Parasitic manipulation and neuroinflammation: Evidence from the system Microphallus papillorobustus (Trematoda) - Gammarus (Crustacea)

Contrasting alterations in brain chemistry in a crustacean intermediate host of two acanthocephalan parasites

The dynamic properties of neural systems throughout life can be hijacked by so-called manipulative parasites. This study investigated changes in the brain chemistry of the amphipod Gammarus fossarum in response to infection with two trophically-transmitted helminth parasites known to induce distinct behavioral alterations: the bird acanthocephalan Polymorphus minutus and the fish acanthocephalan Pomphorhynchus tereticollis. We quantified brain antioxidant capacity as a common marker of homeostasis and neuroprotection, and brain total protein, on 72 pools of six brains. We analyzed the concentration of serotonin (5HT), dopamine (DA) and tyramine in 52 pools of six brains, by using ultrafast high performance liquid chromatography with electrochemical detection (UHPLC-ECD). Brain total protein concentration scaled hypo-allometrically to dry body weight, and was increased in infected gammarids compared to uninfected ones. The brain of gammarids infected with P. minutus had significantly lower total antioxidant capacity relative to total proteins. Infection with P. tereticollis impacted DA level compared to uninfected ones, and in opposite direction between spring and summer. Brain 5HT level was higher in summer compared to spring independently of infection status, and was decreased by infection after correcting for brain total protein concentration estimated from dry whole-body weight. The potential implication of 5HT/DA balance in parasite manipulation, as a major modulator of the reward-punishment axis, is discussed. Taken together, these findings highlight the need to consider both brain homeostatic and/or structural changes (antioxidant and total protein content) together with neurotransmission balance and flexibility, in studies investigating the impact of parasites on brain and behavior.

Effects of Cymatocarpus solearis (Trematoda: Brachycoeliidae) on its second intermediate host, the Caribbean spiny lobster Panulirus argus

Many digenean trematodes require three hosts to complete their life cycle. For Cymatocarpus solearis (Brachycoeliidae), the first intermediate host is unknown; the Caribbean spiny lobster Panulirus argus is a second intermediate host, and the loggerhead turtle Caretta caretta, a lobster predator, is the definitive host. Trophically-transmitted parasites may alter the behavior or general condition of intermediate hosts in ways that increase the hosts' rates of consumption by definitive hosts. Here, we examined the effects of infection by C. solearis on P. argus by comparing several physiological and behavioral variables among uninfected lobsters (0 cysts) and lobsters with light (1-10 cysts), moderate (11-30 cysts), and heavy (>30 cysts) infections. Physiological variables were hepatosomatic index, growth rate, hemocyte count, concentration in hemolymph of cholesterol, protein, albumin, glucose, dopamine (DA) and serotonin (5-HT). Behavioral variables included seven components of the escape response (delay to escape, duration of swimming bout, distance traveled in a swimming bout, swim velocity, acceleration, force exerted, and work performed while swimming). There was no relationship between lobster size or sex and number of cysts. Significant differences among the four lobster groups occurred only in concentration of glucose (lower in heavily infected lobsters) and 5-HT (higher in heavily and moderately infected lobsters) in plasma. As changes in 5-HT concentration can modify the host's activity patterns or choice of microhabitat, our results suggest that infection with C. solearis may alter the behavior of spiny lobsters, potentially increasing the likelihood of trophic transmission of the parasite to the definitive host.

Adaptive host responses to infection can resemble parasitic manipulation

Using a dynamic optimisation model for juvenile fish in stochastic food environments, we investigate optimal hormonal regulation, energy allocation and foraging behaviour of a growing host infected by a parasite that only incurs an energetic cost. We find it optimal for the infected host to have higher levels of orexin, growth and thyroid hormones, resulting in higher activity levels, increased foraging and faster growth. This growth strategy thus displays several of the fingerprints often associated with parasite manipulation: higher levels of metabolic hormones, faster growth, higher allocation to reserves (i.e. parasite-induced gigantism), higher risk-taking and eventually higher predation rate. However, there is no route for manipulation in our model, so these changes reflect adaptive host compensatory responses. Interestingly, several of these changes also increase the fitness of the parasite. Our results call for caution when interpreting observations of gigantism or risky host behaviours as parasite manipulation without further testing.

Parasite infection and the movement of the aquatic snail Potamopyrgus antipodarum along a depth cline

Parasite species that use two or more host species during their life cycle depend on successful transmission between these species. These successive host species may have different habitat requirements. For example, one host species may be aquatic while the other is terrestrial. To overcome this complicating factor in transmission, a wide diversity of parasite species have adaptations that alter the habitat preference in one host species to facilitate transmission to the next host species.Two common trematode parasites in New Zealand, Atriophallophorus winterbourni and Notocotylus spp., both have a life cycle with two host species. The aquatic snail Potamopyrgus antipodarum is the intermediate host, from which the parasites require transmission to dabbling ducks or other waterfowl. Of these parasites, A. winterbourni is most frequently found in snails from the shallow-water margin. This may indicate parasite-induced movement of infected snails into the foraging habitat of dabbling ducks.To test whether the parasites manipulate the snails to move into shallow water, we stretched tubular mesh cages across depth-specific ecological habitat zones in a lake. Both infected and healthy snails were released into the cages. After 11 days, significantly higher infection frequencies of A. winterbourni were retrieved from the shallowest end of the cages, while Notocotylus spp. frequencies did not vary with depth.The hypothesis that A. winterbourni induces its snail host to move into the shallow-water habitat cannot be rejected based on the experimental results. Although further research is needed to address alternative explanations, the depth preference of infected snails may be due to a parasite adaptation that facilitates trophic transmission of parasites to dabbling ducks.

Trypanosoma cruzi Parasite Load Modulates the Circadian Activity Pattern of Triatoma infestans

Simple Summary

We studied the locomotor activity of one of the kissing bug species that transmit the Chagas disease-causing parasite in humans, which usually bites during the night. To date, no other reports researching its behavior take into account the amount of parasites inside the kissing bug; however, some studies have demonstrated that the presence of parasites modifies the activity of some kissing bug species. We recorded their movements in light and dark conditions after part of the insects fed on mammals that had the parasite and others fed on those that did not have the parasite. Later, their amounts of parasites were quantified. We found that, compared with insects with no parasites, kissing bugs with higher parasite amounts increase the number of times they move and the distance they travel, especially during daylight hours. This could imply that the insect increases its time searching for a food source when it is infected with a higher number of parasites, and this could increase the risk of transmission of the parasite to people by the kissing bug.

Abstract

American trypanosomiasis is a disease caused by the flagellate protozoan Trypanosoma cruzi, which is transmitted mainly in endemic areas by blood-sucking triatomine vectors. Triatoma infestans is the most important vector in the southern cone of South America, exhibiting a nocturnal host-seeking behavior. It has been previously documented that the parasite produces changes in some triatomine species, but this is the first time that the behavior of a vector has been evaluated in relation to its parasite load. After comparing the movement events and distance traveled of infected and non-infected T. infestans, we evaluated the change produced by different T. cruzi parasite loads on its circadian locomotor activity. We observed differences between infected and non-infected triatomines, and a significant relation between the parasite load and the increase in locomotor activity of T. infestans, which was accentuated during the photophase. This could have direct implications on the transmission of T. cruzi, as the increased movement and distance traveled could enhance the contact of the vector with the host, while increasing the predation risk for the vector, which could both constitute a risk for vectorial and oral transmission to mammals.

Invasion history shapes host transcriptomic response to a body-snatching parasite

By shuffling biogeographical distributions, biological invasions can both disrupt long-standing associations between hosts and parasites and establish new ones. This creates natural experiments with which to study the ecology and evolution of host-parasite interactions. In estuaries of the Gulf of Mexico, the white-fingered mud crab (Rhithropanopeus harrisii) is infected by a native parasitic barnacle, Loxothylacus panopaei (Rhizocephala), which manipulates host physiology and behaviour. In the 1960s, L. panopaei was introduced to the Chesapeake Bay and has since expanded along the southeastern Atlantic coast, while host populations in the northeast have so far been spared. We use this system to test the host's transcriptomic response to parasitic infection and investigate how this response varies with the parasite's invasion history, comparing populations representing (i) long-term sympatry between host and parasite, (ii) new associations where the parasite has invaded during the last 60 years and (iii) naïve hosts without prior exposure. A comparison of parasitized and control crabs revealed a core response, with widespread downregulation of transcripts involved in immunity and moulting. The transcriptional response differed between hosts from the parasite's native range and where it is absent, consistent with previous observations of increased susceptibility in populations lacking exposure to the parasite. Crabs from the parasite's introduced range, where prevalence is highest, displayed the most dissimilar response, possibly reflecting immune priming. These results provide molecular evidence for parasitic manipulation of host phenotype and the role of gene regulation in mediating host-parasite interactions.

Insect Behavioral Change and the Potential Contributions of Neuroinflammation-A Call for Future Research

Many organisms are able to elicit behavioral change in other organisms. Examples include different microbes (e.g., viruses and fungi), parasites (e.g., hairworms and trematodes), and parasitoid wasps. In most cases, the mechanisms underlying host behavioral change remain relatively unclear. There is a growing body of literature linking alterations in immune signaling with neuron health, communication, and function; however, there is a paucity of data detailing the effects of altered neuroimmune signaling on insect neuron function and how glial cells may contribute toward neuron dysregulation. It is important to consider the potential impacts of altered neuroimmune communication on host behavior and reflect on its potential role as an important tool in the "neuro-engineer" toolkit. In this review, we examine what is known about the relationships between the insect immune and nervous systems. We highlight organisms that are able to influence insect behavior and discuss possible mechanisms of behavioral manipulation, including potentially dysregulated neuroimmune communication. We close by identifying opportunities for integrating research in insect innate immunity, glial cell physiology, and neurobiology in the investigation of behavioral manipulation.

Gene Duplication and Gain in the Trematode Atriophallophorus winterbourni Contributes to Adaptation to Parasitism

Gene duplications and novel genes have been shown to play a major role in helminth adaptation to a parasitic lifestyle because they provide the novelty necessary for adaptation to a changing environment, such as living in multiple hosts. Here we present the de novo sequenced and annotated genome of the parasitic trematode Atriophallophorus winterbourni and its comparative genomic analysis to other major parasitic trematodes. First, we reconstructed the species phylogeny, and dated the split of A. winterbourni from the Opisthorchiata suborder to approximately 237.4 Ma (±120.4 Myr). We then addressed the question of which expanded gene families and gained genes are potentially involved in adaptation to parasitism. To do this, we used hierarchical orthologous groups to reconstruct three ancestral genomes on the phylogeny leading to A. winterbourni and performed a GO (Gene Ontology) enrichment analysis of the gene composition of each ancestral genome, allowing us to characterize the subsequent genomic changes. Out of the 11,499 genes in the A. winterbourni genome, as much as 24% have arisen through duplication events since the speciation of A. winterbourni from the Opisthorchiata, and as much as 31.9% appear to be novel, that is, newly acquired. We found 13 gene families in A. winterbourni to have had more than ten genes arising through these recent duplications; all of which have functions potentially relating to host behavioral manipulation, host tissue penetration, and hiding from host immunity through antigen presentation. We identified several families with genes evolving under positive selection. Our results provide a valuable resource for future studies on the genomic basis of adaptation to parasitism and point to specific candidate genes putatively involved in antagonistic host-parasite adaptation.

High parasite diversity in the amphipod Gammarus lacustris in a subarctic lake

Amphipods are often key species in aquatic food webs due to their functional roles in the ecosystem and as intermediate hosts for trophically transmitted parasites. Amphipods can also host many parasite species, yet few studies address the entire parasite community of a gammarid population, precluding a more dynamic understanding of the food web. We set out to identify and quantify the parasite community of Gammarus lacustris to understand the contributions of the amphipod and its parasites to the Takvatn food web. We identified seven parasite taxa: a direct life cycle gregarine, Rotundula sp., and larval stages of two digenean trematode genera, two cestodes, one nematode, and one acanthocephalan. The larval parasites use either birds or fishes as final hosts. Bird parasites predominated, with trematode Plagiorchis sp. having the highest prevalence (69%) and mean abundance (2.7). Fish parasites were also common, including trematodes Crepidostomum spp., nematode Cystidicola farionis, and cestode Cyathocephalus truncatus (prevalences 13, 6, and 3%, respectively). Five parasites depend entirely on G. lacustris to complete their life cycle. At least 11.4% of the overall parasite diversity in the lake was dependent on G. lacustris, and 16% of the helminth diversity required or used the amphipod in their life cycles. These dependencies reveal that in addition to being a key prey item in subarctic lakes, G. lacustris is also an important host for maintaining parasite diversity in such ecosystems.

Comparative transcriptome analysis of Chinese grass shrimp (Palaemonetes sinensis) infected with isopod parasite Tachaea chinensis

Parasitic isopods negatively affect reproduction and ingestion in several commercially important crustaceans; however, little is known about such parasite-host interactions. Therefore, we performed high-throughput sequencing of cDNA samples from Chinese grass shrimp Palaemonetes sinensis infected by parasitic isopod Tachaea chinensis and a non-infected control. We randomly assembled 46,858,882 and 41,110,746 clean reads from the parasitized and control groups, respectively. From these, we identified 1323 differentially expressed genes (DEGs) (p < 0.05), of which 702 were up-regulated and 621 were down-regulated after T. chinensis infection, respectively. The up-regulated genes were enriched in 'ribosome', 'purine metabolism', and 'pyrimidine metabolism' signalling pathways, suggesting altered host nucleotide metabolite levels, possibly through the action of intracellular parasites transported by T. chinensis. Additionally, 14 representative DEGs involved in reproduction were down-regulated after parasitisation, indicating T. chinensis causes cascading effects in P. sinensis. Overall, parasitisation appeared to affect host immune response, metabolism, and gonadal development. In conclusion, the present study improves our understanding on the molecular mechanisms underlying interactions between isopod parasites and their crustacean hosts.

Immunity, host physiology, and behaviour in infected vectors

When infection alters host behaviour such that the pathogen benefits, the behaviour is termed a manipulation. There are several examples of this fascinating phenomenon in many different systems. Vector-borne diseases are no exception. In some instances, as the term implies, pathogens directly interfere with host processes to control behaviour. However, host response to infection and host physiology are likely to play important roles in these phenotypes. We highlight the importance of considering host response and physiology from recent work on altered host-seeking in malaria parasite-infected mosquitoes and argue that this general approach will provide useful insights across vector-borne disease systems.

Parasite-related modification of mating behaviour and refuge use in the aquatic isopod Caecidotea intermedius: neurological correlates

The acanthocephalan Acanthocephalus dirus is a trophically transmitted parasite that infects freshwater isopods as intermediate hosts and fish as definitive hosts. Using a laboratory-based experiment, we examined if parasite infection was associated with changes in mating behaviour, refuge use and neurochemical levels of infected isopods (Caecidotea intermedius). Infected isopods were less likely to engage in mating behaviour and more likely to be located in the open than uninfected isopods. Infected isopods also contained lower levels of serotonin (5-HT) and dopamine (DA) and had a greater mass of neural tissue (CNS) than uninfected isopods. We propose that the parasite-related changes in mating behaviour and refuge use may be modulated by the serotonergic and dopaminergic systems. We also suggest that the parasites could potentially be modulating these behavioural changes by exploiting the neural-immune system of the hosts through their neuroinflammatory responses.

Experimental test of host specificity in a behaviour-modifying trematode

Host behavioural modification by parasites is a common and well-documented phenomenon. However, knowledge on the complexity and specificity of the underlying mechanisms is limited, and host specificity among manipulating parasites has rarely been experimentally verified. We tested the hypothesis that the ability to infect and manipulate host behaviour is restricted to phylogenetically closely related hosts. Our model system consisted of the brain-encysting trematode Euhaplorchis sp. A and six potential fish intermediate hosts from the Order Cyprinodontiformes. Five co-occurring cyprinids were examined for naturally acquired brain infections. Then we selected three species representing three levels of taxonomic relatedness to a known host to experimentally evaluate their susceptibility to infection, and the effect of infection status on behaviours presumably linked to increased trophic transmission. We found natural brain infections of Euhaplorchis sp. A metacercariae in three cyprinids in the shallow sublittoral zone. Of the three experimentally exposed species, Fundulus grandis and Poecilia latipinna acquired infections and displayed an elevated number of conspicuous behaviours in comparison with uninfected controls. Euhaplorchis sp. A was able to infect and manipulate fish belonging to two different families, suggesting that ecological similarity rather than genetic relatedness determines host range in this species.

Impacts of a newly identified behaviour-altering trematode on its host amphipod: from the level of gene expression to population

Changes to host behaviour induced by some trematode species, as a means of increased trophic transmission, represents one of the seminal examples of host manipulation by a parasite. The amphipod Echinogammarus marinus (Leach, 1815) is infected with a previously undescribed parasite, with infected individuals displaying positive phototaxic and negative geotaxic behaviour. This study reveals that the unknown parasite encysts in the brain, nerve cord and the body cavity of E. marinus, and belongs to the Microphallidae family. An 18 month population study revealed that host abundance significantly and negatively correlated with parasite prevalence. Investigation of the trematode's influence at the transcriptomic level revealed genes with putative neurological functions, such as serotonin receptor 1A, an inebriated-like neurotransmitter, tryptophan hydroxylase and amino acid decarboxylase, present consistent altered expression in infected animals. Therefore, this study provides one of the first transcriptomic insights into the neuronal gene pathways altered in amphipods infected with a trematode parasite associated with changes to its host's behaviour and population structure.

Position of larval tapeworms, Polypocephalus sp., in the ganglia of shrimp, Litopenaeus setiferus

Parasites that invade the nervous system of their hosts have perhaps the best potential to manipulate their host's behavior, but how they manipulate the host, if they do at all, could depend on their position within the host's nervous system. We hypothesize that parasites that live in the nervous system of their host will be randomly distributed if they exert their influence through non-specific effects (i.e., general pathology), but that their position in the nervous system will be non-random if they exert their influence by targeting specific neural circuits. We recorded the position of larval tapeworms, Polypocephalus sp., in the abdominal ganglia of white shrimp, Litopenaeus setiferus. Tapeworms are more common within ganglia than in the section of the nerve cord between ganglia, even though the nerve cord has a greater volume than the ganglia. The tapeworms are also more abundant in the periphery of the ganglia. Because most synaptic connections are within the central region of the ganglion, such positioning may represent a trade-off between controlling the nervous system and damaging it.

A trematode parasite alters growth, feeding behavior, and demographic success of invasive rusty crayfish (Orconectes rusticus)

Nonindigenous species can cause major changes to community interactions and ecosystem processes. The strong impacts of these species are often attributed to their high demographic success. While the importance of enemy release in facilitating invasions has often been emphasized, few studies have addressed the role of parasites in the invasive range in controlling demographic success of potential invaders. Here we examine whether a trematode parasite (Microphallus spp.) can contribute to previously documented alternate states in the abundance of invasive rusty crayfish (Orconectes rusticus) in north temperate lakes in Wisconsin, USA. Microphallus infect O. rusticus after emerging from their first intermediate host, a hydrobiid snail. As previously documented, O. rusticus reduce densities of hydrobiid snails through direct predation and destruction of macrophyte habitat. Therefore, if Microphallus substantially reduce O. rusticus fitness, these parasites may reinforce a state of low crayfish abundance, and, at the other extreme, abundant crayfish may repress these parasites, reinforcing a state of high crayfish abundance. From samples collected from 109 sites in 16 lakes, we discovered (1) a positive relationship between crayfish infection intensity and hydrobiid snail abundance, (2) a negative relationship between parasite prevalence and crayfish abundance, and (3) a negative relationship between parasite prevalence and crayfish population growth. With experiments, we found that infection with Microphallus reduced foraging behavior and growth in O. rusticus, which may be the mechanisms responsible for the population reductions we observed. Overall results are consistent with the hypothesis that Microphallus contributes to alternate states in the abundance and impacts of O. rusticus.

Parasites: evolution’s neurobiologists

For millions of years, parasites have altered the behaviour of their hosts. Parasites can affect host behaviour by: (1) interfering with the host’s normal immune–neural communication, (2) secreting substances that directly alter neuronal activity via non-genomic mechanisms and (3) inducing genomic- and/or proteomic-based changes in the brain of the host. Changes in host behaviour are often restricted to particular behaviours, with many other behaviours remaining unaffected. Neuroscientists can produce this degree of selectivity by targeting specific brain areas. Parasites, however, do not selectively attack discrete brain areas. Parasites typically induce a variety of effects in several parts of the brain. Parasitic manipulation of host behaviour evolved within the context of the manipulation of other host physiological systems (especially the immune system) that was required for a parasite’s survival. This starting point, coupled with the fortuitous nature of evolutionary innovation and evolutionary pressures to minimize the costs of parasitic manipulation, likely contributed to the complex and indirect nature of the mechanisms involved in host behavioural control. Because parasites and neuroscientists use different tactics to control behaviour, studying the methods used by parasites can provide novel insights into how nervous systems generate and regulate behaviour. Studying how parasites influence host behaviour will also help us integrate genomic, proteomic and neurophysiological perspectives on behaviour.

Parasite-induced alterations of sensorimotor pathways in gammarids: collateral damage of neuroinflammation?

Some larval helminths alter the behavior of their intermediate hosts in ways that favor the predation of infected hosts, thus enhancing trophic transmission. Gammarids (Crustacea: Amphipoda) offer unique advantages for the study of the proximate factors mediating parasite-induced behavioral changes. Indeed, amphipods infected by distantly related worms (acanthocephalans, cestodes and trematodes) encysted in different microhabitats within their hosts (hemocoel, brain) present comparable, chronic, behavioral pathologies. In order to evaluate the potential connection between behavioral disturbances and immune responses in parasitized gammarids, this Review surveys the literature bearing on sensorimotor pathway dysfunctions in infected hosts, on the involvement of the neuromodulator serotonin in altered responses to environmental stimuli, and on systemic and neural innate immunity in arthropods. Hemocyte concentration and phenoloxidase activity associated with melanotic encapsulation are depressed in acanthocephalan-manipulated gammarids. However, other components of the arsenal deployed by crustaceans against pathogens have not yet been investigated in helminth-infected gammarids. Members of the Toll family of receptors, cytokines such as tumor necrosis factors (TNFs), and the free radical nitric oxide are all implicated in neuroimmune responses in crustaceans. Across animal phyla, these molecules and their neuroinflammatory signaling pathways are touted for their dual beneficial and deleterious properties. Thus, it is argued that neuroinflammation might mediate the biochemical events upstream of the serotonergic dysfunction observed in manipulated gammarids – a parsimonious hypothesis that could explain the common behavioral pathology induced by distantly related parasites, both hemocoelian and cerebral.

Comparing mechanisms of host manipulation across host and parasite taxa

Parasites affect host behavior in several ways. They can alter activity, microhabitats or both. For trophically transmitted parasites (the focus of our study), decreased activity might impair the ability of hosts to respond to final-host predators, and increased activity and altered microhabitat choice might increase contact rates between hosts and final-host predators. In an analysis of trophically transmitted parasites, more parasite groups altered activity than altered microhabitat choice. Parasites that infected vertebrates were more likely to impair the host’s reaction to predators, whereas parasites that infected invertebrates were more likely to increase the host’s contact with predators. The site of infection might affect how parasites manipulate their hosts. For instance, parasites in the central nervous system seem particularly suited to manipulating host behavior. Manipulative parasites commonly occupy the body cavity, muscles and central nervous systems of their hosts. Acanthocephalans in the data set differed from other taxa in that they occurred exclusively in the body cavity of invertebrates. In addition, they were more likely to alter microhabitat choice than activity. Parasites in the body cavity (across parasite types) were more likely to be associated with increased host contact with predators. Parasites can manipulate the host through energetic drain, but most parasites use more sophisticated means. For instance, parasites target four physiological systems that shape behavior in both invertebrates and vertebrates: neural, endocrine, neuromodulatory and immunomodulatory. The interconnections between these systems make it difficult to isolate specific mechanisms of host behavioral manipulation.

Investigating candidate neuromodulatory systems underlying parasitic manipulation: concepts, limitations and prospects

Studies addressing the functional basis of parasitic manipulation suggest that alteration of the neuromodulatory system is a common feature of manipulated hosts. Screening of the neuromodulatory system has so far been carried out by performing ethopharmacological analysis, biochemical quantification of neurotransmitters and neuromodulators, and/or immunocytochemistry. Here, we review the advantages and limitations of such approaches through the analysis of case studies. We further address whether the analysis of candidate neuromodulatory systems fits the current view of manipulation as being multidimensional. The benefits in combining ethopharmacology with more recent molecular tools to investigate candidate neuromodulatory pathways is also emphasized. We conclude by discussing the value of a multidisciplinary study of parasitic manipulation, combining evolutionary (parasite transmission), behavioural (syndrome of manipulation) and neuroimmunological approaches.