Parasite Transmission Success Research Articles

Evolution of parasites in the Anthropocene: new pressures, new adaptive directions.

The Anthropocene is seeing the human footprint rapidly spreading to all of Earth's ecosystems. The fast-changing biotic and abiotic conditions experienced by all organisms are exerting new and strong selective pressures, and there is a growing list of examples of human-induced evolution in response to anthropogenic impacts. No organism is exempt from these novel selective pressures. Here, we synthesise current knowledge on human-induced evolution in eukaryotic parasites of animals, and present a multidisciplinary framework for its study and monitoring. Parasites generally have short generation times and huge fecundity, features that predispose them for rapid evolution. We begin by reviewing evidence that parasites often have substantial standing genetic variation, and examples of their rapid evolution both under conditions of livestock production and in serial passage experiments. We then present a two-step conceptual overview of the causal chain linking anthropogenic impacts to parasite evolution. First, we review the major anthropogenic factors impacting parasites, and identify the selective pressures they exert on parasites through increased mortality of either infective stages or adult parasites, or through changes in host density, quality or immunity. Second, we discuss what new phenotypic traits are likely to be favoured by the new selective pressures resulting from altered parasite mortality or host changes; we focus mostly on parasite virulence and basic life-history traits, as these most directly influence the transmission success of parasites and the pathology they induce. To illustrate the kinds of evolutionary changes in parasites anticipated in the Anthropocene, we present a few scenarios, either already documented or hypothetical but plausible, involving parasite taxa in livestock, aquaculture and natural systems. Finally, we offer several approaches for investigations and real-time monitoring of rapid, human-induced evolution in parasites, ranging from controlled experiments to the use of state-of-the-art genomic tools. The implications of fast-evolving parasites in the Anthropocene for disease emergence and the dynamics of infections in domestic animals and wildlife are concerning. Broader recognition that it is not only the conditions for parasite transmission that are changing, but the parasites themselves, is needed to meet better the challenges ahead.

Factores de relevancia en la dinámica de transmisión de malaria aviar: patrón de alimentación de los mosquitos y capacidad de desarrollo de Plasmodium spp.

Gutierrez-Lopez, R. Martinez-de la Puente, J. 2020. Relevant factors in the transmission dynamics of avian malaria: mosquito feeding patterns and Plasmodium spp. development capacity. Ecosistemas 29(2):1964. https://doi.org/10.7818/ECOS.1964 The transmission dynamics of malaria parasites depends on the parasites and vectors interactions, which are influenced by the environmental conditions where are developed. Therefore, in order to understand the ecology and evolution of the transmission of these diseases, it is essential to study the factors that determine the interactions between mosquitoes, birds and parasites that infect them in natural ecosystems. There are two processes determining parasite transmission success, i) the contact rates between mosquitoes and infected or susceptible birds, and ii) the capacity of development of the parasite inside the mosquito (vector competence). Different studies have evidenced the role of the mosquitoes feeding behaviour on the transmission dynamics of avian Plasmodium. Likewise, the development capabilities of avian Plasmodium parasites within species of mosquitoes have been studied. However, both processes together in natural conditions have been scarcely studied. Here, it is exposed how differences in feeding rates of mosquitoes and in the development capacity of avian Plasmodium determine the transmission dynamics of avian malaria in natural ecosystems. In addition, future research lines are also exposed, in order to improve the knowledge on the transmission dynamics of avian Plasmodium. The consideration of vectors in the studies on the ecology of parasitism will allow a better understanding of the complex interactions between vertebrate host, mosquito and parasite in natural conditions.

The Consequences of Mixed-Species Malaria Parasite Co-Infections in Mice and Mosquitoes for Disease Severity, Parasite Fitness, and Transmission Success.

The distributions of human malaria parasite species overlap in most malarious regions of the world, and co-infections involving two or more malaria parasite species are common. Little is known about the consequences of interactions between species during co-infection for disease severity and parasite transmission success. Anti-malarial interventions can have disproportionate effects on malaria parasite species and may locally differentially reduce the number of species in circulation. Thus, it is important to have a clearer understanding of how the interactions between species affect disease and transmission dynamics. Controlled competition experiments using human malaria parasites are impossible, and thus we assessed the consequences of mixed-species infections on parasite fitness, disease severity, and transmission success using the rodent malaria parasite species Plasmodium chabaudi, Plasmodium yoelii, and Plasmodium vinckei. We compared the fitness of individual species within single species and co-infections in mice. We also assessed the disease severity of single vs. mixed infections in mice by measuring mortality rates, anemia, and weight loss. Finally, we compared the transmission success of parasites in single or mixed species infections by quantifying oocyst development in Anopheles stephensi mosquitoes. We found that co-infections of P. yoelii with either P. vinckei or P. chabaudi led to a dramatic increase in infection virulence, with 100% mortality observed in mixed species infections, compared to no mortality for P. yoelii and P. vinckei single infections, and 40% mortality for P. chabaudi single infections. The increased mortality in the mixed infections was associated with an inability to clear parasitaemia, with the non-P. yoelii parasite species persisting at higher parasite densities than in single infections. P. yoelii growth was suppressed in all mixed infections compared to single infections. Transmissibility of P. vinckei and P. chabaudi to mosquitoes was also reduced in the presence of P. yoelii in co-infections compared to single infections. The increased virulence of co-infections containing P. yoelii (reticulocyte restricted) and P. chabaudi or P. vinckei (predominantly normocyte restricted) may be due to parasite cell tropism and/or immune modulation of the host. We explain the reduction in transmission success of species in co-infections in terms of inter-species gamete incompatibility.

Can differences between continental and insular habitats influence the parasites communities associated with the endemic frog Haddadus binotatus?

Habitats characterized by geographic isolation such as islands have been studied using different organisms as models for understanding the dynamic and insular patterns of biodiversity. Determinants of parasite richness in insular host populations have been conducted mainly with mammals and birds, showing that parasite richness decreases in insular areas. In the present study, we predicted that the type of environment (insular or continental) can influence the richness, diversity and abundance of parasites associated with the endemic frog Haddadus binotatus (Spix, 1824). We sampled frogs in two insular and two mainland fragments to survey their helminth parasites. The total richness was composed of 15 taxa of Nematoda and two of Acanthocephala, and the community composition of the two islands had more similarities between them than the two mainland localities. The insular effect was positive for richness and abundance of helminths, and no significant effect was observed on helminth diversity - even the mean diversity presented high numbers for the islands. We presumed that insular hosts could have lost some parasites in the colonization process when these continental islands were separated from the mainland, approximately 11,000 years ago. However, the high richness and abundance on islands can be explained by an epidemiological argument, which considers high population density due to insularity and other features of the host as factors that increase parasite transmission success among individuals.

Alternative host identity and lake morphometry drive trematode transmission in brook charr.

Both biotic and abiotic factors have been invoked to explain the large variations observed in the prevalence and abundance of parasites in aquatic ecosystems. However, we have only a poor knowledge of the potential interplay among these factors in natural systems. It is, therefore, important to analyze the effects of multiple potential environmental drivers together to get an integrated view of their influence on the prevalence and abundance of parasites. To this end, we selected two genera of digenean trematode parasites that require at least two hosts to complete their life cycle and use two different transmission strategies. Crepidostomum moves through a trophic pathway via consumption of infected prey by the host, while Apophallus infects its hosts via direct penetration of their skin. This study was conducted in 23 Canadian Shield lakes exhibiting orthogonal gradients of biotic (fish species richness and biomass) and abiotic (morphometry, physico-chemical) variables. We quantified prevalence and abundance of these parasites in the skin and intestine of brook charr (Salvelinus fontinalis). Our results show that biotic factors are key drivers of parasite abundance and prevalence, with Apophallus being negatively associated with the fish species richness-biomass gradient, and Crepidostomum responding more to identity of host than to the diversity gradient. Among the abiotic variables, lake areawas found to be positively related to both prevalence and abundance in Apophallus. Our results suggest that taking into account the interplay of both biotic and abiotic factors is crucial for understanding patterns of parasite transmission success in boreal lakes.

Cercarial Behavior Determines Risk of Predation

The potential for local biodiversity to affect transmission success of parasites has been shown to be particularly important in trematodes, where non-host organisms can feed on and 'dilute' free-living infective stages (cercariae). Earlier studies have analyzed the effects of various predators on transmission stages of single trematode species, but not how cercariae of different species react to predation pressure. Here, we tested whether cercariae with different host-searching movement patterns show varying susceptibility to predation by non-host species with different feeding habits. For this study, we performed a set of predation experiments with 6 species of trematode cercariae (Coitocaecum parvum, Maritrema poulini, Apatemon sp., Telogaster opisthorchis, Plagiorchioid sp. I, and Aporocotylid sp. II) that represent 2 groups of host-searching behavior, free-swimming vs. bottom-dwelling, and 2 predators ( Sphaerium sp., Physa acuta) with distinct feeding modes, a filter feeder and a grazer. Our results show that cercarial susceptibility to predation is highly dependent on the interspecific interaction between dispersal behavior of cercariae and feeding behavior of non-host organisms: Filter feeders only diluted free-swimming cercarial stages, not bottom-dwelling ones; grazers on the other hand, had no effect on free-swimming cercariae but reduced bottom-dwelling cercariae in 1 trematode species. Our findings give further support to the hypothesis that the transmission dynamics of trematodes do not simply depend on local biodiversity but rather on the species-specific interactions between parasite transmission stages and free-living organisms in the ecosystem. This has important implications for disease dynamics in ecological communities (e.g., the parasites' infection success), and for ecosystem energetics, as cercariae constitute important food items.

Differential patterns of definitive host use by two fish acanthocephalans occurring in sympatry: Pomphorhynchus laevis and Pomphorhynchus tereticollis.

Parasites with complex life-cycles and trophic transmission are expected to show low specificity towards final hosts. However, testing this hypothesis may be hampered by low taxonomic resolution, particularly in helminths. We investigated this issue using two intestinal fish parasites with similar life-cycles and occurring in sympatry, Pomphorhynchus laevis and Pomphorhynchus tereticollis (Acanthocephala). We used species-specific ITS1 length polymorphism to discriminate parasite species from 910 adult acanthocephalans collected in 174 individual hosts from 12 fish species. Both P. laevis and P. tereticollis exhibited restricted host range within the community of available fish host species, and transmission bias compared to their relative abundance in intermediate hosts. The two parasites also exhibited low niche overlap, primarily due to their contrasting use of bentho-pelagic (P. laevis) and benthic (P. tereticollis) fish. Furthermore, parasite prevalence in intermediate hosts appeared to increase with taxonomic specificity in definitive host use. Comparison of P. laevis and P. tereticollis adult size in the two main definitive hosts, barbel and chub, suggested lower compatibility towards the fish species with the lowest parasite abundance, in particular in P. laevis. The determinants of low niche overlap between these two sympatric acanthocephalan species, and the contribution of definitive host range diversity to parasite transmission success, are discussed.

Host density increases parasite recruitment but decreases host risk in a snail-trematode system.

Most species aggregate in local patches. High host density in patches increases contact rate between hosts and parasites, increasing parasite transmission success. At the same time, for environmentally transmitted parasites, high host density can decrease infection risk to individual hosts, because infective stages are divided among all hosts in a patch, leading to safety in numbers. We tested these predictions using the California horn snail, Cerithideopsis californica (=Cerithidea californica), which is the first intermediate host for at least 19 digenean trematode species in California estuaries. Snails become infected by ingesting trematode eggs or through penetration by free-swimming miracidia that hatch from trematode eggs deposited with final-host (bird or mammal) feces. This complex life cycle decouples infective-stage production from transmission, raising the possibility of an inverse relationship between host density and infection risk at local scales. In a field survey, higher snail density was associated with increased trematode (infected snail) density, but decreased trematode prevalence, consistent with either safety in numbers, parasitic castration, or both. To determine the extent to which safety in numbers drove the negative snail-density-trematode-prevalence association, we manipulated uninfected snail density in 83 cages at eight sites within Carpinteria Salt Marsh (California, USA). At each site, we quantified snail density and used data on final-host (bird and raccoon) distributions to control for between-site variation in infective-stage supply. After three months, overall trematode infections per cage increased with snail biomass density. For egg-transmitted trematodes, per-snail infection risk decreased with snail biomass density in the cage and surrounding area, whereas per-snail infection risk did not decrease for miracidium-transmitted trematodes. Furthermore, both trematode recruitment and infection risk increased with infective-stage input, but this was significant only for miracidium-transmitted species. A model parameterized with our experimental results and snail densities from 524 field transects estimated that safety in numbers, when combined with patchy host density, halved per capita infection risk in this snail population. We conclude that, depending on transmission mode, host density can enhance parasite recruitment and reduce per capita infection risk.

Non-linear effects of ocean acidification on the transmission of a marine intertidal parasite

High levels of atmospheric carbon dioxide are driving the acidification of the world's oceans, with considerable and generally negative impacts on the physiology, performance and survival of marine organisms. The differential and often idiosyncratic responses shown by differ- ent taxa suggest that interspecific interactions may be drastically affected by ocean acidification. Here, we quantified the transmission success of the trematode Maritrema novaezealandense to its intertidal amphipod intermediate host Paracalliope novizealandiae, as well as the host's survival, under acidified conditions. We used a custom-built system to simulate ocean acidification with 3 different seawater treatments: 8.1 pH, corresponding to current average ocean surface waters, as well as 7.6 and 7.4 pH, the levels predicted for the years 2100 and 2300, respectively. In 2 separate experiments, parasite transmission success tended to peak in the most acidified conditions (7.4 pH), although this was only statistically significant when a wide range of infection doses was used. Because the survival of the parasite's transmission stages decreases with decreasing pH, this pattern suggests that host susceptibility remains unaffected at 7.6 pH and is only compromised with further acidification. Amphipod mortality was not affected by pH levels, though it tended to be lowest at 7.6 pH, where the longevity of parasite transmission stages was reduced but host sus- ceptibility was unaffected. These results suggest that ocean acidification could change the dynamics of parasite transmission with possible consequences for intertidal community structure, and emphasise the need to consider the transmission and severity of marine parasites and diseases in ocean acidification research.

Quantifying larval trematode infections in hosts: A comparison of method validity and implications for infection success

Accurately estimating parasite transmission success and subsequent infection load has important ramifications for a wide range of disease-related questions and research disciplines. Recent interest in the role of parasites in amphibian population declines and deformities, for instance, has prompted increased interest in approaches to quantify infection and pathology. Here, we introduce a novel method of fluorescently labeling trematode cercariae and optimize its application to interactions between the pathogenic trematode Ribeiroia ondatrae and amphibian hosts (Lithobates sphenocephalus). We then compare the efficacy of this method with two other approaches commonly used to assess infection in second intermediate hosts – necropsy and tissue clearing – with a focus on accuracy, precision, and bias. Dye (Invitrogen, BODIPY® FL C12) concentrations of <200 nM and DMSO solvent concentrations <0.2% offered a highly visible, long-lasting marker with no detectable effects on cercariae survival or metacercariae establishment. Among methods, the necropsy approach yielded the highest proportion of detected parasites and the lowest standard error around the mean. However, the fluorescent labeling method offered highly similar results (r = 0.99), with an estimated 75% of administered parasites establishing successfully. At low to moderate parasite exposure dosages, tissue clearing was comparable to the other two methods, but tended to underpredict infection at the higher exposures (i.e., ‘proportional bias’), likely because individual parasites became more difficult to distinguish. Conceptually, these findings illustrate that initial infection within amphibian hosts is a consistent, linear function of exposure dosage, suggesting that parasite density dependence does not regulate initial establishment. From an applied standpoint, our results offer a methodological foundation for subsequent research using fluorescently labeled parasites, which offer distinct advantages such as allowing in vivo parasite tracking within living hosts.

No uniform associations between parasite prevalence and environmental nutrients

The resource quality of the host has been shown to affect parasite transmission success, prevalence, and virulence. Seasonal availability of environmental nutrients alters density and stoichiometric quality (carbon–nutrient ratios) of both producers and consumers, suggesting that nutrient availability may drive fluctuations in parasite prevalence patterns observed in nature. We examined the interactions between the population dynamics of a keystone herbivore, Daphnia, and its parasites, and their associations with water nutrient concentrations, resource quantity and quality, and other environmental variables (temperature, pH, oxygen concentration) in a small lake, using general linear models. We found that the prevalence of two gut endoparasites was positively related to food source and quality as well as nitrogen content of Daphnia, whereas the prevalence of an epibiont and overall parasite species richness was negatively related to phosphorus content of Daphnia. When only endoparasite species richness was considered, no connections to nutrients were found. Daphnia density was not connected to parasites, but we found interactions between Daphnia fecundity and parasite prevalence. Overall, our results suggest that environmental nutrient concentrations and stoichiometric quality of the host have the potential to affect seasonality in parasite epidemics, but the connections between environmental carbon : nutrient ratios and parasite prevalence patterns are diverse and species specific.

Biogeography of parasitism in freshwater fish: spatial patterns in hot spots of infection

Contrary to species occurrence, little is known about the determinants of spatial patterns of intraspecific variation in abundance, particularly for parasitic organisms. In this study, we provide a multi‐faceted overview of spatial patterns in parasite abundance and examine several potential underlying processes. We first tested for a latitudinal gradient in local abundance of the regionally most common parasite species and whether these species achieve higher abundances at the same localities (shared hot spots of infection). Secondly, we tested whether intraspecific similarity in local abundance between sites follows a spatial distance decay pattern or is better explained by variation in extrinsic biotic and abiotic factors between localities related to local parasite transmission success. We examined the infection landscape of a model fish host system (common and upland bullies, genusGobiomorphus: Eleotridae) across its entire distributional range. We applied general linear models to test the effect of latitude on each species local abundance independently, including the abundance of each co‐infecting species as another predictor. We computed multiple regressions on distance matrices among localities based on abundance of each of the four most common trematode species, as well as for geographic distance, biotic and abiotic distinctness of the localities. Our results showed that the most widely distributed parasites of bullies also achieve the highest mean local abundances, following the abundance – occupancy relationship. Variation in local abundance of any focal parasite species was independent of latitude, the abundance of co‐occurring species and spatial distance or disparity in biotic attributes between localities. For only one parasite species, similarity of abundance between sites covaried with the extent of abiotic differences between sites. The lack of association between hot spots of infection for co‐occurring species reinforces the geographic mosaic scenario in which hosts and parasites coevolve by suggesting non‐deterministic, species‐specific variation in parasite abundance across space.

Inferring host range dynamics from comparative data: the protozoan parasites of new world monkeys.

Uncovering the ecological determinants of parasite host range is a central goal of comparative parasitology and infectious disease ecology. But while parasites are often distributed nonrandomly across the host phylogeny, such patterns are difficult to interpret without a genealogy for the parasite samples and without knowing what sorts of ecological dynamics might lead to what sorts of nonrandomness. We investigated inferences from comparative data, using presence/absence records from protozoan parasites of the New World monkeys. We first demonstrate several distinct types of phylogenetic signal in these data, showing, for example, that parasite species are clustered on the host tree and that closely related host species harbor similar numbers of parasite species. We then show that all of these patterns can be generated by a single, simple dynamical model, in which parasite host range changes more rapidly than host speciation/extinction and parasites preferentially colonize uninfected host species that are closely related to their existing hosts. Fitting this model to data, we then estimate its parameters. Finally, we caution that quite different ecological processes can lead to similar signatures but show how phylogenetic variation in host susceptibility can be distinguished from a tendency for parasites to colonize closely related hosts. Our new process-based analyses, which estimate meaningful parameters, should be useful for inferring the determinants of parasite host range and transmission success.

An experimental test of the effects of behavioral and immunological defenses against vectors: do they interact to protect birds from blood parasites?

BackgroundBlood-feeding arthropods can harm their hosts in many ways, such as through direct tissue damage and anemia, but also by distracting hosts from foraging or watching for predators. Blood-borne pathogens transmitted by arthropods can further harm the host. Thus, effective behavioral and immunological defenses against blood-feeding arthropods may provide important fitness advantages to hosts if they reduce bites, and in systems involving pathogen transmission, if they lower pathogen transmission rate.MethodsWe tested whether Rock Pigeons (Columba livia) have effective behavioral and immunological defenses against a blood-feeding hippoboscid fly (Pseudolynchia canariensis) and, if so, whether the two defenses interact. The fly vectors the blood parasite Haemoproteus columbae; we further tested whether these defenses reduced the transmission success of blood parasites when birds were exposed to infected flies. We compared four experimental treatments in which hosts had available both purported defenses, only one of the defenses, or no defenses against the flies.ResultsWe found that preening and immunological defenses were each effective in decreasing the survival and reproductive success of flies. However, the two defenses were additive, rather than one defense enhancing or decreasing the effectiveness of the other defense. Neither defense reduced the prevalence of H. columbae, nor the intensity of infection in birds exposed to infected flies.ConclusionsFlies experience reduced fitness when maintained on hosts with immunological or preening defenses. This suggests that if vectors are given a choice among hosts, they may choose hosts that are less defended, which could impact pathogen transmission in a system where vectors can choose among hosts.

Consistent Pattern of Local Adaptation during an Experimental Heat Wave in a Pipefish-Trematode Host-Parasite System

Extreme climate events such as heat waves are expected to increase in frequency under global change. As one indirect effect, they can alter magnitude and direction of species interactions, for example those between hosts and parasites. We simulated a summer heat wave to investigate how a changing environment affects the interaction between the broad-nosed pipefish (Syngnathus typhle) as a host and its digenean trematode parasite (Cryptocotyle lingua). In a fully reciprocal laboratory infection experiment, pipefish from three different coastal locations were exposed to sympatric and allopatric trematode cercariae. In order to examine whether an extreme climatic event disrupts patterns of locally adapted host-parasite combinations we measured the parasite's transmission success as well as the host's adaptive and innate immune defence under control and heat wave conditions. Independent of temperature, sympatric cercariae were always more successful than allopatric ones, indicating that parasites are locally adapted to their hosts. Hosts suffered from heat stress as suggested by fewer cells of the adaptive immune system (lymphocytes) compared to the same groups that were kept at 18°C. However, the proportion of the innate immune cells (monocytes) was higher in the 18°C water. Contrary to our expectations, no interaction between host immune defence, parasite infectivity and temperature stress were found, nor did the pattern of local adaptation change due to increased water temperature. Thus, in this host-parasite interaction, the sympatric parasite keeps ahead of the coevolutionary dynamics across sites, even under increasing temperatures as expected under marine global warming.

Genome Comparison of Human and Non-Human Malaria Parasites Reveals Species Subset-Specific Genes Potentially Linked to Human Disease

Genes underlying important phenotypic differences between Plasmodium species, the causative agents of malaria, are frequently found in only a subset of species and cluster at dynamically evolving subtelomeric regions of chromosomes. We hypothesized that chromosome-internal regions of Plasmodium genomes harbour additional species subset-specific genes that underlie differences in human pathogenicity, human-to-human transmissibility, and human virulence. We combined sequence similarity searches with synteny block analyses to identify species subset-specific genes in chromosome-internal regions of six published Plasmodium genomes, including Plasmodium falciparum, Plasmodium vivax, Plasmodium knowlesi, Plasmodium yoelii, Plasmodium berghei, and Plasmodium chabaudi. To improve comparative analysis, we first revised incorrectly annotated gene models using homology-based gene finders and examined putative subset-specific genes within syntenic contexts. Confirmed subset-specific genes were then analyzed for their role in biological pathways and examined for molecular functions using publicly available databases. We identified 16 genes that are well conserved in the three primate parasites but not found in rodent parasites, including three key enzymes of the thiamine (vitamin B1) biosynthesis pathway. Thirteen genes were found to be present in both human parasites but absent in the monkey parasite P. knowlesi, including genes specifically upregulated in sporozoites or gametocytes that could be linked to parasite transmission success between humans. Furthermore, we propose 15 chromosome-internal P. falciparum-specific genes as new candidate genes underlying increased human virulence and detected a currently uncharacterized cluster of P. vivax-specific genes on chromosome 6 likely involved in erythrocyte invasion. In conclusion, Plasmodium species harbour many chromosome-internal differences in the form of protein-coding genes, some of which are potentially linked to human disease and thus promising leads for future laboratory research.

Determinants of distribution and prevalence of avian malaria in blue tit populations across Europe: separating host and parasite effects

Although avian malarial parasites are globally distributed, the factors that affect the geographical distribution and local prevalence of different parasite lineages across host populations or species are still poorly understood. Based on the intense screening of avian malarial parasites in nine European blue tit populations, we studied whether distribution ranges as well as local adaptation, host specialization and phylogenetic relationships can determine the observed prevalences within populations. We found that prevalence differed consistently between parasite lineages and host populations, indicating that the transmission success of parasites is lineage specific but is partly shaped by locality-specific effects. We also found that the lineage-specific estimate of prevalence was related to the distribution range of parasites: lineages found in more host populations were generally more prevalent within these populations. Additionally, parasites with high prevalence that were also widely distributed among blue tit populations were also found to infect more host species. These findings suggest that parasites reaching high local prevalence can also realize wide distribution at a global scale that can have further consequences for host specialization. Although phylogenetic relationships among parasites did not predict prevalence, we detected a close match between a tree based on the geographic distance of the host populations and the parasite phylogenetic tree, implying that neighbouring host populations shared a related parasite fauna.

Intra-host dynamics of mixed species malaria parasite infections in mice and mosquitoes

Background The distributions of human malaria parasite species overlap in most regions of the world where malaria is present, and co-infections involving two or more malaria parasites are common. Currently, very little is known about the consequences of any interactions that may occur between species during co-infection for disease severity and parasite transmission success. However, current anti-malarial interventions such as vector control and drug interventions and the future application of vaccines will and do have disproportionate effects on some species compared to others; with the ultimate consequence of reducing the number of species in circulation in any one area. We believe that such a situation warrants a clearer understanding of how the interactions between species affect malaria disease and transmission dynamics.

Effect of cadmium on the susceptibility of Tubifex tubifex to Myxobolus cerebralis (Myxozoa), the causative agent of whirling disease

Environmental pollutants alter a wide range of host-parasite interactions in various ways. In some cases, pollution leads to a significant increase in parasite abundance, causing epidemics of parasitic diseases. In other cases, toxicants restrict the transmission success of parasites, resulting in reduction of their abundance. However, very little is known regarding whether and to what extent aquatic pollution affects myxozoan obligate parasites commonly found in fish. We investigated the effect of cadmium (Cd) on the aquatic oligochaete Tubifex tubifex infected with the myxozoan Myxobolus cerebralis. The oligochaetes were experimentally exposed to M. cerebralis myxospores and kept in various concentrations of Cd for 4 mo. Neither survival nor reproduction of the worms was affected by the metal, but infection prevalence and numbers of triactinosmyxon spores produced by individual worms were higher in the Cd-exposed group than the unexposed control. A comparative assay of a lethal Cd concentration (LC50) on infected and non-infected T. tubifex revealed that infected worms are more resistant to the acute toxicity of Cd, probably because uptake of Cd was reduced by the infection. These results suggest that the abundance of M. cerebralis likely increases in polluted waters and escalates the risk of whirling disease in the respective area.

Host manipulation as a parasite transmission strategy when manipulation is exploited by non-host predators

Trophically transmitted parasites often alter their intermediate host's phenotype, thereby predisposing hosts to increased predation. This is generally considered to be a parasite strategy evolved to enhance transmission to the next host. However, the adaptive value of host manipulation is not clear, as it may be associated with costs, such as increased susceptibility to predator species that are unsuitable next hosts for the parasites. Thus, it has been proposed that, to be adaptive, manipulation should be specific by predisposing hosts more strongly to predation by target hosts (next host in the life cycle) than to non-hosts. Here we formally evaluate this prediction, and show that manipulation does not have to be specific to be adaptive. However, when manipulation is nonspecific, it needs to effectively increase the overall predation risk of infected hosts if it is to increase the parasite transmission probability. Thus, when initial predation risk is low, even highly nonspecific manipulation strategies can be adaptive. However, when initial predation risk is high, manipulation needs to be more specific to increase parasite transmission success. Therefore, nonspecific host manipulation may evolve in nature, but the adaptive value of a certain manipulation strategy can vary among different parasite populations depending on the variation in initial predation risk.