[Show abstract][Hide abstract] ABSTRACT: Similar to parasites, cancer cells depend on their hosts for sustenance, proliferation and reproduction, exploiting the hosts for energy and resources, and thereby impairing their health and fitness. Because of this lifestyle similarity, it is predicted that cancer cells could, like numerous parasitic organisms, evolve the capacity to manipulate the phenotype of their hosts to increase their own fitness. We claim that the extent of this phenomenon and its therapeutic implications are, however, underappreciated. Here, we review and discuss what can be regarded as cases of host manipulation in the context of cancer development and progression. We elaborate on how acknowledging the applicability of these principles can offer novel therapeutic and preventive strategies. The manipulation of host phenotype by cancer cells is one more reason to adopt a Darwinian approach in cancer research.
[Show abstract][Hide abstract] ABSTRACT: Parasites often have direct impacts on their host physiology or function, which can in turn have indirect effects on interspecific interactions and ecosystem structure. The present study investigates the effect of trematode parasite infection on the ecologically and commercially important venerid clam Austrovenus stutchburyi. Although the indirect impacts of clam infection on the broader benthic community, mediated by impaired burrowing of parasitized clams, have been well documented before, the more direct impacts on the clam itself remain poorly studied. The consequence of parasite infection on clam growth rate, mortality, body condition and foot length was quantified in a three-month laboratory experiment, in which juvenile clams were infected with varying levels of the echinostome trematode Curtuteria australis. Although mortality was unaffected by parasite infection, greater numbers of parasites deleteriously affected the growth rate, body condition and foot length of clams. This may result in delayed maturity and a lower filtration rate for infected individuals, as well as a reduced ability to bury into the sediment. Consequently, increased parasite infection not only has subsequent broader impacts on the surrounding ecological community, but can also affect the clam host directly and lower its value as a harvested species.
Full-text · Article · Jan 2016 · Journal of Experimental Marine Biology and Ecology
[Show abstract][Hide abstract] ABSTRACT: The species richness of freshwater environments is disproportionately high compared with that of the oceans, given their respective sizes. If diversification rates are higher in freshwaters because they are isolated and heterogeneous, this should apply to parasites as well. Using 14 large datasets comprising 677 species of freshwater and marine fish, the hypothesis that freshwater parasites experience higher rates of diversification than marine ones is tested by contrasting the relative numbers of species per parasite genus between the regional endohelminth faunas of fish in both environments. The relationship between the number of parasite genera and the number of parasite species per host was well described by a power function, in both environments; although the exponent of this function was slightly lower for freshwater parasite faunas than marine ones, the difference was not significant. However, the ratio between the number of parasite species and the number of parasite genera per host species was significantly higher in freshwater fish than in marine ones. These findings suggest fundamental differences between the way parasite faunas diversify in freshwater versus marine habitats, with the independent evolution of conspecific parasite populations in isolated host populations being a more common phenomenon in freshwater environments.
No preview · Article · Jan 2016 · International journal for parasitology
[Show abstract][Hide abstract] ABSTRACT: Ocean acidification is already having measurable impacts on marine ecosystems. Intraspecific variation in the responses of marine organisms to ocean acidification can reveal genetic differences in tolerance to low pH conditions and determine the potential for a species to adapt to a changing environment. This study tests for the existence of genetic variation in both the transmission success of the trematode Maritrema novaezealandense to its second intermediate amphipod host, Paracalliope novizealandiae, and the extent of parasite-induced mortality in that host, in response to decreasing pH. Eight parasite genotypes were tested in a custom-built ocean acidification simulation system, at 8.1 pH (current ocean conditions) and under conditions of 7.4 pH (worst-case scenario future prediction). The parasites had significantly higher infection success in the more acidic treatment, but there was no significant difference among genotypes in how infection success was affected by pH. In contrast, some parasite genotypes induced higher mortality in amphipods than other genotypes, but this genetic effect was also independent of pH. Overall, our results reveal no significant intergenotype variation in how the parasite responds to ocean acidification with respect to two key traits, infection success and parasite-induced host mortality, suggesting limited potential for adaptation in the face of acidifying conditions.
[Show abstract][Hide abstract] ABSTRACT: Host-parasite co-evolution can lead to genetic differentiation among isolated host-parasite populations and local adaptation between parasites and their hosts. However, tests of local adaptation rarely consider multiple fitness-related traits although focus on a single component of fitness can be misleading. Here, we concomitantly examined genetic structure and co-divergence patterns of the trematode Coitocaecum parvum and its crustacean host Paracalliope fluviatilis among isolated populations using the mitochondrial cytochrome oxidase I gene (COI). We then performed experimental cross-infections between two genetically divergent host-parasite populations. Both hosts and parasites displayed genetic differentiation among populations, although genetic structure was less pronounced in the parasite. Data also supported a co-divergence scenario between C. parvum and P. fluviatilis potentially related to local co-adaptation. Results from cross-infections indicated that some parasite lineages seemed to be locally adapted to their sympatric (home) hosts in which they achieved higher infection and survival rates than in allopatric (away) amphipods. However, local, intrinsic host and parasite characteristics (host behavioural or immunological resistance to infections, parasite infectivity or growth rate) also influenced patterns of host-parasite interactions. For example, overall host vulnerability to C. parvum varied between populations, regardless of parasite origin (local vs. foreign), potentially swamping apparent local co-adaptation effects. Furthermore, local adaptation effects seemed trait specific; different components of parasite fitness (infection and survival rates, growth) responded differently to cross-infections. Overall, data show that genetic differentiation is not inevitably coupled with local adaptation, and that the latter must be interpreted with caution in a multi-trait context.
Full-text · Article · Dec 2015 · Biological Journal of the Linnean Society
[Show abstract][Hide abstract] ABSTRACT: Variations in levels of parasitism among individuals in a population of hosts underpin the importance of parasites as an evolutionary or ecological force. Factors influencing parasite richness (number of parasite species) and load (abundance and biomass) at the individual host level ultimately form the basis of parasite infection patterns. In fish, diet range (number of prey taxa consumed) and prey selectivity (proportion of a particular prey taxon in the diet) have been shown to influence parasite infection levels. However, fish diet is most often characterized at the species or fish population level, thus ignoring variation among conspecific individuals and its potential effects on infection patterns among individuals. Here, we examined parasite infections and stomach contents of New Zealand freshwater fish at the individual level. We tested for potential links between the richness, abundance and biomass of helminth parasites and the diet range and prey selectivity of individual fish hosts. There was no obvious link between individual fish host diet and helminth infection levels. Our results were consistent across multiple fish host and parasite species and contrast with those of earlier studies in which fish diet and parasite infection were linked, hinting at a true disconnect between host diet and measures of parasite infections in our study systems. This absence of relationship between host diet and infection levels may be due to the relatively low richness of freshwater helminth parasites in New Zealand and high host-parasite specificity.
[Show abstract][Hide abstract] ABSTRACT: Host-parasite checklists are essential resources in ecological parasitology, and are regularly used as sources of data in comparative studies of parasite species richness across host species, or of host specificity among parasite species. However, checklists are only useful datasets if they are relatively complete, that is, close to capturing all host–parasite associations occurring in a particular region. Here, we use three approaches to assess the completeness of 25 checklists of metazoan parasites in vertebrate hosts from various geographic regions. First, treating checklists as interaction networks between a set of parasite species and a set of host species, we identify networks with a greater connectance (proportion of realized host–parasite associations) than expected for their size. Second, assuming that the cumulative rise over time in the number of known host–parasite associations in a region tends toward an asymptote as their discovery progresses, we attempt to extrapolate the estimated total number of existing associations. Third, we test for a positive correlation between the number of published reports mentioning an association and the time since its first record, which is expected because observing and reporting host–parasite associations are frequency-dependent processes. Overall, no checklist fared well in all three tests, and only three of 25 passed two of the tests. These results suggest that most checklists, despite being useful syntheses of regional host–parasite associations, cannot be used as reliable sources of data for comparative analyses.
[Show abstract][Hide abstract] ABSTRACT: The interactive effects of ocean acidification (OA) and parasitic infection have the potential to alter the performance of many marine organisms. Parasitic infection can affect host organisms' response to abiotic stressors, and vice versa, while the response of marine organisms to stressors associated with OA can vary within and between taxonomic groups (host or parasite). Accordingly, it seems likely that the combination of infection stress and the novel stressors associated with OA could alter previously stable host-parasite interactions. This study is a detailed investigation into the changes to shell growth, dissolution, and tensile strength in the New Zealand mud snail Zeacumantus subcarinatus caused by trematode infection in combination with exposure to simulated OA conditions. This study also tests the effects of reduced pH on snails infected by 3 different trematode species to investigate potential species-specific effects of infection. After a 90 d exposure to 3 pH treatments (pH 8.1, 7.6, and 7.4), acidified seawater caused significant reductions in shell growth, length, and tensile strength in all snails. Trematode infected snails displayed increased shell growth and dissolution and reduced shell strength relative to uninfected conspecifics. In all measured variables, there were also significant differences between snails maintained at the same pH but infected by different species of parasite. These results indicate that parasitic infection has the potential to alter host organisms' response to OA and that the magnitude of this effect varies among parasite species.
Full-text · Article · Oct 2015 · Marine Ecology Progress Series
[Show abstract][Hide abstract] ABSTRACT: The ability to manipulate host behaviour is among the most fascinating and best-studied adaptations of parasites. In this opinion article, we highlight trends and biases in the study of this phenomenon that may cloud or limit our understanding of its evolution. For instance, reviews and theoretical studies have shown a disproportionately sharp increase in the past decade relative to empirical studies. We also discuss taxonomic biases in the host-parasite systems investigated, as well as recent declines in the use of experimental infections and in the proportion of these systems in which fitness benefits for the parasites have been confirmed. We finish this opinion article by offering recommendations for the continued success of research in this area.
No preview · Article · Oct 2015 · Trends in Parasitology
[Show abstract][Hide abstract] ABSTRACT: 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 different 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 susceptibility 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.
[Show abstract][Hide abstract] ABSTRACT: Species of Apatemon Szidat, 1928 and Australapatemon Sudarikov, 1959 are reported from New Zealand for the first time, and their life cycles are resolved using molecular sequence data (28S and ITS rDNA regions and mitochondrial COI). The metacercaria of Apatemon sp. 'jamiesoni' ex Gobiomorphus cotidianus and its cercaria ex Potamopyrgus antipodarum are described in detail. Its adult, found in Anas platyrhynchos and Phalacrocorax punctatus, is identified by molecular sequence data. Apatemon sp. 'jamiesoni' uses a different species of snail host, exhibits consistent differences in the genetic markers examined and its single described adult differs from known species so as to be considered distinct, but its formal description awaits additional adult specimens. Australapatemon niewiadomski n. sp. is described from Anas platyrhynchos. It is distinguished morphologically by the absence of a ringnapf and its overall smaller size compared to most other Australapatemon spp. except Au. magnacetabulum and Au. minor, which are smaller in nearly all features than the new species. Au. niewiadomski n. sp. metacercaria and its intermediate host (Barbronia weberi) are identified via matching of molecular sequence data. The status of Apatemon and Australapatemon as distinct genera is confirmed based on their respective monophyly, and genetic divergence between them is comparable to other well-established genera in the Strigeidae. The diagnosis of Australapatemon is emended. Life history data, accurate patterns of host specialisation and distribution, alongside concurrent molecular and morphological evidence would be useful for an integrative taxonomical approach towards the elucidation of species diversity in this group.
Full-text · Article · Sep 2015 · Parasitology Research
[Show abstract][Hide abstract] ABSTRACT: When hosts experience environmental stress, the quantity and quality of resources they provide for parasites may be diminished, and host longevity may be decreased. Under stress, parasites may adopt alternative strategies to avoid fitness reductions. Trematode parasites typically have complex life cycles, involving asexual reproduction in a gastropod first intermediate host. A rare phenomenon, briefly mentioned in the literature, and termed 'precocious encystment' involves the next stage in the parasites' life cycle (metacercarial cyst) forming within the preceding stage (redia), while still inside the snail. In the trematode Parorchis sp. NZ using rocky shore snails exposed to long periods outside water, we hypothesised this might be an adaptive strategy against desiccation, preventing parasite emergence from the snail. To test this, we first investigated the effect of prolonged desiccation on the survival of two species of high intertidal snails. Secondly, we measured the reproductive output (cercarial production) of the parasite under wet and dry conditions. Finally, we quantified the influence of desiccation stress on the occurrence of precocious encystment. Snail mortality was higher under dry conditions, indicating stress, and it was somewhat exacerbated for infected snails. Parasite reproductive output differed between wet and dry conditions, with parasites of snails kept in dry conditions producing more cercariae when placed in water. Little variation was observed in the occurrence of precocious encystment, although some subtle patterns emerged. Given the stresses associated with living in high intertidal environments, we discuss precocious encystment as a possible stress response in this trematode parasite.
Full-text · Article · Sep 2015 · Parasitology International
[Show abstract][Hide abstract] ABSTRACT: In many host–parasite systems, infection risk can be reduced by high local biodiversity, though the mitigating effects of diversity are context dependent and not universal.In aquatic ecosystems, local fauna can reduce the transmission success of parasite free-swimming infective stages by preying on them, acting as decoy hosts, or physically interfering with transmission. However, most prior research has focused on the effect of a single non-host organism at a time and/or has been performed under simplified and artificial conditions.Here, using data on 11 trematode species sampled in different New Zealand lakes, we test whether local biodiversity affects infection risk in target second intermediate hosts, as well as total parasite population size (number of parasites per m2), under natural conditions. We considered four components of local biodiversity: total biomass of non-host fish species, diversity (Simpson index) of non-host benthic invertebrates, total density of zooplankton and macrophyte biomass. Our analyses also accounted for host density, a known determinant of parasite prevalence, intensity of infection and total parasite population density.The only influence of local biodiversity we detected was a negative effect of the diversity of non-host benthic invertebrates on the prevalence achieved by trematodes in their second intermediate hosts: that is, the proportion of individual hosts that are infected. Interestingly, this effect was discernible in all 11 trematode species considered here, even if very weak within some species.Our findings suggest that higher non-host benthic diversity may generally decrease infection risk for target hosts including snails, arthropods and fish. However, reduced infection success did not automatically mean smaller overall parasite population size, as other factors can maintain the parasite population in the face of high local diversity of non-hosts.
[Show abstract][Hide abstract] ABSTRACT: The standing crop biomass of different populations or trophic levels reflects patterns of energy flow through an ecosystem. The contribution of parasites to total biomass is often considered negligible; recent evidence suggests otherwise, although it comes from a narrow range of natural systems. Quantifying how local parasite biomass, whether that of a single species or an assemblage of species sharing the same host, varies across localities with host population biomass, is critical to determine what constrains parasite populations. We use an extensive dataset on all free-living and parasitic metazoan species from multiple sites in New Zealand lakes to measure parasite biomass and test how it covaries with host biomass. In all lakes, trematodes had the highest combined biomass among parasite taxa, ranging from about 0.01 to 0.25 g m−2, surpassing the biomass of minor free-living taxa. Unlike findings from other studies, the life stage contributing the most to total trematode biomass was the metacercarial stage in the second intermediate host, and not sporocysts or rediae within snail first intermediate hosts, possibly due to low prevalence and small snail sizes. For populations of single parasite species, we found no relationship between host and parasite biomass for either juvenile or adult nematodes. In contrast, all life stages of trematodes had local biomasses that correlated positively with those of their hosts. For assemblages of parasite species sharing the same host, we found strong relationships between local host population biomass and the total biomass of parasites supported. In these host–parasite biomass relationships, the scaling factor (slope in log-log space) suggests that parasites may not be making full use of available host resources. Host populations appear capable of supporting a little more parasite biomass, and may be open to expansion of existing parasites or invasion by new ones.
[Show abstract][Hide abstract] ABSTRACT: Within any parasite species, abundance varies spatially, reaching higher values in certain localities than in others, presumably reflecting the local availability of host resources or the local suitability of habitat characteristics for free-living stages. In the absence of strong interactions between two species of helminths with complex life cycles, we might predict that the degree to which their abundances covary spatially is determined by their common resource requirements, i.e. how many host species they share throughout their life cycles. We test this prediction using five trematode species, all with a typical three-host cycle, from multiple lake sampling sites in New Zealand's South Island: Stegodexamene anguillae, Telogaster opisthorchis, Coitocaecum parvum, Maritrema poulini, and an Apatemon sp. Pairs of species from this set of five share the same host species at either one, two, or all three life cycle stages. Our results show that when two trematode species share the same host species at all three life stages, they show positive spatial covariation in abundance (of metacercarial and adult stages) across localities. When they share hosts at two life stages, they show positive spatial covariation in abundance in some cases but not others. Finally, if two trematode species share only one host species, at a single life stage, their abundances do not covary spatially. These findings indicate that the extent of resource sharing between parasite species can drive the spatial match-mismatch between their abundances, and thus influence their coevolutionary dynamics and the degree to which host populations suffer from additive or synergistic effects of multiple infections.
Full-text · Article · Jun 2015 · Parasitology Research
[Show abstract][Hide abstract] ABSTRACT: Zooplankton and phytoplankton communities play host to a wide diversity of parasites, which have been found to play a significant role in a number of ecosystem processes, such as facilitating energy transfer and promoting species succession through altering interspecific competition. Yet we know little about the mechanisms that drive parasite dynamics in aquatic ecosystems. Recent mathematical models have shown how habitat can shape parasite dynamics through influencing the efficacy of parasite transmission; however, these predictions have yet to be tested at larger ecological scales. Here, we present a comparative analysis of parasitism in planktonic communities, assembling data from a range of host and parasite taxa, habitat types, and geographic regions. Our results suggest that the prominent depth-prevalence relationship observed in studies on Daphnia in temperate lakes of North America is applicable to a wide range of aquatic habitats, hosts, and parasites; however, differences in transmission strategies between parasites can lead to considerable variation in parasite dynamics. Observational studies which incorporate a diversity of habitat types will be important in uncovering the mechanisms which underlie this relationship. In particular, more experimental work on transmission stage survivability and infectivity in aquatic environments will be necessary before we can make accurate predictive models of parasite spread in these ecosystems.
[Show abstract][Hide abstract] ABSTRACT: Theory predicts the bottom–up coupling of resource and consumer densities, and epidemiological models make the same prediction for host–parasite interactions. Empirical evidence that spatial variation in local host density drives parasite population density remains scarce, however. We test the coupling of consumer (parasite) and resource (host) populations using data from 310 populations of metazoan parasites infecting invertebrates and fish in New Zealand lakes, spanning a range of transmission modes. Both parasite density (no. parasites per m2) and intensity of infection (no. parasites per infected hosts) were quantified for each parasite population, and related to host density, spatial variability in host density and transmission mode (egg ingestion, contact transmission or trophic transmission). The results show that dense and temporally stable host populations are exploited by denser and more stable parasite populations. For parasites with multi-host cycles, density of the ‘source’ host did not matter: only density of the current host affected parasite density at a given life stage. For contact-transmitted parasites, intensity of infection decreased with increasing host density. Our results support the strong bottom–up coupling of consumer and resource densities, but also suggest that intraspecific competition among parasites may be weaker when hosts are abundant: high host density promotes greater parasite population density, but also reduces the number of conspecific parasites per individual host.