INTRASPECIFIC COMPETITION AND THE EVOLUTION OF VIRULENCE IN A PARASITIC TREMATODE

Reaching the World Health Organization elimination targets for schistosomiasis: the importance of a One Health perspective

The past three years has seen the launch of a new World Health Organization (WHO) neglected tropical diseases (NTDs) roadmap, together with revised control and elimination guidelines. Across all, there is now a clear emphasis on the need to incorporate a One Health approach, recognizing the critical links between human and animal health and the environment. Schistosomiasis, caused by Schistosoma spp. trematodes, is a NTD of global medical and veterinary importance, with over 220 million people and untold millions of livestock currently infected. Its burden remains extremely high in certain regions, particularly within sub-Saharan Africa, despite over two decades of mass preventive chemotherapy (mass drug administration), predominantly to school-aged children. In Africa, in contrast to Asia, any zoonotic component of schistosomiasis transmission and its implications for disease control has, until recently, been largely ignored. Here, we review recent epidemiological, clinical, molecular, and modelling work across both Asia and Africa. We outline the evolutionary history and transmission dynamics of Schistosoma species, and emphasize the emerging risk raised by both wildlife reservoirs and viable hybridization between human and animal schistosomes. To achieve the 2030 WHO roadmap elimination targets, a truly multi-disciplinary One Health perspective must be implemented. This article is part of the theme issue 'Challenges and opportunities in the fight against neglected tropical diseases: a decade from the London Declaration on NTDs'.

Metabolic efficiency reshapes the seminal relationship between pathogen growth rate and virulence

A cornerstone of classical virulence evolution theories is the assumption that pathogen growth rate is positively correlated with virulence, the amount of damage pathogens inflict on their hosts. Such theories are key for incorporating evolutionary principles into sustainable disease management strategies. Yet, empirical evidence raises doubts over this central assumption underpinning classical theories, thus undermining their generality and predictive power. In this paper, we identify a key component missing from current theories which redefines the growth-virulence relationship in a way that is consistent with data. By modifying the activity of a single metabolic gene, we engineered strains of Magnaporthe oryzae with different nutrient acquisition and growth rates. We conducted in planta infection studies and uncovered an unexpected non-monotonic relationship between growth rate and virulence that is jointly shaped by how growth rate and metabolic efficiency interact. This novel mechanistic framework paves the way for a much-needed new suite of virulence evolution theories.

Assessing rates of parasite coinfection and spatiotemporal strain variation via metabarcoding: insights for the conservation of European Turtle Doves Streptopelia turtur

Understanding the frequency, spatiotemporal dynamics and impacts of parasite coinfections is fundamental to developing control measures and predicting disease impacts. The European turtle dove (Streptopelia turtur) is one of Europe's most threatened bird species. High prevalence of infection by the protozoan parasite Trichomonas gallinae has previously been identified, but the role of this and other coinfecting parasites in turtle dove declines remains unclear. Using a high‐throughput sequencing approach, we identified seven strains of T. gallinae, including two novel strains, from ITS1/5.8S/ITS2 ribosomal sequences in turtle doves on breeding and wintering grounds, with further intrastrain variation and four novel subtypes revealed by the iron‐hydrogenase gene. High spatiotemporal turnover was observed in T. gallinae strain composition, and infection was prevalent in all populations (89%–100%). Coinfection by multiple Trichomonas strains was rarer than expected (1% observed compared to 38.6% expected), suggesting either within‐host competition, or high mortality of coinfected individuals. In contrast, coinfection by multiple haemosporidians was common (43%), as was coinfection by haemosporidians and T. gallinae (90%), with positive associations between strains of T. gallinae and Leucocytozoon suggesting a mechanism such as parasite‐induced immune modulation. We found no evidence for negative associations between coinfections and host body condition. We suggest that longitudinal studies involving the recapture and investigation of infection status of individuals over their lifespan are crucial to understand the epidemiology of coinfections in natural populations.

Host exposure history and priority effects impact the development and reproduction of a dominant parasite

Within a single organism, numerous parasites often compete for space and resources. This competition, together with a parasite's ability to locate and successfully establish in a host, can contribute to the distribution and prevalence of parasites. Coinfection with trematodes in snail intermediate hosts is rarely observed in nature, partly due to varying competitive abilities among parasite taxa. Using a freshwater snail host (Biomphalaria glabrata), we studied the ability of a competitively dominant trematode, Echinostoma caproni, to establish and reproduce in a host previously infected with a less competitive trematode species, Schistosoma mansoni. Snails were exposed to S. mansoni and co-exposed to E. caproni either simultaneously or 1 week, 4 weeks, or 6 weeks post S. mansoni exposure. Over the course of infection, we monitored the competitive success of the dominant trematode through infection prevalence, parasite development time, and parasite reproductive output. Infection prevalence of E. caproni did not differ among co-exposed groups or between co-exposed and single exposed groups. However, E. caproni infections in co-exposed hosts took longer to reach maturity when the timing between co-exposures increased. All co-exposed groups had higher E. caproni reproductive output than single exposures. We show that although timing of co-exposure affects the development time of parasite transmission stages, it is not important for successful establishment. Additionally, co-exposure, but not priority effects, increases the reproductive output of the dominant parasite.

Schistosomiasis Morbidity Hotspots: Roles of the Human Host, the Parasite and Their Interface in the Development of Severe Morbidity

Schistosomiasis is the second most important human parasitic disease in terms of socioeconomic impact, causing great morbidity and mortality, predominantly across the African continent. For intestinal schistosomiasis, severe morbidity manifests as periportal fibrosis (PPF) in which large tracts of macro-fibrosis of the liver, visible by ultrasound, can occlude the main portal vein leading to portal hypertension (PHT), sequelae such as ascites and collateral vasculature, and ultimately fatalities. For urogenital schistosomiasis, severe morbidity manifests as pathology throughout the urinary system and genitals, and is a definitive cause of squamous cell bladder carcinoma. Preventative chemotherapy (PC) programmes, delivered through mass drug administration (MDA) of praziquantel (PZQ), have been at the forefront of schistosomiasis control programmes in sub-Saharan Africa since their commencement in Uganda in 2003. However, despite many successes, 'biological hotspots' (as distinct from 'operational hotspots') of both persistent high transmission and morbidity remain. In some areas, this failure to gain control of schistosomiasis has devastating consequences, with not only persistently high infection intensities, but both "subtle" and severe morbidity remaining prevalent. These hotspots highlight the requirement to revisit research into severe morbidity and its mechanisms, a topic that has been out of favor during times of PC implementation. Indeed, the focality and spatially-structured epidemiology of schistosomiasis, its transmission persistence and the morbidity induced, has long suggested that gene-environmental-interactions playing out at the host-parasite interface are crucial. Here we review evidence of potential unique parasite factors, host factors, and their gene-environmental interactions in terms of explaining differential morbidity profiles in the human host. We then take the situation of schistosomiasis mansoni within the Albertine region of Uganda as a case study in terms of elucidating the factors behind the severe morbidity observed and the avenues and directions for future research currently underway within a new research and clinical trial programme (FibroScHot).

Sequential infection can decrease virulence in a fish-bacterium-fluke interaction: Implications for aquaculture disease management

Hosts are typically infected with multiple strains or genotypes of one or several parasite species. These infections can take place simultaneously, but also at different times, i.e. sequentially, when one of the parasites establishes first. Sequential parasite dynamics are common in nature, but also in intensive farming units such as aquaculture. However, knowledge of effects of previous exposures on virulence of current infections in intensive farming is very limited. This is critical as consecutive epidemics and infection history of a host could underlie failures in management practices and medical intervention of diseases. Here, we explored effects of timing of multiple infections on virulence in two common aquaculture parasites, the bacterium Flavobacterium columnare and the fluke Diplostomum pseudospathaceum. We exposed fish hosts first to flukes and then to bacteria in two separate experiments, altering timing between the infections from few hours to several weeks. We found that both short-term and long-term differences in timing of the two infections resulted in significant, genotype-specific decrease in bacterial virulence. Second, we developed a mathematical model, parameterized from our experimental results, to predict the implications of sequential infections for epidemiological progression of the disease, and levels of fish population suppression, in an aquaculture setting. Predictions of the model showed that sequential exposure of hosts can decrease the population-level impact of the bacterial epidemic, primarily through the increased recovery rate of sequentially infected hosts, thereby substantially protecting the population from the detrimental impact of infection. However, these effects depended on bacterial strain-fluke genotype combinations, suggesting the genetic composition of the parasite populations can greatly influence the degree of host suppression. Overall, these results suggest that host infection history can have significant consequences for the impact of infection at host population level, potentially shaping parasite epidemiology, disease dynamics and evolution of virulence in farming environments.

Outcome of within-host competition demonstrates that parasite virulence doesn't equal success in a myxozoan model system

Within-host competition can affect outcomes of infections when parasites occupy the same niche. We investigated within-host competition and infection outcomes in Chinook salmon exposed to two genotypes of Ceratonova shasta (myxozoan parasite). We assessed i) virulence (host mortality, median days to death), ii) within-host competition (abundance in host), and iii) success (spore production, proportion of myxospore-producing hosts) following concurrent and sequential exposures to single or mixed-genotype treatments. In single treatments, genotype-I replicated faster, and caused higher and earlier host mortality (higher virulence) but genotype-II produced more myxospores (higher success). In mixed treatments, costs of competition were observed for both genotypes evidenced by reduced replication or myxospore production following concurrent exposures, but only the less-virulent genotype suffered costs of competition when hosts were exposed to genotypes sequentially. To understand potential host effects on competition outcomes, we characterized systemic (spleen) and local (intestine) cytokine and immunoglobulin expression in single and mixed infections. We observed delayed systemic and immunosuppressive responses to the virulent genotype (I), rapid, localized and non-suppressive responses to the less-virulent genotype (II), and a combination of responses to mixed-genotypes. Thus, competition outcomes favoring the virulent genotype may be partially explained by the localized response to genotype-II that facilitates myxospore production (success) offsetting the systemic response to genotype-I that results in early inflammation and immunosuppression (that increases onset of mortality). This evidence for different but simultaneous responses to each genotype suggests selection should favor the exclusion of the weaker competitor and the evolution of increased virulence in the stronger competitor because the outcome was generally more costly for the less-virulent genotype. With caveats, our results are relevant for understanding infection outcomes in commercially and ecologically important salmonids in C. shasta endemic regions where mixed infections are commonplace.

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Competition between two genotypes of Ceratonova shasta was asymmetric in Chinook salmon hosts.

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Genotype I was more virulent but genotype-II was more successful (produced more myxospores).

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Costs of competition differed between genotypes, may be mediated by host immune response.

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Host immune response to genotype-I was delayed systemic and immunosuppressive.

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Host immune response to genotype-II was rapid, localized and non-suppressive.

Genetic Crosses and Linkage Mapping in Schistosome Parasites

Linkage mapping - utilizing experimental genetic crosses to examine cosegregation of phenotypic traits with genetic markers - is now 100 years old. Schistosome parasites are exquisitely well suited to linkage mapping approaches because genetic crosses can be conducted in the laboratory, thousands of progeny are produced, and elegant experimental work over the last 75 years has revealed heritable genetic variation in multiple biomedically important traits such as drug resistance, host specificity, and virulence. Application of this approach is timely because the improved genome assembly for Schistosoma mansoni and developing molecular toolkit for schistosomes increase our ability to link phenotype with genotype. We describe current progress and potential future directions of linkage mapping in schistosomes.

Of mice and worms: are co-infections with unrelated parasite strains more damaging to definitive hosts?

Intraspecific competition between co-infecting parasites can influence the amount of virulence, or damage, they do to their host. Kin selection theory dictates that infections with related parasite individuals should have lower virulence than infections with unrelated individuals, because they benefit from inclusive fitness and increased host longevity. These predictions have been tested in a variety of microparasite systems, and in larval stage macroparasites within intermediate hosts, but the influence of adult macroparasite relatedness on virulence has not been investigated in definitive hosts. This study used the human parasite Schistosoma mansoni to determine whether definitive hosts infected with related parasites experience lower virulence than hosts infected with unrelated parasites, and to compare the results from intermediate host studies in this system. The presence of unrelated parasites in an infection decreased parasite infectivity, the ability of a parasite to infect a definitive host, and total worm establishment in hosts, impacting the less virulent parasite strain more severely. Unrelated parasite co-infections had similar virulence to the more virulent of the two parasite strains. We combine these findings with complementary studies of the intermediate snail host and describe trade-offs in virulence and selection within the life cycle. Damage to the host by the dominant strain was muted by the presence of a competitor in the intermediate host, but was largely unaffected in the definitive host. Our results in this host-parasite system suggest that unrelated infections may select for higher virulence in definitive hosts while selecting for lower virulence in intermediate hosts.

Assessing S. mansoni prevalence in Biomphalaria snails in the Gombe ecosystem of western Tanzania: the importance of DNA sequence data for clarifying species identification

Background

Snails are essential for the transmission and maintenance of schistosomiasis in endemic areas, as they serve as intermediate hosts for schistosome parasites. A clear understanding of the snail species present, their local distribution and infection status is therefore a prerequisite for effective control of schistosomiasis. The purpose of this study was to establish the infection status and distribution of Schistosoma mansoni in snails in the Gombe area along the shores of Lake Tanganyika in western Tanzania, using both detection of cercarial shedding and molecular approaches.

Methods

Snails were collected from streams located close to human settlements in Gombe National Park, as well as from nearby villages (Kiziba, Mtanga, Mwamgongo and Bugamba) and the largest town in the region (Kigoma). Snails were individually exposed to light to induce shedding of schistosome larvae, which were examined using a compound light microscope. Additionally, the internal transcribed spacer (ITS) region of the ribosomal RNA gene cluster was simultaneously amplified in both snails and their trematodes using a single polymerase chain reaction (PCR) and sequenced to confirm species identification.

Results

Snails morphologically identified as Biomphalaria pfeifferi were present in all streams except at Mtanga but their distribution was patchy in both time and space. Sequencing of PCR products indicated that not all snails were B. pfeifferi. None of the snails from Gombe or Bugamba shed schistosome larvae, while larvae were shed at all other sites. Overall, an infection prevalence of only 12% was observed in snails based on cercarial shedding. While 47% of the snails were PCR-positive for the 500 bp ITS fragment, which was predicted to indicate infection with S. mansoni, sequence data demonstrated that these bands are not species-specific and can be amplified from other trematode infections. In addition, a 1000 bp band was amplified in 14% of samples, which was identified as a trematode in the family Derogenidae.

Conclusions

The results support the previous assumption that B. pfeifferi snails may be involved in transmitting schistosomiasis in the area but suggest that the community structure of both snails and trematodes may be more complicated than previously thought. This emphasises the importance of confirming species identifications using sequencing, rather than relying only on PCR-based diagnostics or cercarial shedding.

Electronic supplementary material

The online version of this article (10.1186/s13071-017-2525-6) contains supplementary material, which is available to authorized users.

Multiple-genotype infections and their complex effect on virulence

Multiple infections are common. Although in recent years our understanding of multiple infections has increased significantly, it has also become clear that a diversity of aspects has to be considered to understand the interplay between co-infecting parasite genotypes of the same species and its implications for virulence and epidemiology, resulting in high complexity. Here, we review different interaction mechanisms described for multiple infections ranging from competition to cooperation. We also list factors influencing the interaction between co-infecting parasite genotypes and their influence on virulence. Finally, we emphasise the importance of between-host effects and their evolution for understanding multiple infections and their implications.

Interactions among bacterial strains and fluke genotypes shape virulence of co-infection

Most studies of virulence of infection focus on pairwise host-parasite interactions. However, hosts are almost universally co-infected by several parasite strains and/or genotypes of the same or different species. While theory predicts that co-infection favours more virulent parasite genotypes through intensified competition for host resources, knowledge of the effects of genotype by genotype (G × G) interactions between unrelated parasite species on virulence of co-infection is limited. Here, we tested such a relationship by challenging rainbow trout with replicated bacterial strains and fluke genotypes both singly and in all possible pairwise combinations. We found that virulence (host mortality) was higher in co-infections compared with single infections. Importantly, we also found that the overall virulence was dependent on the genetic identity of the co-infecting partners so that the outcome of co-infection could not be predicted from the respective virulence of single infections. Our results imply that G × G interactions among co-infecting parasites may significantly affect host health, add to variance in parasite fitness and thus influence evolutionary dynamics and ecology of disease in unexpected ways.

The Influence of Infective Dose on the Virulence of a Generalist Pathogen in Rainbow Trout (Oncorhynchus mykiss) and Zebra Fish (Danio rerio)

Pathogen density and genetic diversity fluctuate in the outside-host environment during and between epidemics, affecting disease emergence and the severity and probability of infections. Although the importance of these factors for pathogen virulence and infection probability has been acknowledged, their interactive effects are not well understood. We studied how an infective dose in an environmentally transmitted opportunistic fish pathogen, Flavobacterium columnare, affects its virulence both in rainbow trout, which are frequently infected at fish farms, and in zebra fish, a host that is not naturally infected by F. columnare. We used previously isolated strains of confirmed high and low virulence in a single infection and in a co-infection. Infection success (measured as host morbidity) correlated positively with dose when the hosts were exposed to the high-virulence strain, but no response for the dose increase was found when the hosts were exposed to the low-virulence strain. Interestingly, the co-infection resulted in poorer infection success than the single infection with the high-virulence strain. The rainbow trout were more susceptible to the infection than the zebra fish but, in both species, the effects of the doses and the strains were qualitatively similar. We suggest that as an increase in dose can lead to increased host morbidity, both the interstrain interactions and differences in infectivity in different hosts may influence the severity and consequently the evolution of disease. Our results also confirm that the zebra fish is a good laboratory model to study F. columnare infection.

The distribution of parasite strains among hosts affects disease spread in a social insect

Social insects present highly interesting and experimentally amenable systems for the study of disease transmission because they naturally live in dense groups of frequently interacting individuals. Using experimental inoculations of five trypanosomatid strains into groups of its natural host, the bumblebee Bombus terrestris, we investigate the effects of the initial parasite strain distribution across group members on the establishment and transmission success of the different strains to new hosts. For a given number of parasite strains circulating within a host group, transmission to new hosts was increased when the strains were initially inoculated as mixed infections (as opposed to separate single infections), presumably because mixed infections generally favored fast replicating strains. In contrast, separate single infections reduced transmission at least in part through a precedence effect, whereby weak strains appeared to persist by making their host unavailable to superinfection. These results suggest that host groups could benefit from 'compartmentalizing' infections by different parasite strains across different group members, which might be achieved in social insects, for example, by division of labor.

The consequences of co-infections for parasite transmission in the mosquito Aedes aegypti

Co-infections may modify parasite transmission opportunities directly as a consequence of interactions in the within-host environment, but also indirectly through changes in host life history. Furthermore, host and parasite traits are sensitive to the abiotic environment with variable consequences for parasite transmission in co-infections. We investigate how co-infection of the mosquito Aedes aegypti with two microsporidian parasites (Vavraia culicis and Edhazardia aedis) at two levels of larval food availability affects parasite transmission directly, and indirectly through effects on host traits. In a laboratory infection experiment, we compared how co-infection, at low and high larval food availability, affected the probability of infection, within-host growth and the transmission potential of each parasite, compared to single infections. Horizontal transmission was deemed possible for both parasites when infected hosts died harbouring horizontally transmitting spores. Vertical transmission was judged possible for E. aedis when infected females emerged as adults. We also compared the total input number of spores used to seed infections with output number, in single and co-infections for each parasite. The effects of co-infection on parasite fitness were complex, especially for V. culicis. In low larval food conditions, co-infection increased the chances of mosquitoes dying as larvae or pupae, thus increasing opportunities for V. culicis' horizontal transmission. However, co-infection reduced larval longevity and hence time available for V. culicis spore production. Overall, there was a negative net effect of co-infection on V. culicis, whereby the number of spores produced was less than the number used to seed infection. Co-infections also negatively affected horizontal transmission of the more virulent parasite, E. aedis, through reduced longevity of pre-adult hosts. However, its potential transmission suffered less relative to V. culicis. Our results show that co-infection can negatively affect parasite transmission opportunities, both directly as well as indirectly via effects on host life history. We also find that transmission is contingent on the combined effect of the abiotic environment.

Diet quality determines interspecific parasite interactions in host populations

The widespread occurrence of multiple infections and the often vast range of nutritional resources for their hosts allow that interspecific parasite interactions in natural host populations might be determined by host diet quality. Nevertheless, the role of diet quality with respect to multispecies parasite interactions on host population level is not clear. We here tested the effect of host population diet quality on the parasite community in an experimental study using Daphnia populations. We studied the effect of diet quality on Daphnia population demography and the interactions in multispecies parasite infections of this freshwater crustacean host. The results of our experiment show that the fitness of a low-virulent microsporidian parasite decreased in low, but not in high-host-diet quality conditions. Interestingly, infections with the microsporidium protected Daphnia populations against a more virulent bacterial parasite. The observed interspecific parasite interactions are discussed with respect to the role of diet quality-dependent changes in host fecundity. This study reflects that exploitation competition in multispecies parasite infections is environmentally dependent, more in particular it shows that diet quality affects interspecific parasite competition within a single host and that this can be mediated by host population-level effects.

Can host ecology and kin selection predict parasite virulence?

Parasite virulence, or the damage a parasite does to its host, is measured in terms of both host costs (reductions in host growth, reproduction and survival) and parasite benefits (increased transmission and parasite numbers) in the literature. Much work has shown that ecological and genetic factors can be strong selective forces in virulence evolution. This review uses kin selection theory to explore how variations in host ecological parameters impact the genetic relatedness of parasite populations and thus virulence. We provide a broad overview of virulence and population genetics studies and then draw connections to existing knowledge about natural parasite populations. The impact of host movement (transporting parasites) and host resistance (filtering parasites) on the genetic structure and virulence of parasite populations is explored, and empirical studies of these factors using Plasmodium and trematode systems are proposed.

Loss of competition in the outside host environment generates outbreaks of environmental opportunist pathogens

Environmentally transmitted pathogens face ecological interactions (e.g., competition, predation, parasitism) in the outside-host environment and host immune system during infection. Despite the ubiquitousness of environmental opportunist pathogens, traditional epidemiology focuses on obligatory pathogens incapable of environmental growth. Here we ask how competitive interactions in the outside-host environment affect the dynamics of an opportunist pathogen. We present a model coupling the classical SI and Lotka–Volterra competition models. In this model we compare a linear infectivity response and a sigmoidal infectivity response. An important assumption is that pathogen virulence is traded off with competitive ability in the environment. Removing this trade-off easily results in host extinction. The sigmoidal response is associated with catastrophic appearances of disease outbreaks when outside-host species richness, or overall competition pressure, decreases. This indicates that alleviating outside-host competition with antibacterial substances that also target the competitors can have unexpected outcomes by providing benefits for opportunist pathogens. These findings may help in developing alternative ways of controlling environmental opportunist pathogens.

Specific detection and quantification of virulent/avirulent Phytophthora infestans isolates using a real-time PCR assay that targets polymorphisms of the Avr3a gene

Molecular tools that allow intraspecific quantification and discrimination of pathogen isolates are useful to assess fitness of competitors during mixed infections. However, methods that were developed for quantifying Phytophthora infestans are only specific at the species level. Here, we reported a TaqMan-based real-time PCR assay allowing, according to the specificity of the used probes, an accurate quantification of different proportions of two genetically distinct clones of P. infestans in mixed fractions. Indeed, in addition to a primer specific to P. infestans, two primers and two TaqMan(®) probes that target single-nucleotide polymorphisms located in the Avr3a/avr3a virulence gene sequence were designed. The reliability of the method was tested on serially diluted fractions containing plasmid DNA with either the Avr3a or the avr3a sequences at concentrations ranging from 10(2) to 10(8)  copies per μl. Based on its specificity, sensitivity and repeatability, the proposed assay allowed a quantification of the targeted DNA sequence in fractions with a Avr3a/avr3a ratio in the range 1/99 to 99/1. The reliability of the test was also checked for counting zoospores. Applications for future research in P. infestans/host quantitative interactions were also discussed.

Multiple infections and the evolution of virulence

Infections that consist of multiple parasite strains or species are common in the wild and are a major public health concern. Theory suggests that these infections have a key influence on the evolution of infectious diseases and, more specifically, on virulence evolution. However, we still lack an overall vision of the empirical support for these predictions. We argue that within-host interactions between parasites largely determine how virulence evolves and that experimental data support model predictions. Then, we explore the main limitation of the experimental study of such 'mixed infections', which is that it draws conclusions on evolutionary outcomes from studies conducted at the individual level. We also discuss differences between coinfections caused by different strains of the same species or by different species. Overall, we argue that it is possible to make sense out of the complexity inherent to multiple infections and that experimental evolution settings may provide the best opportunity to further our understanding of virulence evolution.

Molecular assessment of trematode co-infection and intraspecific competition in molluscan intermediate hosts

In natural populations of the human parasite Schistosoma mansoni, parasite distribution among snail intermediate hosts is generally overdispersed, such that a small proportion of hosts harbor the majority of parasite genotypes. Within these few infected snails, researchers have found that it can be common for hosts to harbor multiple parasite genotypes, creating circumstances in which co-infecting parasites are faced with potential competition over limited host resources. Much theoretical modeling has focused on parasite competition, especially regarding the influence of co-infection on parasite exploitation strategy evolution. However, particularly in the case of intra-molluscan intermediate stages, empirical investigations of parasite-parasite competition have often hinged on the untested assumption that co-exposure produces co-infection. That is, infected hosts exposed to multiple strains have been assumed to harbor multiple strains, regardless of the true nature of the infection outcome. Here we describe a real-time quantitative PCR method to distinguish the conditions of multiple- versus single-strain infection, as well as quantify the relative larval output of co-infecting strains. We applied the method to an empirical investigation of intraspecific parasite competition between S. mansoni strains within the intermediate snail host Biomphalaria glabrata, assessing co-exposure's effects on parasite infectivity and productivity and the concomitant effects on host fitness. Overall, there was no effect of parasite co-infection on snail life history traits relative to single-strain infection. Parasite infectivity significantly increased as a result of increasing overall miracidial dose, rather than co-exposure, though strain-specific productivity was significantly reduced in co-infections in manner consistent with resource competition. Moreover, we show that less than half of infected, co-exposed hosts had patent co-infections and demonstrate the utility of this molecular tool for the study of trematode life history variation in molluscan hosts.

Probing mixed-genotype infections II: high multiplicity in natural infections of the trypanosomatid, Crithidia bombi, in its host, Bombus spp

Mixed-genotype infections have major consequences for many essential elements of host-parasite interactions. With genetic exchange between co-infecting parasite genotypes increased diversity among parasite offspring and the emergence of novel genotypes from infected hosts is possible. We here investigated mixed- genotype infections using the host, Bombus spp. and its trypanosome parasite Crithidia bombi as our study case. The natural infections of C. bombi were genotyped with a novel method for a representative sample of workers and spring queens in Switzerland. We found that around 60% of all infected hosts showed mixed-genotype infections with an average of 2.47±0.22 (S.E.) and 3.65±1.02 genotypes per worker or queen, respectively. Queens, however, harboured up to 29 different genotypes. Based on the genotypes of co-infecting strains, these could be putatively assigned to either ‘primary’ and ‘derived’ genotypes - the latter resulting from genetic exchange among the primary genotypes. High genetic relatedness among co-infecting derived but not primary genotypes supported this scenario. Co-infection in queens seems to be a major driver for the diversity of genotypes circulating in host populations.

Do parasites adopt different strategies in different intermediate hosts? Host size, not host species, influences Coitocaecum parvum (Trematoda) life history strategy, size and egg production

Host exploitation induces host defence responses and competition between parasites, resulting in individual parasites facing highly variable environments. Alternative life strategies may thus be expressed in context-dependent ways, depending on which host species is used and intra-host competition between parasites. Coitocaecum parvum (Trematode) can use facultative progenesis in amphipod intermediate hosts, Paracalliope fluviatilis, to abbreviate its life cycle in response to such environmental factors. Coitocaecum parvum also uses another amphipod host, Paracorophium excavatum, a species widely different in size and ecology from P. fluviatilis. In this study, parasite infection levels and strategies in the two amphipod species were compared to determine whether the adoption of progenesis by C. parvum varied between these two hosts. Potential differences in size and/or egg production between C. parvum individuals according to amphipod host species were also investigated. Results show that C. parvum life strategy was not influenced by host species. In contrast, host size significantly affected C. parvum strategy, size and egg production. Since intra-host interactions between co-infecting parasites also influenced C. parvum strategy, size and fecundity, it is highly likely that within-host resource limitations affect C. parvum life strategy and overall fitness regardless of host species.

The fundamental role of competition in the ecology and evolution of mutualisms

Mutualisms are interspecific interactions that yield reciprocal benefits. Here, by adopting a consumer-resource perspective, we show how considering competition is necessary in order to understand the evolutionary and ecological dynamics of mutualism. We first review the ways in which competition shapes the ecology of mutualisms, using a graphical framework based on resource flows rather than net effects to highlight the opportunities for competition. We then describe the known mechanisms of competition and show how it is a critical driver of the evolutionary dynamics, persistence, and diversification of mutualism. We argue that empirical and theoretical research on the ecology and evolution of mutualisms will jointly progress by addressing four key points: (i) the existence and shape of physiological trade-offs among cooperation, competition, and other life-history and functional traits; (ii) the capacity for individuals to express conditional responses to variation in their mutualistic and competitive environment; (iii) the existence of heritable variation for mutualistic and competitive traits and their potentially conditional expression; and (iv) the structure of the network of consumer-resource interactions in which individuals are embedded.

Interactions among virulence, coinfection and drug resistance in a complex life-cycle parasite

Motivated by relatively recent empirical studies on Schistosoma mansoni, we use a mathematical model to investigate the impacts of drug treatment of the definitive human host and coinfection of the intermediate snail host by multiple parasite strains on the evolution of parasites' drug resistance. Through the examination of evolutionarily stable strategies (ESS) of parasites, our study suggests that higher levels of drug treatment rates (which usually tend to promote monomorphism as the evolutionary equilibrium) favor parasite strains that have a higher level of drug resistance. Our study also shows that whether coinfection of intermediate hosts affects the levels of drug resistance at ESS points and their stability depends on the assumptions on the cost of parasites paid for drug resistance, coinfection functions and parasites' reproduction within coinfected hosts. This calls for more empirical studies on the parasite.

To be or not to be solitary: Phytophthora infestans' dilemma for optimizing its reproductive fitness in multiple infections

The success of parasitic life lies in an optimal exploitation of the host to satisfy key functions directly involved in reproductive fitness. Resource availability generally decreases over time with host mortality, but also during multiple infections, where different strains of parasite share host resources. During multiple infections, the number of parasite strains and their genetic relatedness are known to influence their reproductive rates. Using infections of the potato plant Solanum tuberosum with the parasite Phytophthora infestans, we set up an experimental design to separate dose effects (double- vs. single-site infections) from genetic relatedness (different vs. identical genotypes) on the reproductive fitness of competing parasite genotypes. We showed the existence of two basic response patterns--increase or decrease in reproductive fitness in multiple infections- depending on the parasite genotype. In all cases, the intensity of the response of any genotype depended on the genotype of the competing strain. This diversity of responses to multiple infections is probably maintained by the fluctuating frequencies of multiple infections in nature, arising from variations in disease pressure over the course of an epidemic and between successive epidemics. It allows a rapid response of parasitic populations to changing environments, which are particularly intense in agricultural systems.

Cheating, trade-offs and the evolution of aggressiveness in a natural pathogen population

The evolutionary dynamics of pathogens are critically important for disease outcomes, prevalence and emergence. In this study we investigate ecological conditions that may promote the long-term maintenance of virulence polymorphisms in pathogen populations. Recent theory predicts that evolution towards increased virulence can be reversed if less-aggressive social 'cheats' exploit more aggressive 'cooperator' pathogens. However, there is no evidence that social exploitation operates within natural pathogen populations. We show that for the bacterium Pseudomonas syringae, major polymorphisms for pathogenicity are maintained at unexpectedly high frequencies in populations infecting the host Arabidopsis thaliana. Experiments reveal that less-aggressive strains substantially increase their growth potential in mixed infections and have a fitness advantage in non-host environments. These results suggest that niche differentiation can contribute to the maintenance of virulence polymorphisms, and that both within-host and between-host growth rates modulate cheating and cooperation in P. syringae populations.

Encystment patterns and metacercarial size of an opecoelid trematode in two polychaete hosts

The growth, and thus to some extent the fitness, of parasites may vary among individuals based on a range of factors including which host species they infect or how many other parasites share the host with them. Here, we investigate the determinants of the size attained by metacercariae of an opecoelid trematode within two species of intertidal polychaetes serving as second intermediate host, Heteromastus filiformis and Abarenicola affinis. Metacercarial cysts were found predominantly in the anterior region of H. filiformis, particularly in segments 3-6, whereas no apparent pattern of encystment was observed in A. affinis. No relationship was found between the volume achieved by an individual metacercarial cyst and either host body volume, total number of cysts per host, the segment number in which a cyst was located, or the number of other cysts sharing a distinct segment; this was true for parasites in either polychaete species. However, cysts were found to be significantly larger, on average, within A. affinis than within H. filiformis. It remains to be determined whether the polychaete species in which larger sizes are achieved is also the one in which transmission to fish hosts is maximized.

Virulence and competitiveness: testing the relationship during inter- and intraspecific mixed infections

Understanding the reasons why different parasites cause different degrees of harm to their hosts is an important objective in evolutionary biology. One group of models predicts that if hosts are infected with more than one strain or species of parasite, then competition between the parasites will select for higher virulence. While this idea makes intuitive sense, empirical data to support it are rare and equivocal. We investigated the relationship between fitness and virulence during both inter- and intraspecific competition for a fungal parasite of insects, Metarhizium anisopliae. Contrary to theoretical expectations, competition favored parasite strains with either a lower or a higher virulence depending on the competitor: when in interspecific competition with an entomopathogenic nematode, Steinernema feltiae, less virulent strains of the fungus were more successful, but when competing against conspecific fungi, more virulent strains were better competitors. We suggest that the nature of competition (direct via toxin production when competing against the nematode, indirect via exploitation of the host when competing against conspecific fungal strains) determines the relationship between virulence and competitive ability.

In vitro praziquantel test capable of detecting reduced in vivo efficacy in Schistosoma mansoni human infections

Although great reductions in human schistosomiasis have been observed after praziquantel (PZQ) mass drug administration (MDA), some individuals remain infected after multiple treatments. Many MDA programs now require monitoring for drug efficacy as a key component. No molecular tools for PZQ resistance currently exist and investigations into the dose of PZQ required to kill 50% of adult worms in vivo (ED(50)) present ethical, logistical, and temporal restraints. We, therefore, assessed the feasibility and accuracy of a rapid, inexpensive in vitro PZQ test in the laboratory and directly in the field in Uganda under MDA in conjunction with highly detailed infection intensity, clearance, and reinfection data. This test strongly differentiated between subsequently cleared and uncleared infections as well as differences between parasite populations pre- and post-PZQ treatments, advocating its use for on-the-spot monitoring of PZQ efficacy in natural foci. After only a few treatments, uncleared parasites were identified to be phenotypically different from drug-sensitive parasites, emphasizing the urgent need for monitoring of these repeatedly PZQ-treated populations.

Cloning of the unculturable parasite Pasteuria ramosa and its Daphnia host reveals extreme genotype-genotype interactions

The degree of specificity in host-parasite interactions has important implications for ecology and evolution. Unfortunately, specificity can be difficult to determine when parasites cannot be cultured. In such cases, studies often use isolates of unknown genetic composition, which may lead to an underestimation of specificity. We obtained the first clones of the unculturable bacterium Pasteuria ramosa, a parasite of Daphnia magna. Clonal genotypes of the parasite exhibited much more specific interactions with host genotypes than previous studies using isolates. Clones of P. ramosa infected fewer D. magna genotypes than isolates and host clones were either fully susceptible or fully resistant to the parasite. Our finding enhances our understanding of the evolution of virulence and coevolutionary dynamics in this system. We recommend caution when using P. ramosa isolates as the presence of multiple genotypes may influence the outcome and interpretation of some experiments.

Strain filtering and transmission of a mixed infection in a social insect

Mixed-genotype infections have attracted considerable attention as drivers of pathogen evolution. However, experimental approaches often overlook essential features of natural host-parasite interactions, such as host heterogeneity, or the effects of between-host selection during transmission. Here, following inoculation of a mixed infection, we analyse the success of different strains of a trypanosome parasite throughout the colony cycle of its bumblebee host. We find that most colonies efficiently filter the circulating infection before it reaches the new queens, the only offspring that carry infections to the next season. A few colonies with a poor filtering ability thus contributed disproportionately to the parasite population in the next season. High strain diversity but not high infection intensity within colony was associated with an increased probability of transmission of the infection to new queens. Interestingly, the representation of the different strains changed dramatically over time, so that long-term parasite success could not be predicted from short-term observations. These findings highlight the shaping of within-colony parasite diversity through filtering as a crucial determinant of year-to-year pathogen transmission and emphasize the importance of host ecology and heterogeneity for disease dynamics.

Parasite and host assemblages: embracing the reality will improve our knowledge of parasite transmission and virulence

Interactions involving several parasite species (multi-parasitized hosts) or several host species (multi-host parasites) are the rule in nature. Only a few studies have investigated these realistic, but complex, situations from an evolutionary perspective. Consequently, their impact on the evolution of parasite virulence and transmission remains poorly understood. The mechanisms by which multiple infections may influence virulence and transmission include the dynamics of intrahost competition, mediation by the host immune system and an increase in parasite genetic recombination. Theoretical investigations have yet to be conducted to determine which of these mechanisms are likely to be key factors in the evolution of virulence and transmission. In contrast, the relationship between multi-host parasites and parasite virulence and transmission has seen some theoretical investigation. The key factors in these models are the trade-off between virulence across different host species, variation in host species quality and patterns of transmission. The empirical studies on multi-host parasites suggest that interspecies transmission plays a central role in the evolution of virulence, but as yet no complete picture of the phenomena involved is available. Ultimately, determining how complex host-parasite interactions impact the evolution of host-parasite relationships will require the development of cross-disciplinary studies linking the ecology of quantitative networks with the evolution of virulence.

Applying evolutionary genetics to schistosome epidemiology

We review how molecular markers and evolutionary analysis have been applied to the study of schistosome parasites, important pathogens that infect over 200 million people worldwide. Topics reviewed include phylogenetics and biogeography, hybridization, infection within snails, mating systems, and genetic structure. Some interesting generalizations include that schistosome species hybridize frequently and have switched definitive hosts repeatedly in evolutionary time. We show that molecular markers can be used to infer epidemiologically relevant processes such as spatial variation in transmission, or to reveal complex patterns of mate choice. Analysis of genetic structure data shows that transmission foci can be structured by watershed boundaries, habitat types, and host species. We also discuss sampling and analytical problems that arise when using larvae to estimate genetic parameters of adult schistosome populations. Finally, we review pitfalls in methodologies such as genotyping very small individuals, statistical methods for identifying clonemates or for identifying sibling groups, and estimating allele frequencies from pooled egg samples.

Presence of multiparasite infections within individual colonies of leaf-cutter ants

Host-parasite dynamics can be altered when a host is infected by multiple parasite genotypes. The different strains of parasite are expected to compete for the limited host resources, potentially affecting the survival and reproduction of the host as well as the infecting parasites. Fungus-growing ants, including the well-known leaf-cutters, are an emerging model system for studying the evolution and ecology of symbiosis and host-parasite dynamics. We examine whether the fungus gardens of leaf-cutter ants can be simultaneously infected by multiple strains of the fungal pathogen Escovopsis. Intensive sampling of Escovopsis was conducted from individual gardens, as well as between different garden chambers within individual colonies of leaf-cutting ants. Isolates obtained were genotyped by DNA sequencing. We found that, minimally, 67% of the individual colonies of the leaf-cutter ant genera Atta and Acromyrmex and 50% of the At. colombica garden chambers studied were simultaneously infected by multiple distinct Escovopsis strains. Experimental challenges showed that different Escovopsis strains do not exhibit obvious antagonism toward each other, suggesting that coinfecting strains of the parasite do not engage in interference competition, although interactions were not studied at the cellular level. Further research is needed to understand interparasite interactions between coinfecting Escovopsis strains and to understand the impact of multiparasite infections on the survival of leaf-cutter ant gardens.

Low genetic diversity in a snail intermediate host (Biomphalaria pfeifferi Krass, 1848) and schistosomiasis transmission in the Senegal River Basin

Population genetic perturbations of intermediate hosts, often a consequence of human pressure on environmental resources, can precipitate unexpectedly severe disease outbreaks. Such disturbances are set to become increasingly common following range changes concomitant with climate shifts, dwindling natural resources and major infrastructure changes such as hydroprojects. Construction of the Diama dam in the Senegal River Basin (SRB) reduced river salinity, enabling the freshwater snail intermediate host Biomphalaria pfeifferi to rapidly expand its distribution. A serious public health problem ensued, with an epidemic of intestinal schistosomiasis occurring in the previously schistosome-free Richard-Toll region within 2 years. The current study aimed to assess the population variability of B. pfeifferi in the SRB, and speculate upon its subsequent impact on host-parasite interactions following such engineered ecological change. Genetic variation at nine polymorphic microsatellite loci revealed little population differentiation in SRB snails compared with those from natural habitats in Zimbabwe, where Schistosoma mansoni transmission is much lower. 'Open' SRB habitats are associated with greater water contact, smaller population sizes and less genetic diversity, with sites downstream of Richard-Toll showing greater inter- and intrapopulation variation, concomitant with less frequent human contact. These observations may be explained by rapid expansion into pristine habitat selecting for high fecundity genotypes at the expense of schistosome resistance, presenting S. mansoni with genetically homogenous highly fecund susceptible populations around the focal point, promoting development of a highly compatible host-parasite relationship. Longitudinal study of such systems may prove important in predicting public health risks engendered by future environmental engineering projects.

Genetic structure of Schistosoma mansoni in western Kenya: The effects of geography and host sharing

We examined the spatial structure of Schistosoma mansoni, a parasite of humans, from natural infections at two levels: across the Lake Victoria basin of Kenya and among snail hosts. Using 20 microsatellite markers we examined geographic patterns of relatedness and population structure of cercariae and found weak, but significant structure detected by some, but not all analyses. We hypothesise structure created by aggregations of clonal individuals or adherence of hosts to local transmission sites is eroded by high amounts of gene flow in the region. This finding also supports previous hypotheses concerning the evolution of drug resistance in the region. Intrasnail dynamics were investigated in the context of aggregation and kin selection theory to determine how relatedness and also sex influence host sharing and host exploitation. Cercarial production did not differ significantly between snails with one or two genotypes suggesting that mixed infections resulted in decreased individual fitness and provides a framework for reproductive competition. Coinfection patterns in snails were independent of parasite relatedness indicating that schistosomes were not aggregated according to their relatedness and that kin selection was not influencing host sharing. Additionally, host exploitation in coinfections (measured by cercarial production) was not negatively correlated with relatedness, as predicted by classical models due to increased competition and thus exploitation when parasites are unrelated. Because of the low levels of relatedness within the population, schistosomes may rarely encounter close relatives and kin selection mechanisms that influence the distribution of individuals within snails or the virulence mode of the parasites may simply have not evolved.

Within-host competitive exclusion among species of the anther smut pathogen

BACKGROUND

Host individuals represent an arena in which pathogens compete for resources and transmission opportunities, with major implications for the evolution of virulence and the structure of populations. Studies to date have focused on competitive interactions within pathogen species, and the level of antagonism tends to increase with the genetic distance between competitors. Anther-smut fungi, in the genus Microbotryum, have emerged as a tractable model for within-host competition. Here, using two pathogen species that are frequently found in sympatry, we investigated whether the antagonism seen among genotypes of the same species cascades up to influence competition among pathogen species.

RESULTS

Sequential inoculation of hosts showed that a resident infection most often excludes a challenging pathogen genotype, which is consistent with prior studies. However, the challenging pathogen was significantly more likely to invade the already-infected host if the resident infection was a conspecific genotype compared to challenges involving a closely related species. Moreover, when inter-specific co-infection occurred, the pathogens were highly segregated within the host, in contrast to intra-specific co-infection.

CONCLUSION

We show evidence that competitive exclusion during infection can be greater among closely related pathogen species than among genotypes within species. This pattern follows from prior studies demonstrating that genetic distance and antagonistic interactions are positively correlated in Microbotryum. Fungal vegetative incompatibility is a likely mechanism of direct competitive interference, and has been shown in some fungi to be effective both within and across species boundaries. For systems where related pathogen species frequently co-occur in the same host populations, these competitive dynamics may substantially impact the spatial segregation of pathogen species.

Successful parasitism of vector snail Biomphalaria glabrata by the human blood fluke (trematode) Schistosoma mansoni: a 2009 assessment

Schistosomiasis, caused by infections by human blood flukes (Trematoda), continues to disrupt the lives of over 200,000,000 people in over 70 countries, inflicting misery and precluding the individuals' otherwise reasonable expectations of productive lives. Infection requires contact with freshwater in which infected snails (the intermediate hosts of schistosomes) have released cercariae larvae. Habitats suitable for the host snails continue to expand as a consequence of water resource development. No vaccine is available, and resistance has emerged towards the single licensed schistosomicide drug. Since human infections would cease if parasite infections in snails were prevented, efforts are being made to discover requirements of intra-molluscan development of these parasites. Wherever blood flukes occur, naturally resistant conspecific snails are present. To understand the mechanisms used by parasites to ensure their survival in immunocompetent hosts, one must comprehend the interior defense mechanisms that are available to the host. For one intermediate host snail (Biomphalaria glabrata) and trematodes for which it serves as vector, molecular genetic and proteomic surveys for genes and proteins influencing the outcomes on infections are yielding lists of candidates. A comparative approach drawing on data from studies in divergent species provides a robust basis for hypothesis generation to drive decisions as to which candidates merit detailed further investigation. For example, reactive oxygen and nitrogen species are known mediators or effectors in battles between infectious agents and their hosts. An approach targeting genes involved in relevant pathways has been fruitful in the Schistosoma mansoni -- B. glabrata parasitism, leading to discovery of a functionally relevant gene set (encoding enzymes responsible for the leukocyte respiratory burst) that associates significantly with host resistance phenotype. This review summarizes advances in the understanding of strategies used by both this trematode parasite and its molluscan host to ensure their survival.

The influence of clonal diversity and intensity-dependence on trematode infections in an amphipod

Individual animals are often infected not only by different parasite species, but also by multiple genotypes of the same parasite species. Genetic relatedness among parasites sharing a host is expected to modulate their strategies of resource exploitation, growth and virulence. We experimentally examined the effects that genetic diversity and infection intensity had on host mortality, infectivity and growth of the marine trematode Maritrema novaezealandensis in amphipod hosts. The presence of 2 versus 1 parasite genotype during infection did not influence subsequent host mortality, had different effects on infectivity among genotypes and did not influence growth or variation in parasite growth. Density-dependent growth reductions revealed that the number of parasites infecting a host was more important than their genetic relatedness. Temperature, host size, and host sex influenced the degree to which density-dependent factors affected parasite growth. Our results suggest that the effects of parasite relatedness vary among parasite genotypes in this trematode species, and reveal that many factors play an important role during parasite development and transmission.