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Myxozoa represent a diverse group of microscopic cnidarian endoparasites alternating between invertebrate and vertebrate hosts. Of the approximately 2,600 species described predominantly from teleost fish, only 1.8% have been reported from cartilaginous fishes (Elasmobranchii). As ancestral vertebrate hosts of myxozoans, elasmobranchs may have played an important role in myxozoan evolution, however, they are also some of the largest vertebrate hosts known for this group of parasites. We screened 50 elasmobranchs belonging to nine species and seven families, from various geographical areas, for myxozoan infection. We found a 22% overall prevalence of myxozoans in elasmobranchs and describe five species new to science. We investigated, for the first known time, the evolution of spore size within three phylogenetic clades, Ceratomyxa, Sphaerospora sensu stricto and Parvicapsula. We found that spores from elasmobranch-infecting myxozoans were on average 4.8× (Ceratomyxa), 2.2× (Parvicapsula clade) and 1.8× (Sphaerospora sensu stricto except polysporoplasmic Sphaerospora spp.) larger than those from teleosts. In all analysed clades, spore size was correlated with phylogenetic position. In ceratomyxids, it was further strongly positively correlated with fish body size and habitat depth, independent of cellular composition of the spores and phylogenetic position in the tree. While in macroparasites a host size-correlated increase in parasite size occurs on a large scale and is often related to improved exploitation of host resources, in microscopic parasites size ranges vary at the scale of a few micrometres, disproportionate to the available additional space in a large host. We discuss the ecological role of these changes with regard to transmission under high pressure and an invertebrate fauna that is adapted to deeper marine habitats.

Infection intensity can dictate disease outcomes but is typically ignored when modeling infection dynamics of microparasites (e.g., bacteria, virus, and fungi). However, for a number of pathogens of wildlife typically categorized as microparasites, accounting for infection intensity and within-host infection processes is critical for predicting population-level responses to pathogen invasion. Here, we develop a modeling framework we refer to as reduced-dimension host-parasite integral projection models (reduced IPMs) that we use to explore how within-host infection processes affect the dynamics of pathogen invasion and virulence evolution. We find that individual-level heterogeneity in pathogen load-a nearly ubiquitous characteristic of host-parasite interactions that is rarely considered in models of microparasites-generally reduces pathogen invasion probability and dampens virulence-transmission trade-offs in host-parasite systems. The latter effect likely contributes to widely predicted virulence-transmission trade-offs being difficult to observe empirically. Moreover, our analyses show that intensity-dependent host mortality does not always induce a virulence-transmission trade-off, and systems with steeper than linear relationships between pathogen intensity and host mortality rate are significantly more likely to exhibit these trade-offs. Overall, reduced IPMs provide a useful framework to expand our theoretical and data-driven understanding of how within-host processes affect population-level disease dynamics.

Major Histocompatibility Complex (MHC) genes code for proteins that recognize foreign protein antigens to initiate T‐cell‐mediated adaptive immune responses. They are often the most polymorphic genes in vertebrate genomes. How evolution maintains this diversity remains of debate. Three main hypotheses seek to explain the maintenance of MHC diversity by invoking pathogen‐mediated selection: heterozygote advantage, frequency‐dependent selection, and fluctuating selection across landscapes or through time. Here, we use a large‐scale field parasite survey in a stickleback metapopulation to test predictions derived from each of these hypotheses. We identify over 1000 MHC IIβ variants (alleles spanning paralogous genes) and find that many of them covary positively or negatively with parasite load, suggesting that these genes contribute to resistance or susceptibility. However, despite our large sample‐size, we find no evidence for the widely cited stabilizing selection on MHC heterozygosity, in which individuals with an intermediate number of MHC variants have the lowest parasite burden. Nor do we observe a rare‐variant advantage, or widespread fluctuating selection across populations. In contrast, we find that MHC diversity is best predicted by neutral genome‐wide heterozygosity and between‐population genomic divergence, suggesting neutral processes are important in shaping the pattern of metapopulation MHC diversity. Thus, although MHC IIβ is highly diverse and relevant to the type and intensity of macroparasite infection in these populations of stickleback, the main models of MHC evolution still provide little explanatory power in this system.

As global trade of live animals expands, there is increasing need to assess the risks of invasive organisms, including pathogens, that can accompany these translocations. The movement and release of live baitfish by recreational anglers has been identified as a particularly high-risk pathway for the spread of aquatic diseases in the United States. To provide risk-based decision support for preventing and managing disease invasions from baitfish release, we developed a hazard identification and ranking tool to identify the pathogens that pose the highest risk to wild fish via this pathway. We created a screening protocol and semi-quantitative stochastic risk ranking framework, combining published data with expert elicitation (n = 25) and applied the framework to identify high-priority pathogens for the bait supply in Minnesota, USA. Normalized scores were developed for seven risk criteria (likelihood of transfer, prevalence in bait supply, likelihood of colonization, current distribution, economic impact if established, ecological impact if established and host species) to characterize a pathogen's ability to persist in the bait supply and cause impacts to wild fish species of concern. The generalist macroparasite Schizocotyle acheilognathi was identified as presenting highest overall threat, followed by the microsporidian Ovipleistophora ovariae, and viral haemorrhagic septicaemia virus. Our findings provide risk-based decision support for managers charged with maintaining both the recreational fishing industry and sustainable, healthy natural resources. Particularly, the identification of several high-risk but currently unregulated pathogens suggests that focusing risk management on pathogens of concern in all potential host species could reduce disease introduction risk. The ranking process, implemented here for a single state case study, provides a conceptual framework for integrating expert opinion and sparse available data that could be scaled up and applied across jurisdictions to inform risk-based management of the live baitfish pathway.

World-wide, infectious diseases represent a major source of mortality in humans and livestock. For wildlife populations, disease-induced mortality is likely even greater, but remains notoriously difficult to estimate-especially for endemic infections. Approaches for quantifying wildlife mortality due to endemic infections have historically been limited by an inability to directly observe wildlife mortality in nature. Here we address a question that can rarely be answered for endemic pathogens of wildlife: what are the population- and landscape-level effects of infection on host mortality? We combined laboratory experiments, extensive field data and novel mathematical models to indirectly estimate the magnitude of mortality induced by an endemic, virulent trematode parasite (Ribeiroia ondatrae) on hundreds of amphibian populations spanning four native species. We developed a flexible statistical model that uses patterns of aggregation in parasite abundance to infer host mortality. Our model improves on previous approaches for inferring host mortality from parasite abundance data by (i) relaxing restrictive assumptions on the timing of host mortality and sampling, (ii) placing all mortality inference within a Bayesian framework to better quantify uncertainty and (iii) accommodating data from laboratory experiments and field sampling to allow for estimates and comparisons of mortality within and among host populations. Applying our approach to 301 amphibian populations, we found that trematode infection was associated with an average of between 13% and 40% population-level mortality. For three of the four amphibian species, our models predicted that some populations experienced >90% mortality due to infection, leading to mortality of thousands of amphibian larvae within a pond. At the landscape scale, the total number of amphibians predicted to succumb to infection was driven by a few high mortality sites, with fewer than 20% of sites contributing to greater than 80% of amphibian mortality on the landscape. The mortality estimates in this study provide a rare glimpse into the magnitude of effects that endemic parasites can have on wildlife populations and our theoretical framework for indirectly inferring parasite-induced mortality can be applied to other host-parasite systems to help reveal the hidden death toll of pathogens on wildlife hosts.

The interaction of pathogens between wild and farmed aquatic animal populations is a concern that remains unclear and controversial. Ichthyophthirius multifiliis, a ciliated protozoan parasite, is a pathogen of freshwater finfish species with geographic and host range that causes significant economic losses in aquaculture. Flow-through farming systems may facilitate the transfer of such a parasite with free-living stages between farmed and wild stocks. Here, experimental and field study infection data are used to describe the infection dynamics of Ichthyophthirius multifiliis in rainbow trout using a simple macroparasite model by including host resistance. The study considered flow-through farming systems with a single or two age-class compartments and simulated the transfer of the parasite between farmed and wild fish populations. Results suggest that aquaculture can promote the prevalence of the resistance in wild stocks by increasing the parasite population in the wild environment. At the same time, acquired resistance in the farmed fish population may protect the wild fish population from lethal effects of the parasite by reducing the total parasite population. This study offers a promising mathematical basis for understanding the effects of freshwater aquaculture in disease spread in wildlife, developing risk assessment modeling, and exploring new ways of aquaculture management.

Helminths are parasites that cause disease at considerable cost to public health and present a risk for emergence as novel human infections. Although recent research has elucidated characteristics conferring a propensity to emergence in other parasite groups (e.g. viruses), the understanding of factors associated with zoonotic potential in helminths remains poor. We applied an investigator-directed learning algorithm to a global dataset of mammal helminth traits to identify factors contributing to spillover of helminths from wild animal hosts into humans. We characterized parasite traits that distinguish between zoonotic and non-zoonotic species with 91% accuracy. Results suggest that helminth traits relating to transmission (e.g. definitive and intermediate hosts) and geography (e.g. distribution) are more important to discriminating zoonotic from non-zoonotic species than morphological or epidemiological traits. Whether or not a helminth causes infection in companion animals (cats and dogs) is the most important predictor of propensity to cause human infection. Finally, we identified helminth species with high modelled propensity to cause zoonosis (over 70%) that have not previously been considered to be of risk. This work highlights the importance of prioritizing studies on the transmission of helminths that infect pets and points to the risks incurred by close associations with these animals. This article is part of the theme issue ‘Infectious disease macroecology: parasite diversity and dynamics across the globe’.

With over 10000 species of Amphipoda currently described, this order is one of the most diverse groups of freshwater and marine Crustacea. Members of this group are globally distributed, and many are keystone species and ecosystem engineers within their respective ecologies. As with most organisms, disease is a key factor that can alter population size, behaviour, survival, invasion potential and physiology of amphipod hosts. This review explores symbiont diversity and pathology in amphipods by coalescing a range of current and historical literature to provide the first full review of our understanding of amphipod disease. The review is broken into 2 parts. The first half explores amphipod microparasites, which include data pertaining to viruses, bacteria, fungi, oomycetes, microsporidians, dinoflagellates, myxozoans, ascetosporeans, mesomycetozoeans, apicomplexans and ciliophorans. The second half reports the metazoan macroparasites of Amphipoda, including rotifers, trematodes, acanthocephalans, nematodes, cestodes and parasitic Crustacea. In all cases we have endeavoured to provide a complete list of known species that cause disease in amphipods, while also exploring the effects of parasitism. Although our understanding of disease in amphipods requires greater research efforts to better define taxonomic diversity and host effects of amphipod symbionts, research to date has made huge progress in cataloguing and experimentally determining the effects of disease upon amphipods. For the future, we suggest a greater focus on developing model systems that use readily available amphipods and diseases, which can be comparable to the diseases in other Crustacea that are endangered, economically important or difficult to house.

Infectious diseases of domesticated animals impact human well-being via food insecurity, loss of livelihoods, and human infections. While much research has focused on parasites that infect single host species, most parasites of domesticated mammals infect multiple species. The impact of multihost parasites varies across hosts; some rarely result in death, whereas others are nearly always fatal. Despite their high ecological and societal costs, we currently lack theory for predicting the lethality of multihost parasites. Here, using a global dataset of >4,000 case-fatality rates for 65 infectious diseases (caused by microparasites and macroparasites) and 12 domesticated host species, we show that the average evolutionary distance from an infected host to other mammal host species is a strong predictor of disease-induced mortality. We find that as parasites infect species outside of their documented phylogenetic host range, they are more likely to result in lethal infections, with the odds of death doubling for each additional 10 million years of evolutionary distance. Our results for domesticated animal diseases reveal patterns in the evolution of highly lethal parasites that are difficult to observe in the wild and further suggest that the severity of infectious diseases may be predicted from evolutionary relationships among hosts.

The adaptive immune system is critical to an effective, long-lasting ability to respond to infection in vertebrates and T-helper (Th) cells play a key role in orchestrating the adaptive immune response. Laboratory studies show that functionally distinct Th responses provide protection against different kinds of parasites (i.e., Th1 responses against microparasites and Th2 against macroparasites).Natural populations must deal with challenges from a wide range of infectious agents and co-infection with different types of parasite is the norm, so different Th responses are likely to play an important and dynamic role in maintaining host health and fitness. However, the relationship between T helper immune phenotypes and infection with different types of parasites remains poorly understood in wild animals.In this study, we characterised variation in functionally distinct Th responses (Th1, Th2, Th17 and regulatory responses) in a wild population of Soay sheep using flow cytometry to detect Th-subset specific transcription factors, and ex vivo lymphocyte stimulation to quantify release of Th-associated cytokines. We specifically tested the prediction that raised Th1 and Th2 responses should predict reduced apicomplexan (coccidian) and helminth (nematode) parasite burdens, respectively.Cell counts of different Th subsets measured by flow cytometry did not vary with age or sex. However, all measures of Th-associated ex vivo cytokine production increased with age, and Th17- and regulatory Th-associated cytokine production increased more rapidly with age in males than females.Independent of age and sex, Th2-associated immune measures negatively predicted gastro-intestinal strongyle nematode faecal egg count, while production of the Th1-associated cytokine IFN-γ negatively predicted coccidian faecal oocyst count.Our results provide important support from outside the laboratory that Th1 and Th2 responses confer resistance to different kinds of parasites (micro- and macro-parasites, respectively). They also add to mounting evidence from wild populations that Th1/Th2 trade-offs often observed in controlled laboratory experiments may not readily translate to more complex natural systems.Our study illustrates that harnessing more specific reagents and tools from laboratory immunology has the potential to illuminate our understanding of epidemiology and host-parasite co-evolution in the wild.

Genomic approaches hold great promise for resolving unanswered questions about transmission patterns and responses to control efforts for schistosomiasis and other neglected tropical diseases. However, the cost of generating genomic data and the challenges associated with obtaining sufficient DNA from individual schistosome larvae (miracidia) from mammalian hosts have limited the application of genomic data for studying schistosomes and other complex macroparasites. Here, we demonstrate the feasibility of utilizing whole genome amplification and sequencing (WGS) to analyze individual archival miracidia. As an example, we sequenced whole genomes of 22 miracidia from 11 human hosts representing two villages in rural Sichuan, China, and used these data to evaluate patterns of relatedness and genetic diversity. We also down-sampled our dataset to test how lower coverage sequencing could increase the cost effectiveness of WGS while maintaining power to accurately infer relatedness. Collectively, our results illustrate that population-level WGS datasets are attainable for individual miracidia and represent a powerful tool for ultimately providing insight into overall genetic diversity, parasite relatedness, and transmission patterns for better design and evaluation of disease control efforts., Author summary Schistosomiasis is a neglected tropical disease caused by helminths within the genus Schistosoma, which affects over 200 million people globally. Here, we demonstrate the ability to obtain whole genome sequences from single S. japonicum miracidia that have been archived for years, and the promise of how these data may be used to answer major questions to understand and eventually help control schistosomiasis. To demonstrate the approach, we generated a moderate-sized dataset of 22 individual S. japonicum genomes and use these data to infer fine-scale patterns of relatedness among parasites from human hosts from two villages in Sichuan, China. We highlight how this type of genomic information may be useful for understanding the genetic variation among individual parasites with high precision, which could ultimately provide high resolution for understanding transmission pathways and inform control efforts with larger sample sizes. We also show that it is possible to use less data per individual than we obtained here, and still maintain high levels of accuracy; this would enable greater sample sizes by lowering individual sample cost. Collectively our results demonstrate that sampling genomes of individual schistosome parasites is now feasible on a population-level scale, opening new avenues for studying relatedness, tracing parasite spread, and studying parasite biology.

Infection by macroparasites, such as nematodes, varies within vertebrate host systems; elevated infection is commonly observed in juveniles and males, and, for females, with different reproductive states. However, while such patterns are widely recognized in short-lived model systems, how they apply to long-lived hosts is comparatively understudied. Here, we investigated how infection varies with host age, sex, and female reproduction in a semi-captive population of individually marked Asian elephants Elephas maximus. We carried out 1,977 faecal egg counts (FECs) across five years to estimate nematode loads for 324 hosts. Infection patterns followed an established age-infection curve, whereby calves (5 years) exhibited the highest FECs and adults (45 years) the lowest. However, males and females had similar FECs across their long lifespan, despite distinct differences in life-history strategy and clear sexual dimorphism. Additionally, although mothers invest two years in pregnancy and a further three to five years into lactation, nematode load did not vary with four different measures of female reproduction. Our results provide a much-needed insight into the host-parasite dynamics of a long-lived host; determining host-specific associations with infection in such systems is important for broadening our knowledge of parasite ecology and provides practical applications for wildlife medicine and management.

Native foxes face serious threats related to anthropic activities and the uncontrolled pets. However, the transmission of several pathogens such as parasites by domestic dogs is an important conservation issue with limited attention in Chile. The lack of a proper identification of the helminths of wild canids in the country, based mostly in coprological studies, limited our knowledge in potential interspecific transmission. Thus, the aim of the present survey was to identify the parasitic fauna of two native canids, the Andean fox (Lycalopex culpaeus) (n=49) and Chilla (Lycalopex griseus) (n=17), from several localities in Chile through a complete parasitological survey. Sixteen different species were identified, 9 of which are new host records for Andean fox and two for Chilla fox in South America, and three are recorded for first time in Chile. Also, five of them are of zoonotic concern and six are known to use domestic dogs as their principal hosts. These findings have implications for public health and the conservation of canids whose interspecific transmission of parasites with domestic dogs is confirmed through these findings. The need for an exhaustive surveillance of wild carnivores is emphasized considering these hosts as important sources of micro- and macroparasites. Resumo As raposas nativas enfrentam sérias ameaças devido às atividades antrópicas e contato com animais de estimação sem controle. Entretanto, a transmissão de patógenos, tais como parasitas de cães domésticos, é uma questão importante na conservação das raposas que têm sido pouco abordadas no Chile. A falta de uma identificação adequada dos helmintos de canídeos silvestres no país, que se baseia principalmente, em estudos coprológicos, limita o conhecimento sobre uma potencial transmissão interespecífica de parasitas. Assim, o objetivo do presente estudo foi identificar a fauna parasitária em dois canídeos nativos: a raposa andina (Lycalopex culpaeus) (n = 49) e a raposa chilla (Lycalopex griseus) (n = 17), provenientes de várias localidades do Chile, por meio de um levantamento parasitológico completo. Foram identificadas 16 espécies de parasitas, nove correspondem a novos registros de hospedeiros para a raposa andina e dois para a raposa chilla na América do Sul. Três espécies de parasitas são registrados pela primeira vez no Chile, cinco são motivo de preocupação zoonótica e seis são conhecidos por usar cães domésticos como seus principais hospedeiros. Esses achados têm implicações em saúde pública e na conservação de canídeos, pois confirmam a transmissão interespecífica de parasitas desde cães domésticos. A necessidade de vigilância exaustiva dos carnívoros selvagens é enfatizada considerando-se esses hospedeiros como importantes fontes de micro e macroparasitas.

Intraspecific competition between co-infecting parasites can influence the amount of virulence, or damage, they do to their host. Kin selection theory dictates that infections with related parasite individuals should have lower virulence than infections with unrelated individuals, because they benefit from inclusive fitness and increased host longevity. These predictions have been tested in a variety of microparasite systems, and in larval stage macroparasites within intermediate hosts, but the influence of adult macroparasite relatedness on virulence has not been investigated in definitive hosts. This study used the human parasite Schistosoma mansoni to determine whether definitive hosts infected with related parasites experience lower virulence than hosts infected with unrelated parasites, and to compare the results from intermediate host studies in this system. The presence of unrelated parasites in an infection decreased parasite infectivity, the ability of a parasite to infect a definitive host, and total worm establishment in hosts, impacting the less virulent parasite strain more severely. Unrelated parasite co-infections had similar virulence to the more virulent of the two parasite strains. We combine these findings with complementary studies of the intermediate snail host and describe trade-offs in virulence and selection within the life cycle. Damage to the host by the dominant strain was muted by the presence of a competitor in the intermediate host, but was largely unaffected in the definitive host. Our results in this host-parasite system suggest that unrelated infections may select for higher virulence in definitive hosts while selecting for lower virulence in intermediate hosts.

Cerastoderma edule (edible cockle) is among the most exploited bivalves in Europe playing an important socio-economic role. Cockles live in estuaries and lagoons where their population is controlled by several environmental factors including parasitism. Parasites represent an important part of the world known biodiversity but are often neglected. Trematodes are the most prevalent macroparasites of cockles being able to exert an impact both at the individual and population levels. Therefore, it is of prime relevance to recognize and understand the parasite-host system dynamics in order to better predict potential conservation threats to bivalve populations and to maximize the success of stock and disease episodes management. Cockle monitoring was conducted in 2012 and 2016, in six and eight stations, respectively, at the Ria de Aveiro coastal lagoon, Portugal. Cockles were sampled in one single occasion in 2012 and seasonally in 2016. The tested hypothesis is that the trematode community in cockles was spatially and seasonally heterogeneous but stable over time. The main result showed that despite a relative homogeneity of the parasite community structure in cockles, the among-years heterogeneity of trematode communities was higher than among-stations and among-seasons heterogeneity rejecting the postulated hypothesis. Results demonstrated that trematode communities from the Ria de Aveiro are characterized by low abundance, which resulted in a spatial and seasonal trematode homogeneity (despite an overall channel difference and a slight downstream-upstream gradient). The interannual analysis showed a worrisome loss of trematode diversity and prevalence which consequently indicates an important loss of overall diversity and/or environmental conditions reflecting the negative effects of global change (mean temperature rise and overharvesting, among others). The present study highlighted the importance of trematodes in characterising their associated environment and respective biodiversity which might be helpful to assess ecosystem ecological status and to identify threatened areas.

Classical research on animal toxicity has focused on the role of toxins in protection against predators, but recent studies suggest these same compounds can offer a powerful defense against parasites and infectious diseases. Newts in the genus Taricha are brightly coloured and contain the potent neurotoxin, tetrodotoxin (TTX), which is hypothesized to have evolved as a defense against vertebrate predators such as garter snakes. However, newt populations often vary dramatically in toxicity, which is only partially explained by predation pressure. The primary aim of this study was to evaluate the relationships between TTX concentration and infection by parasites. By systematically assessing micro- and macroparasite infections among 345 adult newts (sympatric populations of Taricha granulosa and T. torosa), we detected 18 unique taxa of helminths, fungi, viruses and protozoans. For both newt species, per-host concentrations of TTX, which varied from undetectable to >60 μg/cm2 skin, negatively predicted overall parasite richness as well as the likelihood of infection by the chytrid fungus, Batrachochytrium dendrobatidis, and ranavirus. No such effect was found on infection load among infected hosts. Despite commonly occurring at the same wetlands, T. torosa supported higher parasite richness and average infection load than T. granulosa. Host body size and sex (females > males) tended to positively predict infection levels in both species. For hosts in which we quantified leucocyte profiles, total white blood cell count correlated positively with both parasite richness and total infection load. By coupling data on host toxicity and infection by a broad range of micro- and macroparasites, these results suggest that-alongside its effects on predators-tetrodotoxin may help protect newts against parasitic infections, highlighting the importance of integrative research on animal chemistry, immunological defenses and natural enemy ecology.

As a part of a comprehensive survey of macroparasites of commercially exploited fish species off the coast of Otago, New Zealand, the parasite fauna of the New Zealand sole Peltorhamphus novaezeelandiae Günther was recently studied. Steringotrema robertpoulini n. sp. is described from this host and compared with known species of Steringotrema Odhner, 1911. The new species is readily distinguished from all of its congeners, except for S. divergens (Rudolphi, 1809) Odhner, 1911, by having the follicular vitellarium divided in four zones rather than two, and can be differentiated from S. divergens mainly by the posterior extent of the intestinal caeca in the hindbody, as well as by host association and geographical distribution. DNA sequences of the 28S ribosomal gene were generated and phylogenetic analyses were undertaken using maximum likelihood and Bayesian inference to assess the phylogenetic position of the new species within the family Fellodistomidae Nicoll, 1909. Analyses included the available sequences for 14 species of the family distributed among eight genera, along with nine species of other members of the order Plagiorchiida La Rue, 1957 as outgroups. The resulting topology shows that the new species of Steringotrema is nested as the sister species of Steringophorus dorsolineatus (Reimer, 1985) Bray, 1995. However, low nodal support indicates that relationships among these species are not fully resolved and require further revision and denser taxon sampling for more detailed molecular work. More information is required to draw further conclusions about the taxonomic status of the genera Steringotrema and Steringophorus Odhner, 1905.

Understanding pathogen transmission is crucial for predicting and managing disease. Nonetheless, experimental comparisons of alternative functional forms of transmission remain rare, and those experiments that are conducted are often not designed to test the full range of possible forms.To differentiate among 10 candidate transmission functions, we used a novel experimental design in which we independently varied four factors—duration of exposure, numbers of parasites, numbers of hosts and parasite density—in laboratory infection experiments.We used interactions between amphibian hosts and trematode parasites as a model system and all candidate models incorporated parasite depletion. An additional manipulation involving anaesthesia addressed the effects of host behaviour on transmission form.Across all experiments, nonlinear transmission forms involving either a power law or a negative binomial function were the best‐fitting models and consistently outperformed the linear density‐dependent and density‐independent functions. By testing previously published data for two other host–macroparasite systems, we also found support for the same nonlinear transmission forms.Although manipulations of parasite density are common in transmission studies, the comprehensive set of variables tested in our experiments revealed that variation in density alone was least likely to differentiate among competing transmission functions. Across host–pathogen systems, nonlinear functions may often more accurately represent transmission dynamics and thus provide more realistic predictions for infection., The authors tested 10 candidate functions representing the transmission of trematode parasites to amphibian hosts using laboratory experiments varying the duration of exposure, numbers of parasites, numbers of hosts and parasite density. Nonlinear functions outperformed classical frequency‐ and density‐dependent transmission functions and may provide more realistic predictions for infection.

Highlights • Simple, yet flexible, model of human movement patterns. • Analytic formalism which can be used to derive important spatial scales. • Introduces a novel drift–diffusion approximation for stochastic reservoirs. • A new critical spatial scale predicted for helminth reservoirs of infection. • The necessary data needed to test these predictions is outlined in detail., The life cycle of parasitic organisms that are the cause of much morbidity in humans often depend on reservoirs of infection for transmission into their hosts. Understanding the daily, monthly and yearly movement patterns of individuals between reservoirs is therefore of great importance to implementers of control policies seeking to eliminate various parasitic diseases as a public health problem. This is due to the fact that the underlying spatial extent of the reservoir of infection, which drives transmission, can be strongly affected by inputs from external sources, i.e., individuals who are not spatially attributed to the region defined by the reservoir itself can still migrate and contribute to it. In order to study the importance of these effects, we build and examine a novel theoretical model of human movement between spatially-distributed focal points for infection clustered into regions defined as ‘reservoirs of infection’. Using our model, we vary the spatial scale of human moment defined around focal points and explicitly calculate how varying this definition can influence the temporal stability of the effective transmission dynamics – an effect which should strongly influence how control measures, e.g., mass drug administration (MDA), define evaluation units (EUs). Considering the helminth parasites as our main example, by varying the spatial scale of human movement, we demonstrate that a critical scale exists around infectious focal points at which the migration rate into their associated reservoir can be neglected for practical purposes. This scale varies by species and geographic region, but is generalisable as a concept to infectious reservoirs of varying spatial extents and shapes. Our model is designed to be applicable to a very general pattern of infectious disease transmission modified by the migration of infected individuals between clustered communities. In particular, it may be readily used to study the spatial structure of hosts for macroparasites with temporally stationary distributions of infectious focal point locations over the timescales of interest, which is viable for the soil-transmitted helminths and schistosomes. Additional developments will be necessary to consider diseases with moving reservoirs, such as vector-born filarial worm diseases.

Biological invasions have the capacity to introduce non-native parasites. This study aimed to determine whether the invasive green crab population, Carcinus spp., on the Southwestern Atlantic coast of Argentina harbours any symbionts, and whether these may spillover or spillback between native crabs, Cyrtograpsus altimanus and C. angulatus. Macroscopy, histology, and molecular analyses of some parasites were used to describe and compare their diversity across the three species of crab. We also evaluated the susceptibility of invasive Carcinus spp. to a native digenean, Maritrema madrynense, via experimental infections (exposure and cohabitation). Our results revealed that the green crab pathobiome included similar symbiotic groups to native crabs. This included putative viral, bacterial, and protozoan parasites. Haplosporidium-like observations were recorded in all crab species, and a single green crab was found to be parasitized by an Agmasoma-like microsporidium. Metagenomic analysis of one individual revealed additional symbiotic diversity (46 bacteria, 5 eukaryotic species). The green crabs were infected by more microparasite taxa than the native crabs (5:3). Wild populations of Carcinus spp. were free of metazoan parasites and are shown not to be susceptible to M. madryense under experimental conditions. Our results suggest a reduction/escape of macroparasites (trematode Maritrema madrynense; acanthocephalan Profilicollis chasmagnathi) in invasive Carcinus spp. compared to their native competitors.

Parasite virulence is a key trait in host-parasite interactions and plays a crucial role in infection dynamics. Our study system offers the rare opportunity to study the virulence of an individual macroparasite (Schistocephalus solidus) in its vertebrate fish host (Gasterosteus aculeatus). The size of the tapeworm in the fish can be regarded as a good proxy for individual parasite virulence, as parasite size correlates negatively with fitness traits of the stickleback host (i.e. the bigger the parasite, the lower the host's reproductive success) as well as directly with the number of parasite offspring to be expected. To investigate how virulence is inherited, laboratory bred, parasite-naïve stickleback were infected with a cross of two S. solidus populations of either high or low virulence, as well as one hybrid cross between the two. The relative weight of the parasite as expressed in the parasite index served as a measure of virulence. Furthermore, we measured several condition and immune related traits in the fish host to assess parasite impact on the stickleback. We hypothesized that parasite virulence is to a large extent genetically determined and correlated with several fitness traits in the stickleback host. We found that virulence is inherited additively in S. solidus, with hybrids of high and low virulence parasites displaying intermediate levels. However, contrary to expectation, infection rate of S. solidus in three-spined stickleback is not related to virulence. Even though the presence of the parasite caused differences in host condition, these were indistinguishable between the different levels of virulence in this experiment. Fish immune traits also showed a response to infection but had no correlation with level of parasite virulence. With this experiment we have taken the first step towards understanding how virulence is inherited and how it is driven in the Schistocephalus-stickleback system, even though virulence, as measured here, does not directly translate into cost for the host. A better understanding of the costs inflicted on the host by S. solidus infection is needed to understand this interaction in greater detail.

Parasitic castration is an adaptive strategy where parasites usurp the hosts' reproductive physiology to complete their life cycle. The alterations in the host traits vary in their magnitude, from subtle changes in the host morpho-physiology and behaviour to the production of complex aberrant phenotypes, which often depend on the host gender. The strepsipteran macroparasite Xenos vesparum induces dramatic behavioural and physiological changes in its female host, the paper wasp Polistes dominula, while its effect on the male phenotype is largely unknown. In this study we investigated how a single X. vesparum parasite influences the functional morphology of P. dominula male reproductive apparatus. We performed morphometry and ultrastructure characterization of corpora allata, testes, seminal vesicles and accessory glands in parasitized and unparasitized males, and also in young and old males to control for the effect of age on the natural deterioration of these organs. Our results show that age significantly affects the development of male reproductive apparatus. A low parasite load - one parasite per host is the common prevalence in the field - has only a marginal impact on the reproductive morphology of P. dominula males, affecting quantitatively but not qualitatively the protein content of male accessory glands. Thus, in male P. dominula wasps, X. vesparum appears to behave as a true "parasite", in clear opposition to the role of "parasitoid" that it takes in female hosts where castration causes the reproductive death.

Summary Coinfections are increasingly recognized as important drivers of disease dynamics. Consequently, greater emphasis has been placed on integrating principles from community ecology with disease ecology to understand within-host interactions among parasites. Using larval amphibians and two amphibian parasites (ranaviruses and the trematode Echinoparyphium sp.), we examined the influence of coinfection on disease outcomes. Our first objective was to examine how priority effects (the timing and sequence of parasite exposure) influence infection and disease outcomes in the laboratory. We found that interactions between the parasites were asymmetric; prior infection with Echinoparyphium reduced ranaviral loads by 9% but there was no reciprocal effect of prior ranavirus infection on Echinoparyphium load. Additionally, survival rates of hosts (larval gray treefrogs; Hyla versicolor) infected with Echinoparyphium 10 days prior to virus exposure were 25% greater compared to hosts only exposed to virus. Our second objective was to determine whether these patterns were generalizable to multiple amphibian species under more natural conditions. We conducted a semi-natural mesocosm experiment consisting of four larval amphibian hosts [gray treefrogs, American toads (Anaxyrus americanus), leopard frogs (Lithobates pipiens) and spring peepers (Pseudacris crucifer)] to examine how prior Echinoparyphium infection influenced ranavirus transmission within the community, using ranavirus-infected larval wood frogs (Lithobates sylvaticus) as source of ranavirus. Consistent with the laboratory experiment, we found that prior Echinoparyphium infection reduced ranaviral loads by 19 to 28% in three of the four species. Collectively, these results suggest that macroparasite infection can reduce microparasite replication rates across multiple amphibian species, possibly through cross-reactive immunity. Although the immunological mechanisms driving this outcome are in need of further study, trematode infections appear to benefit hosts that are exposed to ranaviruses. Additionally, these results suggest that consideration of priority effects and timing of exposure are vital for understanding parasite interactions within hosts and disease outcomes.

Despite the fact that tapeworms comprise the bulk of parasite communities of sharks in marine ecosystems, little is known about their life cycles and, more specifically, about the potential intermediate hosts they utilize as transmission routes. In the absence of morphological features required for specific identification of larval tapeworms from potential intermediate hosts, recent molecular advances have contributed to linking larval and adult parasites and, in some instances, uncovering unknown trophic links. Host–parasite checklists are often the first source of information consulted to assess the diversity and host specificity of parasites, and provide insights into parasite identification. However, these host–parasite checklists are only useful if they encompass the full spectrum of associations between hosts and parasites. A checklist of New Zealand fishes and their parasites has been published, but recent parasitological examinations of commercial fish species reveal that the checklist appears to be far from complete. We focused our current study on a comprehensive survey of macroparasites of a commercial species, the New Zealand sole (Peltorhamphus novaezeelandiae) off the coast of Otago, New Zealand. Specifically, we were expecting to recover marine tapeworms using sharks as their definitive hosts that are generally underreported in parasite surveys. The parasites recovered included tapeworms, flukes, round worms and thorny-headed worms. Surprisingly, a large proportion of the non-tapeworm parasites we recovered were not previously reported from this fish species. A discussion on the potential ecological roles played by this fish species in the transmission of parasites is included.

Background The red fox Vulpes vulpes is the most common mesocarnivore in Europe and with a wide geographical distribution and a high density in most terrestrial habitats of the continent. It is fast urbanising species, which can harbor high numbers of different tick species, depending on the region. Here we present the results of a large-scale study, trying to disentangle the intricate relationship between environmental factors and the species composition of ectoparasites in red foxes. The samples were collected in Transylvania (Romania), a region with a diverse geography and high biodiversity. The dead foxes (collected primarily through the National Surveillance Rabies Program) were examined carefully for the presence of ticks. Results Ticks (n = 4578) were found on 158 foxes (out of 293 examined; 53.9%). Four species were identified: Dermacentor marginatus, Ixodes canisuga, I. hexagonus and I. ricinus. The most common tick species was I. hexagonus (mean prevalence 37.5%, mean intensity 32.2), followed by I. ricinus (15.0%; 4.86), I. canisuga (4.8%; 7.71) and D. marginatus (3.7%; 3.45). Co-occurrence of two or more tick species on the same host was relatively common (12.6%), the most common co-occurrence being I. hexagonus - I. ricinus. For D. marginatus and I. canisuga the highest prevalence was recorded in lowlands, for I. hexagonus in hilly areas, while for I. ricinus in mountains. Conclusions Altitude influenced the intensity of parasitism, with highest intensity observed for all Ixodes species in hilly areas. Dermacentor marginatus occurred only in lowlands, I. canisuga in lowlands and hilly areas while the other two species occurred in all of the regions studied. Foxes from lower altitudes had the most tick species associated, with most incidences of co-parasitism also recorded here. Land use affected tick-species composition, with the presence of D. marginatus strongly associated with the extension of arable areas and lack of forests. The presence of I. hexagonus was determined only by the extent of arable lands. As foxes are frontrunners of wildlife urbanization process, with a continuous increase of their numbers in urban areas, the knowledge of their ticks’ ecology (and the pathogens vectored by these) is of utmost importance.

During the last century, emerging diseases have increased in number, posing a severe threat for human health. Zoonoses, in particular, represent the 60% of emerging diseases, and are a big challenge for public health due to the complexity of their dynamics. Mathematical models, by allowing an a priori analysis of dynamic systems and the simulation of different scenarios at once, may represent an efficient tool for the determination of factors and phenomena involved in zoonotic infection cycles, but are often underexploited in public health. In this context, we developed a deterministic mathematical model to compare the efficacy of different intervention strategies aimed at reducing environmental contamination by macroparasites, using raccoons (Procyon lotor) and their zoonotic parasite Bayilsascaris procyonis as a model system. The three intervention strategies simulated are raccoon depopulation, anthelmintic treatment of raccoons and faeces removal. Our results show that all these strategies are able to eliminate the parasite egg population from the environment, but they are effective only above specific threshold coverages. Host removal and anthelmintic treatment showed the fastest results in eliminating the egg population, but anthelmintic treatment requires a higher effort to reach an effective result compared to host removal. Our simulations show that mathematical models can help to shed light on the dynamics of communicable infectious diseases, and give specific guidelines to contain B. procyonis environmental contamination in native, as well as in new, areas of parasite emergence. In particular, the present study highlights that identifying in advance the appropriate treatment coverage is fundamental to achieve the desired results, allowing for the implementation of cost- and time-effective intervention strategies., Author summary Emerging infectious diseases involve complex systems on which background information is often lacking. Mathematical modelling, by providing a dynamic analysis of interactions between populations, can help our understanding of a disease transmission and maintenance, allowing for the simulation and comparison of intervention strategies aimed at controlling disease spread. We applied mathematical modelling to baylisascariasis, a disease caused by accidental ingestion of Baylisascaris procyonis eggs by humans, with consequences including fatal or severe neurologic disease. B. procyonis is an intestinal roundworm of raccoons, which shed eggs in the environment with their faeces. We modelled this system to simulate three different ways to reduce egg contamination and exposure risk for humans: raccoon depopulation, anthelmintic treatment of raccoons and faeces removal. Our models revealed that raccoon depopulation is the most efficient strategy to eliminate B. procyonis eggs, providing specific information on the minimum effort required to achieve their elimination, and showing that mathematical modelling is an efficient tool to identify the best strategy to control a disease beforehand, without wasting time and resources.

The longstanding impression that hybrid mice are more highly parasitized and therefore less fit than parentals persists despite the findings of recent studies. Working across a novel transect of the European House Mouse hybrid zone we assessed intracellular infections byEimeria, a parasite of high pathogenicity, and infections by pinworms, assumed to be less pathogenic. ForEimeriawe found lower intensities in hybrid hosts than in parental mice but no evidence of lowered probability of infection in the centre of the hybrid zone. This means ecological and epidemiological factors are very unlikely to be responsible for the reduced load of infected hybrids. Focussing on parasite intensity (load in infected hosts) we also corroborated reduced pinworm loads reported for hybrid mice in previous studies. In addition we questioned whether differences in body condition during infection would indicate different impacts on hybrid vs. parental hosts’ health. We couldn’t show such an effect. We conclude that intensity of diverse parasites, including the previously unstudiedEimeria, is reduced in hybrid mice compared to parental subspecies. We suggest caution in extrapolating this to differences in hybrid host fitness in the absence of, for example, evidence for a link between parasitemia and health.

Understanding the effects of invasive alien species (IAS) on native host–parasite relationships is of importance for enhancing ecological theory and IAS management. When IAS and their parasite(s) invade a guild, the effects of interspecific resource competition and/or parasite‐mediated competition can alter existing native host–parasite relationships and the dependent biological traits such as native species' behaviour. We used a natural experiment of populations of native red squirrels Sciurus vulgaris that were colonized by the alien grey squirrel Sciurus carolinensis, comparing repeated measurements of red squirrel parasite infection and personality with those taken in sites where only the native species occurred. We explored two alternative hypotheses: (a) individual differences in personality traits (activity and/or sociability) of native red squirrel positively affect the probability of macroparasite spillover and thus the likelihood to acquire the alien's parasitic helminth Strongyloides robustus; (b) the combined effects of grey squirrel presence and parasite infection result in a reduction of costly personality traits (activity and/or exploration). Using data from 323 arena tests across three experimental (native species and IAS) and three control sites (only native species), we found negative correlations between native species' activity and infection with S. robustus in the sites invaded by the alien species. Activity was also negatively correlated with infection by its native helminth Trypanoxyuris sciuri but only when grey squirrels were present, while in the red‐only sites there was no relationship of T. sciuri infection with any of the personality traits. Moreover, individuals that acquired S. robustus during the study reduced their activity after infection, while this was not the case for animals that remained uninfected. Our results show that parasite‐mediated competition is costly, reducing activity in individuals of the native species, and altering the native host–native parasite relationships.

1. Parasites are important selective agents with the potential to limit gene flow between host populations by shaping local host immunocompetence. 2. We report on a contact zone between lake and river three‐spined sticklebacks (Gasterosteus aculeatus) that offers the ideal biogeographic setting to explore the role of parasite‐mediated selection on reproductive isolation. A waterfall acts as a natural barrier and enforces unidirectional migration from the upstream river stickleback population to the downstream river and lake populations. 3. We assessed population genetic structure and parasite communities over four years. In a set of controlled experimental infections, we compared parasite susceptibility of upstream and downstream fish by exposing laboratory‐bred upstream river and lake fish, as well as hybrids, to two common lake parasite species: a generalist trematode parasite, Diplostomum pseudospathaceum, and a host‐specific cestode, Schistocephalus solidus. 4. We found consistent genetic differentiation between upstream and downstream populations across four sampling years, even though the downstream river consisted of ~10% first‐generation migrants from the upstream population as detected by parentage analysis. Fish in the upstream population had lower genetic diversity and were strikingly devoid of macroparasites. Through experimental infections, we demonstrated that upstream fish and their hybrids had higher susceptibility to parasite infections than downstream fish. Despite this, naturally sampled upstream migrants were less infected than downstream residents. Thus, migrants coming from a parasite‐free environment may enjoy an initial fitness advantage, but their descendants seem likely to suffer from higher parasite loads. 5. Our results suggest that adaptation to distinct parasite communities can influence stickleback invasion success and may represent a barrier to gene flow, even between close and connected populations. publishedVersion

Very few studies have characterized the concentrations of pollutants in bioturbating species. These species are considered as ecosystem engineers and characterizing stressors, such as contaminants, that impact them could lead to a better understanding of the functioning of ecosystems. In addition to contaminants, bioturbators are affected by a wide range of stressors, which can influence their physiological status and their ability to accumulate pollutants. Among these stressors, parasitism is of particular concern due to the ubiquity of parasites in natural environments and their influence on the fitness of their host. This study aims to assess the relationship between parasitism and metal accumulation in the bioturbating mud shrimp Upogebia cf. pusilla. A one-year seasonal survey was conducted in Arcachon Bay, France, with the aims of (1) characterizing the levels of metals in the mud shrimp and (2) evaluating the influence of two macroparasites (a bopyrid isopod and a trematode) on the variation of the metal content in mud shrimp. The bopyrid parasite castrates its female host and a particular attention has therefore been paid to the reproductive cycle of female mud shrimp by quantifying the expression of the vitellogenin gene that encodes the major yolk protein in female crustaceans. The levels of contaminants in mud shrimp appeared low compared to those reported in other crustaceans in areas of higher pollution. Even at these low contamination levels, we observed a significant impact by the bopyrid parasite that depends on season: bopyrid-infested organisms are generally more contaminated than their uninfested conspecifics except in summer when the opposite trend was observed. We suggest that the bopyrid indirectly interferes with the metal accumulation process by altering the reproductive capabilities of the mud shrimp. On the opposite, very low influence of the trematode parasite on the metal content of the host was found.

Parasites are a major component of all animal populations. Males and females often differ in their levels of parasite prevalence, potentially leading to sex differences in the impact of parasitism on fitness, with important implications for the evolution of parasite and host traits including resistance, tolerance, and virulence. However, quantitative measures of the impact of parasitism under free‐living conditions are extremely rare, as they require detailed host demographic data with measures of parasite burden over time. Here, we use endoscopy for direct quantification of natural‐parasite burdens and relate these to reproductive success over 7 yr in a wild population of seabirds. Contrary to predictions, only female burdens were associated with negative impacts of parasitism on breeding success, despite males having significantly higher burdens. Female reproductive success declined by 30% across the range of natural parasite burdens. These effects persisted when accounting for interannual population differences in breeding success. Our results provide quantitative estimates of profound sub‐lethal effects of parasitism on the population. Importantly, they highlight how parasites act unpredictably to shape ecological and evolutionary processes in different components of the same population, with implications for demography and selection on host and parasite traits.

Parasites may have strong eco‐evolutionary interactions with their hosts. Consequently, they may contribute to host diversification. The radiation of cichlid fish in Lake Victoria provides a good model to study the role of parasites in the early stages of speciation. We investigated patterns of macroparasite infection in a community of 17 sympatric cichlids from a recent radiation and 2 older species from 2 nonradiating lineages, to explore the opportunity for parasite‐mediated speciation. Host species had different parasite infection profiles, which were only partially explained by ecological factors (diet, water depth). This may indicate that differences in infection are not simply the result of differences in exposure, but that hosts evolved species‐specific resistance, consistent with parasite‐mediated divergent selection. Infection was similar between sampling years, indicating that the direction of parasite‐mediated selection is stable through time. We morphologically identified 6 Cichlidogyrus species, a gill parasite that is considered a good candidate for driving parasite‐mediated speciation, because it is host species‐specific and has radiated elsewhere in Africa. Species composition of Cichlidogyrus infection was similar among the most closely related host species (members of the Lake Victoria radiation), but two more distantly related species (belonging to nonradiating sister lineages) showed distinct infection profiles. This is inconsistent with a role for Cichlidogyrus in the early stages of divergence. To conclude, we find significant interspecific variation in parasite infection profiles, which is temporally consistent. We found no evidence that Cichlidogyrus‐mediated selection contributes to the early stages of speciation. Instead, our findings indicate that species differences in infection accumulate after speciation., Species composition of Cichlidogyrus infection was similar among the closely related cichlid species of the Lake Victoria radiation (in orange), but differed between this lineage and two distantly related species (belonging to nonradiating sister lineages, in blue).

Over a billion people on earth are infected with helminth parasites and show remarkable variation in parasite burden and chronicity. These parasite distributions are captured well by classic statistics, such as the negative binomial distribution. But the within-host processes underlying this variation are not well understood. In this study, we explain variation in macroparasite infection outcomes on the basis of resource flows within hosts. Resource flows realize the interactions between parasites and host immunity and metabolism. When host metabolism is modulated by parasites, we find a positive feedback of parasites on their own resources. While this positive feedback results in parasites improving their resource availability at high burdens, giving rise to chronic infections, it also results in a threshold biomass required for parasites to establish in the host, giving rise to acute infections when biomass fails to clear the threshold. Our finding of chronic and acute outcomes in bistability contrasts with classic theory, yet is congruent with the variation in helminth burdens observed in human and wildlife populations.

Few hosts have many parasites while many hosts have few parasites. This axiom of macroparasite aggregation is so pervasive it is considered a general law in disease ecology, with important implications for the dynamics of host-parasite systems. Because of these dynamical implications, a significant amount of work has explored both the various mechanisms leading to parasite aggregation patterns and how to infer mechanism from these patterns. However, as many disease mechanisms can produce similar aggregation patterns, it is not clear whether aggregation itself provides any additional information about mechanism. Here we apply a "constraint-based" approach developed in macroecology that allows us to explore whether parasite aggregation contains any additional information beyond what is provided by mean parasite load. We tested two constraint-based null models, both of which were constrained on the total number of parasites P and hosts H found in a sample, using data from 842 observed amphibian host-trematode parasite distributions. We found that constraint-based models captured ~85% of the observed variation in host-parasite distributions, suggesting that the constraints P and H contain much of the information about the shape of the host-parasite distribution. However, we also found that extending the constraint-based null models can identify the potential role of known aggregating mechanisms (such as host heterogeneity) and disaggregating mechanisms (such as parasite-induced host mortality) in constraining host-parasite distributions. Thus, by providing robust null models, constraint-based approaches can help guide investigations aimed at detecting biological processes that directly affect parasite aggregation above and beyond those that indirectly affect aggregation through P and H.

SUMMARYRecent results suggest that bivalves can play an important role in restraining the spread of various aquatic infections. However, the ability of mussels to remove free-living stages of macroparasites and reduce their transmission is still understudied, especially for freshwater ecosystems. We investigated the influence of the common freshwater mussel (Anodonta anatina) on the transmission of a trematode (eye fluke, Diplostomum pseudospathaceum), which frequently infects fish in farms and natural habitats. In our experiments, mussels caused a significant decrease (P < 0·001) in the abundance of trematode free-living stages, from 6520 to 1770 cercariae L−1 on average (about 4-fold in 2 h). Individual clearance rates of mussels were 0·6‒3·7 L per hour (mean 1·9). These tests were followed by experimental infections of rainbow trout (Oncorhynchus mykiss) with different doses of D. pseudospathaceum cercariae in the presence or absence of mussels. Exposure of fish to cercariae in the presence of mussels significantly (P < 0·05) reduced the infection intensities in fish (by 30–40%) at all exposure doses. Our results indicate that freshwater bivalves can markedly reduce local cercariae densities and could be useful in mitigation of trematodoses harmful to fish farming.

SUMMARYParasite infection in young animals can affect host traits related to demographic processes such as survival and reproduction, and is therefore crucial to population viability. However, variation in infection among juvenile hosts is poorly understood. Experimental studies have indicated that effects of parasitism can vary with host sex, hatching order and hatch date, yet it remains unclear whether this is linked to differences in parasite burdens. We quantified gastrointestinal nematode burdens of wild juvenile European shags (Phalacrocorax aristotelis) using two in situ measures (endoscopy of live birds and necropsy of birds that died naturally) and one non-invasive proxy measure (fecal egg counts (FECs)). In situ methods revealed that almost all chicks were infected (98%), that infections established at an early age and that older chicks hosted more worms, but FECs underestimated prevalence. We found no strong evidence that burdens differed with host sex, rank or hatch date. Heavier chicks had higher burdens, demonstrating that the relationship between burdens and their costs is not straightforward. In situ measures of infection are therefore a valuable tool in building our understanding of the role that parasites play in the dynamics of structured natural populations.

Infection with the abomasal nematode Haemonchus contortus is responsible for considerable production loss in small ruminants globally, and especially in warm, summer-rainfall regions. Previous attempts to predict infection levels have followed the traditional framework for macroparasite models, i.e . tracking parasite population sizes as a function of host and climatic factors. Targeted treatment strategies, in which patho-physiological indices are used to identify the individuals most affected by parasites, could provide a foundation for alternative, incidence-based epidemiological models. In this paper, an elaboration of the classic susceptible-infected-recovered (SIR) model framework for microparasites was adapted to haemonchosis and used to predict disease in Merino sheep on a commercial farm in South Africa. Incidence was monitored over a single grazing season using the FAMACHA scoring system for conjunctival mucosal coloration, which indicates high burdens of H. contortus , and used to fit the model by estimating transmission parameters. The model predicted force of infection (FOI) between sequential FAMACHA monitoring events in groups of dry, pregnant and lactating ewes, and related FOI to factors including climate (temperature, rainfall and rainfall entropy), using a random effects model with reproductive status group as the cluster variable. Temperature and rainfall in the seven days prior to monitoring significantly predicted the interval FOI (p ≤ 0.002), while rainfall entropy did not (p = 0.289). Differences across the three groups accounted for approximately 90% of the variability in the interval FOI over the period of investigation. Maintained FOI during targeted treatment of cases of haemonchosis suggests strong underlying transmission from sub-clinically infected animals, and/or limited impact on pre-existing pasture contamination by removal of clinical worm burdens later in the grazing season. The model has the potential to contribute to the sustainable management of H. contortus by predicting periods of heightened risk, and hence to focus and optimise limited resources for monitoring and treatment. SIR-type model frameworks are an alternative to classic abundance-based compartmental models of macroparasite epidemiology, and could be useful where incidence data are available. Significant challenges remain, however, in the ability to calibrate such models to field data at spatial and temporal scales that are useful for decision support at farm level.

Host–parasite coevolution has rarely been observed in natural systems. Its study often relies on microparasitic infections introducing a potential bias in the estimation of the evolutionary change of host and parasite traits. Using biological invasions as a tool to study host–parasite coevolution in nature can overcome these biases. We demonstrate this with a cross-infection experiment in the invasive macroparasite Mytilicola intestinalis and its bivalve host, the blue mussel Mytilus edulis. The invasion history of the parasite is well known for the southeastern North Sea and is characterised by two separate invasion fronts that reached opposite ends of the Wadden Sea (i.e. Texel, The Netherlands and Sylt, Germany) in a similar time frame. The species’ natural history thus makes this invasion an ideal natural experiment to study host–parasite coevolution in nature. We infected hosts from Texel, Sylt and Kiel (Baltic Sea, where the parasite is absent) with parasites from Texel and Sylt, to form sympatric, allopatric and naïve infestation combinations, respectively. We measured infection rate, host condition and parasite growth to show that sympatric host–parasite combinations diverged in terms of pre- and post-infection traits within

Aggregation is commonly observed for macroparasites, but its adaptive value remains unclear. Heavy infestations intensities may lead to a decrease in some fitness-related traits of parasites (e.g. parasite fecundity or survival). However, to a dioecious parasite, increased aggregation could also increase the chance of finding individuals of the opposite sex. In a laboratory experiment, we tested if previous experience with salmon lice (Lepeophtheirus salmonis) affected susceptibility of Atlantic salmon (Salmo salar) to later exposure to the same parasite species. We found that currently infected fish got higher intensities of new lice than naive fish. This suggests that hosts already carrying parasites are more susceptible to new lice infections. For this dioecious parasite, such positive density dependence might be adaptive, ensuring successful reproduction under conditions of low lice densities by increasing the probability of both sexes infecting the same host.

SUMMARYModelling wildlife disease poses some unique challenges. Wildlife disease systems are data poor in comparison with human or livestock disease systems, and the impact of disease on population size is often the key question of interest. This review concentrates specifically on the application of dynamic models to evaluate and guide management strategies. Models have proved useful particularly in two areas. They have been widely used to evaluate vaccination strategies, both for protecting endangered species and for preventing spillover from wildlife to humans or livestock. They have also been extensively used to evaluate culling strategies, again both for diseases in species of conservation interest and to prevent spillover. In addition, models are important to evaluate the potential of parasites and pathogens as biological control agents. The review concludes by identifying some key research gaps, which are further development of models of macroparasites, deciding on appropriate levels of complexity, modelling genetic management and connecting models to data.

Small rodents serve as reservoir hosts for tick-borne pathogens, such as the spirochetes causing Lyme disease. Whether natural coinfections with other macroparasites alter the success of tick feeding, antitick immunity, and the host's reservoir competence for tick-borne pathogens remains to be determined. In a parasitological survey of wild mice in Berlin, Germany, approximately 40% of Ixodes ricinus -infested animals simultaneously harbored a nematode of the genus Heligmosomoides . We therefore aimed to analyze the immunological impact of the nematode/tick coinfection as well as its effect on the tick-borne pathogen Borrelia afzelii . Hosts experimentally coinfected with Heligmosomoides polygyrus and larval/nymphal I. ricinus ticks developed substantially stronger systemic type 2 T helper cell (Th2) responses, on the basis of the levels of GATA-3 and interleukin-13 expression, than mice infected with a single pathogen. During repeated larval infestations, however, anti-tick Th2 reactivity and an observed partial immunity to tick feeding were unaffected by concurrent nematode infections. Importantly, the strong systemic Th2 immune response in coinfected mice did not affect susceptibility to tick-borne B. afzelii . An observed trend for decreased local and systemic Th1 reactivity against B. afzelii in coinfected mice did not result in a higher spirochete burden, nor did it facilitate bacterial dissemination or induce signs of immunopathology. Hence, this study indicates that strong systemic Th2 responses in nematode/tick-coinfected house mice do not affect the success of tick feeding and the control of the causative agent of Lyme disease.

Fedorov's lumpsucker, Eumicrotremus fedorovi Mandrytsa, 1991, is native to the Kuril Islands area. Two sets of 69 and 30 adult specimens of E. fedorovi caught on the continental shelf and slope of Simushir Island in the Pacific Ocean were dissected and studied for macroparasites. The studied specimens were heavily infected with plerocercoids of Nybelinia surmenicola and Ascarophis pacifica. For the first time, 15 parasite taxa were recorded for E. fedorovi. None of the parasites found are solely specific to the E. fedorovi or to the representatives of the family Cyclopteridae. The infection of E. fedorovi could be dramatically different in different parts of the Simushir Island area in terms of parasite species composition and intensity of infection.

Knowledge about the identity of parasites in vertebrates is relevant because of their influence on ecological processes and health of their hosts. This is particularly important for groups of animals currently facing conservation issues, such as reptiles. The diversity of species and supra-specific taxa of microparasites and macroparasites (such as helminths and arthropods) present in non-avian reptiles in Chile was analyzed through a systematic review. A total of 49 scientific documents (thesis projects, abstracts in congresses, book chapters and peer-reviewed articles) concerning parasites, taxonomic descriptions and parasite ecology were included in this review. The suborder Iguania was among the most studied group with 35 native species recognized as hosts of parasites and 39 species of parasites reported up to the category of species or genus. Liolaemus tenuis was the species with the most taxa of recorded parasites (11 taxa, and only three to species level), but Liolaemus chillanensis had the highest number of identified species of parasites. In addition, only one native species of Serpentes, one of Gymnophthalmoidea, and two of Gekkota, as well as some reports of exotic species, were recorded as hosts. Among parasites, Trombidiformes was the richest order with 10 species from the superfamily Pterygosomatoidea and 16 species from Trombiculoidea. Current knowledge about the richness of helminths is very limited and there were only a few records of microparasites. In general, there is an urgent need for the development of collaborative works between specialists in reptile taxonomy and epidemiology in parasitology destined to evaluate the consequences that reptiles and their parasites will suffer due to the ongoing processes of habitat loss, climate change and the still present taxonomic issues of the native reptiles.

Macroparasites in general, and ectoparasites in particular, have the potential to regulate host population dynamics. In this context, this study addresses the tick parasitism traits of the Mediterranean spur-thighed tortoise (Testudo graeca) in the core area of its distribution range (northwestern Morocco, Maamora). It was discovered that 92.5% of the tortoises were parasitized by ticks in spring, with an infestation intensity and an abundance of 6.7 and 6.2 tick/tortoise, respectively. The observed parasitization rates were among the highest reported worldwide for T. graeca, which could relate to density-depended effects driving host-parasite interactions. The main tick species that parasitized the tortoises were Hyalomma aegyptium (95.6% of the ticks and in the 100% of the parasitized tortoises), Hy. marginatum, Hy. excavatum and Hy. scupense. Individual predictors for the tortoises, such as age, sex and the interaction between body condition and sex, were significantly related to tick abundance. Age-related behavioural differences might favour a higher host-tick effective contact in adults than in juveniles. The fact that males are more active in spring - the breeding season - might explain the observed male-bias in tick abundance and may also be responsible for the negative effect of male body condition on tick infestation rate in contrast to females. Given the potential role played by parasites as regards modulating population dynamics, our results suggest that ticks should be taken into account in the conservation and management programmes of this tortoise species., PA and FRF are supported by MICINN-UCLM through a “Ramón y Cajal” contract (RYC-2012-11970 and RYC-2013-12528, respectively). FRF also acknowledged funding by MICINN research grant CGL2017-89866-R.

Anthropogenic activities, especially those involving substances that pollute the environment can interfere with bivalve populations, as well as parasitism, a fundamental ecological interaction often neglected. In marine environments, organisms are concomitantly exposed to pollutants and parasites, a combination with synergistic, antagonistic or additive effects representing a potential threat to aquatic communities sustainability. In the present study, Cerastoderma edule (the edible cockle)–Himasthla elongata (trematode) was used as host–parasite model. Cockles are worldwide recognized as good sentinel and bioindicator species and can be infected by several trematodes, the most abundant macroparasites in coastal waters. Tested hypotheses were: 1) cockles exposed to increasing parasite pressure will present greater stress response; 2) cockles exposure to arsenic (single concentration test: 5.2 μg L−1) will change parasite infection success and cockles stress response to infection. Arsenic was used for being one of the most common pollutants in the world and stress response assessed using biochemical markers of glycogen content, metabolism, antioxidant activity and cellular damage. Results showed that intensity of parasite pressure was positively correlated to biochemical response, mainly represented by higher metabolic requirements. Contamination did not affect parasite infection success. Compared to arsenic, trematode infection alone exerted a stronger impact: higher glycogen storage, metabolism and cellular damage and antioxidant activity inhibition. In interaction, parasitism and arsenic reduced hosts metabolism and cellular damage. Therefore, to a certain extent and in a contamination scenario, cockles may benefit from trematode infection, working as a protection for the pollutant accumulation in the organisms, reducing overall ROS production, which can consequently led to less toxic effects. These findings highlighted the deleterious effects of trematode infection in their hosts and showed the importance of including parasitology in ecotoxicological studies.

Understanding factors that facilitate interspecific pathogen transmission is a central issue for conservation, agriculture, and human health. Past work showed that host phylogenetic relatedness and geographical proximity can increase cross-species transmission, but further work is needed to examine the importance of host traits, and species interactions such as predation, in determining the degree to which parasites are shared between hosts. Here we consider the factors that predict patterns of parasite sharing across a diverse assemblage of 116 wild ungulates (i.e., hoofed mammals in the Artiodactyla and Perissodactyla) and nearly 900 species of micro- and macroparasites, controlling for differences in total parasite richness and host sampling effort. We also consider the effects of trophic links on parasite sharing between ungulates and carnivores. We tested for the relative influence of range overlap, phylogenetic distance, body mass, and ecological dissimilarity (i.e., the distance separating species in a Euclidean distance matrix based on standardized traits) on parasite sharing. We also tested for the effects of variation in study effort as a potential source of bias in our data, and tested whether carnivores reported to feed on ungulates have more ungulate parasites than those that use other resources. As in other groups, geographical range overlap and phylogenetic similarity predicted greater parasite community similarity in ungulates. Ecological dissimilarity showed a weak negative relationship with parasite sharing. Counter to our expectations, differences, not similarity, in host body mass predicted greater parasite sharing between pairs of ungulate hosts. Pairs of well-studied host species showed higher overlap than poorly studied species, although including sampling effort did not reduce the importance of biological traits in our models. Finally, carnivores that feed on ungulates harboured a greater richness of ungulate helminths. Overall, we show that the factors that predict parasite sharing in wild ungulates are similar to those known for other mammal groups, and demonstrate the importance of controlling for heterogeneity in host sampling effort in future analyses of parasite sharing. We also show that ecological interactions, in this case trophic links via predation, can allow sharing of some parasite species among distantly related host species.

The round goby, Neogobius melanostomus, is a Ponto-Caspian fish considered as an invasive species in a wide range of aquatic ecosystems. To understand the role that parasites may play in its successful invasion across Western Europe, we investigated the parasitic diversity of the round goby along its invasion corridor, from the Danube to the Upper Rhine rivers, using data from literature and a molecular barcoding approach, respectively. Among 1666 parasites extracted from 179 gobies of the Upper Rhine, all of the 248 parasites barcoded on the c oxidase subunit I gene were identified as Pomphorhynchus laevis. This lack of macroparasite diversity was interpreted as a loss of parasites along its invasion corridor without spillback compensation. The genetic diversity of P. laevis was represented by 33 haplotypes corresponding to a haplotype diversity of 0·65 ± 0·032, but a weak nucleotide diversity of 0·0018 ± 0·00015. Eight of these haplotypes were found in 88·4% of the 248 parasites. These haplotypes belong to a single lineage so far restricted to the Danube, Vistula and Volga rivers (Eastern Europe). This result underlines the exotic status of this Ponto-Caspian lineage in the Upper Rhine, putatively disseminated by the round goby along its invasion corridor.

Removal of parasite free-living stages by predators has previously been suggested an important factor controlling parasite transmission in aquatic habitats. Experimental studies of zooplankton predation on macroparasite larvae are, however, scarce. We tested whether trematode cercariae, which are often numerous in shallow waters, are suitable prey for syntopic zooplankters. Feeding rates and survival of freshwater cyclopoids (Megacyclops viridis, Macrocyclops distinctus), calanoids (Arctodiaptomus paulseni), cladocerans (Sida crystallina) and rotifers Asplanchna spp., fed with cercariae of Diplostomum pseudospathaceum, a common fish trematode, were studied. In additional long-term experiments, we studied reproduction of cyclopoids fed with cercariae. All tested zooplankton species consumed cercariae. The highest feeding rates were observed for cyclopoids (33 ± 12 cercariae ind−1 h−1), which actively reproduced (up to one egg clutch day−1) when fed ad libitum with cercariae. Their reproductive characteristics did not change significantly with time, indicating that cercariae supported cyclopoids’ dietary needs. Mortality of rotifers and cladocerans was high (25–28% individuals) when exposed to cercariae in contrast to cyclopoids and calanoids (

Studies of amphibian macroparasites are relevant for the investigation of parasite community ecology and disease dynamics. Here, the parasite communities of five anuran species (Hypsiboas raniceps, Phyllomeduza azurea, Pseudis paradoxa, Leptodactylus fuscus and Leptodactylus podicipinus) are described from two habitats with different levels of preservation (pasture versus nature reserve). Specifically, we used mixed-effect models to test whether helminth species richness, prevalence and abundance differ between the two host collection sites. A total of 120 anuran individuals and 25 helminth parasite taxa were collected. Helminth communities differed between collection sites and among host species. In general, helminth species richness, prevalence and abundance were higher in hosts collected in the pond from the nature reserve. In all, these data help fill the gap in parasite biodiversity knowledge in a changing area, within a sensitive group of vertebrate hosts.

Recent climate change has resulted in changes to the phenology and distribution of invertebrates worldwide. Where invertebrates are associated with disease, climate variability and changes in climate may also affect the spatio-temporal dynamics of disease. Due to its significant impact on sheep production and welfare, the recent increase in diagnoses of ovine haemonchosis caused by the nematode Haemonchus contortus in some temperate regions is particularly concerning. This study is the first to evaluate the impact of climate change on H. contortus at a continental scale. A model of the basic reproductive quotient of macroparasites, Q0, adapted to H. contortus and extended to incorporate environmental stochasticity and parasite behaviour, was used to simulate Pan-European spatio-temporal changes in H. contortus infection pressure under scenarios of climate change. Baseline Q0 simulations, using historic climate observations, reflected the current distribution of H. contortus in Europe. In northern Europe, the distribution of H. contortus is currently limited by temperatures falling below the development threshold during the winter months and within-host arrested development is necessary for population persistence over winter. In southern Europe, H. contortus infection pressure is limited during the summer months by increased temperature and decreased moisture. Compared with this baseline, Q0 simulations driven by a climate model ensemble predicted an increase in H. contortus infection pressure by the 2080s. In northern Europe, a temporal range expansion was predicted as the mean period of transmission increased by 2-3 months. A bimodal seasonal pattern of infection pressure, similar to that currently observed in southern Europe, emerges in northern Europe due to increasing summer temperatures and decreasing moisture. The predicted patterns of change could alter the epidemiology of H. contortus in Europe, affect the future sustainability of contemporary control strategies, and potentially drive local adaptation to climate change in parasite populations.