development within the insect vector
  12. Transmission and restraint
  13. Developmental time within the insect vector: why so long?
  14. Infections in context
  15. Sex allocation and conflict between conspecific clones
  16. Cooperation between conspecific clones
  17. Concluding remarks
  18. Box 1. Malaria parasite life cycle
  19. Acknowledgements
  20. References

RBCs are not homogeneous packets of parasite resource and differ globally (the haemoglobinopathies such as sickle cell anaemia, the thalassemiae and ovalocytosis), locally (e.g. blood groups) and within an individual (RBCs mature over time and when old, are cleared by the spleen). During a Plasmodium infection, the blood environment changes rapidly; in a naive host only a small fraction of the RBCs are reticulocytes (young RBCs), but which increases rapidly with malaria-associated anaemia. Several Plasmodium spp. have seemingly developed sophisticated mechanisms for adjusting to this variable environment and maintain the infection (Snounou et al. 2000). For example, in P. yoelii (a rodent parasite), the daughter merozoites of a single asexually replicating mother cell (schizont) expressed different variant proteins dictating RBC preference (Preiser et al. 1999). Such phenotypic plasticity in RBC preference would enable the parasite to rapidly adapt to the heterogeneous nature of the blood environment, whether due to its own action on the host or as a result of previously established parasites of the same or other species (Fig. 2). Several other multigene families are implicated in parasite survival. Plasmodium falciparum expresses variant proteins on the surface of the infected RBC (reviewed in Kyes et al. 2001) that enable parasite sequestration, thus avoiding destruction in the spleen. The expression of these adhesion proteins on the RBC surface exposes them to immune recognition. Within a single parasite genome there can be more than 50 copies of the var gene encoding for the variant proteins. However, any one parasite expresses only a single variant. Antigenic variation in clonal populations results in the switching of these variant proteins, such that in each asexual generation (48 h) only a small fraction of parasites exhibit a new variant and thus avoid immune targeting. This enables the parasite to persist but the switching is surprisingly low (order of few percentage express a novel variant each generation). Why risk sacrificing 98% of the parasite population to immune recognition and destruction each generation? One explanation is that as well as enabling sequestration and avoidance of the spleen, the immuno-dominant nature of this protein serves as a death flag, which results in auto-regulation of parasite densities and avoidance of runaway exploitation of the host (Saul 1999).

image

Figure 2. Parasite strategies responding to changes in vertebrate host blood environment as infection progresses. Strategy concerning red blood cell (RBC) preference: phenotypic plasticity in individual capacity (parasite with variant protein a or b or c) to invade RBCs of differing ages (mature vs. reticulocytes). Strategies governing sex allocation: total allocation in sexual gametocytes and allocation into male vs. female. At the beginning of an infection there is little anaemia, low production of erythropoietin hormone (Epo) and few reticulocytes. Parasites expressing variants for reticulocytes (b and c) do not find suitable RBCs. There is low investment in gametocytes and males particularly. As the infection progresses, there is increased anaemia, Epo and reticulocytes. Parasite variants favouring reticulocyte invasion proliferate, there is increased gametocyte production, especially in males to assure fertilization despite an increasing host antibody response. Epo is directly or indirectly involved in sex allocation.

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Self-restraint, or prudent host exploitation, is contrary to current evolutionary thought on host–parasite interactions, largely because mutant unrestrained parasites would spread rapidly through the population. Such an eventuality may almost certainly be expected to arise for the human malaria parasites, P. falciparum and P. vivax, that cause 500 million new infections every year and generate several million parasites in each case. Indeed, as discussed above, there is some evidence that malaria parasites employ host exploitation strategies that maximize transmission success; by contrast, there has been no formal approach to the evolution of self-restraint in malaria parasites. However, there are several striking features of Plasmodium life history traits concerning the blood stage infection phase that are suggestive of self-restraint. Plasmodium spp. exhibit differential preferences for RBCs of differing ages. In the human parasite species, for example, P. vivax and P. ovale have a predilection for reticulocytes, P. malariae for mature RBCs and P. falciparum for all types. Similar variability in RBC preferences is found among all Plasmodium spp. Plasmodium spp. with reticulocyte predilections will initially grow slowly. By the time parasite-induced anaemia has induced increased reticulocyte densities, and thus potential for more rapid asexual replication, the host immune response would be expected to control the parasite. Those parasites targeting mature RBCs will be expected to rapidly deplete their RBC source and thus density will be checked by the RBC turnover. Parasite spp. with more catholic tastes, such as P. falciparum, abnegate resource-mediated control. Notably, however, the variant proteins enabling host exploitation via avoidance of the spleen (e.g. P. falciparum) or variant RBC specificity (e.g. P. yoelii) are highly immunogenic. Thus, although several parasite spp. have developed mechanisms to prolong infection and increase host exploitation, this facility is apparently accompanied by the induction of a strong immune response. This seems to go against the grain, as Plasmodium is known for the immune evasion structure of other surface molecules. Is it therefore possible that such antigenic variation simultaneously incorporates a self-restraint mechanism? Thus, as well as evading the immune response or adapting to the RBC population, does antigenic variation serve the pleiotropic role of reducing over-exploitation to maintain an infectious reservoir? Formal proof that such pleiotropic effects could evolve and be maintained is lacking. However, there is some empirical data that suggest this area warrants further thought. A selected lethal clone of the rodent parasite P. yoelii capable of invading all RBCs was restricted to reticulocytes in the presence of antibodies against this variant protein, and both the host and the parasite survived considerably longer. Similarly, under such a self-restraint perspective, reticulophilic parasites should generally be expected to invoke a greater immunological response to avoid runaway exploitation of the growing reticulocyte population (resulting from host response to parasite-induced anaemia). Comparative data is lacking, but P. vivax, for example, does elicit a strong fever response at a lower parasite density than P. falciparum (Boyd 1949). Finally, it should be remembered that the transmission success of the parasite does not solely rest on its interaction with its host vertebrate; the parasite needs the presence of the insect vector. It is thus feasible that optimum schedules of host exploitation will vary significantly between areas where the insect vectors are permanently active and where activity is highly seasonal. For example, prolonging the infection in the vertebrate host would be beneficial in regions of seasonal mosquito activity, in order to maintain an infectious reservoir. Despite considerable progress in our understanding of transmission from vertebrate to vector (as discussed next), the role vector population dynamics may play in shaping parasite host exploitation strategies remains unknown but potentially fruitful.

Anaemia and transmission

  1. Top of page
  2. Abstract
  3. Introduction
  4. Invasion of the host vertebrate, exoerythrocytic expansion and latency
  5. Expansion and transmission – the erythrocytic parasite development
  6. Why bother with asexual proliferation in the blood?
  7. Red blood cells: survival vs. self-restraint
  8. Anaemia and transmission
  9. Parasite responses to the host
  10. Host responses to the parasite – extending its phenotype?
  11. Fertilization and transmission – the development within the insect vector
  12. Transmission and restraint
  13. Developmental time within the insect vector: why so long?
  14. Infections in context
  15. Sex allocation and conflict between conspecific clones
  16. Cooperation between conspecific clones
  17. Concluding remarks
  18. Box 1. Malaria parasite life cycle
  19. Acknowledgements
  20. References

Transmission of Plasmodium from the vertebrate host to the insect vector is mediated solely by the sexual stages, the gametocytes. Plasmodium blood stage parasites are haploid and there are no sex chromosomes. Therefore a clone of Plasmodium, which is thus a simultaneous hermaphrodite, must make several decisions: when to produce gametocytes (i.e. infection maturity) (Eisen & Schall 2000), what proportion to allocate to gametocyte stages and how many females vs. males? The optimal sex allocation strategy will be that which produces the greatest fitness returns. This may be fixed for a given habitat, or there may be considerable phenotypic plasticity according to the precise conditions encountered. Gametocyte production appears to have an adaptive basis: it increases when conditions for asexual proliferation worsen (e.g. immunological stress and chemotherapy) (Carter & Graves 1988; Buckling et al. 1997; Buckling & Read 2001). Recently, an analysis of parasite life history strategies during the erythrocytic cycle has determined that variation in only three traits describes Plasmodium infection in lizards: the rate of increase and peak parasitaemia (both asexual and sexual stages), the duration of infection growth and the delay before gametocytes are produced (i.e. infection maturity). Importantly, there was a genetic basis for the first two traits but not for maturity (Eisen & Schall 2000). This confirms that, whilst the first two traits underline the importance of host and parasite genotypes in infection outcome, parasites show great plasticity in their age of maturity. This would be predicted if environmental conditions determine, to a large extent, transmisssion success. Parasites would, therefore, be expected to exhibit phenotypic responses to changes in the blood environment that affect their transmission success to mosquitoes. An understanding of how the ‘quality’ of the RBC system alters over the course of an infection is thus central to interpreting potential parasite phenotypic responses.

Anaemia and immunological humoral and fever responses are the most evident consequences of a Plasmodium infection. Fever, however, is triggered only at certain threshold parasite densities and tolerance can develop with repeated infection in humans. Parasite-induced anaemia, on the contrary, has multiple origins, arising as a consequence of at least three distinct processes: (1) infected RBC destruction by the infecting parasite; (2) uninfected and infected erythrocyte destruction by the host; and (3) dyserythropoiesis where the parasite interferes with the production of new RBCs. The first process will simply reflect the parasite density and dyserythropoiesis is considered to be of minor importance in primary infections and probably plays a more significant role in chronic long-term low density infections. The host destruction of RBCs is believed to be the major determinant of anaemia. This immune-mediated component of anaemia simultaneously signifies a worsening RBC environment in terms of resource availability and the development of an immunological response to infection. Evidence suggests that the parasite responds to both these host constraints by adaptive sex allocation.

Parasite responses to the host

  1. Top of page
  2. Abstract
  3. Introduction
  4. Invasion of the host vertebrate, exoerythrocytic expansion and latency
  5. Expansion and transmission – the erythrocytic parasite development
  6. Why bother with asexual proliferation in the blood?
  7. Red blood cells: survival vs. self-restraint
  8. Anaemia and transmission
  9. Parasite responses to the host
  10. Host responses to the parasite – extending its phenotype?
  11. Fertilization and transmission – the development within the insect vector
  12. Transmission and restraint
  13. Developmental time within the insect vector: why so long?
  14. Infections in context
  15. Sex allocation and conflict between conspecific clones
  16. Cooperation between conspecific clones
  17. Concluding remarks
  18. Box 1. Malaria parasite life cycle
  19. Acknowledgements
  20. References

There is growing evidence linking parasite sex allocation to the haematological state of the host suggesting that the parasite is capable of adaptive facultative investment. The nature of the Plasmodium spp. response, however, depends on their RBC preferences. For example, in Plasmodium spp. that have catholic RBC tastes, such as P. chabaudi in rodents and P. falciparum in humans, increasing reticulocyte density results in an increasing gametocyte density (Gautret et al. 1996; Price et al. 1999). That is the parasites increase their proportional allocation to sexual transmission stages per se as the blood composition changes. In a bird and a rodent Plasmodium spp. that both have mature RBC preferences, increasing reticulocyte density reduces asexual proliferation rate and, whereas gametocyte density does not increase disproportionately, the gametocyte sex ratio alters. Indeed, the initially female-biased gametocyte sex ratio approached sexual equality with increasing anaemia (Paul et al. 2000) (Fig. 2). The parasite seemingly increases its male investment, at the expense of female investment, to ensure fertilization in the face of an increasing ‘transmission-blocking’ immune response (Paul et al. 2002a). Such an adaptive strategy advocates an important role for mating assurance (Shutler & Read 1998; West et al. 2002). Such mating assurance models have been previously termed low-density models that were derived as explanations for the adaptive significance of simultaneous hermaphroditism, where individuals mating at random have low encounter probabilities (Ghiselin 1974). Mating assurance may, therefore, be important to Plasmodium when its gametocyte densities are very low (Paul et al. 2002a; West et al. 2002). Interestingly, although probably fortuitous rather than selected, such transmission-blocking immunity may play a beneficial role. At low antibody concentrations transmission success is actually enhanced (Ponnudurai et al. 1987); antibody agglutination of the gametes may aid in the maintenance of physical contact between the gametes in the turbulent environment of the bloodmeal.

Host responses to the parasite – extending its phenotype?

  1. Top of page
  2. Abstract
  3. Introduction
  4. Invasion of the host vertebrate, exoerythrocytic expansion and latency
  5. Expansion and transmission – the erythrocytic parasite development
  6. Why bother with asexual proliferation in the blood?
  7. Red blood cells: survival vs. self-restraint
  8. Anaemia and transmission
  9. Parasite responses to the host
  10. Host responses to the parasite – extending its phenotype?
  11. Fertilization and transmission – the development within the insect vector
  12. Transmission and restraint
  13. Developmental time within the insect vector: why so long?
  14. Infections in context
  15. Sex allocation and conflict between conspecific clones
  16. Cooperation between conspecific clones
  17. Concluding remarks
  18. Box 1. Malaria parasite life cycle
  19. Acknowledgements
  20. References

A second strategy for optimizing transmission would be to manipulate vertebrate host behaviour. Does Plasmodium alter its vertebrate host to increase its transmission success to insect vectors? Changes in host physiology, increased body temperatures and CO2 (Murphy et al. 2001) as a result of the infection may make the host more susceptible and more attractive to the insect vector. Indeed pregnant women are seemingly more attractive to mosquitoes (Ansell et al. 2002). The general torpor associated with ill health also reduces mosquito-repellent behaviour (Rossignol et al. 1985). In addition, parasite-induced anaemia reduces blood viscosity that may facilitate blood ingestion, and decrease the vector's risk of dying during the bloodmeal (Taylor & Hurd 2001). However, all these potential benefits are a direct result of infection pathology rather than any direct manipulation of the host. Although it is feasible that increasingly virulent pathology is selected for increased transmission to the insect vector, it is more likely that these are simply examples of side-effects, rather than selected traits.

Fertilization and transmission – the development within the insect vector

  1. Top of page
  2. Abstract
  3. Introduction
  4. Invasion of the host vertebrate, exoerythrocytic expansion and latency
  5. Expansion and transmission – the erythrocytic parasite development
  6. Why bother with asexual proliferation in the blood?
  7. Red blood cells: survival vs. self-restraint
  8. Anaemia and transmission
  9. Parasite responses to the host
  10. Host responses to the parasite – extending its phenotype?
  11. Fertilization and transmission – the development within the insect vector
  12. Transmission and restraint
  13. Developmental time within the insect vector: why so long?
  14. Infections in context
  15. Sex allocation and conflict between conspecific clones
  16. Cooperation between conspecific clones
  17. Concluding remarks
  18. Box 1. Malaria parasite life cycle
  19. Acknowledgements
  20. References

Within the insect vector, the parasite is faced with the same challenges of infection, proliferation and immune evasion but with the additional problem of short host longevity (Fig. 3). Indeed, one of the most surprising features of parasite development within the insect (the sporogonic development) is its comparatively long duration. The rate of sporogonic development increases with ambient temperature but generally takes 1–2 weeks (some species are seemingly ‘cool’-adapted, e.g. P. berghei which does not develop above 21 °C and hence takes longer). Given the average lifespan of insects in natural settings, the probability that an insect survives sufficiently long to enable sporogonic development is small. Thus the parasite is extremely sensitive to mosquito population densities and dynamics, which is why over the short term, control methods targeting the adult mosquito have been so successful, especially in zones on the limit of Plasmodium spp. ranges (Robert & Boudin 2003).

image

Figure 3. Plasmodium development within the insect vector. Following parasite fertilization within the insect bloodmeal, the zygote transforms into a mobile ookinete responsible for invasion of the insect midgut wall. (1) Apoptosis of ookinetes occurs and may reduce the parasite impact on the insect vector. Ookinetes penetrate the midgut wall; (2) there is suppression of the insect immune encapsulation response both directly by the parasite and via the effects of the blood stage infection on the host vertebrate immune response (Igs: immunoglobulins). During oocyst development (c. 10 days); (3) the insect vector feeds less often and has reduced fecundity, both of which increase insect survivorship. Following parasite maturation and invasion of the salivary glands by parasite sporozoites; and (4) feeding facility is disrupted and the insect probes longer and more often to gain a bloodmeal, thus increasing parasite transmission.

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Transmission and restraint

  1. Top of page
  2. Abstract
  3. Introduction
  4. Invasion of the host vertebrate, exoerythrocytic expansion and latency
  5. Expansion and transmission – the erythrocytic parasite development
  6. Why bother with asexual proliferation in the blood?
  7. Red blood cells: survival vs. self-restraint
  8. Anaemia and transmission
  9. Parasite responses to the host
  10. Host responses to the parasite – extending its phenotype?
  11. Fertilization and transmission – the development within the insect vector
  12. Transmission and restraint
  13. Developmental time within the insect vector: why so long?
  14. Infections in context
  15. Sex allocation and conflict between conspecific clones
  16. Cooperation between conspecific clones
  17. Concluding remarks
  18. Box 1. Malaria parasite life cycle
  19. Acknowledgements
  20. References

The parasite is faced with the trade-off of infecting sufficient mosquitoes but reducing its own virulent effect on the insect vector so as not to exacerbate the already short lifespan. One reason postulated for the relatively low numbers of gametocytes was that it is the strategy optimizing the trade-off between transmission and mosquito mortality (Taylor & Read 1997). Evidence that parasites may attempt to reduce their impact on their vector has only recently come to light: parasite ookinetes, the stages responsible for penetration of the insect stomach wall, undergo apoptosis (programmed cell death), resulting in a 50% reduction in their number (Al-Olayan et al. 2002). Reducing ookinete number is expected to lessen the damage inflicted upon the insect and, importantly, to reduce subsequent opportunist infection by midgut bacteria. However, that there is an actual negative impact of the parasite on the insect vector during this early phase of infection remains controversial. Indeed a recent survey of all available studies concluded that, in natural parasite–vector insect associations, the parasite only induces mortality on a timescale approaching that needed for the completion of a sporogonic development (Ferguson & Read 2002).

The danger of over-infecting the insect with an excess of ookinetes may in fact be more perceived than real and not actually applicable to the majority of natural infections where parasite densities are low. Indeed, although transmission efficiency increases with overall gametocyte density, vertebrate host immune responses cause considerable parasite death and few gametocytes may actually ever be viable. By contrast, the parasite seems to have developed several mechanisms to maximize its fertilization efficiency and infection of the mosquito, especially when gametocyte densities are low. As previously noted, low titres of transmission-blocking antibodies actually enhance transmission success (Ponnudurai et al. 1987). Gametocytes are distributed non-randomly within the bloodmeals, which will increase the probability of both male and female gametocytes being in the same midgut (Pichon et al. 2000). In addition, extensive studies in rodent Plasmodium spp. strongly suggest that infectious gametocyte stages are preferentially located in sub-dermal blood capillaries, where the insect vectors take their bloodmeal (Gautret et al. 1996). Furthermore, the parasite, and specifically the ookinete stage, actively suppresses the insect immune melanization response (Boëte et al. 2002). Thus, current limited data suggest that the parasite has adapted to overcome the mosquito rather than evolved restraint to avoid harmful impact. The absence of a significant negative impact of the parasite on the vector may result in insufficient selection pressure to initiate a co-evolutionary arms race.

Developmental time within the insect vector: why so long?

  1. Top of page
  2. Abstract
  3. Introduction
  4. Invasion of the host vertebrate, exoerythrocytic expansion and latency
  5. Expansion and transmission – the erythrocytic parasite development
  6. Why bother with asexual proliferation in the blood?
  7. Red blood cells: survival vs. self-restraint
  8. Anaemia and transmission
  9. Parasite responses to the host
  10. Host responses to the parasite – extending its phenotype?
  11. Fertilization and transmission – the development within the insect vector
  12. Transmission and restraint
  13. Developmental time within the insect vector: why so long?
  14. Infections in context
  15. Sex allocation and conflict between conspecific clones
  16. Cooperation between conspecific clones
  17. Concluding remarks
  18. Box 1. Malaria parasite life cycle
  19. Acknowledgements
  20. References

The majority of the sporogonic cycle is spent in asexual multiplication within the oocyst to produce sporozoite stages (about 2000 sporozoites are produced from one zygote that successfully develops into a mature oocyst). At sporozoite injection during the blood feed, however, only tens of sporozoites are injected. Thus it seems paradoxical that the parasite should risk investing great periods of time in amplifying its number of sporozoites when only few are injected. The constancy of the long duration of the sporogonic development across a wide range of Plasmodium and insect vector spp. suggests that producing a large number of sporozoites is important. One explanation is that many sporozoites are needed in the salivary glands to transmit even few to the vertebrate host; indeed, very broadly the number injected does increase with the number in the salivary glands. However, an alternative explanation is that the parasite manipulates its vector to increase transmission – i.e. when sporozoites are mature the parasite causes an increase in vector-feeding behaviour. Field data do indeed suggest that not only does biting frequency increase in sporozoite-infected mosquitoes, but also that it increases with total sporozoite load in the salivary glands (Koella & Packer 1996; Koella et al. 1998). Increased feeding-related mortality seems to be a major virulent effect of the parasite on the vector. The presence of sporozoites in the salivary glands increases probing number and time, both of which increase transmission but also render the vector more susceptible to feeding-associated death (Anderson et al. 2000). If, however, biting rate increases with sporozoite load but the associated cost of increased mortality does not increase more than linearly with biting rate, parasites would maximize transmission success with increasing sporozoite load (Koella 1999). The developmental process whereby the parasite replicates asexually to produce many sporozoites may be a mechanism for manipulating vector-biting behaviour to increase its transmission success. Thus selection would promote a long productive sporogonic cycle.

Given the presumed intrinsic parasite developmental requirement for sufficient sporozoite production, parasites might be expected to attempt either to accelerate its development or to prolong the lifespan of mosquitoes until sporozoite maturity (Schwartz & Koella 2001). The parasite appears to do both. A detailed field study on the thermal ecology of the lizard malaria parasite P. mexicanum suggests that the parasite may actually alter the insect vector's thermoregulatory behaviour. The insect was found to have an increase in preferred ambient temperature of 2 °C following an infected bloodmeal, resulting in an increase in the rate of sporozoite development (Fialho & Schall 1995). Prolonging vector lifespan could be achieved by reducing mosquito fecundity (and the longevity costs of reproduction) (Hamilton & Hurd 2002) and/or feeding frequency. As predicted, parasite infection does reduce both vector fecundity (Rossignol et al. 1986) and feeding frequency during the developing oocyst stage of infection (Koella et al. 2002).

In this very brief appraisal of host–Plasmodium life cycle interactions, the parasite does appear to have developed many strategies enabling survival and increasing its transmission in the spatially and temporally heterogeneous vertebrate and insect vector host environments. In natural populations, however, Plasmodium infections will rarely be composed of one parasite clone or even one species. Rather, parasites will find themselves in hosts with varying concurrent parasitic infections and with a variable history of infection. To what extent are parasite strategies influenced by both the direct and indirect consequences of co-infecting parasites over both the short- and long-term?

Infections in context

  1. Top of page
  2. Abstract
  3. Introduction
  4. Invasion of the host vertebrate, exoerythrocytic expansion and latency
  5. Expansion and transmission – the erythrocytic parasite development
  6. Why bother with asexual proliferation in the blood?
  7. Red blood cells: survival vs. self-restraint
  8. Anaemia and transmission
  9. Parasite responses to the host
  10. Host responses to the parasite – extending its phenotype?
  11. Fertilization and transmission – the development within the insect vector
  12. Transmission and restraint
  13. Developmental time within the insect vector: why so long?
  14. Infections in context
  15. Sex allocation and conflict between conspecific clones
  16. Cooperation between conspecific clones
  17. Concluding remarks
  18. Box 1. Malaria parasite life cycle
  19. Acknowledgements
  20. References

Mixed species infections of Plasmodium spp. occur widely throughout animal and human populations (Richie 1988; McKenzie & Bossert 1997a) and many Plasmodium spp. co-exist in sympatry. Plasmodium spp. may facilitate or inhibit each other within the host. Plasmodium agamae, a parasite of lizards, for example, facilitates infection with P. giganteum a reticulocyte specialist, probably because of the reticulocyte-rich environment generated as a consequence of the infection (Schall & Bromwich 1994). However, P. falciparum infections, despite inducing anaemia, may generate considerable dyserythropoiesis, thereby preventing the production of reticulocytes and so suppressing growth by P. vivax, a reticulophile. Plasmodium spp. can also certainly have negative impacts upon one another through the general host immune response to infection – i.e. there is apparent competition (Read & Taylor 2001). Thus, parasite spp. do interact within the host, but what factors determine malaria parasite community structure? The favoured hypothesis, for parasite communities in general, is that the diversity and observed patterns of parasite spp. abundance reflect the individual species life history strategies, notably those pertaining to fecundity and transmission success. Longitudinal analyses of natural infections in human populations have revealed alternating patterns of species dominance, strongly suggesting the potential for strong interspecific pressures affecting species fitness (Bruce et al. 2000). Such interactions may have considerable influence on species transmission rates through suppression or facilitation of co-infecting spp., or by directly affecting transmission. Indeed, the prevalence of multiple parasite species in naturally infected mosquitoes was less than that expected if each species was transmitting independently (McKenzie & Bossert 1997b). The transmission outcome of the interaction, however, depends crucially on the biology of the species involved. Co-infection (or intriguingly prior infection) of man with P. malariae increased P. falciparum gametocyte production (McKenzie et al. 2002). Conversely P. gallinaceum transmission was seemingly negatively affected by co-infecting P. juxtanucleare (Paul et al. 2002b). Such effects can be interpreted as non-adaptive side-effects of co-infection. Whether there is sufficient selection for the development of adaptive, context-dependent responses by Plasmodium spp. remains a question to be rigorously addressed. However, adaptive strategies governing more flexible life history traits, such as maturation rate and extent of gametocyte production may be more amenable to rapid, context-dependent selection. With the current emphasis on human malaria parasites, comparative Plasmodium spp. life history data across the spectrum of transmission facies may become accessible and yield useful insights into the question of short-term adaptive parasite evolution.

The importance of conspecific interactions is, by contrast, both more accessible and probably more pertinent, and has been the subject of considerable interest (Tibayrenc 1999; Read & Taylor 2001; Read et al. 2002). The average number of co-infecting genotypes in a population (the parasite population structure) will depend broadly on the transmission intensity and determines the degree of parasite inbreeding (Nee et al. 2002), which has important clinical, epidemiological and evolutionary consequences (Tibayrenc 1999; Read et al. 2002). The dawn of Darwinian medicine (Williams & Ness 1991), whose premise is the application of adaptionist argumentation to infectious diseases, has encouraged evolutionary biologists to take an active role in infectious disease research. To what extent do Plasmodium populations really yield to classical adaptionist reasoning? As an initial test case, several authors considered the extent to which sex allocation theory, one of the best verified evolutionary theories (Charnov 1982), can explain observed sex allocation strategies in Plasmodium (Ghiselin 1974; Read et al. 1992).

Sex allocation and conflict between conspecific clones

  1. Top of page
  2. Abstract
  3. Introduction
  4. Invasion of the host vertebrate, exoerythrocytic expansion and latency
  5. Expansion and transmission – the erythrocytic parasite development
  6. Why bother with asexual proliferation in the blood?
  7. Red blood cells: survival vs. self-restraint
  8. Anaemia and transmission
  9. Parasite responses to the host
  10. Host responses to the parasite – extending its phenotype?
  11. Fertilization and transmission – the development within the insect vector
  12. Transmission and restraint
  13. Developmental time within the insect vector: why so long?
  14. Infections in context
  15. Sex allocation and conflict between conspecific clones
  16. Cooperation between conspecific clones
  17. Concluding remarks
  18. Box 1. Malaria parasite life cycle
  19. Acknowledgements
  20. References

Plasmodium spp. have been the subject of several studies examining sex ratio with respect to population structure (review in West et al. 2001). In single clone malaria parasite infections the optimal sex ratio will be that which maximizes R0, in this case via maximization of fertilization and hence transmission success to mosquitoes (Read et al. 1992). When multiple clones can co-infect a host, the optimal sex ratio will become less female-biased because clones investing more in males will monopolize the abundance of females (Read et al. 1992). This principle of adjusting sex allocation according to the number of foundresses or in this case co-infecting clones, is known as local mate competition (LMC) (Hamilton 1967; Read et al. 1992). The probability of inbreeding, which depends upon the parasite population structure, is therefore predicted to determine the optimal sex ratio (Nee et al. 2002). There is some field evidence from human, bird and lizard populations that LMC applies to malaria and other apicomplexan parasites, where the gametocyte sex ratio broadly reflects parasite population structure and the number of co-infecting clones (Read et al. 1992, 1995; Paul et al. 1995; Robert et al. 1996; Shutler & Read 1998; West et al. 2000, 2001). Notable exceptions were those parasites which undergo syzygy, whereby single male and female cells unite prior to fertilization, thereby removing competition for mating opportunities and hence selection for female-biased sex ratios under LMC (West et al. 2000).

If parasites are able to respond to both long-term population structure and short-term changes in the vertebrate host haematological state, by adopting appropriate sex allocation strategies, the parasite may be expected to be able to respond to short-term changes in co-infecting parasite genotype number (Taylor et al. 1998). Indeed, the number of co-infecting parasite genotypes can vary significantly and unpredictably even at a local scale. Under such circumstances parasites may be able to facultatively alter their sex ratio according to co-infecting clone number – i.e. facultative LMC where parasites adopt a conditional strategy (Shutler & Read 1998). Current experimental evidence is inconclusive but consistent with facultative LMC (Taylor 1997; Osgood et al. 2002).

Cooperation between conspecific clones

  1. Top of page
  2. Abstract
  3. Introduction
  4. Invasion of the host vertebrate, exoerythrocytic expansion and latency
  5. Expansion and transmission – the erythrocytic parasite development
  6. Why bother with asexual proliferation in the blood?
  7. Red blood cells: survival vs. self-restraint
  8. Anaemia and transmission
  9. Parasite responses to the host
  10. Host responses to the parasite – extending its phenotype?
  11. Fertilization and transmission – the development within the insect vector
  12. Transmission and restraint
  13. Developmental time within the insect vector: why so long?
  14. Infections in context
  15. Sex allocation and conflict between conspecific clones
  16. Cooperation between conspecific clones
  17. Concluding remarks
  18. Box 1. Malaria parasite life cycle
  19. Acknowledgements
  20. References

Demonstrations of cooperation between genotypes are expected to be rare, but one such example seems to occur during invasion of the vertebrate. Co-inoculation of sporozoites from two different clones leads to increased infection success via a mechanism of immune peptide antagonism (Gilbert et al. 1998). If there was a true advantage to such cooperation one would expect to see a propensity for co-transmission and mixed clone infections even in regions of limited transmission intensity and low infection prevalence. This was indeed observed in Thailand where individuals receive less than one infectious bite per year and where the lifespan of P. falciparum in these regions is a year or less. Contrary to the expected single clone infections, the majority of the infected population (representing only 4% of the human population) were found to carry two clone infections and an over-representation of particular genotype combinations (Paul et al. 1999). However, this data is equally consistent with the observed increased transmission success to mosquitoes from mixed vs. single clone infections in the vertebrate host (Taylor et al. 1997). The importance of cooperation between related and unrelated parasites opens up new avenues of research into the adaptive forces shaping host–parasite interactions and is currently on the agenda of evolutionary biologists involved in the application of evolutionary biology to public health issues (Williams & Ness 1991; Read et al. 2002).

Concluding remarks

  1. Top of page
  2. Abstract
  3. Introduction
  4. Invasion of the host vertebrate, exoerythrocytic expansion and latency
  5. Expansion and transmission – the erythrocytic parasite development
  6. Why bother with asexual proliferation in the blood?
  7. Red blood cells: survival vs. self-restraint
  8. Anaemia and transmission
  9. Parasite responses to the host
  10. Host responses to the parasite – extending its phenotype?
  11. Fertilization and transmission – the development within the insect vector
  12. Transmission and restraint
  13. Developmental time within the insect vector: why so long?
  14. Infections in context
  15. Sex allocation and conflict between conspecific clones
  16. Cooperation between conspecific clones
  17. Concluding remarks
  18. Box 1. Malaria parasite life cycle
  19. Acknowledgements
  20. References

Assessing the extent to which observed parasite life history traits are evolved strategies in response to imposed environmental pressures has yielded considerable information about the aspects of the life cycle crucial to the parasite. Identifying the key selection pressures is the first step in a constructive approach to disease control. By concentrating on the steps in the life cycle under the most extreme selective pressures, we may find novel ways to unsettle the parasite. In this way evolutionary ecologists can make a considerable contribution in highlighting potential parasite weak points or indicating erroneous paths. For example, methods designed to shorten insect vector longevity will reduce transmission in the short term but may select for a more rapid sporogonic cycle. However, the fact that Plasmodium spp. have not already evolved such accelerated development to expand their global distribution suggests that developmental processes constrain the rate of sporozoite production. In addition, more rapid development may not generate sufficient sporozoites and thus Plasmodium may be severely restricted in their evolutionary potential. This hypothetical scenario can be contrasted with the recent interest in transgenic mosquitoes. Not only may transgenic mosquitoes be less fit for a variety of reasons, but there is also no reason to expect that the parasite will not evolve another mechanism to bypass or suppress a transgenic refractory mosquito (Boëte & Koella 2003). That the parasite has already one such mechanism gives, in contrast to the previous scenario, good reason to believe that it is not developmentally restrained and further co-evolution will occur. Similarly, vaccine development proceeds in isolation of evolutionary considerations. Yet recent work has shown the value of combining evolutionary and epidemiology theory in predicting the long-term outcome of differing vaccine strategies (Gandon et al. 2001).

The current focus on P. falciparum and the immense public health burden it imposes on human populations globally, emphasizes the virulent nature of the parasite. However, despite the enormous number of new infections every year, by far the majority do not provoke clinical disease, which is a specific human interpretation of parasite pathogenicity. The temptation to address parasite strategies with respect to the minority symptomatic cases detracts from the majority of parasite–human interactions that may have more impact on parasite evolution. In addition, inherent in much of our evolutionary interpretation of parasite strategies is the assumption that the host and parasite populations have attained an equilibrium. For P. falciparum in particular, this is far from obvious. The human population explosion over the last 1000 years has certainly resulted in a rapidly expanding niche for P. falciparum. Malthusian parasite population growth in response to such rapid niche expansion would have liberated P. falciparum from strong directional selection, thus resulting in the observed parasite genomic diversity. This would be consistent with the deep root of P. falciparum within the mammalian Plasmodium spp. phylogeny (Perkins & Schall 2002). An interesting parallel is the highly variable genome of HIV, that has colonized the human population in just 50 years, compared with its ancestor simian immunodeficiency virus, that has co-evolved with its simian hosts for considerably longer. Thus, it is not clear the extent to which we would expect P. falciparum to have evolved stable strategies, and this species of Plasmodium may not necessarily be so amenable to classical evolutionary interpretation. However, the general conformity of Plasmodium spp., including P. falciparum, to sex allocation theory provides optimistic grounds for expanding adaptionist reasoning to more complex and perhaps more pertinent medical phenotypes, such as virulence (Pickering et al. 2000).

The global and local distribution of human malaria is changing with the evolution of our society. Malaria is no longer a disease restricted to rural communities characterized by high transmission intensities where individuals develop varying degrees of protective immunity. With >60% of the human population in Africa now living in towns and cities, the transmission picture of malaria has changed and mosquito vectors are likely to adapt to an urban and peri-urban way of life. Although overall transmission intensity is decreasing (Robert et al. 2003), P. falciparum will transmit among the largely naive dense urban populations, thus potentially generating epidemics. Therefore, evolving from an historically endemic disease, that imposed a considerable mortality and morbidity burden on very young children, malaria may take on an additional urban nature and resemble classic urban epidemic vector-borne diseases such as dengue and yellow fever. Metapopulation concepts applied to falciparum malaria suggest that low endemicity will provide a heterogeneous environment leading to high spot biodiversity, thus enabling rapid selection of highly adapted parasites (Ariey et al. 2003). Understanding how individual parasite clones can rapidly adapt and the extent of their phenotypic plasticity, is fundamental to both the public health consequences of urbanization and the consequences of globalization. If parasites are phenotypically very plastic, local adaptation to the novel transmission setting (e.g. peri-urban) may be expected to occur rapidly following introduction, with little selection for specific genotypes. If parasites are adapted to their local environment, such as suggested by their long-term responses to sex ratio, considerable parasite diversity may disappear with the changing epidemiology of human malaria, favouring the rapid expansion of locally selected genotypes. Being able to describe broadly how we expect parasites to adapt to very novel environments will provide a firm basis upon which we can address issues concerning appropriate control methods and hence optimize expenditure of the limited public health funds.

Box 1. Malaria parasite life cycle

  1. Top of page
  2. Abstract
  3. Introduction
  4. Invasion of the host vertebrate, exoerythrocytic expansion and latency
  5. Expansion and transmission – the erythrocytic parasite development
  6. Why bother with asexual proliferation in the blood?
  7. Red blood cells: survival vs. self-restraint
  8. Anaemia and transmission
  9. Parasite responses to the host
  10. Host responses to the parasite – extending its phenotype?
  11. Fertilization and transmission – the development within the insect vector
  12. Transmission and restraint
  13. Developmental time within the insect vector: why so long?
  14. Infections in context
  15. Sex allocation and conflict between conspecific clones
  16. Cooperation between conspecific clones
  17. Concluding remarks
  18. Box 1. Malaria parasite life cycle
  19. Acknowledgements
  20. References

When an infected female insect vector takes a bloodmeal, she injects saliva into the vertebrate host, which contains an anaesthetic and an anti-coagulant and, if infected with Plasmodium, will also, unwittingly, inject the parasite sporozoite stages which invade vertebrate host cells (e.g. the liver in mammals and macrophages in birds). These sporozoites undergo asexual proliferation in these host cells producing many tens of thousands of merozoite stage parasites about 1 week later. These merozoites invade erythrocytes wherein they grow, divide asexually to produce further merozoites and burst out from the cell to invade further erythrocytes, a cycle generally occurring every 24–72 h (although potentially longer in some lizard malaria spp.) according to the Plasmodium spp.. These asexual blood stages are responsible for disease. At some point during the course of the infection, most notably when such asexual proliferation is slowed, the merozoite stages grow but do not divide and produce the sexual stages, the gametocytes, which are gamete precursors. Transmission from the vertebrate host to the insect vector is mediated solely by these sexual stages of the parasite, which are distinguishable as males and females. Mature gametocytes are arrested in G0 of the cell cycle in the vertebrate host blood until they are taken up in the bloodmeal by another female insect, whereupon they transform into gametes: each male gametocyte undergoes exflagellation, by which process up to eight male gametes are produced; each female gametocyte produces only one female gamete. Such gametogenesis occurs within 10–15 min following uptake in the bloodmeal in response to the drops in temperature and pH associated with the different hosts (vertebrate and insect vector) and mosquito factors. Within 30 min the male must actively swim to find and fertilize the female gamete. The subsequent zygote transforms into a mobile ookinete which penetrates the mosquito stomach wall where it encysts. After 8–15 days (depending on the Plasmodium spp.), this mature oocyst releases several thousand sporozoites which invade the salivary glands of the mosquito and are injected into the vertebrate host during her next bloodmeal.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Invasion of the host vertebrate, exoerythrocytic expansion and latency
  5. Expansion and transmission – the erythrocytic parasite development
  6. Why bother with asexual proliferation in the blood?
  7. Red blood cells: survival vs. self-restraint
  8. Anaemia and transmission
  9. Parasite responses to the host
  10. Host responses to the parasite – extending its phenotype?
  11. Fertilization and transmission – the development within the insect vector
  12. Transmission and restraint
  13. Developmental time within the insect vector: why so long?
  14. Infections in context
  15. Sex allocation and conflict between conspecific clones
  16. Cooperation between conspecific clones
  17. Concluding remarks
  18. Box 1. Malaria parasite life cycle
  19. Acknowledgements
  20. References
  • Al-Olayan, E.B., Williams, G.T. & Hurd, H. (2002). Apoptosis in the malaria protozoan, Plasmodium berghei: a possible mechanism for limiting intensity of infection in the mosquito. Int. J. Parasitol., 32, 11331143.
  • Anderson, R.M. & May, R.M. (1991). Infectious Diseases of Humans: Dynamics and Control. Oxford University Press, Oxford.
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