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

Coinfection frequency in water flea populations is a mere reflection of parasite diversity

In nature, parasite species often coinfect the same host. Yet, it is not clear what drives the natural dynamics of coinfection prevalence. The prevalence of coinfections might be affected by interactions among coinfecting species, or simply derive from parasite diversity. Identifying the relative impact of these parameters is crucial for understanding patterns of coinfections. We studied the occurrence and likelihood of coinfections in natural populations of water fleas (Daphnia magna). Coinfection prevalence was within the bounds expected by chance and parasite diversity had a strong positive effect on the likelihood of coinfections. Additionally, coinfection prevalence increased over the season and became as common as a single infection. Our results demonstrate how patterns of coinfection, and particularly their temporal variation, are affected by overlapping epidemics of different parasites. We suggest that monitoring parasite diversity can help predict where and when coinfection prevalence will be high, potentially leading to increased health risks to their hosts.

Interactions between microsporidia and other members of the microbiome

The microbiome is the collection of microbes that are associated with a host. Microsporidia are intracellular eukaryotic parasites that can infect most types of animals. In the last decade, there has been much progress to define the relationship between microsporidia and the microbiome. In this review, we cover an increasing number of reports suggesting that microsporidia are common components of the microbiome in both invertebrates and vertebrates. These microsporidia infections can range from mutualistic to pathogenic, causing several physiological phenotypes, including death. Infection with microsporidia often causes a disruption in the normal microbiome, with both increases and decreases of bacterial, fungal, viral, and protozoan species being observed. This impact on the microbiome can occur through upregulation and downregulation of innate immunity as well as morphological changes to tissues that impact interactions with these microbes. Other microbes, particularly bacteria, can inhibit microsporidia and have been exploited to control microsporidia infections. These bacteria can function through regulating immunity, secreting anti-microsporidia compounds, and, in engineered versions, expressing double-stranded RNA targeting microsporidia genes. We end this review by discussing potential future directions to further understand the complex interactions between microsporidia and the other members of the microbiome.

Increased virulence due to multiple infection in Daphnia leads to limited growth in 1 of 2 co-infecting microsporidian parasites

Recent outbreaks of various infectious diseases have highlighted the ever-present need to understand the drivers of the outbreak and spread of disease. Although much of the research investigating diseases focuses on single infections, natural systems are dominated by multiple infections. These infections may occur simultaneously, but are often acquired sequentially, which may alter the outcome of infection. Using waterfleas (Daphnia magna) as a model organism, we examined the outcome of sequential and simultaneous multiple infections with 2 microsporidian parasites (Ordospora colligata and Hamiltosporidium tvaerminnensis) in a fully factorial design with 9 treatments and 30 replicates. We found no differences between simultaneous and sequential infections. However, H. tvaerminnensis fitness was impeded by multiple infection due to increased host mortality, which gave H. tvaerminnensis less time to grow. Host fecundity was also reduced across all treatments, but animals infected with O. colligata at a younger age produced the fewest offspring. As H. tvaerminnensis is both horizontally and vertically transmitted, this reduction in offspring may have further reduced H. tvaerminnensis fitness in co-infected treatments. Our findings suggest that in natural populations where both species co-occur, H. tvaerminnensis may evolve to higher levels of virulence following frequent co-infection by O. colligata.

Age Structure Eliminates the Impact of Coinfection on Epidemic Dynamics in a Freshwater Zooplankton System

AbstractParasites often coinfect host populations and, by interacting within hosts, might change the trajectory of multiparasite epidemics. However, host-parasite interactions often change with host age, raising the possibility that within-host interactions between parasites might also change, influencing the spread of disease. We measured how heterospecific parasites interacted within zooplankton hosts and how host age changed these interactions. We then parameterized an epidemiological model to explore how age effects altered the impact of coinfection on epidemic dynamics. In our model, we found that in populations where epidemiologically relevant parameters did not change with age, the presence of a second parasite altered epidemic dynamics. In contrast, when parameters varied with host age (based on our empirical measures), there was no longer a difference in epidemic dynamics between singly infected and coinfected populations, indicating that variable age structure within a population eliminates the impact of coinfection on epidemic dynamics. Moreover, infection prevalence of both parasites was lower in populations where epidemiologically relevant parameters changed with age. Given that host population age structure changes over time and space, these results indicate that age effects are important for understanding epidemiological processes in coinfected systems and that studies focused on a single age group could yield inaccurate insights.

Control of pest ants by pathogenic fungi: state of the art

Pest ants are known for their damage to biodiversity, harm to agriculture, and negative impact on human welfare. Ants thrive when environmental opportunities arise, becoming pests and/or invading non-native areas. As social insects, they are extremely difficult to control using sustainable methods like biological control. The latter, although safer to the environment, acts slowly allowing the ants to use their individual and social defenses. Among biocontrol agents, fungal pathogens were proposed as promising, however, it is difficult to ascertain their success when the bibliography has not been reviewed and condensed. Therefore, this paper is the first in performing such task by analyzing publications mainly from 2000 to 2022 about the control of pest ants by fungi. From 85 publications selected, 77% corresponded to laboratory studies. Beauveria and Metarhizium were the genera most used in laboratory and field studies. Most of them included Acromyrmex and Atta leaf-cutter ants (LCA), and Solenopsis fire ants. From laboratory experiments, we evaluated how ant net mortality was affected by ant and fungal species, and also by origin, concentration, and inoculation technique of the fungal strains tested. Beauveria bassiana and Metarhizium anisopliae produced the greatest mortality, along with the inoculation spray technique and fungal strains collected from ants. There was a positive relationship between ant mortality and fungal concentration only for those studies which evaluated more than one concentration. Twenty field experimental studies were found, covering 13 pest species, mainly LCA and Solenopsis invicta. Only B. bassiana was tested on Solenopsis, M. anisopliae was mostly used for Acromyrmex, and M. anisopliae or Trichoderma were mainly used with Atta species. The median control field efficiency varied from 20% to 85% for different fungi and ant genera. When grouping all fungal species together, the median control efficiency seemed to be better for Acromyrmex (67%) than for Atta and Solenopsis (both 43%). Our review shows that, at this stage of knowledge, it is very difficult to extrapolate any result. We offer suggestions to improve and standardize laboratory and field experimental studies in order to advance more efficiently in the fungal control of pest ants.

Coinfection with chytrid genotypes drives divergent infection dynamics reflecting regional distribution patterns

By altering the abundance, diversity, and distribution of species-and their pathogens-globalization may inadvertently select for more virulent pathogens. In Brazil's Atlantic Forest, a hotspot of amphibian biodiversity, the global amphibian trade has facilitated the co-occurrence of previously isolated enzootic and panzootic lineages of the pathogenic amphibian-chytrid (Batrachochytrium dendrobatidis, 'Bd') and generated new virulent recombinant genotypes ('hybrids'). Epidemiological data indicate that amphibian declines are most severe in hybrid zones, suggesting that coinfections are causing more severe infections or selecting for higher virulence. We investigated how coinfections involving these genotypes shapes virulence and transmission. Overall, coinfection favored the more virulent and competitively superior panzootic genotype, despite dampening its transmission potential and overall virulence. However, for the least virulent and least competitive genotype, coinfection increased both overall virulence and transmission. Thus, by integrating experimental and epidemiological data, our results provide mechanistic insight into how globalization can select for, and propel, the emergence of introduced hypervirulent lineages, such as the globally distributed panzootic lineage of Bd.

Consequences of microsporidian prior exposure for virus infection outcomes and bumble bee host health

Host-parasite interactions do not occur in a vacuum, but in connected multi-parasite networks that can result in co-exposures and coinfections of individual hosts. These can affect host health and disease ecology, including disease outbreaks. However, many host-parasite studies examine pairwise interactions, meaning we still lack a general understanding of the influence of co-exposures and coinfections. Using the bumble bee Bombus impatiens, we study the effects of larval exposure to a microsporidian Nosema bombi, implicated in bumble bee declines, and adult exposure to Israeli Acute Paralysis Virus (IAPV), an emerging infectious disease from honey bee parasite spillover. We hypothesize that infection outcomes will be modified by co-exposure or coinfection. Nosema bombi is a potentially severe, larval-infecting parasite, and we predict that prior exposure will result in decreased host resistance to adult IAPV infection. We predict double parasite exposure will also reduce host tolerance of infection, as measured by host survival. Although our larval Nosema exposure mostly did not result in viable infections, it partially reduced resistance to adult IAPV infection. Nosema exposure also negatively affected survival, potentially due to a cost of immunity in resisting the exposure. There was a significant negative effect of IAPV exposure on survivorship, but prior Nosema exposure did not alter this survival outcome, suggesting increased tolerance given the higher IAPV infections in the bees previously exposed to Nosema. These results again demonstrate that infection outcomes can be non-independent when multiple parasites are present, even when exposure to one parasite does not result in a substantial infection.

Polystyrene nanoplastics differentially influence the outcome of infection by two microparasites of the host Daphnia magna

The accumulation of micro- and nanoplastic particles in freshwater bodies has given rise to much concern regarding their potential adverse effects on aquatic biota. Beyond their known effects on single species, recent experimental evidence suggests that host-parasite interactions can also be affected by environmental concentrations of micro- and nanoplastics. However, investigating the effects of contaminants in simplified infection settings (i.e. one host, one parasite) may understate their ecological relevance, considering that co-infections are common in nature. We exposed the cladoceran Daphnia magna to a fungal parasite of the haemolymph (Metschnikowia bicuspidata) and a gut microsporidium (Ordospora colligata), either in single or co-infection. In addition, Daphnia were raised individually in culture media containing 0, 5 or 50 mg l^-1 of polystyrene nanoplastic beads (100 nm). Only few infections were successful at the higher nanoplastic concentration, due to increased mortality of the host. While no significant effect of the low concentration was detected on the microsporidium, the proportion of hosts infected by the fungal parasite increased dramatically, leading to more frequent co-infections under nanoplastic exposure. These results indicate that nanoplastics can affect the performance of distinct pathogens in diverging ways, with the potential to favour parasite coexistence in a common zooplanktonic host. This article is part of the theme issue 'Infectious disease ecology and evolution in a changing world'.

A road map for in vivo evolution experiments with blood-borne parasitic microbes

Laboratory experiments in which blood-borne parasitic microbes evolve in their animal hosts offer an opportunity to study parasite evolution and adaptation in real time and under natural settings. The main challenge of these experiments is to establish a protocol that is both practical over multiple passages and accurately reflects natural transmission scenarios and mechanisms. We provide a guide to the steps that should be considered when designing such a protocol, and we demonstrate its use via a case study. We highlight the importance of choosing suitable ancestral genotypes, treatments, number of replicates per treatment, types of negative controls, dependent variables, covariates, and the timing of checkpoints for the experimental design. We also recommend specific preliminary experiments to determine effective methods for parasite quantification, transmission, and preservation. Although these methodological considerations are technical, they also often have conceptual implications. To this end, we encourage other researchers to design and conduct in vivo evolution experiments with blood-borne parasitic microbes, despite the challenges that the work entails.

Daphnia as a versatile model system in ecology and evolution

Water fleas of the genus Daphnia have been a model system for hundreds of years and is among the best studied ecological model organisms to date. Daphnia are planktonic crustaceans with a cyclic parthenogenetic life-cycle. They have a nearly worldwide distribution, inhabiting standing fresh- and brackish water bodies, from small temporary pools to large lakes. Their predominantly asexual reproduction allows for the study of phenotypes excluding genetic variation, enabling us to separate genetic from non-genetic effects. Daphnia are often used in studies related to ecotoxicology, predator-induced defence, host–parasite interactions, phenotypic plasticity and, increasingly, in evolutionary genomics. The most commonly studied species are Daphnia magna and D. pulex, for which a rapidly increasing number of genetic and genomic tools are available. Here, I review current research topics, where the Daphnia model system plays a critical role.

Factors That Determine Microsporidia Infection and Host Specificity

Microsporidia are a large phylum of obligate intracellular parasites that infect an extremely diverse range of animals and protists. In this chapter, we review what is currently known about microsporidia host specificity and what factors influence microsporidia infection. Extensive sampling in nature from related hosts has provided insight into the host range of many microsporidia species. These field studies have been supported by experiments conducted in controlled laboratory environments which have helped to demonstrate host specificity. Together, these approaches have revealed that, while examples of generalist species exist, microsporidia specificity is often narrow, and species typically infect one or several closely related hosts. For microsporidia to successfully infect and complete their life cycle within a compatible host, several steps must occur, including spore germination, host cell invasion, and proliferation of the parasite within the host tissue. Many factors influence infection, including temperature, seasonality, nutrient availability, and the presence or absence of microbes, as well as the developmental stage, sex, and genetics of the host. Several studies have identified host genomic regions that influence resistance to microsporidia, and future work is likely to uncover molecular mechanisms of microsporidia host specificity in more detail.

Sequential infection of Daphnia magna by a gut microsporidium followed by a haemolymph yeast decreases transmission of both parasites

Over the course of seasonal epidemics, populations of susceptible hosts may encounter a wide variety of parasites. Parasite phenology affects the order in which these species encounter their hosts, leading to sequential infections, with potentially strong effects on within-host growth and host population dynamics. Here, the cladoceran Daphnia magna was exposed sequentially to a haemolymph-infecting yeast (Metschnikowia bicuspidata) and a gut microsporidium (Ordospora colligata), with experimental treatments reflecting two possible scenarios of parasite succession. The effects of single and co-exposure were compared on parasite infectivity, spore production and the overall virulence experienced by the host. We show that neither parasite benefited from coinfection; instead, when hosts encountered Ordospora, followed by Metschnikowia, higher levels of host mortality contributed to an overall decrease in the transmission of both parasites. These results showcase an example of sequential infections generating unilateral priority effects, in which antagonistic interactions between parasites can alleviate the intensity of infection and coincide with maladaptive levels of damage inflicted on the host.

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.

Sequential co-infections drive parasite competition and the outcome of infection

Co-infections by multiple parasites are common in natural populations. Some of these are likely to be the result of sequential rather than simultaneous infections. The timing of the co-infections may affect their competitive interactions, thereby influencing the success of the parasites and their impact on the host. This may have important consequence for epidemiological and eco-evolutionary dynamics. We examined in two ecological conditions the effect of sequential co-infection on the outcome of infection by two microsporidians, Vavraia culicis and Edhazardia aedis, that infect the mosquito Aedes aegypti. The two parasites have different transmission strategies: V. culicis is transmitted horizontally either among larvae or from adults to larvae, while E. aedis can be transmitted horizontally among larvae or vertically from females to their eggs. We investigated how the timing and order of the co-infection and how the host's food availability affected the parasite's transmission potential (the percentage of individuals that harboured transmissible spores) and the host's juvenile survival, its age at emergence and its longevity. The outcome of co-infection was strongly affected by the order at which the parasites arrived. In co-infections, V. culicis had greater horizontal transmission if it arrived early, whereas the transmission potential of E. aedis, either vertical or horizontal, was not affected by the competitor V. culicis. The availability of food determined the duration of infection leading to variation in mortality and in the transmission potential. For both parasites low food decreased juvenile survival, delayed emergence to adulthood and increased horizontal transmission potential. High food increased juvenile survival and the probability of emergence with higher vertical transmission for E. aedis. Overall, our results suggest that early infection favours transmission and that (a) V. culicis plastically responded to co-infection, (b) E. aedis was not affected by co-infection but it was more susceptible to factors extending or decreasing the time it spent in the host (time of infection and food). Our results emphasize the complexity of the impact of co-infection on host-parasite interactions. In particular, the timing and order of sequential co-infections can result in different within-host dynamics and modify infection outcomes.

Molecular Evidence that Lysiphlebia japonica Regulates the Development and Physiological Metabolism of Aphis gossypii

Lysiphlebia japonica Ashmead (Hymenoptera, Braconidae) is an endophagous parasitoid and Aphis gossypii Glover (Hemiptera, Aphididae) is a major pest in cotton. The relationship between insect host-parasitoids and their hosts involves complex physiological, biochemical and genetic interactions. This study examines changes in the development and physiological metabolism of A. gossypii regulated by L. japonica. Our results demonstrated that both the body length and width increased compared to non-parasitized aphids. We detected significantly increases in the developmental period as well as severe reproductive castration following parasitization by L. japonica. We then used proteomics to characterize these biological changes, and when combined with transcriptomes, this analysis demonstrated that the differential expression of mRNA (up or downregulation) captured a maximum of 48.7% of the variations of protein expression. We assigned these proteins to functional categories that included immunity, energy metabolism and transport, lipid metabolism, and reproduction. We then verified the contents of glycogen and 6-phosphate glucose, which demonstrated that these important energy sources were significantly altered following parasitization. These results uncover the effects on A. gossypii following parasitization by L. japonica, additional insight into the mechanisms behind insect-insect parasitism, and a better understanding of host-parasite interactions.

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.

Genetic resistance and specificity in sister taxa of Daphnia: insights from the range of host susceptibilities

BACKGROUND

Host genetic diversity can affect various aspects of host-parasite interactions, including individual-level effects on parasite infectivity, production of transmission stages and virulence, as well as population-level effects that reduce disease spread and prevalence, and buffer against widespread epidemics. However, a key aspect of this diversity, the genetic variation in host susceptibility, has often been neglected in interpreting empirical data and in theoretical studies. Daphnia similis naturally coexists with its competitor Daphnia magna and is more resistant to the endoparasitic microsporidium Hamiltosporidium tvaerminnensis, as suggested by a previous survey of waterbodies, which detected this parasite in D. magna, but not in D. similis. However, under laboratory conditions D. similis was sometimes found to be susceptible. We therefore asked if there is genetic variation for disease trait expression, and if the genetic variation in disease traits in D. similis is different from that of D. magna.

METHODS

We exposed ten clones of D. similis and ten clones of D. magna to three isolates of H. tvaerminnensis, and measured infection rates, parasite-induced host mortality and parasite spore production.

RESULTS

The two Daphnia species differ in the range and variation of their susceptibilities. The parasite produced on average two-fold more spores when growing in D. magna clones than in D. similis clones.

CONCLUSIONS

We confirm that D. similis is indeed much more resistant than D. magna and suggest that this could create a dilution effect in habitats where both species coexist.

Genomics of host-pathogen interactions: challenges and opportunities across ecological and spatiotemporal scales

Evolutionary genomics has recently entered a new era in the study of host-pathogen interactions. A variety of novel genomic techniques has transformed the identification, detection and classification of both hosts and pathogens, allowing a greater resolution that helps decipher their underlying dynamics and provides novel insights into their environmental context. Nevertheless, many challenges to a general understanding of host-pathogen interactions remain, in particular in the synthesis and integration of concepts and findings across a variety of systems and different spatiotemporal and ecological scales. In this perspective we aim to highlight some of the commonalities and complexities across diverse studies of host-pathogen interactions, with a focus on ecological, spatiotemporal variation, and the choice of genomic methods used. We performed a quantitative review of recent literature to investigate links, patterns and potential tradeoffs between the complexity of genomic, ecological and spatiotemporal scales undertaken in individual host-pathogen studies. We found that the majority of studies used whole genome resolution to address their research objectives across a broad range of ecological scales, especially when focusing on the pathogen side of the interaction. Nevertheless, genomic studies conducted in a complex spatiotemporal context are currently rare in the literature. Because processes of host-pathogen interactions can be understood at multiple scales, from molecular-, cellular-, and physiological-scales to the levels of populations and ecosystems, we conclude that a major obstacle for synthesis across diverse host-pathogen systems is that data are collected on widely diverging scales with different degrees of resolution. This disparity not only hampers effective infrastructural organization of the data but also data granularity and accessibility. Comprehensive metadata deposited in association with genomic data in easily accessible databases will allow greater inference across systems in the future, especially when combined with open data standards and practices. The standardization and comparability of such data will facilitate early detection of emerging infectious diseases as well as studies of the impact of anthropogenic stressors, such as climate change, on disease dynamics in humans and wildlife.

The dynamics between limited-term and lifelong coinfecting bacterial parasites in wild rodent hosts

Interactions between coinfecting parasites may take various forms, either direct or indirect, facilitative or competitive, and may be mediated by either bottom-up or top-down mechanisms. Although each form of interaction leads to different evolutionary and ecological outcomes, it is challenging to tease them apart throughout the infection period. To establish the first step towards a mechanistic understanding of the interactions between coinfecting limited-term bacterial parasites and lifelong bacterial parasites, we studied the coinfection of Bartonella sp. (limited-term) and Mycoplasma sp. (lifelong), which commonly co-occur in wild rodents. We infected Bartonella- and Mycoplasma-free rodents with each species, and simultaneously with both, and quantified the infection dynamics and host responses. Bartonella benefited from the interaction; its infection load decreased more slowly in coinfected rodents than in rodents infected with Bartonella alone. There were no indications for bottom-up effects, but coinfected rodents experienced various changes, depending on the infection stage, in their body mass, stress levels and activity pattern, which may further affect bacterial replication and transmission. Interestingly, the infection dynamics and changes in the average coinfected rodent traits were more similar to the chronic effects of Mycoplasma infection, whereas coinfection uniquely impaired the host's physiological and behavioral stability. These results suggest that parasites with distinct life history strategies may interact, and their interaction may be asymmetric, non-additive, multifaceted and dynamic through time. Because multiple, sometimes contrasting, forms of interactions are simultaneously at play and their relative importance alternates throughout the course of infection, the overall outcome may change under different ecological conditions.

Importance of Sequence and Timing in Parasite Coinfections

Coinfections by multiple parasites predominate in the wild. Interactions between parasites can be antagonistic, neutral, or facilitative, and they can have significant implications for epidemiology, disease dynamics, and evolution of virulence. Coinfections commonly result from sequential exposure of hosts to different parasites. We argue that the sequential nature of coinfections is important for the consequences of infection in both natural and man-made environments. Coinfections accumulate during host lifespan, determining the structure of the parasite infracommunity. Interactions within the parasite community and their joint effect on the host individual potentially shape evolution of parasite life-history traits and transmission biology. Overall, sequential coinfections have the potential to change evolutionary and epidemiological outcomes of host-parasite interactions widely across plant and animal systems.

Ants avoid superinfections by performing risk-adjusted sanitary care

Being cared for when sick is a benefit of sociality that can reduce disease and improve survival of group members. However, individuals providing care risk contracting infectious diseases themselves. If they contract a low pathogen dose, they may develop low-level infections that do not cause disease but still affect host immunity by either decreasing or increasing the host's vulnerability to subsequent infections. Caring for contagious individuals can thus significantly alter the future disease susceptibility of caregivers. Using ants and their fungal pathogens as a model system, we tested if the altered disease susceptibility of experienced caregivers, in turn, affects their expression of sanitary care behavior. We found that low-level infections contracted during sanitary care had protective or neutral effects on secondary exposure to the same (homologous) pathogen but consistently caused high mortality on superinfection with a different (heterologous) pathogen. In response to this risk, the ants selectively adjusted the expression of their sanitary care. Specifically, the ants performed less grooming and more antimicrobial disinfection when caring for nestmates contaminated with heterologous pathogens compared with homologous ones. By modulating the components of sanitary care in this way the ants acquired less infectious particles of the heterologous pathogens, resulting in reduced superinfection. The performance of risk-adjusted sanitary care reveals the remarkable capacity of ants to react to changes in their disease susceptibility, according to their own infection history and to flexibly adjust collective care to individual risk.

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.

Can Wolbachia modulate the fecundity costs of Plasmodium in mosquitoes?

Vertically transmitted parasites (VTPs) such as Wolbachia are expected not only to minimize the damage they inflict on their hosts, but also to protect their hosts against the damaging effects of coinfecting parasites. By modifying the fitness costs of the infection, VTPs can therefore play an important role in the evolution and epidemiology of infectious diseases.Using a natural system, we explore the effects of a Wolbachia-Plasmodium co-infection on mosquito fecundity. While Plasmodium is known to frequently express its virulence by partially castrating its mosquito vectors, the effects of Wolbachia infections on mosquito fecundity are, in contrast, highly variable. Here, we show that Plasmodium drastically decreases the fecundity of mosquitoes by ca. 20%, and we provide the first evidence that this decrease is independent of the parasite's burden. Wolbachia, on the other hand, increases fecundity by roughly 10%, but does not alter the tolerance (fecundity-burden relationship) of mosquitoes to Plasmodium infection.Although Wolbachia-infected mosquitoes fare overall better than uninfected ones, Wolbachia does not confer a sufficiently high reproductive boost to mosquitoes to compensate for the reproductive losses inflicted by Plasmodium. We discuss the potential mechanisms and implications underlying the conflicting effects of these two parasites on mosquito reproduction.

The Tick Microbiome: Why Non-pathogenic Microorganisms Matter in Tick Biology and Pathogen Transmission

Ticks are among the most important vectors of pathogens affecting humans and other animals worldwide. They do not only carry pathogens however, as a diverse group of commensal and symbiotic microorganisms are also present in ticks. Unlike pathogens, their biology and their effect on ticks remain largely unexplored, and are in fact often neglected. Nonetheless, they can confer multiple detrimental, neutral, or beneficial effects to their tick hosts, and can play various roles in fitness, nutritional adaptation, development, reproduction, defense against environmental stress, and immunity. Non-pathogenic microorganisms may also play a role in driving transmission of tick-borne pathogens (TBP), with many potential implications for both human and animal health. In addition, the genetic proximity of some pathogens to mutualistic symbionts hosted by ticks is evident when studying phylogenies of several bacterial genera. The best examples are found within members of the Rickettsia, Francisella, and Coxiella genera: while in medical and veterinary research these bacteria are traditionally recognized as highly virulent vertebrate pathogens, it is now clear to evolutionary ecologists that many (if not most) Coxiella, Francisella, and Rickettsia bacteria are actually non-pathogenic microorganisms exhibiting alternative lifestyles as mutualistic ticks symbionts. Consequently, ticks represent a compelling yet challenging system in which to study microbiomes and microbial interactions, and to investigate the composition, functional, and ecological implications of bacterial communities. Ultimately, deciphering the relationships between tick microorganisms as well as tick symbiont interactions will garner invaluable information, which may aid in the future development of arthropod pest and vector-borne pathogen transmission control strategies.

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.

A Population Biology Perspective on the Stepwise Infection Process of the Bacterial Pathogen Pasteuria ramosa in Daphnia

The infection process of many diseases can be divided into series of steps, each one required to successfully complete the parasite's life and transmission cycle. This approach often reveals that the complex phenomenon of infection is composed of a series of more simple mechanisms. Here we demonstrate that a population biology approach, which takes into consideration the natural genetic and environmental variation at each step, can greatly aid our understanding of the evolutionary processes shaping disease traits. We focus in this review on the biology of the bacterial parasite Pasteuria ramosa and its aquatic crustacean host Daphnia, a model system for the evolutionary ecology of infectious disease. Our analysis reveals tremendous differences in the degree to which the environment, host genetics, parasite genetics and their interactions contribute to the expression of disease traits at each of seven different steps. This allows us to predict which steps may respond most readily to selection and which steps are evolutionarily constrained by an absence of variation. We show that the ability of Pasteuria to attach to the host's cuticle (attachment step) stands out as being strongly influenced by the interaction of host and parasite genotypes, but not by environmental factors, making it the prime candidate for coevolutionary interactions. Furthermore, the stepwise approach helps us understanding the evolution of resistance, virulence and host ranges. The population biological approach introduced here is a versatile tool that can be easily transferred to other systems of infectious disease.

Interspecific competition in honeybee intracellular gut parasites is asymmetric and favours the spread of an emerging infectious disease

There is increasing appreciation that hosts in natural populations are subject to infection by multiple parasite species. Yet the epidemiological and ecological processes determining the outcome of mixed infections are poorly understood. Here, we use two intracellular gut parasites (Microsporidia), one exotic and one co-evolved in the western honeybee (Apis mellifera), in an experiment in which either one or both parasites were administered either simultaneously or sequentially. We provide clear evidence of within-host competition; order of infection was an important determinant of the competitive outcome between parasites, with the first parasite significantly inhibiting the growth of the second, regardless of species. However, the strength of this 'priority effect' was highly asymmetric, with the exotic Nosema ceranae exhibiting stronger inhibition of Nosema apis than vice versa. Our results reveal an unusual asymmetry in parasite competition that is dependent on order of infection. When incorporated into a mathematical model of disease prevalence, we find asymmetric competition to be an important predictor of the patterns of parasite prevalence found in nature. Our findings demonstrate the wider significance of complex multi-host-multi-parasite interactions as drivers of host-pathogen community structure.

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.

Genetic architecture of resistance in Daphnia hosts against two species of host-specific parasites

Understanding the genetic architecture of host resistance is key for understanding the evolution of host-parasite interactions. Evolutionary models often assume simple genetics based on few loci and strong epistasis. It is unknown, however, whether these assumptions apply to natural populations. Using a quantitative trait loci (QTL) approach, we explore the genetic architecture of resistance in the crustacean Daphnia magna to two of its natural parasites: the horizontally transmitted bacterium Pasteuria ramosa and the horizontally and vertically transmitted microsporidium Hamiltosporidium tvaerminnensis. These two systems have become models for studies on the evolution of host-parasite interactions. In the QTL panel used here, Daphnia's resistance to P. ramosa is controlled by a single major QTL (which explains 50% of the observed variation). Resistance to H. tvaerminnensis horizontal infections shows a signature of a quantitative trait based in multiple loci with weak epistatic interactions (together explaining 38% variation). Resistance to H. tvaerminnensis vertical infections, however, shows only one QTL (explaining 13.5% variance) that colocalizes with one of the QTLs for horizontal infections. QTLs for resistance to Pasteuria and Hamiltosporidium do not colocalize. We conclude that the genetics of resistance in D. magna are drastically different for these two parasites. Furthermore, we infer that based on these and earlier results, the mechanisms of coevolution differ strongly for the two host-parasite systems. Only the Pasteuria-Daphnia system is expected to follow the negative frequency-dependent selection (Red Queen) model. How coevolution works in the Hamiltosporidium-Daphnia system remains unclear.

Cooperation and conflict in host manipulation: interactions among macro-parasites and micro-organisms

Several parasite species are known to manipulate the phenotype of their hosts in ways that enhance their own transmission. Co-occurrence of manipulative parasites, belonging to the same species or to more than one species, in a single host has been regularly observed. Little is known, however, on interactions between co-occurring manipulative parasites with same or different transmission routes. Several models addressing this problem have provided predictions on how cooperation and conflict between parasites could emerge from multiple infections. Here, we review the empirical evidence in favor of the existence of synergistic or antagonistic interactions between co-occurring parasites, and highlight the neglected role of micro-organisms. We particularly discuss the actual importance of selective forces shaping the evolution of interactions between manipulative parasites in relation to parasite prevalence in natural populations, efficiency in manipulation, and type of transmission (i.e., horizontal versus vertical), and we emphasize the potential for future research.

Parasites and genetic diversity in an invasive bumblebee

Biological invasions are facilitated by the global transportation of species and climate change. Given that invasions may cause ecological and economic damage and pose a major threat to biodiversity, understanding the mechanisms behind invasion success is essential. Both the release of non-native populations from natural enemies, such as parasites, and the genetic diversity of these populations may play key roles in their invasion success. We investigated the roles of parasite communities, through enemy release and parasite acquisition, and genetic diversity in the invasion success of the non-native bumblebee, Bombus hypnorum, in the United Kingdom. The invasive B. hypnorum had higher parasite prevalence than most, or all native congeners for two high-impact parasites, probably due to higher susceptibility and parasite acquisition. Consequently parasites had a higher impact on B. hypnorum queens' survival and colony-founding success than on native species. Bombus hypnorum also had lower functional genetic diversity at the sex-determining locus than native species. Higher parasite prevalence and lower genetic diversity have not prevented the rapid invasion of the United Kingdom by B. hypnorum. These data may inform our understanding of similar invasions by commercial bumblebees around the world. This study suggests that concerns about parasite impacts on the small founding populations common to re-introduction and translocation programs may be less important than currently believed.

The expression of virulence for a mixed-mode transmitted parasite in a diapausing host

Many parasites survive harsh periods together with their hosts. Without the possibility of horizontal transmission during host diapause, parasite persistence depends entirely on host survival. We therefore hypothesize that a parasite should be avirulent during its host's diapausing stage. In contrast, the parasite may express higher virulence, i.e. parasite-induced fitness reduction of the host, during host life stages with good opportunities for horizontal transmission. Here we study the effects of a vertically and horizontally transmitted microsporidium parasite, Hamiltosporidium tvaerminnensis, on the quantity and survival of resting eggs of its host Daphnia magna. We find that the parasite did not affect egg volume, hatching success and time to hatching of the Daphnia's resting eggs, although it did strongly reduce the number of resting eggs produced by infected females, revealing high virulence during the non-diapause phase of the host's life cycle. These results also explain another aspect of this system - namely the strong decline in natural population prevalence across diapause. This decline is not caused by mortality in infected resting stages, as was previously hypothesized, but because infected female hosts produce lower rates of resting eggs. Together, these results help explain the epidemiological dynamics of a microsporidian disease and highlight the adaptive nature of life stage-dependent parasite virulence.

Bioaccumulation of human waterborne protozoa by zebra mussel (Dreissena polymorpha): interest for water biomonitoring

Cryptosporidium parvum, Giardia duodenalis and Toxoplasma gondii are ubiquitous pathogens, which waterborne transmission has been largely demonstrated. Since they can be found in various watercourses, interactions with aquatic organisms are possible. Protozoan detection for watercourses biomonitoring is currently based on large water filtration. The zebra mussel, Dreissena polymorpha, is a choice biological model in ecotoxicological studies which are already in use to detect chemical contaminations in watercourses. In the present study, the zebra mussel was tested as a new tool for detecting water contamination by protozoa. In vivo exposures were conducted in laboratory experiments. Zebra mussel was exposed to various protozoan concentrations for one week. Detection of protozoa was realized by Taqman real time qPCR. Our experiments evidenced C. parvum, G. duodenalis and T. gondii oocyst bioaccumulation by mussels proportionally to ambient contamination, and significant T. gondii prevalence was observed in muscle tissue. To our knowledge, this is the first study that demonstrates T. gondii oocyst accumulation by zebra mussel. The results from this study highlight the capacity of zebra mussels to reveal ambient biological contamination, and thus to be used as a new effective tool in sanitary biomonitoring of water bodies.

Pathogen regulation of plant diversity via effective specialization

The Janzen-Connell (JC) hypothesis, one of the most influential hypotheses explaining forest diversity, is inconsistent with evidence that tree species share the same natural enemies. Through the discussion of seedling diseases from a pathogen-centered perspective, we expand the JC hypothesis to tie in host-pathogen-environment interactions at three levels: local adaptation, host specificity of the combined effect of multiple infections, and environmental modulation of disease. We present evidence from plant pathology, disease ecology, and host-parasite evolution relevant to (but not commonly associated with) forest species diversity maintenance. This expanded view of the JC hypothesis suggests ways to direct new experiments to integrate research on pathogen local adaptation, co-infection, and environmental effects on infection by using high-throughput molecular techniques and statistical models.

Host adaptation is contingent upon the infection route taken by pathogens

Evolution of pathogen virulence is affected by the route of infection. Also, alternate infection routes trigger different physiological responses on hosts, impinging on host adaptation and on its interaction with pathogens. Yet, how route of infection may shape adaptation to pathogens has not received much attention at the experimental level. We addressed this question through the experimental evolution of an outbred Drosophila melanogaster population infected by two different routes (oral and systemic) with Pseudomonas entomophila. The two selection regimes led to markedly different evolutionary trajectories. Adaptation to infection through one route did not protect from infection through the alternate route, indicating distinct genetic bases. Finally, relatively to the control population, evolved flies were not more resistant to bacteria other than Pseudomonas and showed higher susceptibility to viral infections. These specificities and trade-offs may contribute to the maintenance of genetic variation for resistance in natural populations. Our data shows that the infection route affects host adaptation and thus, must be considered in studies of host-pathogen interaction.

The expression and evolution of virulence in multiple infections: the role of specificity, relative virulence and relative dose

Background

Multiple infections of the same host by different strains of the same microparasite species are believed to play a crucial role during the evolution of parasite virulence. We investigated the role of specificity, relative virulence and relative dose in determining the competitive outcome of multiple infections in the Daphnia magna-Pasteuria ramosa host-parasite system.

Results

We found that infections by P. ramosa clones (single genotype) were less virulent and produced more spores than infections by P. ramosa isolates (possibly containing multiple genotypes). We also found that two similarly virulent isolates of P. ramosa differed considerably in their within-host competitiveness and their effects on host offspring production when faced with coinfecting P. ramosa isolates and clones. Although the relative virulence of a P. ramosa isolate/clone appears to be a good indicator of its competitiveness during multiple infections, the relative dose may alter the competitive outcome. Moreover, spore counts on day 20 post-infection indicate that the competitive outcome is largely decided early in the parasite’s growth phase, possibly mediated by direct interference or apparent competition.

Conclusions

Our results emphasize the importance of epidemiology as well as of various parasite traits in determining the outcome of within-host competition. Incorporating realistic epidemiological and ecological conditions when testing theoretical models of multiple infections, as well as using a wider range of host and parasite genotypes, will enable us to better understand the course of virulence evolution.

Protozoa interaction with aquatic invertebrate: interest for watercourses biomonitoring

Toxoplasma gondii, Cryptosporidium parvum, and Giardia duodenalis are human waterborne protozoa. These worldwide parasites had been detected in various watercourses as recreational, surface, drinking, river, and seawater. As of today, water protozoa detection was based on large water filtration and on sample concentration. Another tool like aquatic invertebrate parasitism could be used for sanitary and environmental biomonitoring. In fact, organisms like filter feeders could already filtrate and concentrate protozoa directly in their tissues in proportion to ambient concentration. So molluscan shellfish can be used as a bioindicator of protozoa contamination level in a site since they were sedentary. Nevertheless, only a few researches had focused on nonspecific parasitism like protozoa infection on aquatic invertebrates. Objectives of this review are twofold: Firstly, an overview of protozoa in worldwide water was presented. Secondly, current knowledge of protozoa parasitism on aquatic invertebrates was detailed and the lack of data of their biological impact was pointed out.

Effect of multiple parasitic infections on the tolerance to pollutant contamination

The horizontally-transmitted acanthocephalan parasite Polymorphus minutus and the vertically-transmitted microsporidian parasite Dictyocoela roeselum have both been shown to influence on the antitoxic responses of mono-infected Gammarus roeseli exposed to cadmium. The present study investigates the effect of this co-infection on the antitoxic defence responses of naturally infected females exposed to cadmium stress. Our results revealed that, depending on the cadmium dose, bi-infection induced only slight, significant increased cell damage in G. roeseli as compared to non-infection. In addition, the antitoxic defence pattern of cadmium-exposed bi-infected hosts was similar to the pattern of cadmium-exposed D. roeselum-infected hosts. Reduced glutathione concentrations, carotenoid levels and γ-glutamylcystein ligase activity decreased, while metallothionein concentrations increased. This similar pattern indicates that host physiology can be controlled to some extent by microsporidia under stress conditions. It supports the hypothesis of a disruption of acanthocephalan effects in the presence of microsporidia. However, the global negative effects of bi-infection on host condition should be tested on more biological models, since competition between parasites depends on life history trade-off.

Virulence of mixed fungal infections in honey bee brood

Introduction

Honey bees, Apis mellifera, have a diverse community of pathogens. Previous research has mostly focused on bacterial brood diseases of high virulence, but milder diseases caused by fungal pathogens have recently attracted more attention. This interest has been triggered by partial evidence that co-infection with multiple pathogens has the potential to accelerate honey bee mortality. In the present study we tested whether co-infection with closely related fungal brood-pathogen species that are either specialists or non-specialist results in higher host mortality than infections with a single specialist. We used a specially designed laboratory assay to expose honey bee larvae to controlled infections with spores of three Ascosphaera species: A. apis, the specialist pathogen that causes chalkbrood disease in honey bees, A. proliperda, a specialist pathogen that causes chalkbrood disease in solitary bees, and A. atra, a saprophytic fungus growing typically on pollen brood-provision masses of solitary bees.

Results

We show for the first time that single infection with a pollen fungus A. atra may induce some mortality and that co-infection with A. atra and A. apis resulted in higher mortality of honey bees compared to single infections with A. apis. However, similar single and mixed infections with A. proliperda did not increase brood mortality.

Conclusion

Our results show that co-infection with a closely related fungal species can either increase or have no effect on host mortality, depending on the identity of the second species. Together with other studies suggesting that multiple interacting pathogens may be contributing to worldwide honey bee health declines, our results highlight the importance of studying effects of multiple infections, even when all interacting species are not known to be specialist pathogens.

Natural distribution and co-infection patterns of microsporidia parasites in the Daphnia longispina complex

Microsporidia are intracellular parasites, frequently infecting the planktonic crustacean Daphnia. Questioning the ability to detect and identify microsporidia with conventional microscopic techniques, we applied molecular methods in order to investigate the distribution and co-infection patterns of this parasite among 8 communities of the Daphnia longispina hybrid complex. Eight microsporidian taxa were detected, including 3 that previously had not been characterized genetically. Microsporidian communities from nearby lakes were found to be more similar to each other, apparently due to short distance dispersal via secondary hosts. Moreover, we detected seasonal (but not interannual) changes in microsporidian community structure. With some microsporidia being host-specific, these changes might have resulted from seasonal changes in host taxon and clonal composition. The 2 dominant and closely related parasite species were found mainly in single infections, whereas another pair of related microsporidians was found predominantly in co-infections; suggesting species-level differences in the ability to colonize infected hosts. By applying molecular methods, we were not only able to unambiguously identify parasite taxa but also to reveal multiple infections that otherwise would have remained undetected. Given the increased level of accuracy and sensitivity, we highly recommend molecular approaches in future parasite surveys of Daphnia infections.