Endohelminth diversity in the invasive Clarias gariepinus (Burchell, 1822) from two freshwater lakes: Naivasha and Ol'Bolossat, Kenya

Fish parasitology contributes to our understanding of the potential risks posed by diverse groups of parasitic organisms on fish stocks in either wild and culture systems. This study was conducted in May 2023 and aimed at assessing the diversity of endohelminths in the invasive North African catfish Clarias gariepinus (Burchell, 1822) obtained from two freshwater lakes, Naivasha and Ol'Bolossat, in Kenya. Parasitological examination of 66 and 35 fish samples collected from the two lakes respectively was achieved using light and scanning electron microscopy methods. Results revealed endohelminth diversity broadly classified as four digeneans, two nematodes, and one cestode. Seven taxa of endohelminths were found in C. gariepinus samples, but only four of these taxa could be identified up to the species level. Six of the taxa (Diplostomum sp., Tylodelphys mashonense, Plagiorchioidea sp., Paracamallanus cyathopharynx, Contracaecum sp., and Tetracampos ciliotheca) were common in samples from the two lakes. Glossidium pedatum only occurred in samples from Lake Ol'Bolossat. Parasite prevalence ranged from 8.6 (T. mashonense) to 100% (Diplostomum sp., T. ciliotheca, and Contracaecum sp.) and mean intensity from 1.4 (T. mashonense) to 16.9 (Diplostomum sp.). The diversity and richness indices were comparatively higher in fish samples from Lake Ol'Bolossat and attributed to the occurrence of G. pedatum in the Ol'Bolossat. However, parasitic infestation of fish samples from the two lakes depicted close similarity, both in diversity and prevalence. These findings form an important baseline data for further follow-up studies, and they suggest the need for further molecular analyses to fully describe three of the taxa only identified up to the genus level.

Parasite-mediated predation determines infection in a complex predator-prey-parasite system

The interplay of host-parasite and predator-prey interactions is critical in ecological dynamics because both predators and parasites can regulate communities. But what is the prevalence of infected prey and predators when a parasite is transmitted through trophic interactions considering stochastic demographic changes? Here, we modelled and analysed a complex predator-prey-parasite system, where parasites are transmitted from prey to predators. We varied parasite virulence and infection probabilities to investigate how those evolutionary factors determine species' coexistence and populations' composition. Our results show that parasite species go extinct when the infection probabilities of either host are small and that success in infecting the final host is more critical for the survival of the parasite. While our stochastic simulations are consistent with deterministic predictions, stochasticity plays an important role in the border regions between coexistence and extinction. As expected, the proportion of infected individuals increases with the infection probabilities. Interestingly, the relative abundances of infected and uninfected individuals can have opposite orders in the intermediate and final host populations. This counterintuitive observation shows that the interplay of direct and indirect parasite effects is a common driver of the prevalence of infection in a complex system.

Selected Wildlife Trematodes

The trematodes are a species-rich group of parasites, with some estimates suggesting that there are more than 24,000 species. However, the complexities associated with their taxonomic status and nomenclature can hinder explorations of the biology of wildlife trematodes, including fundamental aspects such as host use, life cycle variation, pathology, and disease. In this chapter, we review work on selected trematodes of amphibians, birds, mammals, and their snail intermediate hosts, with the goal of providing a tool kit on how to study trematodes of wildlife. We provide a brief introduction to each group of wildlife trematodes, followed by some examples of the challenges each group of trematodes has relative to the goal of their identification and understanding of the biology and interactions these organisms have with their wildlife hosts.

Season and prey identity mediate the effect of predators on parasites in rodents: a test of the healthy herds hypothesis

The healthy herds hypothesis (HHH) suggests that predators decrease parasitism in their prey. Repeated tests of this hypothesis across a range of taxa and ecosystems have revealed significant variation in the effect of predators on parasites in prey. Differences in the response to predators (1) between prey taxa, (2) between seasons, and (3) before and after catastrophic disturbance are common in natural systems, but typically ignored in empirical tests of the HHH. We used a predator exclusion experiment to measure the effect of these heterogeneities on the tri-trophic interaction among predators, parasites and prey. We experimentally excluded mammalian predators from the habitats of hispid cotton rats (Sigmodon hispidus) and cotton mice (Peromyscus gossypinus) and measured the effect of exclusion on gastrointestinal parasites in these rodents. Our experiment spanned multiple seasons and before and after a prescribed burn. We found that the exclusion of the same predators had opposite effects on the parasites of small mammal prey species. Additionally, we found that the effect of mammal exclusion on parasitism differed before versus after fire disturbance. Finally, we saw that the effect of predator exclusion was highly dependent on prey capture season. Significant effects of exclusion emerged primarily in the fall and winter months. The presence of so many different effects in one relatively simple system suggests that predator effects on parasites in prey are highly context dependent.

Can predators stabilize host–parasite interactions? Changes in aquatic predator identity alter amphibian responses and parasite abundance across life stages

The role of parasites can change depending on the food web community. Predators, for instance, can amplify or dilute parasite effects on their hosts. Likewise, exposure to parasites or predators at one life stage can have long‐term consequences on individual performance and survival, which can influence population and disease dynamics. To understand how predators affect amphibian parasite infections across life stages, we manipulated exposure of northern leopard frog (Rana pipiens) tadpoles to three predators (crayfish [Orconectes rusticus], bluegill [Lepomis macrochirus], or mosquitofish [Gambusia affinis]) and to trematode parasites (Echinostoma spp.) in mesocosms and followed juveniles in outdoor terrestrial enclosures through overwintering. Parasites and predators both had strong impacts on metamorphosis with bluegill and parasites individually reducing metamorph survival. However, when fish were present, the negative effects of parasites on survival was not apparent, likely because fish altered community composition via increased algal food resources. Bluegill also reduced snail abundance, which could explain reduced abundance of parasites in surviving metamorphs. Bluegill and parasite exposure increased mass at metamorphosis, which increased metamorph jumping, swimming, and feeding performance, suggesting that larger frogs would experience better terrestrial survival. Effects on size at metamorphosis persisted in the terrestrial environment but did not influence overwintering survival. Based on our results, we constructed stage‐structured population models to evaluate the lethal and sublethal effects of bluegill and parasites on population dynamics. Our models suggested that positive effects of bluegill and parasites on body size may have greater effects on population growth than the direct effects of mortality. This study illustrates how predators can alter the outcome of parasitic infections and highlights the need for long‐term experiments that investigate how changes in host–parasite systems alter population dynamics. We show that some predators reduce parasite effects and have indirect positive effects on surviving individuals potentially increasing host population persistence.

Gastrointestinal parasites of six large mammals in the Wasgomuwa National Park, Sri Lanka

Gastrointestinal (GI) parasites may impose detrimental consequences on wildlife populations due to their capacity to cause mortality and reduce fitness. Additionally, wild animals play an important role in the transmission of zoonoses. Despite this importance, information on GI parasites of tropical wild mammals is critically lacking. The present study aimed to document GI parasites of six wild-dwelling large mammal taxa in Sri Lanka: Asian elephant (Elephas maximus), Sloth bear (Melursus ursinus), civet (Paradoxurus sp.), Leopard (Panthera pardus), Grey langur (Semnopithecus priam) and buffalo (Bubalus sp). Fresh faecal samples (n = 56) collected from the Wasgomuwa National Park, Sri Lanka were subjected to coprological examination using faecal smears, and the brine floatation technique followed by microscopic identification; quantitative data were accrued using the formol-ether method. The survey revealed a high prevalence of GI parasites, where 86% (48/56) of faecal samples screened positive for parasitic infections. Faecal samples of the civet, buffalo and Leopard recorded 100% prevalence, while the lowest (40%) was recorded for the Grey langur. Eight types of GI parasites were documented: protozoan cysts, platyhelminth ova (three types of digenean and a single cyclophillidean type), nematode ova (strongyle, strongyloid, ascarid, and trichuroid types) and rhabditiform larvae. The buffaloes and civets had a comparatively high number and diversity of GI parasites (buffalo: 7 types, H’ = 1.02; civet: 6 types, H’ = 1.52), whilst only a single type (digenean) was detected in the Grey langur. Likewise, parasite loads were also highly variable; highest in the bear (486 per g faeces) and lowest in the monkey (10 per g faeces). The outcome of this survey is important on two accounts; i) to fill the knowledge gap on GI parasites of tropical wild mammals, and ii) the revelation of many first-time parasite-host records for some of the threatened wild-dwelling large mammals in Sri Lanka.

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GI parasites of six native large mammals from the wild recorded from Sri Lanka.

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Novel GI parasite records of locally and globally endangered large mammals reported.

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High prevalence (86%) of infected fecal samples; eight GI parasite types identified.

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Buffaloes and civets had high parasite diversity; highest parasite load in the Sloth bear.

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GI parasite diversity and prevalence differed between wild and captive mammals.

Do predators keep prey healthy or make them sicker? A meta-analysis

Ecological theory suggests that predators can either keep prey populations healthy by reducing parasite burdens or alternatively, increase parasitism in prey. To quantify the overall magnitude and direction of the effect of predation on parasitism in prey observed in practice, we conducted a meta-analysis of 47 empirical studies. We also examined how study attributes, including parasite type and life cycle, habitat type, study design, and whether predators were able to directly consume prey contributed to variation in the predator-prey-parasite interaction. We found that the overall effect of predation on parasitism differed between parasites and parasitoids and that whether consumptive effects were present, and whether a predator was a non-host spreader of parasites, were the most important traits predicting the parasite response. Our results suggest that the mechanistic basis of predator-prey interactions strongly influences the effects of predators on parasites and that these effects, although context dependent, are predictable.

Mechanisms by which predators mediate host-parasite interactions in aquatic systems

It is often assumed that predators reduce disease prevalence and transmission by lowering prey population density and/or by selectively feeding on infected individuals. However, recent studies, many of which come from aquatic systems, suggest numerous alternative mechanisms by which predators can influence disease dynamics in their prey. Here, we review the mechanisms by which predators can mediate host-parasite interactions in aquatic prey. We highlight how life histories of aquatic hosts and parasites influence transmission pathways and describe how such pathways intersect with predation to shape disease dynamics. We also provide recommendations for future studies; experiments that account for multiple effects of predators on host-parasite interactions, and that examine how predator-host-parasite interactions shift under changing environmental conditions, are particularly needed.

The Atrazine Saga and its Importance to the Future of Toxicology, Science, and Environmental and Human Health

The herbicide atrazine is one of the most commonly used, well studied, and controversial pesticides on the planet. Much of the controversy involves the effects of atrazine on wildlife, particularly amphibians, and the ethically questionable decision making of members of industry, government, the legal system, and institutions of higher education, in most cases in an effort to "bend science," defined as manipulating research to advance economic, political, or ideological ends. In this Critical Perspective I provide a timeline of the most salient events in the history of the atrazine saga, which includes a multimillion-dollar smear campaign, lawsuits, investigative reporting, accusation of impropriety against the US Environmental Protection Agency, and a multibillion-dollar transaction. I argue that the atrazine controversy must be more than just a true story of cover-ups, bias, and vengeance. It must be used as an example of how manufacturing uncertainty and bending science can be exploited to delay undesired regulatory decisions and how greed and conflicts of interest-situations where personal or organizational considerations have compromised or biased professional judgment and objectivity-can affect environmental and public health and erode trust in the discipline of toxicology, science in general, and the honorable functioning of societies. Most importantly, I offer several recommendations that should help to 1) prevent the history of atrazine from repeating itself, 2) enhance the credibility and integrity of science, and 3) enrich human and environmental health. Environ Toxicol Chem 2021;40:1544-1558. © 2021 SETAC.

Are Vulnerable Communities Thoroughly Informed on Mosquito Bio-Ecology and Burden?

Mosquitoes account for a significant burden of morbidity and mortality globally. Despite evidence of (1) imminent anthropogenic climate and environmental changes, (2) vector-pathogen spatio-temporal dynamics and (3) emerging and re-emerging mosquito borne infections, public knowledge on mosquito bio-ecology remain scant. In particular, knowledge, attitude and practices (KAPs) on mosquitoes are often neglected despite otherwise expensive remedial efforts against consequent infections and other indirect effects associated with disease burden. To gather baseline KAPs that identify gaps for optimising vector-borne disease control, we surveyed communities across endemic and non-endemic malaria sub-districts (Botswana). The study revealed limited knowledge of mosquitoes and their infections uniformly across endemic and non-endemic areas. In addition, a significant proportion of respondents were concerned about mosquito burdens, although their level of personal, indoor and environmental protection practices varied significantly across sub-districts. Given the limited knowledge displayed by the communities, this study facilitates bridging KAP gaps to minimise disease burdens by strengthening public education. Furthermore, it provides a baseline for future studies in mosquito bio-ecology and desirable control practices across differential spheres of the rural–urban lifestyle, with implications for enhanced livelihoods as a consequence of improved public health.

Effects of different roadway deicing salts on host-parasite interactions: The importance of salt type

The application of roadway deicing salts is increasing the salinity of freshwater systems. Increased salinization from salts, such as NaCl, CaCl₂ and MgCl_2, can have direct, negative impacts on freshwater organisms at concentrations found in nature. Yet, our understanding of how these salts can indirectly impact freshwater organisms by altering important ecological interactions, such as those between hosts and their parasites, is limited. Using a larval amphibian and infectious free-living helminth (i.e. trematode) model, we examined whether exposure to environmentally relevant concentrations of NaCl, CaCl₂ and MgCl₂ 1) influence trematode mortality; 2) alter amphibian-trematode interactions; and 3) alter larval amphibian activity (a behavior associated with parasite avoidance). We found that exposure to CaCl₂ greatly reduced trematode survival across all Cl^- concentrations (230, 500, 860 and 1000 mg Cl^- L^-1) while NaCl and MgCl₂ had no effect. When both host and parasites were exposed to the salts, exposure to NaCl, but not MgCl₂ or CaCl₂, increased infection. The lack of effect of CaCl₂ on infection was likely driven by CaCl₂ reducing trematode survival. Exposure to NaCl increased infection at 500 mg Cl^- L^-1, but not 230 or 860 mg Cl^- L^-1. Increased infection was not due to salt exposure altering tadpole behavior. Our results suggest that NaCl can negatively impact amphibian populations indirectly by increasing trematode infections in tadpole hosts.

Predator density modifies mosquito regulation in increasingly complex environments

BACKGROUND

Predation plays a pivotal role in the composition and functioning of ecosystems. Both habitat complexity and predator density are important contexts which may determine the strength of trophic and non-trophic interactions. In aquatic systems, the efficacy of natural enemies in regulating vector pest species could be modified by such context dependencies. Here, we use a functional response (FR) approach to experimentally quantify conspecific multiple predator effects across a habitat complexity gradient of two notonectids, Anisops sardea and Enithares chinai, towards larvae of the vector mosquito Culex pipiens pipiens.

RESULTS

E. chinai exhibited significantly greater consumption rates than A. sardea across habitat complexities, both as individuals and conspecific pairs. Each predator type displayed Type II FRs across experimental treatments, with synergistic multiple predator effects (i.e. prey risk enhancement) displayed in the absence of habitat complexity. Effects of increasing habitat complexity modified multiple predator effects differentially between species given behavioral differences, with habitat complexity causing significant antagonism (i.e. prey risk reduction) with multiple A. sardea compared to E. chinai.

CONCLUSION

Habitat complexity effects on multiple predator interactions can manifest differently at the species level, suggesting emergent effects which complicate predictions of natural enemy impact in heterogenous environments. Considerations of density, diversity and habitat effects on efficacies of natural enemies should thus be considered by pest management practitioners to better explain biocontrol efficacies in increasingly diverse environments. © 2020 Society of Chemical Industry.

Intraguild predation decreases predator fitness with potentially varying effects on pathogen transmission in a herbivore host

Predators and pathogens often regulate the population dynamics of their prey or hosts. When species interact with both their predators and their pathogens, understanding each interaction in isolation may not capture the system's dynamics. For instance, predators can influence pathogen transmission via consumptive effects, such as feeding on infected prey, or non-consumptive effects, such as changing the prey's susceptibility to infection. A prey species' infection status can, in turn, influence predator's choice of prey and have negative fitness consequences for the predator. To test how intraguild predation (IGP), when predator and pathogen share the same prey/host, affects pathogen transmission, predator preference, and predator fitness, we conducted a series of experiments using a crop pest (Pseudoplusia includens), a generalist predator (Podisus maculiventris), and a generalist pathogen (Autographa californica multicapsid nuclear polyhedrovirus, AcMNPV). Using a field experiment, we quantified the effects of consumptive and non-consumptive predators on pathogen transmission. We found that a number of models provided similar fits to the data. These models included null models showing no effects of predation and models that included a predation effect. We also found that predators consumed infected prey more often when choosing between live infected or live healthy prey. Infected prey also reduced predator fitness. Developmental times of predators fed infected prey increased by 20% and longevity decreased by 45%, compared with those that consumed an equivalent number of non-infected prey. While this research shows an effect of the pathogen on intraguild predator fitness, we found no support that predators affected pathogen transmission.

How intraguild predation affects the host diversity-disease relationship in a multihost community

Broad evidence has shown that host diversity can impede disease invasion and reduce the eventual prevalence, but little is known on how species interactions play in shaping this host diversity-disease relationship. Previous work has illustrated that intraguild predation (IGP), combined with parasite-mediated indirect effects, can have strong influences on parasitic infection. Following this line of thinking, we here examine the role of predatory interactions in the disease transmission within a multihost community. Through varying fractions of IGP in a competitive community, we show that, dependent on the fraction of predatory interactions, species richness can switch from enhancing to inhibiting disease establishment/prevalence. Without IGP interactions, high host species richness can likely weaken the 'dilution effect' and in some cases even enhance the disease establishment (and/or prevalence) due to the existence of alternative sources for infection, whereas IGP can generally heighten the negative diversity-disease relationship due to the reduction of encounter rate between prospective hosts and parasites. Although trait-mediated interactions (captured as the infection-induced changes in predation rate) only weakly affect disease prevalence, density-mediated interactions (captured as the additional infection-induced mortality) can pose a relatively strong influence on disease transmission. Our results thus underline the importance of considering species interactions when investigating the host diversity-disease relationship.

Host resistance and tolerance to parasitism: development-dependent fitness consequences in Common Hourglass Tree Frog (Polypedates cruciger) tadpoles exposed to two larval trematodes

Tolerance and resistance to parasites are defense strategies of host organisms. Here, we tested the development-dependent tolerance and resistance of Polypedates cruciger Blyth, 1852 tadpoles to trematode infection. We exposed the tadpoles at Gosner stages 27, 28–29, and 30–31 to two types of cercariae (furcocercous and pleurolophocercous cercariae of Acanthostomum burminis (Bhalerao, 1926)) under laboratory conditions. To determine tolerance (the ability of a host to limit health effects of a given parasite load), we exposed the tadpoles until all cercariae penetrated the host. As a measure of determining resistance, we exposed tadpoles to cercariae for a limited time and counted the number of cercariae penetrating the tadpoles. The survival of tadpoles exposed at early stages was significantly lower than that of tadpoles exposed at later stages (mixed-effect model, p < 0.05), suggesting an age-dependent tolerance to parasitism. Tadpoles exposed at early stages were also smaller, took longer to metamorphosis, showed lower resistance to parasitism (ANOVA, p < 0.001), and developed axial malformations. In the resistance experiment, fewer parasites penetrated later stage tadpoles than early stage tadpoles. Tadpoles of P. cruciger showed a development-dependent tolerance and resistance to parasitism, resulting in greater survival and fewer malformations when parasitism occurs at late stages.

Disease and the Drying Pond: Examining Possible Links among Drought, Immune Function, and Disease Development in Amphibians

Drought can heavily impact aquatic ecosystems. For amphibian species that rely on water availability for larval development, drought can have direct and indirect effects on larval survival and postmetamorphic fitness. Some amphibian species can accelerate the timing of metamorphosis to escape drying habitats through developmental plasticity. However, trade-offs associated with premature metamorphosis, such as reduced body size and altered immune function in the recently metamorphosed individual, may have downstream effects on susceptibility to disease. Here, we review the physiological mechanisms driving patterns in larval amphibian development under low water conditions. Specifically, we discuss drought-induced accelerated metamorphosis and how it may alter immune function, predisposing juvenile amphibians to infectious disease. In addition, we consider how these physiological and immunological adjustments could play out in a lethal disease system, amphibian chytridiomycosis. Last, we propose avenues for future research that adopt an ecoimmunological approach to evaluate the combined threats of drought and disease for amphibian populations.

Effects of pesticides on exposure and susceptibility to parasites can be generalised to pesticide class and type in aquatic communities

Pesticide pollution can alter parasite transmission, but scientists are unaware if effects of pesticides on parasite exposure and host susceptibility (i.e. infection risk given exposure) can be generalised within a community context. Using replicated temperate pond communities, we evaluate effects of 12 pesticides, nested in four pesticide classes (chloroacetanilides, triazines, carbamates organophosphates) and two pesticide types (herbicides, insecticides) applied at standardised environmental concentrations on larval amphibian exposure and susceptibility to trematode parasites. Most of the variation in exposure and susceptibility occurred at the level of pesticide class and type, not individual compounds. The organophosphate class of insecticides increased snail abundance (first intermediate host) and thus trematode exposure by increasing mortality of snail predators (top-down mechanism). While a similar pattern in snail abundance and trematode exposure was observed with triazine herbicides, this effect was driven by increases in snail resources (periphytic algae, bottom-up mechanism). Additionally, herbicides indirectly increased host susceptibility and trematode infections by (1) increasing time spent in susceptible early developmental stages and (2) suppressing tadpole immunity. Understanding generalisable effects associated with contaminant class and type on transmission is critical in reducing complexities in predicting disease dynamics in at-risk host populations.

Evidence for carry-over effects of predator exposure on pathogen transmission potential

Accumulating evidence indicates that species interactions such as competition and predation can indirectly alter interactions with other community members, including parasites. For example, presence of predators can induce behavioural defences in the prey, resulting in a change in susceptibility to parasites. Such predator-induced phenotypic changes may be especially pervasive in prey with discrete larval and adult stages, for which exposure to predators during larval development can have strong carry-over effects on adult phenotypes. To the best of our knowledge, no study to date has examined possible carry-over effects of predator exposure on pathogen transmission. We addressed this question using a natural food web consisting of the human malaria parasite Plasmodium falciparum, the mosquito vector Anopheles coluzzii and a backswimmer, an aquatic predator of mosquito larvae. Although predator exposure did not significantly alter mosquito susceptibility to P. falciparum, it incurred strong fitness costs on other key mosquito life-history traits, including larval development, adult size, fecundity and longevity. Using an epidemiological model, we show that larval predator exposure should overall significantly decrease malaria transmission. These results highlight the importance of taking into account the effect of environmental stressors on disease ecology and epidemiology.

The distribution of echinostome parasites in ponds and implications for larval anuran survival

Parasites can influence host population dynamics, community composition and evolution. Prediction of these effects, however, requires an understanding of the influence of ecological context on parasite distributions and the consequences of infection for host fitness. We address these issues with an amphibian – trematode (Digenea: Echinostomatidae) host–parasite system. We initially performed a field survey of trematode infection in first (snail) and second (larval green frog, Rana clamitans) intermediate hosts over 5 years across a landscape of 23 ponds in southeastern Michigan. We then combined this study with a tadpole enclosure experiment in eight ponds. We found echinostomes in all ponds during the survey, although infection levels in both snails and amphibians differed across ponds and years. Echinostome prevalence (proportion of hosts infected) in snails also changed seasonally depending on host species, and abundance (parasites per host) in tadpoles depended on host size and prevalence in snails. The enclosure experiment demonstrated that infection varied at sites within ponds, and tadpole survival was lower in enclosures with higher echinostome abundance. The observed effects enhance our ability to predict when and where host–parasite interactions will occur and the potential fitness consequences of infection, with implications for population and community dynamics, evolution and conservation.

Bottom-up and trait-mediated effects of resource quality on amphibian parasitism

Leaf litter subsidies are important resources for aquatic consumers like tadpoles and snails, causing bottom-up effects on wetland ecosystems. Recent studies have shown that variation in litter nutritional quality can be as important as litter quantity in driving these bottom-up effects. Resource subsidies likely also have indirect and trait-mediated effects on predation and parasitism, but these potential effects remain largely unexplored. We generated predictions for differential effects of litter nutrition and secondary polyphenolic compounds on tadpole (Lithobates sylvatica) exposure and susceptibility to Ribeiroia ondatrae, based on ecological stoichiometry and community-ecology theory. We predicted direct and indirect effects on key traits of the tadpole host (rates of growth, development and survival), the trematode parasite (production of the cercaria infective stages) and the parasite's snail intermediate host (growth and reproduction). To test these predictions, we conducted a large-scale mesocosm experiment using a natural gradient in the concentrations of nutrients (nitrogen) and toxic secondary compounds (polyphenolics) of nine leaf litter species. To differentiate between effects on exposure vs. susceptibility to infection, we included multiple infection experiments including one with constant per capita exposure. We found that increased litter nitrogen increased tadpole survival, and also increased cercaria production by the snail intermediate hosts, causing opposing effects on tadpole per capita exposure to trematode infection. Increased litter polyphenolics slowed tadpole development, leading to increased infection by increasing both their susceptibility to infection and the length of time they were exposed to parasites. Based on these results, recent shifts in forest composition towards more nitrogen-poor litter species should decrease trematode infection in tadpoles via density- and trait-mediated effects on the snail intermediate hosts. However, these shifts also involve increased abundance of litter species with high polyphenolic levels, which should increase trematode infection via trait-mediated effects on tadpoles. Future studies will be needed to determine the relative strength of these opposing effects in natural wetland communities. [Correction added after online publication on 5 January 2017: wording changed to 'which should increase trematode infection via trait-mediated effects on tadpoles'.].

Host food resource supplementation increases echinostome infection in larval anurans

Host-parasite interactions are often influenced by environmental factors through multiple mechanisms. For example, changes in host food resources may affect multiple host traits (e.g., body size, behavior, immunocompetence), which may increase or decrease infection levels and the impact of parasites on host fitness. We often lack an understanding of which traits are most important for parasite transmission and fitness effects, posing challenges to predicting consequences of changing environmental conditions (e.g., eutrophication). Here, I examined the effects of food resources and host traits experimentally in a larval frog (Rana clamitans Latreille, 1801)-trematode parasite (Echinostoma revolutum Looss, 1899) system. I hypothesized that higher food resources reduce parasite infection and parasite effects on host growth and survival, due to increased host investment in parasite defenses, which I tested in a laboratory experiment. Contrary to my hypothesis, the results indicated that increased food levels enhanced infection in hosts, while the effect of parasites on survival did not depend on host food resources. A potential explanation for the positive effect of food level on infection was size-dependent infection rates (i.e., higher food levels increased infection through increased host growth), which is supported by a positive relationship between host body size and infection. These findings emphasize the complex relationship between host food resources and parasitism and the importance of environmental context and host traits (i.e., body size) in mediating interactions with parasites. The results also have relevance for conservation in light of rising anthropogenic impacts on aquatic systems and recent amphibian declines.

Effects of Echinostoma trivolvis metacercariae infection during development and metamorphosis of the wood frog (Lithobates sylvaticus)

Many organisms face energetic trade-offs between defense against parasites and other host processes that may determine overall consequences of infection. These trade-offs may be particularly evident during unfavorable environmental conditions or energetically demanding life history stages. Amphibian metamorphosis, an ecologically important developmental period, is associated with drastic morphological and physiological changes and substantial energetic costs. Effects of the trematode parasite Echinostoma trivolvis have been documented during early amphibian development, but effects during later development and metamorphosis are largely unknown. Using a laboratory experiment, we examined the energetic costs of late development and metamorphosis coupled with E. trivolvis infection in wood frogs, Lithobates [=Rana] sylvaticus. Echinostoma infection intensity did not differ between tadpoles examined prior to and after completing metamorphosis, suggesting that metacercariae were retained through metamorphosis. Infection with E. trivolvis contributed to a slower growth rate and longer development period prior to the initiation of metamorphosis. In contrast, E. trivolvis infection did not affect energy expenditure during late development or metamorphosis. Possible explanations for these results include the presence of parasites not interfering with pronephros degradation during metamorphosis or the mesonephros compensating for any parasite damage. Overall, the energetic costs of metamorphosis for wood frogs were comparable to other species with similar life history traits, but differed from a species with a much shorter duration of metamorphic climax. Our findings contribute to understanding the possible role of energetic trade-offs between parasite defense and host processes by considering parasite infection with simultaneous energetic demands during a sensitive period of development.

Combined Effects of Pesticides and Trematode Infections on Hourglass Tree Frog Polypedates cruciger

The impact of widespread and common environmental factors, such as chemical contaminants, on infectious disease risk in amphibians is particularly important because both chemical contaminants and infectious disease have been implicated in worldwide amphibian declines. Here we report on the lone and combined effects of exposure to parasitic cercariae (larval stage) of the digenetic trematode, Acanthostomum burminis, and four commonly used pesticides (insecticides: chlorpyrifos, dimethoate; herbicides: glyphosate, propanil) at ecologically relevant concentrations on the survival, growth, and development of the common hourglass tree frog, Polypedates cruciger Blyth 1852. There was no evidence of any pesticide-induced mortality on cercariae because all the cercariae successfully penetrated each tadpole host regardless of pesticide treatment. In isolation, both cercarial and pesticide exposure significantly decreased frog survival, development, and growth, and increased developmental malformations, such as scoliosis, kyphosis, and also edema and skin ulcers. The combination of cercariae and pesticides generally posed greater risk to frogs than either factor alone by decreasing survival or growth or increasing time to metamorphosis or malformations. The exception was that lone exposure to chlorpyrifos had higher mortality without than with cercariae. Consistent with mathematical models that suggest that stress should increase the impact of generalist parasites, the weight of the evidence from the field and laboratory suggests that ecologically relevant concentrations of agrochemicals generally increase the threat that trematodes pose to amphibians, highlighting the importance of elucidating interactions between anthropogenic activities and infectious disease in taxa of conservation concern.

Density-dependent immunity and parasitism risk in experimental populations of lizards naturally infested by ixodid ticks

When effective immune defenses against parasites are costly and resources limited, individuals are expected to alter their investment in immunity in response to the risk of infection. As an ecological factor that can affect both food abundance and parasite exposure, host density can play an important role in host immunity and host-parasite interactions. High levels of intraspecific competition for food and social stress at high host density may diminish immune defenses and increase host susceptibility to parasites. At the same time, for contagious and environmentally transmitted parasites, parasite exposure often increases with host density, whereas in mobile parasites that actively search for hosts, parasite exposure can decrease with host density due to the "encounter-dilution effect." To unravel these multiple and potentially opposing effects of host density on immunity, we manipulated density of the common lizard Zootoca vivipara and measured local inflammation in response to PHA injection and levels of infestation by the tick Ixodes ricinus, a mobile ectoparasite for which we expected an encounter-dilution effect to occur. Local inflammation strongly decreased with lizard density in adults, but not in yearlings. Tick infestation (abundance and prevalence) was negatively correlated with lizard density in both age classes. Using path analyses, we found independent, direct negative density feedbacks on immunity and parasite exposure in adults, supporting the hypothesis of energy constraints and/or physiological stress acting on immunity at high density. In contrast, for yearlings, the best path model showed that density diluted exposure to parasites, which themselves down-regulated immune defenses in lizards. These results highlight the importance of investigating the pathways among host density, host immunity, and parasite infestation, while accounting for relevant individual traits such as age.

Parasites of Trinidadian guppies: evidence for sex- and age-specific trait-mediated indirect effects of predators

Predation pressure can alter the morphology, physiology, life history, and behavior of prey; each of these in turn can change how surviving prey interact with parasites. These trait-mediated indirect effects may change in direction or intensity during growth or, in sexually dimorphic species, between the sexes. The Trinidadian guppy, Poecilia reticulata presents a unique opportunity to examine these interactions; its behavioral ecology has been intensively studied in wild populations with well-characterized predator faunas. Predation pressure is known to have driven the evolution of many guppy traits; for example, in high-predation sites, females (but not males) tend to shoal, and this anti-predator behavior facilitates parasite transmission. To test for evidence of predator-driven differences in infection in natural populations, we collected 4715 guppies from 62 sites across Trinidad between 2003 and 2009 and screened them for ectosymbionts, including Gyrodactylus. A novel model-averaging analysis revealed that females were more likely to be infected with Gyrodactylus parasites than males, but only in populations with both high predation pressure and high infection prevalence. We propose that the difference in shoaling tendency between the sexes could explain the observed difference in infection prevalence between males and females in high-predation sites. The infection rate of juveniles did not vary with predation regime, probably because juveniles face constant predation pressure from conspecific adults and therefore tend to shoal in both high- and low-predation sites. This represents the first evidence for age- and sex-specific trait-mediated indirect effects of predators on the probability of infection in their prey.

Predator diversity, intraguild predation, and indirect effects drive parasite transmission

Humans are altering biodiversity globally and infectious diseases are on the rise; thus, there is interest in understanding how changes to biodiversity affect disease. Here, we explore how predator diversity shapes parasite transmission. In a mesocosm experiment that manipulated predator (larval dragonflies and damselflies) density and diversity, non-intraguild (non-IG) predators that only consume free-living cercariae (parasitic trematodes) reduced metacercarial infections in tadpoles, whereas intraguild (IG) predators that consume both parasites and tadpole hosts did not. This likely occurred because IG predators reduced tadpole densities and anticercarial behaviors, increasing per capita exposure rates of the surviving tadpoles (i.e., via density- and trait-mediated effects) despite the consumption of parasites. A mathematical model demonstrated that non-IG predators reduce macroparasite infections, but IG predation weakens this "dilution effect" and can even amplify parasite burdens. Consistent with the experiment and model, a wetland survey revealed that the diversity of IG predators was unrelated to metacercarial burdens in amphibians, but the diversity of non-IG predators was negatively correlated with infections. These results are strikingly similar to generalities that have emerged from the predator diversity-pest biocontrol literature, suggesting that there may be general mechanisms for pest control and that biocontrol research might inform disease management and vice versa. In summary, we identified a general trait of predators--where they fall on an IG predation continuum--that predicts their ability to reduce infections and possibly pests in general. Consequently, managing assemblages of predators represents an underused tool for the management of human and wildlife diseases and pest populations.

Trait-Mediated Effects of Parasites on Invader-Native Interactions

Parasites have a variety of behavioural effects on their hosts, which can in turn affect species with which the host interacts. Here we review how these trait-mediated indirect effects of parasites can alter the outcomes of invader-native interactions, illustrating with examples from the literature and with particular regard to the invader-native crustacean systems studied in our laboratories. Parasites may potentially inhibit or exacerbate invasions via their effects on host behaviour, in addition to their direct virulence effects on hosts. In several crustacean systems, we have found that parasites influence both host predation rates on intra- and inter-guild prey and host vulnerability to being preyed upon. These trait effects can theoretically alter invasion impact and patterns of coexistence, as they indirectly affect interactions between predators and prey with the potential for further ramifications to other species in the food web. The fitness consequences of parasite-induced trait-mediated effects are rarely considered in traditional parasitological contexts, but demand attention in the context of ecological communities. We can regard these trait effects as a form of cryptic virulence that only becomes apparent when hosts are examined in the context of the other species with which they interact.

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.

Early-life exposure to a herbicide has enduring effects on pathogen-induced mortality

Exposure to stressors at formative stages in the development of wildlife and humans can have enduring effects on health. Understanding which, when and how stressors cause enduring health effects is crucial because these stressors might then be avoided or mitigated during formative stages to prevent lasting increases in disease susceptibility. Nevertheless, the impact of early-life exposure to stressors on the ability of hosts to resist and tolerate infections has yet to be thoroughly investigated. Here, we show that early-life, 6-day exposure to the herbicide atrazine (mean ± s.e.: 65.9±3.48 µg l(-1)) increased frog mortality 46 days after atrazine exposure (post-metamorphosis), but only when frogs were challenged with a chytrid fungus implicated in global amphibian declines. Previous atrazine exposure did not affect resistance of infection (fungal load). Rather, early-life exposure to atrazine altered growth and development, which resulted in exposure to chytrid at more susceptible developmental stages and sizes, and reduced tolerance of infection, elevating mortality risk at an equivalent fungal burden to frogs unexposed to atrazine. Moreover, there was no evidence of recovery from atrazine exposure. Hence, reducing early-life exposure of amphibians to atrazine could reduce lasting increases in the risk of mortality from a disease associated with worldwide amphibian declines. More generally, these findings highlight that a better understanding of how stressors cause enduring effects on disease susceptibility could facilitate disease prevention in wildlife and humans, an approach that is often more cost-effective and efficient than reactive medicine.

Disentangling the effects of exposure and susceptibility on transmission of the zoonotic parasite Schistosoma mansoni

For all parasites, transmission is composed of two processes: host contact with parasites ('exposure') and risk of infection given such contact ('susceptibility'). Classic models, such as mass action (density-dependent) transmission, lump these processes together. However, separating these processes could enhance predictions for disease dynamics, especially for free-living parasites. Here, we outline three transmission models that partition exposure and susceptibility. Using data from a study of Schistosoma mansoni (trematode) infections in Biomphalaria glabrata snails, we competed these three models against four alternative models, including the mass action model (which lumps exposure and susceptibility). The models that separately accounted for exposure and susceptibility best predicted prevalence across the density gradients of hosts and parasites, outperforming all other models based on Akaike information criterion. When embedded into a dynamic epidemiological model, the exposure-explicit models all predicted lower equilibrium densities of infected snails and human-infectious cercariae. Thus, population-level epidemiological models that utilize the classic mass action transmission model might overestimate human risk of schistosomiasis. More generally, the presented approach for disentangling exposure and susceptibility can distinguish between behavioural and immunological resistance, identify mechanisms of 'disease dilution' and provide a more complete dissection of drivers of parasite transmission.

Linking manipulative experiments to field data to test the dilution effect

The dilution effect, the hypothesis that biodiversity reduces disease risk, has received support in many systems. However, few dilution effect studies have linked mechanistic experiments to field patterns to establish both causality and ecological relevance. We conducted a series of laboratory experiments and tested the dilution effect hypothesis in an amphibian-Batrachochytrium dendrobatidis (Bd) system and tested for consistency between our laboratory experiments and field patterns of amphibian species richness, host identity and Bd prevalence. In our laboratory experiments, we show that tadpoles can filter feed Bd zoospores and that the degree of suspension feeding was positively associated with their dilution potential. The obligate suspension feeder, Gastrophryne carolinensis, generally diluted the risk of chytridiomycosis for tadpoles of Bufo terrestris and Hyla cinerea, whereas tadpoles of B. terrestris (an obligate benthos feeder) generally amplified infections for the other species. In addition, G. carolinensis reduced Bd abundance on H. cinerea more so in the presence than absence of B. terrestris and B. terrestris amplified Bd abundance on H. cinerea more so in the absence than presence of G. carolinensis. Also, when ignoring species identity, species richness was a significant negative predictor of Bd abundance. In our analysis of field data, the presence of Bufo spp. and Gastrophryne spp. were significant positive and negative predictors of Bd prevalence, respectively, even after controlling for climate, vegetation, anthropogenic factors (human footprint), species richness and sampling effort. These patterns of dilution and amplification supported our laboratory findings, demonstrating that the results are likely ecologically relevant. The results from our laboratory and field data support the dilution effect hypothesis and also suggest that dilution and amplification are predictable based on host traits. Our study is among the first to link manipulative experiments, in which a potential dilution mechanism is supported, with analyses of field data on species richness, host identity, spatial autocorrelation and disease prevalence.

Parasite predators exhibit a rapid numerical response to increased parasite abundance and reduce transmission to hosts

Predators of parasites have recently gained attention as important parts of food webs and ecosystems. In aquatic systems, many taxa consume free-living stages of parasites, and can thus reduce parasite transmission to hosts. However, the importance of the functional and numerical responses of parasite predators to disease dynamics is not well understood. We collected host–parasite–predator cooccurrence data from the field, and then experimentally manipulated predator abundance, parasite abundance, and the presence of alternative prey to determine the consequences for parasite transmission. The parasite predator of interest was a ubiquitous symbiotic oligochaete of mollusks, Chaetogaster limnaei limnaei, which inhabits host shells and consumes larval trematode parasites. Predators exhibited a rapid numerical response, where predator populations increased or decreased by as much as 60% in just 5 days, depending on the parasite:predator ratio. Furthermore, snail infection decreased substantially with increasing parasite predator densities, where the highest predator densities reduced infection by up to 89%. Predators of parasites can play an important role in regulating parasite transmission, even when infection risk is high, and especially when predators can rapidly respond numerically to resource pulses. We suggest that these types of interactions might have cascading effects on entire disease systems, and emphasize the importance of considering disease dynamics at the community level.

Interactive effects of competition and predator cues on immune responses of leopard frogs at metamorphosis

Recent hypotheses suggest that immunosuppression, resulting from altered environmental conditions, may contribute to the increased incidence of amphibian disease around the world. Antimicrobial peptides (AMPs) in amphibian skin are an important innate immune defense against fungal, viral and bacterial pathogens. Their release is tightly coupled with release of the stress hormone noradrenaline (norepinephrine). During metamorphosis, AMPs may constitute the primary immune response in the skin of some species because acquired immune functions are temporarily suppressed in order to prevent autoimmunity against new adult antigens. Suppression of AMPs during this transitional stage may impact disease rates. We exposed leopard frog tadpoles (Lithobates pipiens) to a factorial combination of competitor and caged-predator environments and measured their development, growth and production of hydrophobic skin peptides after metamorphosis. In the absence of predator cues, or if the exposure to predator cues was late in ontogeny, competition caused more than a 250% increase in mass-standardized hydrophobic skin peptides. Predator cues caused a decrease in mass-standardized hydrophobic skin peptides when the exposure was late in ontogeny under low competition, but otherwise had no effect. Liquid chromatography tandem mass spectrometry of the skin peptides showed that they include six AMPs in the brevinin and temporin families and at least three of these peptides are previously uncharacterized. Both of these peptide families have previously been shown to inhibit harmful microbes including Batrachochytrium dendrobatidis, the fungal pathogen associated with global amphibian declines. Our study shows that amphibians may be able to adjust their skin peptide defenses in response to stressors that are experienced early in ontogeny and that these effects extend through an important life-history transition.