Experimental evaluation of the relationship between lethal or non-lethal virulence and transmission success in malaria parasite infections

Background  

Evolutionary theory suggests that the selection pressure on parasites to maximize their transmission determines their optimal host exploitation strategies and thus their virulence. Establishing the adaptive basis to parasite life history traits has important consequences for predicting parasite responses to public health interventions. In this study we examine the extent to which malaria parasites conform to the predicted adaptive trade-off between transmission and virulence, as defined by mortality. The majority of natural infections, however, result in sub-lethal virulent effects (e.g. anaemia) and are often composed of many strains. Both sub-lethal effects and pathogen population structure have been theoretically shown to have important consequences for virulence evolution. Thus, we additionally examine the relationship between anaemia and transmission in single and mixed clone infections.     

Experimental manipulation of immune-mediated disease and its fitness costs for rodent malaria parasites

Background  

Explaining parasite virulence (harm to the host) represents a major challenge for evolutionary and biomedical scientists alike. Most theoretical models of virulence evolution assume that virulence arises as a direct consequence of host exploitation, the process whereby parasites convert host resources into transmission opportunities. However, infection-induced disease can be immune-mediated (immunopathology). Little is known about how immunopathology affects parasite fitness, or how it will affect the evolution of parasite virulence. Here we studied the effects of immunopathology on infection-induced host mortality rate and lifetime transmission potential – key components of parasite fitness – using the rodent malaria model, Plasmodium chabaudi chabaudi.     

A correlated response of a parasite’s virulence and life cycle to selection on its host’s life history

We demonstrate a correlated response of the virulence and the mode of transmission of the microsporidian parasite Edhazardia aedis to selection on the age at pupation of its host, the mosquito Aedes aegypti. We selected three lines of mosquitoes each for early or late pupation and exposed the larvae after zero, two and four generations of selection to a low and a high concentration of the parasite’s spores. Before selection the parasites induced a similar level of mortality in the six lines; after four generations of selection mortality was higher in the mosquitoes selected for late pupation than in those selected for early pupation. Overall, parasite-induced mortality was positively correlated with the mean age at pupation of the matching uninfected line. When they died, mosquitoes selected for early pupation harboured mostly binucleate spores, which are responsible for vertical transmission. Mosquitoes selected for late pupation were more likely to harbour uninucleate spores, which are responsible for horizontal transmission. The parasite enhanced this tendency for horizontal transmission by prolonging the larval period in the lines selected for late pupation, but not in the ones selected for early pupation. These results suggest that the genetic basis of the mosquito’s age at pupation helps to determine the parasite’s mode of transmission: parasites in rapidly developing mosquitoes are benign and transmit vertically, while parasites in slowly developing mosquitoes are virulent and transmit horizontally. Thus, as the host’s life history evolves, the parasite’s performance changes, because the host’s evolution changes the environment in which the parasite develops.     

The expression of virulence during double infections by different parasites with conflicting host exploitation and transmission strategies

In many natural populations, hosts are found to be infected by more than one parasite species. When these parasites have different host exploitation strategies and transmission modes, a conflict among them may arise. Such a conflict may reduce the success of both parasites, but could work to the benefit of the host. For example, the less‐virulent parasite may protect the host against the more‐virulent competitor. We examine this conflict using the waterflea Daphnia magna and two of its sympatric parasites: the blood‐infecting bacterium Pasteuria ramosa that transmits horizontally and the intracellular microsporidium Octosporea bayeri that can concurrently transmit horizontally and vertically after infecting ovaries and fat tissues of the host. We quantified host and parasite fitness after exposing Daphnia to one or both parasites, both simultaneously and sequentially. Under conditions of strict horizontal transmission, Pasteuria competitively excluded Octosporea in both simultaneous and sequential double infections, regardless of the order of exposure. Host lifespan, host reproduction and parasite spore production in double infections resembled those of single infection by Pasteuria. When hosts became first vertically (transovarilly) infected with O. bayeri, Octosporea was able to withstand competition with P. ramosa to some degree, but both parasites produced less transmission stages than they did in single infections. At the same time, the host suffered from reduced fecundity and longevity. Our study demonstrates that even when competing parasite species utilize different host tissues to proliferate, double infections lead to the expression of higher virulence and ultimately may select for higher virulence. Furthermore, we found no evidence that the less‐virulent and vertically transmitting O. bayeri protects its host against the highly virulent P. ramosa.     

Virulence and competitive ability in an obligately killing parasite

Mixed infections are thought to have a major influence on the evolution of parasite virulence. During a mixed infection, higher within‐host parasite growth is favored under the assumption that it is critical to the competitive success of the parasite. As within‐host parasite growth may also increase damage to the host, a positive correlation is predicted between virulence and competitive success. However, when parasites must kill their hosts in order be transmitted, parasites may spend energy on directly attacking their host, even at the cost of their within‐host growth. In such systems, a negative correlation between virulence and competitive success may arise. We examined virulence and competitive ability in three sympatric species of obligately killing nematode parasites in the genus Steinernema. These nematodes exist in a mutualistic symbiosis with bacteria in the genus Xenorhabdus. Together the nematodes and their bacteria kill the insect host soon after infection, with reproduction of both species occurring mainly after host death. We found significant differences among the three nematode species in the speed of host killing. The nematode species with the lowest and highest levels of virulence were associated with the same species of Xenorhabdus, indicating that nematode traits, rather than the bacterial symbionts, may be responsible for the differences in virulence. In mixed infections, host mortality rate closely matched that associated with the more virulent species, and the more virulent species was found to be exclusively transmitted from the majority of coinfected hosts. Thus, despite the requirement of rapid host death, virulence appears to be positively correlated with competitive success in this system. These findings support a mechanistic link between parasite growth and both anti‐competitor and anti‐host factors.     

The virulence–transmission relationship in an obligate killer holds under diverse epidemiological and ecological conditions,but where is the tradeoff?

Parasite virulence is a leading theme in evolutionary biology. Modeling the course of virulence evolution holds the promise of providing practical insights into the management of infectious diseases and the implementation of vaccination strategies. A key element of virulence modeling is a tradeoff between parasite transmission rate and host lifespan. This assumption is crucial for predicting the level of optimal virulence. Here, I test this assumption using the water flea Daphnia magna and its castrating and obligate‐killing bacterium Pasteuria ramosa. I found that the virulence–transmission relationship holds under diverse epidemiological and ecological conditions. In particular, parasite genotype, absolute and relative parasite dose, and within‐host competition in multiple infections did not significantly affect the observed trend. Interestingly, the relationship between virulence and parasite transmission in this system is best explained by a model that includes a cubic term. Under this relationship, parasite transmission initially peaks and saturates at an intermediate level of virulence, but then it further increases as virulence decreases, surpassing the previous peak. My findings also highlight the problem of using parasite‐induced host mortality as a “one‐size‐fits‐all” measure of virulence for horizontally transmitted parasites, without considering the onset and duration of parasite transmission as well as other equally virulent effects of parasites (e.g., host castration). Therefore, mathematical models may be required to predict whether these particular characteristics of horizontally transmitted parasites can direct virulence evolution into directions not envisaged by existing models.     

Selection shapes malaria genomes and drives divergence between pathogens infecting hominids versus rodents

Background  

Malaria kills more people worldwide than all inherited human genetic disorders combined. To characterize how the parasites causing this disease adapt to different host environments, we compared the evolutionary genomics of two distinct groups of malaria pathogens in order to identify critical properties associated with infection of different hosts: those parasites infecting hominids (Plasmodium falciparum and P. reichenowi) versus parasites infecting rodent hosts (P. yoelii yoelii, P. berghei, and P. chabaudi). Adaptation by the parasite to its host is likely highly critical to the evolution of these species.     

THE INFLUENCE OF HOST DEMOGRAPHY ON THE EVOLUTION OF VIRULENCE OF A MICROSPORIDIAN GUT PARASITE

It is predicted that host exploitation should evolve to maximize parasite fitness and that virulence (= parasite-induced host mortality) evolves along with the rate of host exploitation. If the life expectancy of a parasite is short, it is expected to evolve a higher rate of host exploitation and therefore higher virulence because the penalty to the parasite for killing the host is reduced. We tested this hypothesis by keeping for 14 months the horizontally transmitted microsporidian parasite Glugoides intestinalis in mono-clonal host cultures (Daphnia magna) under conditions of high and low host background mortality. High host mortality, and thus parasite mortality, was achieved by replacing weekly 70–80% of all hosts in a culture with uninfected hosts from stock cultures (Replacement lines). In the low-mortality treatment no replacement took place. Contrary to our expectation, parasites from the Replacement lines evolved a lower within-host growth rate and virulence than parasites from the Nonreplacement lines. Across lines we found a strong positive correlation between within-host growth rate and virulence. We did further experiments to answer the question why our data did not support the predictions. Sporophorous vesicles (SVs, spore clusters) were smaller in doubly infected than in singly infected host-gut cells, indicating that competition within cells bears costs for the parasite. Due to our experimental protocol, the average life span of infections had been much higher in the Nonreplacement lines. Since the number of parasites inside a host increases with the time since infection, long-lasting infections led to high frequencies of multiply infected host-gut cells. Therefore, we speculated that within-cell competition was more severe in the Nonreplacement lines and may have led to selection for accelerated within-host growth. SVs in the Nonreplacement lines were indeed significantly larger. Our results point out that single-factor explanations for the evolution of virulence can lead to wrong predictions and that multiple infections are an important factor in virulence evolution.     

Life cycle abbreviation in trematode parasites and the developmental time hypothesis: is the clock ticking?

The typical multi‐host life cycle of many parasites, although conferring several advantages, presents the parasites with a highly hazardous transmission route. As a consequence, parasites have evolved various adaptations increasing their chances of transmission between the different hosts of the life cycle. Some trematode species like the opecoelid Coitocaecum parvum have adopted a more drastic alternative strategy whereby the definitive host is facultatively dropped from the cycle, resulting in a shorter, hence easier to complete, life cycle. Like other species capable of abbreviating their life cycle, C. parvum does so through progenetic development within its intermediate host. Laboratory‐reared C. parvum can modulate their developmental strategy inside the second intermediate host according to current transmission opportunities, though this ability is not apparent in natural C. parvum populations. Here we show that this difference is likely due to the time C. parvum individuals spend in their intermediate hosts in the natural environment. Although transmission opportunities, i.e. chemical cues of the presence of definitive hosts, promoted the adoption of a truncated life cycle in the early stages of infection, individuals that remained in their amphipod host for a relatively long time had a similar probability of adopting progenesis and the abbreviated cycle, regardless of the presence or absence of chemical cues from the predator definitive host. These results support the developmental time hypothesis which states that parasites capable of facultative life cycle abbreviation should eventually adopt progenesis regardless of transmission opportunities, and provide further evidence of the adaptive plasticity of parasite transmission strategies.     

Climate,host phylogeny and the connectivity of host communities govern regional parasite assembly

Aim

Identifying barriers that govern parasite community assembly and parasite invasion risk is critical to understand how shifting host ranges impact disease emergence. We studied regional variation in the phylogenetic compositions of bird species and their blood parasites (Plasmodium and Haemoproteus spp.) to identify barriers that shape parasite community assembly.

Location

Australasia and Oceania.

Methods

We used a data set of parasite infections from >10,000 host individuals sampled across 29 bioregions. Hierarchical models and matrix regressions were used to assess the relative influences of interspecies (host community connectivity and local phylogenetic distinctiveness), climate and geographic barriers on parasite local distinctiveness and composition.

Results

Parasites were more locally distinct (co‐occurred with distantly related parasites) when infecting locally distinct hosts, but less distinct (co‐occurred with closely related parasites) in areas with increased host diversity and community connectivity (a proxy for parasite dispersal potential). Turnover and the phylogenetic symmetry of parasite communities were jointly driven by host turnover, climate similarity and geographic distance.

Main conclusions

Interspecies barriers linked to host phylogeny and dispersal shape parasite assembly, perhaps by limiting parasite establishment or local diversification. Infecting hosts that co‐occur with few related species decreases a parasite's likelihood of encountering related competitors, perhaps increasing invasion potential but decreasing diversification opportunity. While climate partially constrains parasite distributions, future host range expansions that spread distinct parasites and diminish barriers to host shifting will likely be key drivers of parasite invasions.     

Tracking costs of virulence in natural populations of the wheat pathogen, Puccinia striiformis f.sp. tritici

Background  

Costs of adaptation play an important role in host-parasite coevolution. For parasites, evolving the ability to circumvent host resistance may trade off with subsequent growth or transmission. Such costs of virulence (sensu plant pathology) limit the spread of all-infectious genotypes and thus facilitate the maintenance of genetic polymorphism in both host and parasite. We investigated costs of three virulence factors in Puccinia striiformis f.sp. tritici, a fungal pathogen of wheat (Triticum aestivum).     

Mosquito mortality and the evolution of malaria virulence

Abstract Several laboratory studies of malaria parasites (Plasmodium sp.) and some field observations suggest that parasite virulence, defined as the harm a parasite causes to its vertebrate host, is positively correlated with transmission. Given this advantage, what limits the continual evolution of higher parasite virulence? One possibility is that while more virulent strains are more infectious, they are also more lethal to mosquitoes. In this study, we tested whether the virulence of the rodent malaria parasite P. chabaudi in the laboratory mouse was correlated with the fitness of mosquitoes it subsequently infected. Mice were infected with one of seven genetically distinct clones of P. chabaudi that differ in virulence. Weight loss and anemia in infected mice were monitored for 16–17 days before Anopheles stephensi mosquitoes were allowed to take a blood meal from them. Infection virulence in mice was positively correlated with transmission to mosquitoes (infection rate) and weakly associated with parasite burden (number of oocysts). Mosquito survival fell with increasing oocyst burden, but there was no overall statistically significant relationship between virulence in mice and mosquito mortality. Thus, there was no evidence that more virulent strains are more lethal to mosquitoes. Both vector survival and fecundity depended on parasite clone, and contrary to expectations, mosquitoes fed on infections more virulent to mice were more fecund. The strong parasite genetic effects associated with both fecundity and survival suggests that vector fitness could be an important selective agent shaping malaria population genetics and the evolution of phenotypes such as virulence in the vector.     

Host and parasite life history interplay to yield divergent population genetic structures in two ectoparasites living on the same bat species

Host–parasite interactions are ubiquitous in nature. However, how parasite population genetic structure is shaped by the interaction between host and parasite life history remains understudied. Studies comparing multiple parasites infecting a single host can be used to investigate how different parasite life history traits interplay with host behaviour and life history. In this study, we used 10 newly developed microsatellite loci to investigate the genetic structure of a parasitic bat fly (Basilia nana). Its host, the Bechstein's bat (Myotis bechsteinii), has a social system and roosting behaviour that restrict opportunities for parasite transmission. We compared fly genetic structure to that of the host and another parasite, the wing‐mite, Spinturnix bechsteini. We found little spatial or temporal genetic structure in B. nana, suggesting a large, stable population with frequent genetic exchange between fly populations from different bat colonies. This contrasts sharply with the genetic structure of the wing‐mite, which is highly substructured between the same bat colonies as well as temporally unstable. Our results suggest that although host and parasite life history interact to yield similar transmission patterns in both parasite species, the level of gene flow and eventual spatiotemporal genetic stability is differentially affected. This can be explained by the differences in generation time and winter survival between the flies and wing‐mites. Our study thus exemplifies that the population genetic structure of parasites on a single host can vary strongly as a result of how their individual life history characteristics interact with host behaviour and life history traits.     

Is the development of falciparum malaria in the human host limited by the availability of uninfected erythrocytes?

Background  

The development and propagation of malaria parasites in their vertebrate host is a complex process in which various host and parasite factors are involved. Sometimes the evolution of parasitaemia seems to be quelled by parasite load. In order to understand the typical dynamics of evolution of parasitaemia, various mathematical models have been developed. The basic premise ingrained in most models is that the availability of uninfected red blood cells (RBC) in which the parasite develops is a limiting factor in the propagation of the parasite population.     

Empirical support for optimal virulence in a castrating parasite

The trade-off hypothesis for the evolution of virulence predicts that parasite transmission stage production and host exploitation are balanced such that lifetime transmission success (LTS) is maximised. However, the experimental evidence for this prediction is weak, mainly because LTS, which indicates parasite fitness, has been difficult to measure. For castrating parasites, this simple model has been modified to take into account that parasites convert host reproductive resources into transmission stages. Parasites that kill the host too early will hardly benefit from these resources, while postponing the killing of the host results in diminished returns. As predicted from optimality models, a parasite inducing castration should therefore castrate early, but show intermediate levels of virulence, where virulence is measured as time to host killing. We studied virulence in an experimental system where a bacterial parasite castrates its host and produces spores that are not released until after host death. This permits estimating the LTS of the parasite, which can then be related to its virulence. We exposed replicate individual Daphnia magna (Crustacea) of one host clone to the same amount of bacterial spores and followed individuals until their death. We found that the parasite shows strong variation in the time to kill its host and that transmission stage production peaks at an intermediate level of virulence. A further experiment tested for the genetic basis of variation in virulence by comparing survival curves of daphniids infected with parasite spores obtained from early killing versus late killing infections. Hosts infected with early killer spores had a significantly higher death rate as compared to those infected with late killers, indicating that variation in time to death was at least in part caused by genetic differences among parasites. We speculate that the clear peak in lifetime reproductive success at intermediate killing times may be caused by the exceptionally strong physiological trade-off between host and parasite reproduction. This is the first experimental study to demonstrate that the production of propagules is highest at intermediate levels of virulence and that parasite genetic variability is available to drive the evolution of virulence in this system.     

Diversity and specificity in the interaction between <Emphasis Type="Italic">Caenorhabditis elegans</Emphasis> and the pathogen <Emphasis Type="Italic">Serratia marcescens</Emphasis>

Background  

Co-evolutionary arms races between parasites and hosts are considered to be of immense importance in the evolution of living organisms, potentially leading to highly dynamic life-history changes. The outcome of such arms races is in many cases thought to be determined by frequency dependent selection, which relies on genetic variation in host susceptibility and parasite virulence, and also genotype-specific interactions between host and parasite. Empirical evidence for these two prerequisites is scarce, however, especially for invertebrate hosts. We addressed this topic by analysing the interaction between natural isolates of the soil nematode Caenorhabditis elegans and the pathogenic soil bacterium Serratia marcescens.     

Within‐host interactions shape virulence‐related traits of trematode genotypes

Within‐host interactions between co‐infecting parasites can significantly influence the evolution of key parasite traits, such as virulence (pathogenicity of infection). The type of interaction is expected to predict the direction of selection, with antagonistic interactions favouring more virulent genotypes and synergistic interactions less virulent genotypes. Recently, it has been suggested that virulence can further be affected by the genetic identity of co‐infecting partners (G × G interactions), complicating predictions on disease dynamics. Here, we used a natural host–parasite system including a fish host and a trematode parasite to study the effects of G × G interactions on infection virulence. We exposed rainbow trout (Oncorhynchus mykiss) either to single genotypes or to mixtures of two genotypes of the eye fluke Diplostomum pseudospathaceum and estimated parasite infectivity (linearly related to pathogenicity of infection, measured as coverage of eye cataracts) and relative cataract coverage (controlled for infectivity). We found that both traits were associated with complex G × G interactions, including both increases and decreases from single infection to co‐infection, depending on the genotype combination. In particular, combinations where both genotypes had low average infectivity and relative cataract coverage in single infections benefited from co‐infection, while the pattern was opposite for genotypes with higher performance. Together, our results show that infection outcomes vary considerably between single and co‐infections and with the genetic identity of the co‐infecting parasites. This can result in variation in parasite fitness and consequently impact evolutionary dynamics of host–parasite interactions.     

A comparison of the Giardia lamblia trophozoite and cyst transcriptome using microarrays

Background  

Compared with many protists, Giardia lamblia has a simple life cycle alternating between cyst and trophozoite. Most research on the molecular biology of Giardia parasites has focused on trophozoites and the processes of excystation and encystation, whereas cysts have attracted less interest. The striking morphological differences between the dormant cyst and the rapidly dividing and motile trophozoite implies profound changes in the metabolism as the parasite encysts in the host's intestine and excysts upon ingestion by a new host.     

Local host competition in the evolution of virulence

The evolution of parasite life histories should usually have correlated effects on host survivorship and/or reproductive success. For example, parasites that reproduce more rapidly might be expected to cause greater reductions in host fitness. Important theoretical advances have recently been made on virulence evolution, but the results are not always consistent. Here I compare two models [ Q. Rev. Biol. 71 (1996) 37 ; Q. Rev. Biol. 75 (2000) 261 ] on the evolution of virulence that get qualitatively different results with respect to the effects of coinfection. I also construct a third model that attempts to connect these two formulations. The results suggest that parasite growth rates should increase as local host competition increases, unless relatedness is at equilibrium. In addition, the qualitative effect of adding coinfections on parasite growth rates depends critically on how the number of coinfections affects transmission success.