Microparasite transmission to <Emphasis Type="Italic">Daphnia magna</Emphasis> decreases in the presence of conspecifics

Single parasite species often have a range of different hosts which vary in their ability to sustain the parasite. When foraging for food, alternative hosts with similar feeding modes may compete for the infective stages of trophically transmitted parasites. If some of the infective stages end up in unsuitable hosts, transmission of the parasite to the focal host is decreased. I studied whether the presence of conspecifics alters the probability of an uninfected susceptible recipient Daphnia becoming infected by a microparasite and if this effect depends on whether the added conspecifics themselves are susceptible or resistant to infection. The presence of both susceptible and resistant conspecifics decreased the probability of infection in recipients. This effect was dependent on the density of the conspecifics but was not found to be related to their size. In addition, when Daphnia were placed in medium derived from crowded Daphnia populations, the probability of infection in recipients decreased as compared to that in standard medium. This implies that decreases in transmission probability are not caused by dilution of spores through food competition only, but also by indirect interference mediated through infochemicals released by Daphnia. Since Daphnia have been found to respond to crowding by decreasing their filtering rate, the decrease in transmission is probably caused by decreased intake of spores in crowded conditions. The presence of conspecifics can thus decrease microparasite transmission in Daphnia which may have important consequences for epidemiology and evolution of Daphnia parasites.     

Food stoichiometry affects the outcome of Daphnia–parasite interaction

Phosphorus (P) is an essential nutrient for growth in consumers. P‐limitation and parasite infection comprise one of the most common stressor pairs consumers confront in nature. We conducted a life‐table study using a Daphnia–microsporidian parasite model, feeding uninfected or infected Daphnia with either P‐sufficient or P‐limited algae, and assessed the impact of the two stressors on life‐history traits of the host. Both infection and P‐limitation negatively affected some life‐history traits tested. However, under P‐limitation, infected animals had higher juvenile growth rate as compared with uninfected animals. All P‐limited individuals died before maturation, regardless of infection. The numbers of spore clusters of the microsporidian parasite did not differ in P‐limited or P‐sufficient hosts. P‐limitation, but not infection, decreased body phosphorus content and ingestion rates of Daphnia tested in separate experiments. As parasite spore production did not suffer even under extreme P‐limitation, our results suggest that parasite was less limited by P than the host. We discuss possible interpretations concerning the stoichiometrical demands of parasite and suggest that our results are explained by parasite‐driven changes in carbon (C) allocation of the hosts. We conclude that the impact of nutrient starvation and parasite infection on consumers depends not only on the stoichiometric demands of host but also those of the parasite.     

Host Density and Competency Determine the Effects of Host Diversity on Trematode Parasite Infection

Variation in host species composition can dramatically alter parasite transmission in natural communities. Whether diverse host communities dilute or amplify parasite transmission is thought to depend critically on species traits, particularly on how hosts affect each other’s densities, and their relative competency as hosts. Here we studied a community of potential hosts and/or decoys (i.e. non-competent hosts) for two trematode parasite species, Echinostoma trivolvis and Ribeiroia ondatrae, which commonly infect wildlife across North America. We manipulated the density of a focal host (green frog tadpoles, Rana clamitans), in concert with manipulating the diversity of alternative species, to simulate communities where alternative species either (1) replace the focal host species so that the total number of individuals remains constant (substitution) or (2) add to total host density (addition). For E. trivolvis, we found that total parasite transmission remained roughly equal (or perhaps decreased slightly) when alternative species replaced focal host individuals, but parasite transmission was higher when alternative species were added to a community without replacing focal host individuals. Given the alternative species were roughly equal in competency, these results are consistent with current theory. Remarkably, both total tadpole and per-capita tadpole infection intensity by E. trivolvis increased with increasing intraspecific host density. For R. ondatrae, alternative species did not function as effective decoys or hosts for parasite infective stages, and the diversity and density treatments did not produce clear changes in parasite transmission, although high tank to tank variation in R. ondatrae infection could have obscured patterns.     

The Relationship between Parasite Fitness and Host Condition in an Insect - Virus System

Research in host-parasite evolutionary ecology has demonstrated that environmental variation plays a large role in mediating the outcome of parasite infection. For example, crowding or low food availability can reduce host condition and make them more vulnerable to parasite infection. This observation that poor-condition hosts often suffer more from parasite infection compared to healthy hosts has led to the assumption that parasite productivity is higher in poor-condition hosts. However, the ubiquity of this negative relationship between host condition and parasite fitness is unknown. Moreover, examining the effect of environmental variation on parasite fitness has been largely overlooked in the host-parasite literature. Here we investigate the relationship between parasite fitness and host condition by using a laboratory experiment with the cabbage looper Trichoplusia ni and its viral pathogen, AcMNPV, and by surveying published host-parasite literature. Our experiments demonstrated that virus productivity was positively correlated with host food availability and the literature survey revealed both positive and negative relationships between host condition and parasite fitness. Together these data demonstrate that contrary to previous assumptions, parasite fitness can be positively or negatively correlated with host fitness. We discuss the significance of these findings for host-parasite population biology.     

Parasite infection alters host stable‐isotope composition under controlled feeding

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Host resistance and tolerance of parasitic gut worms depend on resource availability

Resource availability can significantly alter host–parasite dynamics. Abundant food can provide more resources for hosts to resist infections, but also increase host tolerance of infections by reducing competition between hosts and parasites for food. Whether abundant food favors host resistance or tolerance (or both) might depend on the type of resource that the parasite exploits (e.g., host tissue vs. food), which can vary based on the stage of infection. In our study, we evaluated how low and high resource diets affect Cuban tree frog (Osteopilus septentrionalis) resistance and tolerance of a skin-penetrating, gut nematode Aplectana sp. at each stage of the infection. Compared to a low resource diet, a high resource diet enhanced frog resistance to worm penetration and tolerance while worms traveled to the gut. In contrast, a low resource diet increased resistance to establishment of the infection. After the infection established and worms could access food resources in the gut, a high resource diet enhanced host tolerance of parasites. On a high resource diet, parasitized frogs consumed significantly more food than non-parasitized frogs; when food was then restricted, mass of non-parasitized frogs did not change, whereas mass of parasitized frogs decreased significantly. Thus, a high resource diet increased frog tolerance of established worms because frogs could fully compensate for energy lost to the parasites. Our study shows that host–parasite dynamics are influenced by the effect of resource availability on host resistance and tolerance, which depends on when parasites have access to food and the stage of infection.     

Higher parasite resistance in Daphnia populations with recent epidemics

Natural populations often show genetic variation in parasite resistance, forming the basis for evolutionary response to selection imposed by parasitism. We investigated whether previous epidemics selected for higher resistance to novel parasite isolates in a Daphnia galeata–microparasite system by comparing susceptibility of host clones from populations with varying epidemic history. We manipulated resource availability to evaluate whether diet influences Daphnia susceptibility as epidemics are common in nutrient‐rich lakes. Exposing clones from 10 lakes under two food treatments to an allopatric protozoan parasite, we found that Daphnia originating from lakes (mainly nutrient rich) with previous epidemics better resist infection. Despite this result, there was a tendency of higher susceptibility in the low food treatment, suggesting that higher resistance of clones from populations with epidemic background is not directly caused by lake nutrient level. Rather, our results imply that host populations respond to parasite‐mediated selection by evolving higher parasite resistance.     

Responses of algae, bacteria, Daphnia and natural parasite fauna of Daphnia to nutrient enrichment in mesocosms

Understanding responses of parasites to changes in nutrient regimes is necessary for prediction of their role in aquatic ecosystems under global change in nutrient loading. We studied the response of the natural parasite fauna of Daphnia longispina to nutrient enrichment in mesocosms in a small humic lake. We measured the concentrations of inorganic phosphorus and nitrogen in the water, total nutrients in the seston, algal and bacterial biomass, Daphnia population dynamics, Daphnia stoichiometry, Daphnia stable isotope values and the presence and abundance of parasites in treated mesocosms as compared to three control ones. Incorporation of the nutrient enrichment in the food web was seen as increased nutrient concentrations in the epilimnion and as a decrease in carbon:nutrient ratios and δ¹⁵N values in Daphnia. Nutrient enrichment did not significantly influence algal, bacterial or Daphnia biomass. One of the four parasite species observed, unidentified small gut parasite, had a higher prevalence (percentage of Daphnia infected) in treated mesocosms, but its intensity (number of parasites per infected host) remained the same among treatments. Our results suggest that the effect of nutrient enrichment on host–parasite dynamics is dependent on complex interactions within food webs and on the epidemiological traits of parasites.     

Temperature and host diet jointly influence the outcome of infection in a Daphnia-fungal parasite system

1. Climate change has the potential to shape the future of infectious diseases, both directly and indirectly. In aquatic systems, for example, elevated temperatures can modulate the infectivity of waterborne parasites and affect the immune response of zooplanktonic hosts. Moreover, lake warming causes shifts in the communities of primary producers towards cyanobacterial dominance, thus lowering the quality of zooplankton diet. This may further affect host fitness, resulting in suboptimal resources available for parasite growth.
2. Previous experimental studies have demonstrated the respective effects of temperature and host diet on infection outcomes, using the zooplankter Daphnia and its microparasites as model systems. Although cyanobacteria blooms and heat waves are concurrent events in nature, few attempts have been made to combine both stressors in experimental settings.
3. Here, we raised the zooplankter Daphnia (two genotypes) under a full factorial design with varying levels of temperature (the standard 19°C and elevated 23°C), food quality (Scenedesmus obliquus as high-quality green algae, Microcystis aeruginosa and Planktothrix agardhii as low-quality cyanobacteria) and exposed them to the parasitic yeast Metschnikowia bicuspidata. We recorded life history parameters of the host as well as parasite traits related to transmission.
4. The combination of low-quality cyanobacterial diets and elevated temperature resulted in additive detrimental effects on host fecundity. Low-quality diets reduced parasite output, while temperature effects were context dependent. Overall, we argue that the combined effects of elevated water temperature and poor-quality diets may decrease epidemics of a common fungal parasite under a climate change scenario.

     

Host age modulates within-host parasite competition

In many host populations, one of the most striking differences among hosts is their age. While parasite prevalence differences in relation to host age are well known, little is known on how host age impacts ecological and evolutionary dynamics of diseases. Using two clones of the water flea Daphnia magna and two clones of its bacterial parasite Pasteuria ramosa, we examined how host age at exposure influences within-host parasite competition and virulence. We found that multiply-exposed hosts were more susceptible to infection and suffered higher mortality than singly-exposed hosts. Hosts oldest at exposure were least often infected and vice versa. Furthermore, we found that in young multiply-exposed hosts competition was weak, allowing coexistence and transmission of both parasite clones, whereas in older multiply-exposed hosts competitive exclusion was observed. Thus, age-dependent parasite exposure and host demography (age structure) could together play an important role in mediating parasite evolution. At the individual level, our results demonstrate a previously unnoticed interaction of the host''s immune system with host age, suggesting that the specificity of immune function changes as hosts mature. Therefore, evolutionary models of parasite virulence might benefit from incorporating age-dependent epidemiological parameters.     

The ecology and phylogeny of oomycete infections in Asplanchna rotifers

1. Recently, the potential for parasites to influence the ecology and evolution of their zooplankton hosts has been the subject of increasing study. However, most research to date has focussed on Daphnia hosts, and the potential for parasites to influence other zooplankton taxa remains largely unstudied. 2. During routine sampling of zooplankton in a eutrophic lake, we observed that the rotifer Asplanchna girodi was often infected with a parasitic oomycete. Epidemics of this parasite occurred frequently, with three separate events in a single year. Prevalence at peak infection ranged from 29 to 41% and epidemics lasted from 17 to 56 days. Our data indicate that high densities of the host population are required for epidemics to occur. 3. Our morphological and molecular analyses suggest that this parasite is in the genus Pythium. Most Pythium spp. are plant pathogens, but our study supports recent work on Daphnia, suggesting that Pythium spp. are also important parasites of zooplankton. 4. As the parasite in this study was recalcitrant to cultivation, we developed an alternative method to verify its identity. Our approach used quantitative PCR to show that the ribosomal sequences identified increased with increasing density of infected hosts and, thus, were associated with the parasite. This approach should be generally applicable to other plankton parasites that are difficult to cultivate outside their hosts. 5. Infections significantly reduced host fecundity, lifespan and population growth rate. As a result of the virulence of this parasite, it is likely to influence the population ecology and evolution of its Asplanchna host, and may be a useful model system for studies on host–parasite coevolutionary dynamics.     

Interactions between the parasite's previous and current environment mediate the outcome of parasite infection

The study of parasite virulence has generally focused on the conditions under which virulence is expected to increase or decrease over time and how the interactions between hosts and their environments may mediate the outcome of infection. Recently, parasite traits such as transmission, offspring production, and development have also been shown to be influenced by environmental variation. What is unclear is how variation in the parasite's environment may impact virulence. Recent theory demonstrates that plasticity can promote the evolution of decreased virulence; thus, understanding whether the parasite's environment can mediate virulence can improve predictions regarding the outcome of parasite infection. Here, an obligate mosquito parasite was reared in hosts fed high or low levels of food. Parasite oocysts (offspring) produced in these two host environments were subsequently fed to uninfected hosts. Parasites originating from well-fed hosts were found to be more virulent to these subsequent hosts compared to parasites originating from poorly fed hosts. Additionally, this effect was apparent only when current hosts were food deprived. These results demonstrate that parasite virulence was mediated by a cross-generational effect of the environment and that the overall outcome of infection was modified by variation in both the parasite's and host's environments.     

Chance or choice? Understanding parasite selection and infection in multi-host communities

Ongoing debate over the relationship between biodiversity and disease risk underscores the need to develop a more mechanistic understanding of how changes in host community composition influence parasite transmission, particularly in complex communities with multiple hosts. A key challenge involves determining how motile parasites select among potential hosts and the degree to which this process shifts with community composition. Focusing on interactions between larval amphibians and the pathogenic trematode Ribeiroia ondatrae, we designed a novel, large-volume set of choice chambers to assess how the selectivity of free-swimming infectious parasites varied among five host species and in response to changes in assemblage composition (four different permutations). In a second set of trials, cercariae were allowed to contact and infect hosts, allowing comparison of host-parasite encounter rates (parasite choice) with infection outcomes (successful infections). Cercariae exhibited consistent preferences for specific host species that were independent of the community context; large-bodied amphibians, such as larval bullfrogs (Rana catesbeiana), exhibited the highest level of parasite attraction. However, because host attractiveness was decoupled from susceptibility to infection, assemblage composition sharply affected both per-host infection as well as total infection (summed among co-occurring hosts). Species such as the non-native R. catesbeiana functioned as epidemiological ‘sinks’ or dilution hosts, attracting a disproportionate fraction of parasites relative to the number that established successfully, whereas Taricha granulosa and especially Pseudacris regilla supported comparatively more metacercariae relative to cercariae selection. These findings provide a framework for integrating information on parasite preference in combination with more traditional factors such as host competence and density to forecast how changes within complex communities will affect parasite transmission.     

Manipulation of host-resource dynamics impacts transmission of trophic parasites

Many complex life cycle parasites rely on predator–prey interactions for transmission, whereby definitive hosts become infected via the consumption of an infected intermediate host. As such, these trophic parasites are embedded in the larger community food web. We postulated that exposure to infection and, hence, parasite transmission are inherently linked to host foraging ecology, and that perturbation of the host-resource dynamic will impact parasite transmission dynamics. We employed a field manipulation experiment in which natural populations of the eastern chipmunk (Tamias striatus) were provisioned with a readily available food resource in clumped or uniform spatial distributions. Using replicated longitudinal capture-mark-recapture techniques, replicated supplemented and unsupplemented control sites were monitored before and after treatment for changes in infection levels with three gastro-intestinal helminth parasites. We predicted that definitive hosts subject to food supplementation would experience lower rates of exposure to infective intermediate hosts, presumably because they shifted their diet away from the intermediate host towards the more readily available resource (sunflower seeds). As predicted, prevalence of infection by the trophically transmitted parasite decreased in response to supplemental food treatment, but no such change in infection prevalence was detected for the two directly transmitted parasites in the system. The fact that food supplementation only had an impact on the transmission of the trophically transmitted parasite, and not the directly transmitted parasites, supports our hypothesis that host foraging ecology directly affects exposure to parasites that rely on the ingestion of intermediate hosts for transmission. We concluded that the relative availability of different food resources has important consequences for the transmission of parasites and, more specifically, parasites that are embedded in the food web. The broader implications of these findings for food web dynamics and disease ecology are discussed.     

Notch1 binds and induces degradation of Snail in hepatocellular carcinoma

Background

Infection processes consist of a sequence of steps, each critical for the interaction between host and parasite. Studies of host-parasite interactions rarely take into account the fact that different steps might be influenced by different factors and might, therefore, make different contributions to shaping coevolution. We designed a new method using the Daphnia magna - Pasteuria ramosa system, one of the rare examples where coevolution has been documented, in order to resolve the steps of the infection and analyse the factors that influence each of them.

Results

Using the transparent Daphnia hosts and fluorescently-labelled spores of the bacterium P. ramosa, we identified a sequence of infection steps: encounter between parasite and host; activation of parasite dormant spores; attachment of spores to the host; and parasite proliferation inside the host. The chances of encounter had been shown to depend on host genotype and environment. We tested the role of genetic and environmental factors in the newly described activation and attachment steps. Hosts of different genotypes, gender and species were all able to activate endospores of all parasite clones tested in different environments; suggesting that the activation cue is phylogenetically conserved. We next established that parasite attachment occurs onto the host oesophagus independently of host species, gender and environmental conditions. In contrast to spore activation, attachment depended strongly on the combination of host and parasite genotypes.

Conclusions

Our results show that different steps are influenced by different factors. Host-type-independent spore activation suggests that this step can be ruled out as a major factor in Daphnia - Pasteuria coevolution. On the other hand, we show that the attachment step is crucial for the pronounced genetic specificities of this system. We suggest that this one step can explain host population structure and could be a key force behind coevolutionary cycles. We discuss how different steps can explain different aspects of the coevolutionary dynamics of the system: the properties of the attachment step, explaining the rapid evolution of infectivity and the properties of later parasite proliferation explaining the evolution of virulence. Our study underlines the importance of resolving the infection process in order to better understand host-parasite interactions.     

Immune depression induced by acanthocephalan parasites in their intermediate crustacean host: consequences for the risk of super-infection and links with host behavioural manipulation

Parasite survival in hosts mainly depends on the capacity to circumvent the host immune response. Acanthocephalan infections in gammarids are linked with decreased activity of the prophenoloxidase (ProPO) system, suggesting an active immunosuppression process. Nevertheless, experimental evidence for this hypothesis is lacking: whether these parasites affect several immune pathways is unknown and the consequences of such immune change have not been investigated. In particular, the consequences for other pathogens are not known; neither are the links with other parasite-induced manipulations of the host. Firstly, using experimental infections of Pomphorhynchus laevis we confirmed that the lower immune activity in parasitised Gammarus pulex is induced by the parasite infection. Second, using natural infections of three different parasites, P. laevis, Pomphorhynchus tereticollis and Polymorphus minutus, we showed that acanthocephalan infection was associated with reduction of the activity of the ProPO system and the haemocyte concentration (two major parameters of crustacean immunity) suggesting that immune depression is a phenomenon affecting several immunological activities. This was confirmed by the fact that acanthocephalan infection (whatever the parasite species) was linked to a lower efficiency to eliminate a bacterial infection. The result suggests a cost of parasite immune depression. Finally, acanthocephalans are also known to induce behavioural alterations in the intermediate host which favour their transmission to definitive hosts. We did not find any correlation between behavioural and immunological alterations in both experimentally and naturally-infected gammarids. Overall, this study suggests that whilst immune depression might be beneficial to acanthocephalan survival within the intermediate gammarid host, it might also be costly if it increases host mortality to additional infections before transmission of the parasite.     

Interactions between environmental stressors: the influence of salinity on host–parasite interactions between Daphnia magna and Pasteuria ramosa

Interactions between environmental stressors play an important role in shaping the health of an organism. This is particularly true in terms of the prevalence and severity of infectious disease, as stressors in combination will not always act to simply decrease the immune function of a host, but may instead interact to compound or even oppose the influence of parasitism on the health of an organism. Here, we explore the impact of environmental stress on host–parasite interactions using the water flea Daphnia magna and it is obligate parasite Pasteuria ramosa. Utilising an ecologically relevant stressor, we focus on the combined effect of salinity and P. ramosa on the fecundity and survival of the host, as well as on patterns of infectivity and the proliferation of the parasite. We show that in the absence of the parasite, host fecundity and survival was highest in the low salinity treatments. Once a parasite was introduced into the environment, however, salinity and parasitism acted antagonistically to influence both host survival and fecundity, and these patterns of disease were unrelated to infection rates or parasite spore loads. By summarising the form of interactions found in the broader Daphnia literature, we highlight how the combined effect of stress and parasitism will vary with the type of stressor, the trait used to describe the health of Daphnia and the host–parasite combination under observation. Our results highlight how the context-dependent nature of interactions between stress and parasitism inevitably complicates the link between environmental factors and the prevalence and severity of disease.     

Differential Tolerance to Direct and Indirect Density-Dependent Costs of Viral Infection in Arabidopsis thaliana

Population density and costs of parasite infection may condition the capacity of organisms to grow, survive and reproduce, i.e. their competitive ability. In host–parasite systems there are different competitive interactions: among uninfected hosts, among infected hosts, and between uninfected and infected hosts. Consequently, parasite infection results in a direct cost, due to parasitism itself, and in an indirect cost, due to modification of the competitive ability of the infected host. Theory predicts that host fitness reduction will be higher under the combined effects of costs of parasitism and competition than under each factor separately. However, experimental support for this prediction is scarce, and derives mostly from animal–parasite systems. We have analysed the interaction between parasite infection and plant density using the plant-parasite system of Arabidopsis thaliana and the generalist virus Cucumber mosaic virus (CMV). Plants of three wild genotypes grown at different densities were infected by CMV at various prevalences, and the effects of infection on plant growth and reproduction were quantified. Results demonstrate that the combined effects of host density and parasite infection may result either in a reduction or in an increase of the competitive ability of the host. The two genotypes investing a higher proportion of resources to reproduction showed tolerance to the direct cost of infection, while the genotype investing a higher proportion of resources to growth showed tolerance to the indirect cost of infection. Our findings show that the outcome of the interaction between host density and parasitism depends on the host genotype, which determines the plasticity of life-history traits and consequently, the host capacity to develop different tolerance mechanisms to the direct or indirect costs of parasitism. These results indicate the high relevance of host density and parasitism in determining the competitive ability of a plant, and stress the need to simultaneously consider both factors to understand the selective pressures that drive host–parasite co-evolution.     

Age-biased parasitism and density-dependent distribution of fleas (Siphonaptera) on a desert rodent

Parasites often confront conflicting demands when evaluating and distributing themselves among host individuals, in order to attain maximum reproductive success. We tested two alternative hypotheses about host preference by fleas in relation to the age of their rodent host. The first hypothesis suggests that fleas select adult over juvenile rodents because the latter represent a better nutritional resource (the “well-fed host” hypothesis), whereas the second hypothesis suggests that fleas prefer the weaker and less resistant juveniles because they are easier to colonise and exploit (“poorly fed host” hypothesis). We sampled fleas (Synosternus cleopatrae) on the gerbil (Gerbillus andersoni) in 23 different plots in the Negev desert and found an unequal distribution of fleas between adult and juvenile hosts. Furthermore, flea distribution changed as a function of flea density—from juvenile-biased flea parasitism (the “poorly fed host” hypothesis) at low densities to adult-biased flea parasitism (the “well-fed host” hypothesis) at high densities. Other factors that influenced flea preference were soil temperature and the presence of ticks. These results suggest that host selection is not an explicit alternative choice between adults and juveniles (“well-fed host” versus “poorly fed host” hypotheses), but rather a continuum where the distribution between adults and juveniles depends on host, parasite, and environmentally related factors.