Suppression of predation on the intermediate host by two trophically-transmitted parasites when uninfective

Parasite evolution of host manipulation strategies with fluctuating ecological dynamics

Trophically transmitted parasites often infect an intermediate prey host and manipulate their behaviour to make predation more likely, thus facilitating parasite transmission to the definitive host. However, it is unclear when such a manipulation strategy should be expected to evolve. We develop the first evolutionary invasion model to explore the evolution of manipulation strategies that are in a trade-off with parasite production of free-living spores. We find that the size of the susceptible prey population together with the threat of predation drives manipulation evolution. We find that it is only when the susceptible prey population is large and the threat of predation is relatively small that selection favours manipulation strategies over spore production. We also confirm that the system exhibits cyclic population dynamics, and this can influence the qualitative direction of selection.

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.

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.

The secretome of a parasite alters its host's behaviour but does not recapitulate the behavioural response to infection

Parasites with complex life cycles have been proposed to manipulate the behaviour of their intermediate hosts to increase the probability of reaching their final host. The cause of these drastic behavioural changes could be manipulation factors released by the parasite in its environment (the secretome), but this has rarely been assessed. We studied a non-cerebral parasite, the cestode Schistocephalus solidus, and its intermediate host, the threespine stickleback (Gasterosteus aculeatus), whose response to danger becomes significantly diminished when infected. These altered behaviours appear only during late infection, when the worm is ready to reproduce in its final avian host. Sympatric host-parasite pairs show higher infection success for parasites, suggesting that the secretome effects could differ for allopatric host-parasite pairs with independent evolutionary histories. We tested the effects of secretome exposure on behaviour by using secretions from the early and late infection of S. solidus and by injecting them in healthy sticklebacks from a sympatric and allopatric population. Contrary to our prediction, secretome from late infection worms did not result in more risky behaviours, but secretome from early infection resulted in more cautious hosts, only in fish from the allopatric population. Our results suggest that the secretome of S. solidus contains molecules that can affect host behaviour, that the causes underlying the behavioural changes in infected sticklebacks are multifactorial and that local adaptation between host-parasite pairs may extend to the response to the parasite's secretome content.

Behavioural mechanisms underlying parasite-mediated competition for refuges in a coral reef fish

Parasites have been increasingly recognized as participants in indirect ecological interactions, including those mediated by parasite-induced changes to host behaviour (trait-mediated indirect interactions or TMIIs). In most documented examples, host behaviours altered by parasites increase susceptibility to predation because the predator is also a host (host-manipulation). Here, we test for a TMII in which a parasitic copepod modifies the predator-prey interaction between a small goby host and several larger predatory fish. Gobies compete for crevices in the reef to avoid predation and goby mortality increases more rapidly with increasing refuge shortage for parasitized gobies than for those free of parasites. We found interactive effects of refuge shortage and parasitism on two behaviours we predicted might be associated with parasite-mediated competition for refuges. First, as refuge-shortage increases, the rate of aggression among gobies increases and parasitism intensifies this interaction. Second, goby proximity to refuges increases as refuges become scarce, but parasitism nullifies this increase. In combination, these parasite-induced changes in behaviour may explain why parasitized gobies are poor competitors for refuges. Because the parasite is not trophically transmitted via host manipulation, these altered behaviours in parasitized gobies are likely coincidental to infection.

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.

Behavior out of control: Experimental evolution of resistance to host manipulation

Many parasites alter their host's phenotype in a manner that enhances their own fitness beyond the benefits they would gain from normal exploitation. Such host manipulation is rarely consistent with the host's best interests resulting in suboptimal and often fatal behavior from the host's perspective. In this case, hosts should evolve resistance to host manipulation. The cestode Schistocephalus solidus manipulates the behavior of its first intermediate copepod host to reduce its predation susceptibility and avoid fatal premature predation before the parasite is ready for transmission to its subsequent host. Thereafter, S. solidus increases host activity to facilitate transmission. If successful, this host manipulation is necessarily fatal for the host. I selected the copepod Macrocyclops albidus, a first intermediate host of S. solidus, for resistance or susceptibility to host manipulation to investigate their evolvability. Selection on the host indeed increased host manipulation in susceptible and reduced host manipulation in resistant selection lines. Interestingly, this seemed to be at least partly due to changes in the baseline levels of the modified trait (activity) rather than actual changes in resistance or susceptibility to host manipulation. Hence, hosts seem restricted in how rapidly and efficiently they can evolve resistance to host manipulation.

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.

Crowding in the first intermediate host does not affect infection probability in the second host in two helminths

When many worms co-infect the same host, their average size is often reduced. This negative density-dependent growth is called the crowding effect. Crowding has been reported many times for worms in their intermediate hosts, but rarely have the fitness consequences of crowding been examined. This study tested whether larval crowding reduces establishment success in the next host for two parasites with complex life cycles, the nematode Camallanus lacustris and the cestode Schistocephalus solidus. Infected copepods, the first host, were fed to sticklebacks, the second host. Fish received a constant dose, but the infection intensity in copepods was varied (e.g. giving two singly infected copepods or one doubly infected copepod). Worms from higher-intensity infections did not have significantly reduced infection success in fish. However, crowded treatments had a disproportionate number of low and high infection rates, and although this trend was not significant, it hints at the possibility that multiple worms within a copepod are more likely to either all infect or all die when transmitted to the next host. These results indicate that a smaller larval size due to crowding need not reduce the establishment probability of a worm in the next host.

Two different strategies of host manipulation allow parasites to persist in intermediate-definitive host systems

Trophically transmitted parasites start their development in an intermediate host, before they finish the development in their definitive host when the definitive host preys on the intermediate host. In intermediate-definitive host systems, two strategies of host manipulation have been evolved: increasing the rate of transmission to the definitive host by increasing the chance that the definitive host will prey on the intermediate host, or increasing the lifespan of the parasite in the intermediate host by decreasing the predation chance when the intermediate host is not yet infectious. As the second strategy is less well studied than the first, it is unknown under what conditions each of these strategies is prevailed and evolved. We analysed the effect of both strategies on the presence of parasites in intermediate-definitive host systems with a structured population model. We show that the parasite can increase the parameter space where it can persist in the intermediate-definitive host system using one of these two strategies of host manipulation. We found that when the intermediate host or the definitive host has life-history traits that allow the definitive host to reach large population densities, that is high reproduction rate of the intermediate host or high conversion efficiency of the definitive host (efficiency at which the uninfected definitive host converts caught intermediate hosts into offspring), respectively, evolving manipulation to decrease the predation chance of the intermediate host will be more beneficial than manipulation to increase the predation chance to enhance transmission. Furthermore, manipulation to decrease the predation chance of the intermediate host results in higher population densities of infected intermediate hosts than manipulation that increases the predation chance to enhance transmission. Our study shows that host manipulation in early stages of the parasite development to decrease predation might be a more frequently evolved way of host manipulation than is currently assumed.

Differences between populations in host manipulation by the tapeworm Schistocephalus solidus - is there local adaptation?

Host manipulation whereby a parasite increases its transmission to a subsequent host by altering the behaviour of its current host is very far spread. It also occurs in host-parasite systems that are widely distributed. This offers the potential for local adaptation. The tapeworm Schistocephalus solidus modifies its first intermediate copepod host's predation susceptibility to suit its own needs by reducing its activity before it becomes infective and increasing it thereafter. To investigate potential differences in host manipulation between different populations and test for potential local adaptation with regard to host manipulation, I experimentally infected hosts from two distinct populations with parasites from either population in a fully crossed design. Host manipulation differed between populations mostly once the parasite had reached infectivity. These differences in infective parasites were mostly due to differences between different parasite populations. In not yet infective parasites, however, host population also had a significant effect on host manipulation. There was no evidence of local adaptation; parasites were able to manipulate foreign and local hosts equally well. Likewise, hosts were equally poor at resisting host manipulation by local and foreign parasites.

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.

Positive density-dependent growth supports costs sharing hypothesis and population density sensing in a manipulative parasite

Parasites manipulate their hosts' phenotype to increase their own fitness. Like any evolutionary adaptation, parasitic manipulations should be costly. Though it is difficult to measure costs of the manipulation directly, they can be evaluated using an indirect approach. For instance, theory suggests that as the parasite infrapopulation grows, the investment of individual parasites in host manipulation decreases, because of cost sharing. Another assumption is that in environments where manipulation does not pay off for the parasite, it can decrease its investment in the manipulation to save resources. We experimentally infected rainbow trout Oncorhynchus mykiss with the immature larvae of the trematode Diplostomum pseudospathaceum, to test these assumptions. Immature D. pseudospathaceum metacercariae are known for their ability to manipulate the behaviour of their host enhancing its anti-predator defenses to avoid concomitant predation. We found that the growth rate of individual parasites in rainbow trout increased with the infrapopulation size (positive density-dependence) suggesting cost sharing. Moreover, parasites adjusted their growth to the intensity of infection within the eye lens where they were localized suggesting population density sensing. Results of this study support the hypothesis that macroparasites can adjust their growth rate and manipulation investment according to cost sharing level and infrapopulation size.

A parasite's modification of host behavior reduces predation on its host

Parasite modification of host behavior is common, and the literature is dominated by demonstrations of enhanced predation on parasitized prey resulting in transmission of parasites to their next host. We present a case in which predation on parasitized prey is reduced. Despite theoretical modeling suggesting that this phenomenon should be common, it has been reported in only a few host-parasite-predator systems. Using a system of gregarine endosymbionts in host mosquitoes, we designed experiments to compare the vulnerability of parasitized and unparasitized mosquito larvae to predation by obligate predatory mosquito larvae and then compared behavioral features known to change in the presence of predatory cues. We exposed Aedes triseriatus larvae to the parasite Ascogregarina barretti and the predator Toxohrynchites rutilus and assessed larval mortality rate under each treatment condition. Further, we assessed behavioral differences in larvae due to infection and predation stimuli by recording larvae and scoring behaviors and positions within microcosms. Infection with gregarines reduced cohort mortality in the presence of the predator, but the parasite did not affect mortality alone. Further, infection by parasites altered behavior such that infected hosts thrashed less frequently than uninfected hosts and were found more frequently on or in a refuge within the microcosm. By reducing predation on their host, gregarines may be acting as mutualists in the presence of predation on their hosts. These results illustrate a higher-order interaction, in which a relationship between a species pair (host-endosymbiont or predator-prey) is altered by the presence of a third species.

Conflicts over host manipulation between different parasites and pathogens: Investigating the ecological and medical consequences

When parasites have different interests in regard to how their host should behave this can result in a conflict over host manipulation, i.e. parasite induced changes in host behaviour that enhance parasite fitness. Such a conflict can result in the alteration, or even complete suppression, of one parasite's host manipulation. Many parasites, and probably also symbionts and commensals, have the ability to manipulate the behaviour of their host. Non‐manipulating parasites should also have an interest in host behaviour. Given the frequency of multiple parasite infections in nature, potential conflicts of interest over host behaviour and manipulation may be common. This review summarizes the evidence on how parasites can alter other parasite's host manipulation. Host manipulation can have important ecological and medical consequences. I speculate on how a conflict over host manipulation could alter these consequences and potentially offer a new avenue of research to ameliorate harmful consequences of host manipulation.

Inter- and intraspecific conflicts between parasites over host manipulation

Host manipulation is a common strategy by which parasites alter the behaviour of their host to enhance their own fitness. In nature, hosts are usually infected by multiple parasites. This can result in a conflict over host manipulation. Studies of such a conflict in experimentally infected hosts are rare. The cestode Schistocephalus solidus (S) and the nematode Camallanus lacustris (C) use copepods as their first intermediate host. They need to grow for some time inside this host before they are infective and ready to be trophically transmitted to their subsequent fish host. Accordingly, not yet infective parasites manipulate to suppress predation. Infective ones manipulate to enhance predation. We experimentally infected laboratory-bred copepods in a manner that resulted in copepods harbouring (i) an infective C plus a not yet infective C or S, or (ii) an infective S plus a not yet infective C. An infective C completely sabotaged host manipulation by any not yet infective parasite. An infective S partially reduced host manipulation by a not yet infective C. We hence show experimentally that a parasite can reduce or even sabotage host manipulation exerted by a parasite from a different species.

Does resource availability affect host manipulation? – an experimental test with Schistocephalus solidus

Host manipulation is a common strategy of parasites employed to increase their fitness by changing the phenotype of their hosts. Whether host manipulation might be affected by environmental factors such as resource availability, has received little attention. We experimentally infected laboratory-bred copepods with the cestodeSchistocephalus solidus, submitted infected and uninfected copepods to either a high or a low food treatment, and measured their behaviour. Infection reduced host activity and speed in both feeding treatments. However, the difference between the infected and uninfected copepods was smaller under low food conditions, because uninfected, but not infected, copepods moved slower under these conditions. We suggest that these differences are mediated by the physical condition of copepods rather than changes in how strongly the parasite manipulated host behaviour. Additionally, we measured three fitness-relevant traits (growth, development and infection rate in the next host) of the parasite to identify potential trade-offs with host manipulation. The largest parasites in copepods appeared the least manipulative, i.e. their hosts showed the smallest behavioural alterations, but this may again reflect variation in copepod condition, rather than life history trade-offs between parasite growth and host manipulation. Our results point to the possibility that parasite transmission depends on environmental conditions.

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.

The Behavior Response of Amphipods Infected by Hedruris suttonae (Nematoda) and Pseudocorynosoma sp. (Acanthocephala)

The manipulation of intermediate host behavior may increase chances of parasite transmission to the definitive host. In freshwater environments of the Neotropical Region, studies on behavioral manipulations by parasites are rare, and the majority of these consider only a single parasite species and/or 1 life stage of a particular parasite species. In Andean Patagonian lakes of Argentina, the amphipod Hyalella patagonica is infected by larvae of the fish nematode Hedruris suttonae and by the bird acanthocephalan Pseudocorynosoma sp. The 3 objectives of the present study were to determine whether H. suttonae and Pseudocorynosoma sp. differ in their effects on behavior of H. patagonica , whether such modification is associated with parasite development, and to assess the associations between behavioral traits. From naturally parasitized amphipods, activity (swimming levels) and phototaxis (light preference) was measured. Only in phototaxis trials did larvae of H. suttonae induce significantly higher levels of photophilia, suggesting that they are manipulative. Scores of activity and phototaxis were positive and significantly related for non-parasitized female amphipods and for amphipods parasitized by larvae of Pseudocorynosoma sp. but were not associated in amphipods parasitized with larvae of H. suttonae (infective and non-infective), suggesting that infection separated the relationship between these variables.

When parasites disagree: evidence for parasite-induced sabotage of host manipulation

Host manipulation is a common parasite strategy to alter host behavior in a manner to enhance parasite fitness usually by increasing the parasite's transmission to the next host. In nature, hosts often harbor multiple parasites with agreeing or conflicting interests over host manipulation. Natural selection might drive such parasites to cooperation, compromise, or sabotage. Sabotage would occur if one parasite suppresses the manipulation of another. Experimental studies on the effect of multi-parasite interactions on host manipulation are scarce, clear experimental evidence for sabotage is elusive. We tested the effect of multiple infections on host manipulation using laboratory-bred copepods experimentally infected with the trophically transmitted tapeworm Schistocephalus solidus. This parasite is known to manipulate its host depending on its own developmental stage. Coinfecting parasites with the same aim enhance each other's manipulation but only after reaching infectivity. If the coinfecting parasites disagree over host manipulation, the infective parasite wins this conflict: the noninfective one has no effect. The winning (i.e., infective) parasite suppresses the manipulation of its noninfective competitor. This presents conclusive experimental evidence for both cooperation in and sabotage of host manipulation and hence a proof of principal that one parasite can alter and even neutralize manipulation by another.

Cooperation and conflict in host manipulation: interactions among macro-parasites and micro-organisms

Several parasite species are known to manipulate the phenotype of their hosts in ways that enhance their own transmission. Co-occurrence of manipulative parasites, belonging to the same species or to more than one species, in a single host has been regularly observed. Little is known, however, on interactions between co-occurring manipulative parasites with same or different transmission routes. Several models addressing this problem have provided predictions on how cooperation and conflict between parasites could emerge from multiple infections. Here, we review the empirical evidence in favor of the existence of synergistic or antagonistic interactions between co-occurring parasites, and highlight the neglected role of micro-organisms. We particularly discuss the actual importance of selective forces shaping the evolution of interactions between manipulative parasites in relation to parasite prevalence in natural populations, efficiency in manipulation, and type of transmission (i.e., horizontal versus vertical), and we emphasize the potential for future research.