Host-manipulation by parasites with complex life cycles: adaptive or not?

The human alpha7 nicotinic acetylcholine receptor is a host target for the rabies virus glycoprotein

The rabies virus enters the nervous system by interacting with several molecular targets on host cells to modify behavior and trigger receptor-mediated endocytosis of the virion by poorly understood mechanisms. The rabies virus glycoprotein (RVG) interacts with the muscle acetylcholine receptor and the neuronal α4β2 subtype of the nicotinic acetylcholine receptor (nAChR) family by the putative neurotoxin-like motif. Given that the neurotoxin-like motif is highly homologous to the α7 nAChR subtype selective snake toxin α-bungarotoxin (αBTX), other nAChR subtypes are likely involved. The purpose of this study is to determine the activity of the RVG neurotoxin-like motif on nAChR subtypes that are expressed in brain regions involved in rabid animal behavior. nAChRs were expressed in Xenopus laevis oocytes, and two-electrode voltage clamp electrophysiology was used to collect concentration-response data to measure the functional effects. The RVG peptide preferentially and completely inhibits α7 nAChR ACh-induced currents by a competitive antagonist mechanism. Tested heteromeric nAChRs are also inhibited, but to a lesser extent than the α7 subtype. Residues of the RVG peptide with high sequence homology to αBTX and other neurotoxins were substituted with alanine. Altered RVG neurotoxin-like peptides showed that residues phenylalanine 192, arginine 196, and arginine 199 are important determinants of RVG peptide apparent potency on α7 nAChRs, while serine 195 is not. The evaluation of the rabies ectodomain reaffirmed the observations made with the RVG peptide, illustrating a significant inhibitory impact on α7 nAChR with potency in the nanomolar range. In a mammalian cell culture model of neurons, we confirm that the RVG peptide binds preferentially to cells expressing the α7 nAChR. Defining the activity of the RVG peptide on nAChRs expands our understanding of basic mechanisms in host-pathogen interactions that result in neurological disorders.

Parasitic manipulation or side effects? The effects of past Toxoplasma gondii and Borrelia spp. infections on human personality and cognitive performance are not mediated by impaired health

Bacteria Borrelia burgdorferi s. l. and even more the protist Toxoplasma gondii Nicolle et Manceaux, 1908, are known to affect the behaviour and mental health of their animal and human hosts. Both pathogens infect a significant fraction of human population, both are neurotropic and survive in the host's body for a long time. While latent infections were thought to be clinically asymptomatic, recent studies suggest otherwise, revealing adverse effects on human health. It was hypothesised that the specific behavioural effects of these pathogens may be side effects of general health impairments in infected individuals. This hypothesis was tested using about one hour-long survey consisting of questionnaires and performance tests on a cohort of 7,762 members of the internet population. Results showed that individuals infected with T. gondii reported worse physical and mental health, and those infected with Borrelia spp. reported worse physical health than uninfected controls. Furthermore, infected and noninfected individuals differed in several personality traits, including conscientiousness, pathogen disgust, injury disgust, Machiavellianism, narcissism, tribalism, anti-authoritarianism, intelligence, reaction time, and precision. While the majority of behavioural effects associated with Borrelia infection were similar to those associated with Toxoplasma infection, some differences were observed, such as performance in the Stroop test. Path analyses and nonparametric partial Kendall correlation tests showed that these effects were not mediated by impaired health in infected individuals, contradicting the side effects hypothesis.

Tricks of the puppet masters: morphological adaptations to the interaction with nervous system underlying host manipulation by rhizocephalan barnacle Polyascus polygeneus

Background

Rhizocephalan interaction with their decapod hosts is a superb example of host manipulation. These parasites are able to alter the host's physiology and behavior. Host-parasite interaction is performed, presumably, via special modified rootlets invading the ventral ganglions.

Methods

In this study, we focus on the morphology and ultrastructure of these special rootlets in Polyascus polygeneus (Lützen & Takahashi, 1997), family Polyascidae, invading the neuropil of the host's nervous tissue. The ventral ganglionic mass of the infected crabs were fixed, and the observed sites of the host-parasite interplay were studied using transmission electron microscopy, immunolabeling and confocal microscopy.

Results

The goblet-shaped organs present in the basal families of parasitic barnacles were presumably lost in a common ancestor of Polyascidae and crown "Akentrogonida", but the observed invasive rootlets appear to perform similar functions, including the synthesis of various substances which are transferred to the host's nervous tissue. Invasive rootlets significantly differ from trophic ones in cell layer composition and cuticle thickness. Numerous multilamellar bodies are present in the rootlets indicating the intrinsic cell rearrangement. The invasive rootlets of P. polygeneus are enlaced by the thin projections of glial cells. Thus, glial cells can be both the first hosts' respondents to the nervous tissue damage and the mediator of the rhizocephalan interaction with the nervous cells. One of the potential molecules engaged in the relationships of P. polygeneus and its host is serotonin, a neurotransmitter which is found exclusively in the invasive rootlets but not in trophic ones. Serotonin participates in different biological pathways in metazoans including the regulation of aggression in crustaceans, which is reduced in infected crabs. We conclude that rootlets associated with the host's nervous tissue are crucial for the regulation of host-parasite interplay and for evolution of the Rhizocephala.

The effect of waterfowl signals and Pseudocorynosoma enrietti infection on the behaviour of the amphipod Hyalella patagonica

In the present study, we sought to determine whether i) a waterfowl signal induces avoidance behaviour of the amphipod Hyalella patagonica, ii) infection by the acanthocephalan Pseudocorynosoma enrietti affects the behaviour of the amphipod, and iii) the parasite interferes with the amphipod response to waterfowl. We evaluated amphipod behaviour experimentally by measuring activity levels, phototaxis, geotaxis, and clinging behaviour. The main findings of this study indicate that uninfected amphipods show avoidance behaviour by reducing their activity in the presence of a predator signal. Secondly, infected amphipods show altered behaviour, such as swimming in bright areas near the water surface, which makes them more visible to predators in nature. Lastly, the presence of predatory cues causes infected amphipods to drop to the bottom, which increases their visibility to predators. The present research allows us to perceive the intricate interplay among predators, parasites, and their intermediate hosts and advance our understanding of these complex ecological dynamics.

Possible seasonal and diurnal modulation of Gammarus pulex (Crustacea, Amphipoda) drift by microsporidian parasites

In lotic freshwater ecosystems, the drift or downstream movement of animals (e.g., macroinvertebrates) constitutes a key dispersal pathway, thus shaping ecological and evolutionary patterns. There is evidence that macroinvertebrate drift may be modulated by parasites. However, most studies on parasite modulation of host drifting behavior have focused on acanthocephalans, whereas other parasites, such as microsporidians, have been largely neglected. This study provides new insight into possible seasonal and diurnal modulation of amphipod (Crustacea: Gammaridae) drift by microsporidian parasites. Three 72 h drift experiments were deployed in a German lowland stream in October 2021, April, and July 2022. The prevalence and composition of ten microsporidian parasites in Gammarus pulex clade E varied seasonally, diurnally, and between drifting and stationary specimens of G. pulex. Prevalence was generally higher in drifting amphipods than in stationary ones, mainly due to differences in host size. However, for two parasites, the prevalence in drift samples was highest during daytime suggesting changes in host phototaxis likely related to the parasite's mode of transmission and site of infection. Alterations in drifting behavior may have important implications for G. pulex population dynamics and microsporidians' dispersal. The underlying mechanisms are more complex than previously thought.

How cunning is the puppet-master? Cestode-infected fish appear generally fearless

Trophically transmitted parasites have life cycles that require the infected host to be eaten by the correct type of predator. Such parasites should benefit from an ability to suppress the host’s fear of predators, but if the manipulation is imprecise the consequence may be increased predation by non-hosts, to the detriment of the parasite. Three-spined sticklebacks (Gasterosteus aculeatus) infected by the cestode Schistocephalus solidus express reduced antipredator behaviours, but it is unknown whether this is an example of a highly precise manipulation, a more general manipulation, or if it can even be attributed to mere side effects of disease. In a series of experiments, we investigated several behaviours of infected and uninfected sticklebacks. As expected, they had weak responses to simulated predatory attacks compared to uninfected fish. However, our results suggest that the parasite induced a general fearlessness, rather than a precise manipulation aimed at the correct predators (birds). Infected fish had reduced responses also when attacked from the side and when exposed to odour from a fish predator, which is a “dead-end” for this parasite. We also tested whether the reduced anti-predator behaviours were mere symptoms of a decreased overall vigour, or due to parasite-induced hunger, but we found no support for these ideas. We propose that even imprecise manipulations of anti-predator behaviours may benefit parasites, for example, if other behaviours are altered in a way that increases the exposure to the correct predator.

Life-cycle complexity in helminths: What are the benefits?

Parasitic worms (i.e., helminths) commonly infect multiple hosts in succession. With every transmission step, they risk not infecting the next host and thus dying before reproducing. Given this risk, what are the benefits of complex life cycles? Using a dataset for 973 species of trophically transmitted acanthocephalans, cestodes, and nematodes, we tested whether hosts at the start of a life cycle increase transmission and whether hosts at the end of a life cycle enable growth to larger, more fecund sizes. Helminths with longer life cycles, that is, more successive hosts, infected conspicuously smaller first hosts, slightly larger final hosts, and exploited trophic links with lower predator-prey mass ratios. Smaller first hosts likely facilitate transmission because of their higher abundance and because parasite propagules were the size of their normal food. Bigger definitive hosts likely increase fecundity because parasites grew larger in big hosts, particularly endotherms. Helminths with long life cycles attained larger adult sizes through later maturation, not faster growth. Our results indicate that complex helminth life cycles are ubiquitous because growth and reproduction are highest in large, endothermic hosts that are typically only accessible via small intermediate hosts, that is, the best hosts for growth and transmission are not the same.

Extreme lifespan extension in tapeworm-infected ant workers

Social insects are hosts of diverse parasites, but the influence of these parasites on phenotypic host traits is not yet well understood. Here, we tracked the survival of tapeworm-infected ant workers, their uninfected nest-mates and of ants from unparasitized colonies. Our multi-year study on the ant Temnothorax nylanderi, the intermediate host of the tapeworm Anomotaenia brevis, revealed a prolonged lifespan of infected workers compared with their uninfected peers. Intriguingly, their survival over 3 years did not differ from those of (uninfected) queens, whose lifespan can reach two decades. By contrast, uninfected workers from parasitized colonies suffered from increased mortality compared with uninfected workers from unparasitized colonies. Infected workers exhibited a metabolic rate and lipid content similar to young workers in this species, and they received more social care than uninfected workers and queens in their colonies. This increased attention could be mediated by their deviant chemical profile, which we determined to elicit more interest from uninfected nest-mates in a separate experiment. In conclusion, our study demonstrates an extreme lifespan extension in a social host following tapeworm infection, which appears to enable host workers to retain traits typical for young workers.

Comparative analysis of helminth infectivity: growth in intermediate hosts increases establishment rates in the next host

Parasitic worms (i.e. helminths) commonly infect multiple hosts in succession before reproducing. At each life cycle step, worms may fail to infect the next host, and this risk accumulates as life cycles include more successive hosts. Risk accumulation can be minimized by having high establishment success in the next host, but comparisons of establishment probabilities across parasite life stages are lacking. We compiled recovery rates (i.e. the proportion of parasites recovered from an administered dose) from experimental infections with acanthocephalans, cestodes and nematodes. Our data covered 127 helminth species and 16 913 exposed hosts. Recovery rates increased with life cycle progression (11%, 29% and 46% in first, second and third hosts, respectively), because larger worm larvae had higher recovery, both within and across life stages. Recovery declined in bigger hosts but less than it increased with worm size. Higher doses were used in systems with lower recovery, suggesting that high doses are chosen when few worms are expected to establish infection. Our results indicate that growing in the small and short-lived hosts at the start of a complex life cycle, though dangerous, may substantially improve parasites' chances of completing their life cycles.

The Adaptiveness of Host Behavioural Manipulation Assessed Using Tinbergen's Four Questions

Host organisms show altered phenotypic reactions when parasitised, some of which result from adaptive host manipulation, a phenomenon that has long been debated. Here, we provide an overview and discuss the rationale in distinguishing adaptive versus nonadaptive host behavioural manipulation. We discuss Poulin's criteria of adaptive host behavioural manipulation within the context of Tinbergen's four questions of ethology, while highlighting the importance of both the proximate and evolutionary explanations of such traits. We also provide guidelines for future studies exploring the adaptiveness of host behavioural manipulation. Through this article, we seek to encourage researchers to consider both the proximate and ultimate causes of host behavioural manipulation to infer on the adaptiveness of such traits.

Pathogens Manipulating Tick Behavior-Through a Glass, Darkly

Pathogens can manipulate the phenotypic traits of their hosts and vectors, maximizing their own fitness. Among the phenotypic traits that can be modified, manipulating vector behavior represents one of the most fascinating facets. How pathogens infection affects behavioral traits of key insect vectors has been extensively investigated. Major examples include Plasmodium, Leishmania and Trypanosoma spp. manipulating the behavior of mosquitoes, sand flies and kissing bugs, respectively. However, research on how pathogens can modify tick behavior is patchy. This review focuses on current knowledge about the behavioral changes triggered by Anaplasma, Borrelia, Babesia, Bartonella, Rickettsia and tick-borne encephalitis virus (TBEV) infection in tick vectors, analyzing their potential adaptive significance. As a general trend, being infected by Borrelia and TBEV boosts tick mobility (both questing and walking activity). Borrelia and Anaplasma infection magnifies Ixodes desiccation resistance, triggering physiological changes (Borrelia: higher fat reserves; Anaplasma: synthesis of heat shock proteins). Anaplasma infection also improves cold resistance in infected ticks through synthesis of an antifreeze glycoprotein. Being infected by Anaplasma, Borrelia and Babesia leads to increased tick survival. Borrelia, Babesia and Bartonella infection facilitates blood engorgement. In the last section, current challenges for future studies are outlined.

Parasite-modified behaviour in non-trophic transmission: trematode parasitism increases the attraction between snail intermediate hosts

Many parasites with complex life cycles cause host behavioural changes that increase the likelihood of transmission to the next host. Parasite modification is often found in trophic transmission, but its influence on non-trophic transmission is unclear. In trematodes, transmission from the first to second intermediate host is non-trophic, suggesting that free-swimming larvae (cercariae) emerging in closer proximity to the next host would have higher transmission success. We performed a series of behavioural experiments with echinostome trematodes and their snail hosts to determine if potential second hosts (ramshorn snail, genus Planorbella Haldeman, 1842) were more attracted to parasitized first hosts (marsh pondsnail, Lymnaea elodes Say, 1821). In a Y maze, a responding snail (Planorbella sp.) was placed in the base and its response to five treatments was assessed: no stimulus, turion duckweed (Lemna turionifera Landolt; a food item), non-parasitized L. elodes, parasitized L. elodes, and finally parasitized versus non-parasitized L. elodes. Snails showed some attraction to uninfected snails, but had a stronger response to infected first host snails. These results indicate that potential second host snails were more attracted to parasitized, heterospecific first host snails over non-parasitized heterospecific snails. This study demonstrates that echinostome trematodes alter snail behaviour by changing navigational choices in uninfected potential hosts through a chemical communication mechanism.

Magnitude and direction of parasite-induced phenotypic alterations: a meta-analysis in acanthocephalans

Several parasite species have the ability to modify their host's phenotype to their own advantage thereby increasing the probability of transmission from one host to another. This phenomenon of host manipulation is interpreted as the expression of a parasite extended phenotype. Manipulative parasites generally affect multiple phenotypic traits in their hosts, although both the extent and adaptive significance of such multidimensionality in host manipulation is still poorly documented. To review the multidimensionality and magnitude of host manipulation, and to understand the causes of variation in trait value alteration, we performed a phylogenetically corrected meta-analysis, focusing on a model taxon: acanthocephalan parasites. Acanthocephala is a phylum of helminth parasites that use vertebrates as final hosts and invertebrates as intermediate hosts, and is one of the few parasite groups for which manipulation is predicted to be ancestral. We compiled 279 estimates of parasite-induced alterations in phenotypic trait value, from 81 studies and 13 acanthocephalan species, allocating a sign to effect size estimates according to the direction of alteration favouring parasite transmission, and grouped traits by category. Phylogenetic inertia accounted for a low proportion of variation in effect sizes. The overall average alteration of trait value was moderate and positive when considering the expected effect of alterations on trophic transmission success (signed effect sizes, after the onset of parasite infectivity to the final host). Variation in the alteration of trait value was affected by the category of phenotypic trait, with the largest alterations being reversed taxis/phobia and responses to stimuli, and increased vulnerability to predation, changes to reproductive traits (behavioural or physiological castration) and immunosuppression. Parasite transmission would thereby be facilitated mainly by changing mainly the choice of micro-habitat and the anti-predation behaviour of infected hosts, and by promoting energy-saving strategies in the host. In addition, infection with larval stages not yet infective to definitive hosts (acanthella) tends to induce opposite effects of comparable magnitude to infection with the infective stage (cystacanth), although this result should be considered with caution due to the low number of estimates with acanthella. This analysis raises important issues that should be considered in future studies investigating the adaptive significance of host manipulation, not only in acanthocephalans but also in other taxa. Specifically, the contribution of phenotypic traits to parasite transmission and the range of taxonomic diversity covered deserve thorough attention. In addition, the relationship between behaviour and immunity across parasite developmental stages and host-parasite systems (the neuropsychoimmune hypothesis of host manipulation), still awaits experimental evidence. Most of these issues apply more broadly to reported cases of host manipulation by other groups of parasites.

Neglected Agent Eminent Disease: Linking Human Helminthic Infection, Inflammation, and Malignancy

Helminthic parasitic infection is grossly prevalent across the globe and is considered a significant factor in human cancer occurrence induced by biological agents. Although only three helminths (Schistosoma haematobium, Clonorchis sinensis, and Opisthorchis viverrini) so far have been directly associated with carcinogenesis; there are evidence suggesting the involvement of other species too. Broadly, human helminthiasis can cause chronic inflammation, genetic instability, and host immune modulation by affecting inter- and intracellular communications, disruption of proliferation-anti-proliferation pathways, and stimulation of malignant stem cell progeny. These changes ultimately lead to tumor development through the secretion of soluble factors that interact with host cells. However, the detailed mechanisms by which helminths introduce and promote malignant transformation of host cells are still not clear. Here, we reviewed the current understanding of immune-pathogenesis of helminth parasites, which have been associated with carcinogenesis, and how these infections initiate carcinogenesis in the host.

Zombie bugs? Manipulation of kissing bug behavior by the parasite Trypanosoma cruzi

The parasite manipulation hypothesis states that the parasite modifies host's behavior thereby increasing the probability that the parasite will pass from an intermediate host to its final host. We used the kissing bugs Triatoma pallidipennis and T. longipennis and two isolates of the Trypanosoma cruzi parasite (Chilpancingo and Morelos) to test these ideas. These insects are intermediate hosts of this parasite, which is the causal agent of Chagas disease. The Chilpancingo isolate is more pathogenic than the Morelos isolate, in the bugs. We expected that infected bugs would be more active and likely at detecting human-like odors. Given the differences in pathogenicity between isolates, we expected the Chilpancingo isolate to induce these effects more strongly and lead to higher parasite number than the Morelos isolate. Finally, infected bugs would gain less mass (a mechanism thought to increase bite rate, and thus transmission) than non-infected bugs. Having determined that both isolate haplotypes belong to the Tc1a group, we found that: (a) young instars of both species were more active and likely to detect human odor when they were infected, regardless of the isolate; (b) there was no difference in parasite abundance depending on isolate; and, (c) infected bugs did not end up with less weight than uninfected bugs. These results suggest that T. cruzi can manipulate the bugs, which implies a higher risk to contract Chagas disease than previously thought.

A molecular war: convergent and ontogenetic evidence for adaptive host manipulation in related parasites infecting divergent hosts

Mermithids (phylum Nematoda) and hairworms (phylum Nematomorpha) somehow drive their arthropod hosts into water, which is essential for the worms' survival after egression. The mechanisms behind this behavioural change have been investigated in hairworms, but not in mermithids. Establishing a similar mechanistic basis for host behavioural change between these two distantly related parasitic groups would provide strong convergent evidence for adaptive manipulation and insight into how these parasites modify and/or create behaviour. Here, we search for this convergence, and also contrast changes in physiology between hosts infected with immature and mature mermithids to provide the first ontogenetic evidence for adaptive manipulation by disentangling host response and pathology from the parasite's apparent manipulative effects. We used SWATH-mass spectrometry on brains of Forficula auricularia (earwig) and Bellorchestia quoyana (sandhopper), infected with the mermithids Mermis nigrescens and Thaumamermis zealandica, respectively, at both immature and mature stages of infection, to quantify proteomic changes resulting from mermithid infection. Across both hosts (and hairworm-infected hosts, from earlier studies), the general function of dysregulated proteins was conserved. Proteins involved in energy generation/mobilization were dysregulated, corroborating reports of erratic/hyperactive behaviour in infected hosts. Dysregulated proteins involved in axon/dendrite and synapse modulation were also common to all hosts, suggesting neuronal manipulation is involved in inducing positive hydrotaxis. Furthermore, downregulation of CamKII and associated proteins suggest manipulation of memory also contributes to the behavioural shift.

Podocotyle atomon (Trematoda: Digenea) impacts reproductive behaviour, survival and physiology in Gammarus zaddachi (Amphipoda)

The Trematoda are a group of phylogenetically diverse metazoan parasites that exhibit complex life cycles that often pass through invertebrate and vertebrate hosts. Some trematodes influence their host's behaviour to benefit transmission. Their parasitic influence may impact host population size by inhibiting an individual's reproductive capacity. We assessed the impact of infection by Podocotyle atomon on the reproductive behaviour and fecundity of its amphipod intermediate host, Gammarus zaddachi, using laboratory and field studies. Parasite prevalence was high in the field, with males more likely to be infected (prevalence in males 64%, in females 39%). Males also suffered a higher parasite burden than females. Infected females were less active, but we found no evidence for a reduction in female reproductive success. Infected females also had comparable pairing success to uninfected females. In males, infection reduced survival and fecundity, with mortality being highest, and sperm numbers lowest, in heavily infected individuals. Trematode parasites are sometimes associated with altered host fecundity, but studies often lack the relevant experimental data to explore the evolution of the trait. We discuss this among information specific to the effect of P. atomon infection in G. zaddachi.

Tapeworm manipulation of copepod behaviour: parasite genotype has a larger effect than host genotype

Compared with uninfected individuals, infected animals can exhibit altered phenotypes. The changes often appear beneficial to parasites, leading to the notion that modified host phenotypes are extended parasite phenotypes, shaped by parasite genes. However, the phenotype of a parasitized individual may reflect parasitic manipulation, host responses to infection or both, and disentangling the contribution of parasite genes versus host genes to these altered phenotypes is challenging. Using a tapeworm (Schistocephalus solidus) infecting its copepod first intermediate host, I performed a full-factorial, cross-infection experiment with five host and five parasite genotypes. I found that a behavioural trait modified by infection, copepod activity, was affected by both host and parasite genotype. There was no clear evidence for host genotype by parasite genotype interactions. Several observations indicated that host behaviour was chiefly determined by parasite genes: (i) all infected copepods, regardless of host or parasite genotype, exhibited behavioural changes, (ii) parasitism reduced the differences among copepod genotypes, and (iii) within infected copepods, parasite genotype had twice as large an effect on behaviour as host genotype. I conclude that the altered behaviour of infected copepods primarily represents an extended parasite phenotype, and I discuss how genetic variation in parasitic host manipulation could be maintained.

Host manipulation by parasites as a cryptic driver of energy flow through food webs

Manipulative parasites alter predator-prey interactions, and thus may facilitate, shift or create energy flow pathways through food webs (referred to hereafter as manipulation-mediated energy flow, MMEF). The ecological significance of MMEF would be determined not only by the strength of host manipulation, but also ecological and epidemiological factors, including host biomass, parasite incidence, and trophic position of the host-parasite association in their food webs. While previous theory has predicted that strong manipulation will destabilize host-parasite dynamics, a recently proposed theoretical framework claims that a switching strategy (sequential manipulation from predation suppression to enhancement) should allow parasites to induce strong predation enhancement and thus large MMEF. We formally outline the current and future directions to better understand the causes and consequences of MMEF across biological hierarchies.

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

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

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The fish acanthocephalans P. laevis and P. tereticollis show moderate specificity and low niche overlap.

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Transmission bias from shared intermediate hosts towards either benthic or bentho-pelagic fish is evidenced.

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Decreased taxonomic specificity towards fish hosts matches with lower prevalence in intermediate hosts.

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Lower worm size in the host with the lowest abundance calls for further investigation of compatibility filter.

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Low taxonomic resolution within some parasite species complex hinders accurate estimate of host use pattern.

Experimental evolution of parasitic host manipulation

Host manipulation is a parasite-induced alteration of a host's phenotype that increases parasite fitness. However, if genetically encoded in the parasite, it should be under selection in the parasite. Such host manipulation has often been assumed to be energetically costly, which should restrict its evolution. Evidence of such costs, however, remains elusive. The trophically transmitted cestode Schistocephalus solidus manipulates the activity of its first intermediate copepod host to reduce its predation susceptibility before the parasite is ready for transmission. Thereafter, S. solidus increases host activity to facilitate transmission to its subsequent fish host. I selected S. solidus for or against host manipulation over three generations to investigate the evolvability of manipulation and identify potential trade-offs. Host manipulation responded to selection, confirming that this trait is heritable in the parasite and hence can present an extended phenotype. Changes in host manipulation were not restrained by any obvious costs.

Modulation of the immune response by helminths: a role for serotonin?

The mammalian gut is a remarkable organ: with a nervous system that rivals the spinal cord, it is the body’s largest repository of immune and endocrine cells and houses an immense and complex microbiota. Infection with helminth parasites elicits a conserved program of effector and regulatory immune responses to eradicate the worm, limit tissue damage, and return the gut to homeostasis. Discrete changes in the nervous system, and to a lesser extent the enteroendocrine system, occur following helminth infection but the importance of these adaptations in expelling the worm is poorly understood. Approximately 90% of the body’s serotonin (5-hydroxytryptamine (5-HT)) is made in enterochromaffin (EC) cells in the gut, indicative of the importance of this amine in intestinal function. Signaling via a plethora of receptor subtypes, substantial evidence illustrates that 5-HT affects immunity. A small number of studies document changes in 5-HT levels following infection with helminth parasites, but these have not been complemented by an understanding of the role of 5-HT in the host–parasite interaction. In reviewing this area, the gap in knowledge of how changes in the enteric serotonergic system affects the outcome of infection with intestinal helminths is apparent. We present this as a call-to-action by investigators in the field. We contend that neuronal EC cell–immune interactions in the gut are essential in maintaining homeostasis and, when perturbed, contribute to pathophysiology. The full affect of infection with helminth parasites needs to define, and then mechanistically dissect the role of the enteric nervous and enteroendocrine systems of the gut.

Survival of the feces: Does a nematode lungworm adaptively manipulate the behavior of its cane toad host?

Parasites can enhance their fitness by modifying the behavior of their hosts in ways that increase rates of production and transmission of parasite larvae. We used an antihelminthic drug to experimentally alter infections of lungworms (Rhabdias pseudosphaerocephala) in cane toads (Rhinella marina). We then compared subsequent behaviors of dewormed toads versus toads that retained infections. Both in the laboratory and in the field, the presence of parasites induced hosts to select higher body temperatures (thereby increasing rates of lungworm egg production), to defecate in moister sites, and to produce feces with higher moisture content (thereby enhancing survival of larvae shed in feces). Because those behavioral modifications enhance rather than decrease parasite fitness, they are likely to have arisen as adaptive manipulations of host behavior rather than as host adaptations to combat infection or as nonadaptive consequences of infection on host physiology. However, the mechanisms by which lungworms alter cane toad thermal preference and defecation are not known. Although many examples of host manipulation by parasites involve intermediate hosts facilitating their own demise, our findings indicate that manipulation of definitive hosts can be as subtle as when and where to defecate.

Rabies virus modifies host behaviour through a snake-toxin like region of its glycoprotein that inhibits neurotransmitter receptors in the CNS

Rabies virus induces drastic behaviour modifications in infected hosts. The mechanisms used to achieve these changes in the host are not known. The main finding of this study is that a region in the rabies virus glycoprotein, with homologies to snake toxins, has the ability to alter behaviour in animals through inhibition of nicotinic acetylcholine receptors present in the central nervous system. This finding provides a novel aspect to virus receptor interaction and host manipulation by pathogens in general. The neurotoxin-like region of the rabies virus glycoprotein inhibited acetylcholine responses of α4β2 nicotinic receptors in vitro, as did full length ectodomain of the rabies virus glycoprotein. The same peptides significantly altered a nicotinic receptor induced behaviour in C. elegans and increased locomotor activity levels when injected into the central nervous system of mice. These results provide a mechanistic explanation for the behavioural changes in hosts infected by rabies virus.

Stage-dependent behavioural changes but early castration induced by the acanthocephalan parasite Polymorphus minutus in its Gammarus pulex intermediate host

Multidimensionality in parasite-induced phenotypic alterations (PIPA) has been observed in a large number of host–parasite associations, particularly in parasites with complex life cycles. However, it is still unclear whether such a syndrome is due to the successive activation of independent PIPAs, or results from the synchronous disruption of a single mechanism. The aim of the present study was to investigate the onset and progression of two PIPAs (a behavioural alteration: reversion of geotaxis, and castration) occurring in the crustacean amphipod Gammarus pulex infected with the acanthocephalan Polymorphus minutus, at different parasite developmental stages. Modifications of geotaxis in hosts differed according to the parasite developmental stage. Whereas the cystacanth stage induced a negative geotaxis (exposing the gammarid to predation by birds, the definitive hosts), the acanthella stage, not yet infective for the definitive host, induced a stronger positive geotaxis (presumably protecting gammarids from bird predation). In contrast, castration was almost total at the acanthella stage, with no significant variation in the intensity according to parasite maturation. Finally, no significant correlation was found between the intensity of behavioural changes and the intensity of castration. We discuss our results in relation with current views on the evolution of multidimensionality in PIPA.

Consequences of eye fluke infection on anti-predator behaviours in invasive round gobies in Kalmar Sound

Larvae of the eye fluke, Diplostomum, emerge from snails and infect fish by penetrating skin or gills, then move to the lens where they may impair the vision of the fish. For the fluke to reproduce, a bird must eat the infected fish, and it has been suggested that they therefore actively manipulate the fish's behaviour to increase the risk of predation. We found that round gobies Neogobius melanostomus, a species that was recently introduced to the Kalmar Sound of the Baltic Sea, had an eye fluke prevalence of 90-100%. We investigated how the infection related to behavioural variation in round gobies. Our results showed that the more intense the parasite-induced cataract, the weaker the host's response was to simulated avian attack. The eye flukes did not impair other potentially important anti-predator behaviours, such as shelter use, boldness and the preference for shade. Our results are in accordance with the suggestion that parasites induce changes in host behaviour that will facilitate transfer to their final host.

Small alteration – big impacts: effects of small-scale riparian forest management on host–parasite dynamics in streams

Environmental changes and ecological disturbances can have large and unpredictable effects on parasite dynamics. Increasing human impacts on freshwater ecosystems through land use may thus modify the distribution and abundance of parasites and have cascading effects on host populations. Here we tested the effects of small-scale riparian forest management on the nematode Cystidicoloides ephemeridarum and its insect intermediate host Ephemera danica in forested streams. We assessed the impacts of harvesting riparian trees on parasite prevalence and abundance concomitantly with host densities. We also looked at upstream and downstream reaches to document potential cascading effects on unaltered stream sections mediated by aerial dispersal of adult mayfly or downstream drift of E. danica larvae. We show that host densities and parasite levels (prevalence and abundance) increased significantly following riparian tree removal. Overall, parasite densities showed a 6- to 66-fold increase in harvested reaches compared to upstream, pristine reaches. Similar effects were also clear downstream of the disturbance. Thus, despite the small extent of riparian forest alteration along the study streams, both parasite and intermediate host were strongly affected. Small-scale riparian forest management may thus have large, unforeseen impacts on some aspects of freshwater ecosystem structure and functioning that are often ignored. Generally, understanding how human perturbations influence parasites is vital when trying to predict overall impacts on ecosystem structure and functioning, and how changes in infection dynamics may further affect host species.

The parasite's long arm: a tapeworm parasite induces behavioural changes in uninfected group members of its social host

Parasites can induce alterations in host phenotypes in order to enhance their own survival and transmission. Parasites of social insects might not only benefit from altering their individual hosts, but also from inducing changes in uninfected group members. Temnothorax nylanderi ant workers infected with the tapeworm Anomotaenia brevis are known to be chemically distinct from nest-mates and do not contribute to colony fitness, but are tolerated in their colonies and well cared for. Here, we investigated how tapeworm- infected workers affect colony aggression by manipulating their presence in ant colonies and analysing whether their absence or presence resulted in behavioural alterations in their nest-mates. We report a parasite-induced shift in colony aggression, shown by lower aggression of uninfected nest-mates from parasitized colonies towards conspecifics, potentially explaining the tolerance towards infected ants. We also demonstrate that tapeworm-infected workers showed a reduced flight response and higher survival, while their presence caused a decrease in survival of uninfected nest-mates. This anomalous behaviour of infected ants, coupled with their increased survival, could facilitate the parasites' transmission to its definitive hosts, woodpeckers. We conclude that parasites exploiting individuals that are part of a society not only induce phenotypic changes within their individual hosts, but in uninfected group members as well.

Identifying a key host in an acanthocephalan-amphipod system

Trophically transmitted parasites may use multiple intermediate hosts, some of which may be 'key-hosts', i.e. contributing significantly more to the completion of the parasite life cycle, while others may be 'sink hosts' with a poor contribution to parasite transmission. Gammarus fossarum and Gammarus roeseli are sympatric crustaceans used as intermediate hosts by the acanthocephalan Pomphorhynchus laevis. Gammarus roeseli suffers higher field prevalence and is less sensitive to parasite behavioural manipulation and to predation by definitive hosts. However, no data are available on between-host differences in susceptibility to P. laevis infection, making it difficult to untangle the relative contributions of these hosts to parasite transmission. Based on results from estimates of prevalence in gammarids exposed or protected from predation and laboratory infections, G. fossarum specimens were found to be more susceptible to P. laevis infection. As it is more susceptible to both parasite infection and manipulation, G. fossarum is therefore a key host for P. laevis transmission.

Influence of host nutritional condition on post-infection traits in the association between the manipulative acanthocephalan Pomphorhynchus laevis and the amphipod Gammarus pulex

BACKGROUND

Several parasites with complex life-cycles induce phenotypic alterations in their intermediate hosts. According to the host manipulation hypothesis, such phenotypic alterations are supposed to increase the fitness of the parasite at the expense of that of its intermediate hosts through increasing the probability of transmission to next hosts. Although the phenomenon has received a large attention, the proximate factors modulating the occurrence and intensity of host manipulation remain poorly known. It has however, been suggested that the amount of energy reserves in the intermediate host might be a key parameter, although its precise influence on the intensity of manipulation remains unclear. Dietary depletion in the host may also lead to compromise with other parasite traits, such as probability of establishing or growth or virulence.

METHODS

Here, we address the question through performing experimental infections of the freshwater amphipod Gammarus pulex with two different populations of the acanthocephalan fish parasite Pomphorhynchus laevis, and manipulation of host nutritional condition. Following exposure, gammarids were given either a "standard" diet (consisting of elm leaves and chironomid larvae) or a "deprived" food treatment (deprived in proteins), and infection parameters were recorded. Once parasites reached the stage at which they become infective to their definitive host, refuge use (a behavioural trait presumably implied in trophic transmission) was assessed, and metabolic rate was measured.

RESULTS

Infected gammarids exposed to the deprived food treatment showed a lower metabolic rate, indicative of a lower body condition, compared to those exposed to the standard food treatment. Parasite size was smaller, and, depending on the population of origin of the parasites, intensity of infection was lower or mortality was higher in deprived hosts. However, food treatment had no effect on either the timing or intensity of behavioural modifications.

CONCLUSIONS

Overall, while our results suggest that acanthocephalan parasites develop better in hosts in good condition, no evidence was found for an influence of host nutritional condition on host manipulation by parasites.

Multidimensionality in host manipulation mimicked by serotonin injection

Manipulative parasites often alter the phenotype of their hosts along multiple dimensions. 'Multidimensionality' in host manipulation could consist in the simultaneous alteration of several physiological pathways independently of one another, or proceed from the disruption of some key physiological parameter, followed by a cascade of effects. We compared multidimensionality in 'host manipulation' between two closely related amphipods, Gammarus fossarum and Gammarus pulex, naturally and experimentally infected with Pomphorhynchus laevis (Acanthocephala), respectively. To that end, we calculated in each host-parasite association the effect size of the difference between infected and uninfected individuals for six different traits (activity, phototaxis, geotaxis, attraction to conspecifics, refuge use and metabolic rate). The effects sizes were highly correlated between host-parasite associations, providing evidence for a relatively constant 'infection syndrome'. Using the same methodology, we compared the extent of phenotypic alterations induced by an experimental injection of serotonin (5-HT) in uninfected G. pulex to that induced by experimental or natural infection with P. laevis. We observed a significant correlation between effect sizes across the six traits, indicating that injection with 5-HT can faithfully mimic the 'infection syndrome'. This is, to our knowledge, the first experimental evidence that multidimensionality in host manipulation can proceed, at least partly, from the disruption of some major physiological mechanism.

Eaten alive: cannibalism is enhanced by parasites

Cannibalism is ubiquitous in nature and especially pervasive in consumers with stage-specific resource utilization in resource-limited environments. Cannibalism is thus influential in the structure and functioning of biological communities. Parasites are also pervasive in nature and, we hypothesize, might affect cannibalism since infection can alter host foraging behaviour. We investigated the effects of a common parasite, the microsporidian Pleistophora mulleri, on the cannibalism rate of its host, the freshwater amphipod Gammarus duebeni celticus. Parasitic infection increased the rate of cannibalism by adults towards uninfected juvenile conspecifics, as measured by adult functional responses, that is, the rate of resource uptake as a function of resource density. This may reflect the increased metabolic requirements of the host as driven by the parasite. Furthermore, when presented with a choice, uninfected adults preferred to cannibalize uninfected rather than infected juvenile conspecifics, probably reflecting selection pressure to avoid the risk of parasite acquisition. By contrast, infected adults were indiscriminate with respect to infection status of their victims, probably owing to metabolic costs of infection and the lack of risk as the cannibals were already infected. Thus parasitism, by enhancing cannibalism rates, may have previously unrecognized effects on stage structure and population dynamics for cannibalistic species and may also act as a selective pressure leading to changes in resource use.

Evolution of complex life cycles in trophically transmitted helminths. I. Host incorporation and trophic ascent

Links between parasites and food webs are evolutionarily ancient but dynamic: life history theory provides insights into helminth complex life cycle origins. Most adult helminths benefit by sexual reproduction in vertebrates, often high up food chains, but direct infection is commonly constrained by a trophic vacuum between free-living propagules and definitive hosts. Intermediate hosts fill this vacuum, facilitating transmission to definitive hosts. The central question concerns why sexual reproduction, and sometimes even larval growth, is suppressed in intermediate hosts, favouring growth arrest at larval maturity in intermediate hosts and reproductive suppression until transmission to definitive hosts? Increased longevity and higher growth in definitive hosts can generate selection for larger parasite body size and higher fecundity at sexual maturity. Life cycle length is increased by two evolutionary mechanisms, upward and downward incorporation, allowing simple (one-host) cycles to become complex (multihost). In downward incorporation, an intermediate host is added below the definitive host: models suggest that downward incorporation probably evolves only after ecological or evolutionary perturbations create a trophic vacuum. In upward incorporation, a new definitive host is added above the original definitive host, which subsequently becomes an intermediate host, again maintained by the trophic vacuum: theory suggests that this is plausible even under constant ecological/evolutionary conditions. The final cycle is similar irrespective of its origin (upward or downward). Insights about host incorporation are best gained by linking comparative phylogenetic analyses (describing evolutionary history) with evolutionary models (examining selective forces). Ascent of host trophic levels and evolution of optimal host taxa ranges are discussed.

Friends with social benefits: host-microbe interactions as a driver of brain evolution and development?

The tight association of the human body with trillions of colonizing microbes that we observe today is the result of a long evolutionary history. Only very recently have we started to understand how this symbiosis also affects brain function and behavior. In this hypothesis and theory article, we propose how host-microbe associations potentially influenced mammalian brain evolution and development. In particular, we explore the integration of human brain development with evolution, symbiosis, and RNA biology, which together represent a “social triangle” that drives human social behavior and cognition. We argue that, in order to understand how inter-kingdom communication can affect brain adaptation and plasticity, it is inevitable to consider epigenetic mechanisms as important mediators of genome-microbiome interactions on an individual as well as a transgenerational time scale. Finally, we unite these interpretations with the hologenome theory of evolution. Taken together, we propose a tighter integration of neuroscience fields with host-associated microbiology by taking an evolutionary perspective.

Reevaluating the evidence for Toxoplasma gondii-induced behavioural changes in rodents

The ubiquitous protozoan parasite Toxoplasma gondii has been associated with behavioural changes in various hosts, including humans. In rodents, these behavioural changes are thought to represent adaptive manipulation by T. gondii to enhance transmission from intermediate hosts to the feline definitive host. In this review, we have tabulated evidence of changes in motor coordination, learning, memory, locomotion, anxiety, response to novelty and aversion to feline odour in rodents experimentally infected with T. gondii. In general, there was no consistent indication of the direction or magnitude of behavioural changes in response to infection. This may be due to the use, in these experimental studies, of different T. gondii strains, different host species and sexes and/or different methodologies to measure behaviour. A particular problem with studies of behavioural manipulation is likely to be the validity of behavioural tests, that is, whether they are actually measuring the traits that they were designed to measure. We suggest that future studies can be improved in three major ways. First, they should use multiple tests of behaviour, followed by multivariate data analysis to identify behavioural constructs such as aversion, anxiety and response to novelty. Second, they should incorporate longitudinal measurements on the behaviour of individual hosts before and after infection, so that within-individual and between-individual variances and covariances in behavioural traits can be estimated. Finally, they should investigate how variables such as parasite strain, host species and host sex interact with parasite infection to alter host behaviour, in order to provide a sound foundation for research concerning the proximate and ultimate mechanism(s) responsible for behavioural changes.

Non-specific manipulation of gammarid behaviour by P. minutus parasite enhances their predation by definitive bird hosts

Trophically-transmitted parasites often change the phenotype of their intermediate hosts in ways that increase their vulnerability to definitive hosts, hence favouring transmission. As a "collateral damage", manipulated hosts can also become easy prey for non-host predators that are dead ends for the parasite, and which are supposed to play no role in transmission strategies. Interestingly, infection with the acanthocephalan parasite Polymorphus minutus has been shown to reduce the vulnerability of its gammarid intermediate hosts to non-host predators, whose presence triggered the behavioural alterations expected to favour trophic transmission to bird definitive hosts. Whilst the behavioural response of infected gammarids to the presence of definitive hosts remains to be investigated, this suggests that trophic transmission might be promoted by non-host predation risk. We conducted microcosm experiments to test whether the behaviour of P. minutus-infected gammarids was specific to the type of predator (i.e. mallard as definitive host and fish as non-host), and mesocosm experiments to test whether trophic transmission to bird hosts was influenced by non-host predation risk. Based on the behaviours we investigated (predator avoidance, activity, geotaxis, conspecific attraction), we found no evidence for a specific fine-tuned response in infected gammarids, which behaved similarly whatever the type of predator (mallard or fish). During predation tests, fish predation risk did not influence the differential predation of mallards that over-consumed infected gammarids compared to uninfected individuals. Overall, our results bring support for a less sophisticated scenario of manipulation than previously expected, combining chronic behavioural alterations with phasic behavioural alterations triggered by the chemical and physical cues coming from any type of predator. Given the wide dispersal range of waterbirds (the definitive hosts of P. minutus), such a manipulation whose efficiency does not depend on the biotic context is likely to facilitate its trophic transmission in a wide range of aquatic environments.