Parasite-induced Behavioural Changes Research Articles

Early signals of parasitism expressed through behaviour but modulated by social context

Sickness behaviours are believed to be an adaptive response to infection. However, the degree to which these behaviours can be expressed may be impacted by an individual's social environment. Here we tested, first, whether parasitism reduces the activity behaviour of lambs, Ovis aries , second, whether this occurs prior to other observed costs of parasitism and, third, whether the infection status of other individuals affects the degree to which these behaviours are expressed. Sixty lambs were separated into replicate groups within three treatments: (1) parasitized: all lambs were infected with the parasitic nematode Teladorsagia circumcincta ; (2) nonparasitized; all lambs were dosed with water; (3) mixed: some of the group were infected and some were dosed with water. Activity behaviour was monitored before, during and after parasite infection. Parasitized groups had reduced activity levels following infection, and this occurred before any other impact or measure of parasitism was detected. Infected animals in the mixed groups had reduced activity levels following infection, but the level of change was less than that in animals in the fully parasitized groups. Activity levels remained low until lambs were treated with anthelmintic when activity levels of the groups that had been parasitized returned to the same level as nonparasitized groups. These findings show that parasite-induced behavioural changes occur earlier than other more commonly observed signals of infection, but the infection profile of an individual's group can shape these behavioural responses to infection. • Domestic lambs were experimentally infected with a gastrointestinal nematode. • Parasitized lambs had reduced activity levels following infection. • This occurred before any other measure or impact of parasitism was detected. • However, these behavioural changes were affected by group infection status. • Thus, an individual's social environment can affect their response to infection.

Neophobic behavioural responses of parasitised fish to a potential predator and baited hook

Many juvenile fish show essential avoidance of predators but are also faced with the need to show risky asset-acquiring behaviour, such as trying new food sources, many of which are known to be influenced by parasite infection. Here, we investigated the links between parasite infection, anti-predator behaviour and neophobia towards a novel food presented on a baited hook in fish. Juvenile Spangled perch (Leipotherapon unicolor) undertook two behavioural tests in our laboratory, before being dissected and examined for external and internal parasites. We used an adapted conditioned place avoidance (CPA) paradigm to examine responses to a predator fish and in a separate test, we examined neophobia of a baited hook. Of the 69 Spangled perch studied, 27% of fish were uninfected and at least one of nematodes, Protozoa/Myxozoa or Monogenea were found in the remaining fish. Fish spent less time in the side of the tank where the predator was previously encountered, than before encounters, indicating that fish can learn to avoid locations where predators were previously observed. We found that fish spent longer in the side of the tank where the predator was visible than in the same side of the tank in earlier trials without the predator, supporting previous studies showing that fish actively approach and engage in predator inspection. Fish infected with Protozoa/Myxozoa and camallanid nematodes were less likely to enter the side of the tank with the predator than uninfected fish, suggesting a reluctance to approach a predator in these infected fish. However, Protozoa/Myxozoa and camallanid infected fish bit the baited hook sooner than uninfected fish, possibly indicating less neophobia of the baited hook in fish with these infections. Avoiding to approach a predator yet reduced neophobia to novel foods in some infected fish suggest that parasites do not exert a general, overall, effect on risk aversiveness. Instead, changes in neophobic behaviour appear complex and we discuss the possibility that parasite-induced changes in fish behaviour may be modulated by altered hunger levels and reduced locomotion brought about by illness.

Brain-encysting trematodes (Euhaplorchis californiensis) decrease raphe serotonergic activity in California killifish (Fundulus parvipinnis).

ABSTRACTModulation of brain serotonin (5-HT) signalling is associated with parasite-induced changes in host behaviour, potentially increasing parasite transmission to predatory final hosts. Such alterations could have substantial impact on host physiology and behaviour, as 5-HT serves multiple roles in neuroendocrine regulation. These effects, however, remain insufficiently understood, as parasites have been associated with both increased and decreased serotonergic activity. Here, we investigated effects of trematode Euhaplorchis californiensis metacercariae on post-stress serotonergic activity in the intermediate host California killifish (Fundulus parvipinnis). This parasite is associated with conspicuous behaviour and increased predation of killifish by avian end-hosts, as well as inhibition of post-stress raphe 5-HT activity. Until now, laboratory studies have only been able to achieve parasite densities (parasites/unit host body mass) well below those occurring in nature. Using laboratory infections yielding ecologically relevant parasite loads, we show that serotonergic activity indeed decreased with increasing parasite density, an association likely indicating changes in 5-HT neurotransmission while available transmitter stores remain constant. Contrary to most observations in the literature, 5-HT activity increased with body mass in infected fish, indicating that relationships between parasite load and body mass may in many cases be a real underlying factor for physiological correlates of body size. Our results suggest that parasites are capable of influencing brain serotonergic activity, which could have far-reaching effects beyond the neurophysiological parameters investigated here.

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.

Positive phototropism is accelerated in Biomphalaria glabrata snails by infection with Schistosoma mansoni

Parasite-induced behavioral changes in their hosts favor to complete the lifecycle of parasites. Schistosome infection is also known to cause physiological changes in infected freshwater snail intermediate hosts. Here, we report, a novel phenomenon in which Schistosoma mansoni, a highly debilitating worm affecting millions of people worldwide, alters the phototropic behavior of Biomphalaria glabrata, the vector snail. S. mansoni-infection enhanced positive phototropism of vector snails and infected snails spent significantly more time in light. Possibly, these behavioral changes help the parasite to be released efficiently from the infected intermediate hosts, and to infect mammalian hosts.

Bold, Sedentary Fathead Minnows Have More Parasites.

Parasites that rely on trophic transmission can manipulate the behavior of an intermediate host to compromise the host's antipredator competence and increase the probability of reaching the next host. Selection for parasite manipulation is diminished when there is significant risk of host death to causes other than consumption by a suitable definitive host for the parasite. Consequently, behavioral manipulation by parasites can be expected to be subtle. Ornithodiplostomum ptychocheilus (Op) is a trematode parasite that has a bird-snail-fish host life cycle. Fathead minnows are a common intermediate host of Op, where metacercariae encyst in the minnow brain. In this study, we report a link between metacercarial intensity and behavior in fathead minnows. In the field, we found that roaming distance by free-living minnows over 24 h was negatively correlated with parasite intensity. In the laboratory, we found that boldness in an open field test was positively correlated with parasite intensity. These parasite-induced behavioral changes may render infected minnows more susceptible to predators, which would serve to facilitate trophic transmission of parasites to the bird host.

Host-seeking behaviors of mosquitoes experimentally infected with sympatric field isolates of the human malaria parasite Plasmodium falciparum: no evidence for host manipulation

Previous studies have shown that Plasmodium parasites can manipulate mosquito feeding behaviours such as motivation and avidity to feed on vertebrate hosts, in ways that increase the probability of parasite transmission. These studies, however, have been mainly carried out on non-natural and/or laboratory based model systems and hence may not reflect what occurs in the field. We now need to move closer to the natural setting, if we are to fully capture the ecological and evolutionary consequences of these parasite-induced behavioral changes. As part of this effort, we conducted a series of experiments to investigate the long and short-range behavioural responses to human stimuli in the mosquito Anopheles coluzzii during different stages of infection with sympatric field isolates of the human malaria parasite Plasmodium falciparum in Burkina Faso. First, we used a dual-port olfactometer designed to take advantage of the whole body odor to gauge mosquito long-range host-seeking behaviors. Second, we used a locomotor activity monitor system to assess mosquito short-range behaviors. Compared to control uninfected mosquitoes, P. falciparum infection had no significant effect neither on long-range nor on short-range behaviors both at the immature and mature stages. This study, using a natural mosquito-malaria parasite association, indicates that manipulation of vector behavior may not be a general phenomenon. We speculate that the observed contrasting phenotypes with model systems might result from coevolution of the human parasite and its natural vector. Future experiments, using other sympatric malaria mosquito populations or species are required to test this hypothesis. In conclusion, our results highlight the importance of following up discoveries in laboratory model systems with studies on natural parasite–mosquito interactions to accurately predict the epidemiological, ecological and evolutionary consequences of parasite manipulation of vector behaviors

Parasitic Manipulation of Host Behaviour: Baculovirus SeMNPV EGT Facilitates Tree-Top Disease in Spodoptera exigua Larvae by Extending the Time to Death.

Many parasites enhance their dispersal and transmission by manipulating host behaviour. One intriguing example concerns baculoviruses that induce hyperactivity and tree-top disease (i.e., climbing to elevated positions prior to death) in their caterpillar hosts. Little is known about the underlying mechanisms of such parasite-induced behavioural changes. Here, we studied the role of the ecdysteroid UDP-glucosyltransferase (egt) gene of Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) in tree-top disease in S. exigua larvae. Larvae infected with a mutant virus lacking the egt gene exhibited a shorter time to death and died before the induction of tree-top disease. Moreover, deletion of either the open reading frame or the ATG start codon of the egt gene prevented tree-top disease, indicating that the EGT protein is involved in this process. We hypothesize that SeMNPV EGT facilitates tree-top disease in S. exigua larvae by prolonging the larval time to death. Additionally, we discuss the role of egt in baculovirus-induced tree-top disease.

Calling Behavior of Male Acheta domesticus Crickets Infected with Paragordius varius (Nematomorpha: Gordiida).

It is well established that parasites in the phylum Nematomorpha induce suicide behavior of their insect hosts to bring adult worms to the appropriate habitat for emergence. It is not well established, however, whether other nematomorph-induced behavioral alterations occur before worm emergence. The purpose of our study was to evaluate the effect of the nematomorph Paragordius varius on the calling behavior of the male house cricket Acheta domesticus . We hypothesized that cricket calling, an energetically expensive and risky behavior, would be a potential target for nematomorph-induced behavioral alterations. We assessed if and how infection with P. varius affects A. domesticus calling behavior and whether the presence of wings at time of exposure to P. varius influenced changes in calling behavior. We recorded the calling behavior of male A. domesticus over the course of their infection after exposure to P. various before or after wing development. Additionally, we assessed whether winged crickets were "callers" or "noncallers" before exposure. We found that regardless of cricket developmental stage (or age) at time of infection, infected crickets spent significantly less time calling than their uninfected counterparts but only during the later stages of infection. Developmental stage at infection did affect whether crickets became callers: when infected before wing development significantly more uninfected crickets initiated calling; there was no difference between infected and uninfected crickets when infected as winged adults. Infection was a factor in whether callers stopped calling, with more infected crickets ceasing to call than uninfected crickets. This is the first study to show that infection with nematomorphs affects calling behavior of their insect host. Cricket calling behavior is immensely complex and although it was difficult to elucidate the adaptive nature of these parasite-induced behavioral changes, this study lays the groundwork for future studies to begin teasing out the factors that will help make the determination between side effect of infection or parasite/host adaptation.

Behavioral changes caused by Austrodiplostomum spp. in Hoplias malabaricus from the São Francisco River, Brazil

Traira (Hoplias malabaricus) is a neotropical fish that is widely distributed in freshwater environments in South America. In the present study, we documented the occurrence of metacercariae of Austrodiplostomum spp. (Diplostomidae) in the eyes and cranial cavity of H. malabaricus and described parasite-induced behavioral changes in the host. The fish were collected from the upper São Francisco River, in the Serra da Canastra mountain range, Minas Gerais, transported alive to the laboratory, observed for 2 weeks, and subsequently examined for parasites. Of the 35 fish examined, 28 (80 %) had free metacercariae in the vitreous humor (mean intensity=95.4; mean abundance=76.3), and 24 (68.57 %) had free metacercariae in the cranial cavity, mainly concentrated below the floor of the brain, at the height of the ophthalmic lobe (mean intensity=12.91; mean abundance=8.85). Specimens of H. malabaricus with a high intensity of infection in the brain displayed changes in swimming behavior.

Walking with insects: molecular mechanisms behind parasitic manipulation of host behaviour

Parasitic infections are often followed by changes in host behaviour. Numerous and exquisite examples of such behavioural alterations are known, covering a broad spectrum of parasites and hosts. Most descriptions of such parasite-induced changes in host behaviour are observational reports, while experimentally confirmed examples of parasite genes inducing these changes are limited. In this study, we review changes in invertebrate host behaviour observed upon infection by parasites and discuss such changes in an evolutionary context. We then explore possible mechanisms involved in parasite-induced changes in host behaviour. Genes and pathways known to play a role in invertebrate behaviour are reviewed, and we hypothesize how parasites (may) affect these pathways. This review provides the state of the art in this exciting, interdisciplinary field by exploring possible pathways triggered in hosts, suggesting methodologies to unravel the molecular mechanisms that lead to changes in host behaviour.

Neural parasitology: how parasites manipulate host behaviour

The ability of parasites to alter the behaviour of their hosts fascinates both scientists and non-scientists alike. One reason that this topic resonates with so many is that it touches on core philosophical issues such as the existence of free will. If the mind is merely a machine, then it can be

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.

Parasite-induced behavioural changes to the trade-off between foraging and predator evasion in a marine snail

Trophic interactions in ecological communities not only affect consumer-resource densities, but also induce phenotypic responses that can ripple through a food web to influence prey resources, competitors and other predators (i.e. trait-mediated interactions). Predators affect short-term trade-off decisions in prey species through behavioural alterations, yet the role of parasites in the context of trait-mediated interactions is rarely discussed, despite our extensive knowledge of parasite-induced behavioural alterations. Adding parasites to the picture, the foraging–predation trade-off presents an interesting case where the effects of predator- and parasite-induced behavioural responses occur in conjunction. Empirical studies on the effect of parasites on trait-mediated anti-predator responses are still scarce, especially in marine systems. Our experiments used marine mud snails, Zeacumantus subcarinatus (Batillariidae), infected with the trematode parasites Maritrema novaezealandensis and Philophthalmus sp. to investigate the effects of trematode-induced behavioural alterations on the trade-off between food acquisition and predator avoidance in the presence of a range of predation cues. We found that the time taken to initiate anti-predator responses was overall significantly shorter in M. novaezealandensis-infected snails than in uninfected snails, while it was significantly longer for Philophthalmus-infected snails. In the simultaneous presence of odours from an injured conspecific and a crab predator, however, the reaction time between uninfected and M. novaezealandensis-infected snails did not differ while Philophthalmus-infected snails were still markedly slower than the former groups. Our results demonstrate species-specific parasite-induced behavioural changes in the presence of imminent predation risk. However, effects of the trematodes on overall feeding and fleeing times were not detected in our experiments; thus, the net effect of parasitism on the foraging–predation trade-off remains unknown.

The Neurotropic Parasite Toxoplasma Gondii Increases Dopamine Metabolism

The highly prevalent parasite Toxoplasma gondii manipulates its host's behavior. In infected rodents, the behavioral changes increase the likelihood that the parasite will be transmitted back to its definitive cat host, an essential step in completion of the parasite's life cycle. The mechanism(s) responsible for behavioral changes in the host is unknown but two lines of published evidence suggest that the parasite alters neurotransmitter signal transduction: the disruption of the parasite-induced behavioral changes with medications used to treat psychiatric disease (specifically dopamine antagonists) and identification of a tyrosine hydroxylase encoded in the parasite genome. In this study, infection of mammalian dopaminergic cells with T. gondii enhanced the levels of K+-induced release of dopamine several-fold, with a direct correlation between the number of infected cells and the quantity of dopamine released. Immunostaining brain sections of infected mice with dopamine antibody showed intense staining of encysted parasites. Based on these analyses, T. gondii orchestrates a significant increase in dopamine metabolism in neural cells. Tyrosine hydroxylase, the rate-limiting enzyme for dopamine synthesis, was also found in intracellular tissue cysts in brain tissue with antibodies specific for the parasite-encoded tyrosine hydroxylase. These observations provide a mechanism for parasite-induced behavioral changes. The observed effects on dopamine metabolism could also be relevant in interpreting reports of psychobehavioral changes in toxoplasmosis-infected humans.

S32-02 - Neurological sequalae and toxoplasma infection: Possible mechanisms to explain associations with schizophrenia

Latent infection with the common intracellular protozoan parasite Toxoplasma gondii has been shown to result in altered behaviour of its host. This behaviour manipulation has been proposed to increase predation of the intermediate host (e.g. rodents and birds) enhancing parasite transmission. Other psychologicalsequalae have also been associated with latent toxoplasmosis including human affective disorders, as human are accidental hosts for Toxoplasma. During cyst stages of the life cycle found in the brain and other tissues, there is a complex interaction between the parasite and the host. Our research is concerned with the possibility that the parasite-induced behavioural changes are mediated by neurotransmitters. Potential factors in neurotransmitter levels include the location of the cyst, the host immune response, and direct parasite products. The cyst is found in many brain regions but elevated numbers have been reported in the hippocampus, amygdala, and nucleus accumbens. The host response involves interferon gamma suppressing growth through tryptophan degradation that could decrease serotonin levels. The parasite could directly manipulate the host brain through altering dopamine levels. We have found a change in dopamine associated with infection. Indeed, the parasite itself encodes the rate-limiting enzyme in dopamine synthesis, tyrosine hydroxylase. Dopamine's role is also supported by finding haloperidol, a dopamine antagonist, blocks manipulation of rodents by Toxoplasma. Our current investigations of alterations in neurotransmitter levels during chronic infection and association with brain cysts will be presented. Our data presents an interesting interplay with the ‘dopamine hypothesis’ that has postulated a link between elevated dopamine and schizophrenia.

Parasite-induced aggression and impaired contest ability in a fish host

BackgroundSuccess of trophically transmitted parasites depends to a great extent on their ability to manipulate their intermediate hosts in a way that makes them easier prey for target hosts. Parasite-induced behavioural changes are the most spectacular and diverse examples of manipulation. Most of the studies have been focused on individual behaviour of hosts including fish. We suggest that agonistic interactions and territoriality in fish hosts may affect their vulnerability to predators and thus the transmission efficiency of trophically transmitted parasites. The parasite Diplostomum spathaceum (Trematoda) and juvenile rainbow trout, Oncorhynchus mykiss, were used to study whether infection can alter aggression rates and territorial behaviour of intermediate fish hosts.ResultsThe changes in behaviour of rainbow trout, Oncorhynchus mykiss, infected with an eye fluke Diplostomum spathaceum (Trematoda), was monitored over the course of an experimental infection for 1.5 months. At the beginning of their development, not yet infective D. spathaceum metacercariae decreased the aggressiveness of rainbow trout. By the time that metacercariae were fully infective to their definitive hosts, the aggressiveness increased and exceeded that of control fish. Despite the increased aggressiveness, the experimentally infected fish lost contests for a territory (dark parts of the bottom) against the control fish.ConclusionsThe results obtained indicate that the parasitized fish pay the cost of aggressiveness without the benefit of acquiring a territory that would provide them with better protection against predators. This behaviour should increase transmission of the parasite as expected by the parasite manipulation hypothesis.

Infection with an acanthocephalan manipulates an amphipod’s reaction to a fish predator’s odours

Many parasites with complex life cycles increase the chances of reaching a final host by adapting strategies to manipulate their intermediate host’s appearance, condition or behaviour. The acanthocephalan parasite Pomphorhynchus laevis uses freshwater amphipods as intermediate hosts before reaching sexual maturity in predatory fish. We performed a series of choice experiments with infected and uninfected Gammarus pulex in order to distinguish between the effects of visual and olfactory predator cues on parasite-induced changes in host behaviour. When both visual and olfactory cues, as well as only olfactory cues were offered, infected and uninfected G. pulex showed significantly different preferences for the predator or the non-predator side. Uninfected individuals significantly avoided predator odours while infected individuals significantly preferred the side with predator odours. When only visual contact with a predator was allowed, infected and uninfected gammarids behaved similarly and had no significant preference. Thus, we believe we show for the first time that P. laevis increases its chance to reach a final host by olfactory-triggered manipulation of the anti-predator behaviour of its intermediate host.

Parasite manipulation: stretching the concepts

The parasitological literature is rife with examples of the phenotypic changes that hosts incur as a result of parasitic infections. Some of these changes affect host behaviour in dramatic and bizarre ways that have caught the attention of text book authors and are beloved of both teachers and students of introductory courses in parasitology. Many of these examples are cited by Thomas et al. (2005), who succinctly air the current concerns regarding the interpretation of these observations as examples of parasite manipulation. The debate concerning the adaptive nature of host manipulation, the origin of drive for trait changes, be it parasite or host, and the ancestral history of manipulation is, I agree, difficult to resolve from an experimental standpoint. The design and interpretation of investigations is hampered by our inadequate understanding of the physiological basis of host behaviour even when uninfected. It is thus a pity that the theoretical analysis of host manipulation has focussed to such an extent upon parasite-induced changes in behaviour. Morphological and/or physiological alteration of hosts is so common place as to be almost ubiquitous and we often see a suite of changes associated with the presence of a parasite as, for example, the development of the nurse cell in the cyst of Trichinella spiralis (Despommier, 1993) and the changes in an erythrocyte when infected with the malaria parasite (Paul and Brey, 2003). It may thus be more illuminating to examine parasitic manipulation in the context of physiological changes to the host. I believe that the criteria originally devised by Poulin (1995) can equally be used in the latter context. Furthermore, given the present state of knowledge, it is likely to be much easier to determine the mechanism underlying changes in physiology than behaviour. As with other aspects of the study of host-parasite interactions, an integration of studies concerning the molecular, biochemical and physiological aspects of infection with the evolutionary, ecological and behavioural would be beneficial. I welcome the author’s recognition that host changes that seemingly result from infection by a manipulative parasite may actually be host responses to infection. Parasitologists tend to focus upon the combative nature of the host/parasite relationship that is enshrined in such concepts as the red queen hypothesis. However, in some circumstances selection may favour parasite and host strategies that change the host in a synergistic manner (Hurd, 1998, 2001). We have shown this to occur in the Hymenolepis diminuta/Tenebrio molitor model where infection with a metacestode induces fecundity reduction in the beetle intermediate host. Inhibition of the synthesis of the yolk protein, vitellogenin, is caused directly by a molecule secreted by the tapeworm but uptake of vitellogenin by developing oocytes is inhibited by a molecule of host origin (Cole et al., 2002). Both of these changes result in a reduction in the resources devoted to egg production. Host life span is significantly increased when infected, possibly as a result of repartitioning of resources away from reproduction. The parasite therefore gains an extended period for potential transmission and the host may be capable of replacing lost investment in reproduction by extending the reproductive period (Hurd et al., 2001). In circumstances such as this, where the parasite can successfully evade the host defence system, tolerance on the part of the host is witnessed and strategies may evolve that make the best of the situation. Finally, I firmly endorse the views of the authors that there is still much to be gained from examining host parasite interactions from the standpoint of manipulation, whether it be by the parasite, the host or both partners.

Lack of parasite-mediated sexual selection in a ladybird/sexually transmitted disease system

Despite the clear potential of sexually transmitted diseases (STDs) to affect host mating behaviour via both host and parasite evolution, there have been few explicit tests of the relationships between STDs and sexual behaviour in animals. We investigated the effect of infection on host sexual behaviour within an invertebrate system. Coccipolipus hippodamiae is a sexually transmitted ectoparasite of Adalia bipunctata, the two-spot ladybird. The parasite feeds on host haemolymph, develops rapidly, and is deleterious to A.bipunctata hosts of both sexes. We examined whether infection affected mating success, and whether males or females showed any preferences with respect to infection status. We observed field mating rates of infected and uninfected ladybirds and carried out controlled laboratory experiments. We did not detect any negative effects of parasite infection on host mating vigour, nor any evidence for the existence of a host mating preference based on infection status. In addition, there was no evidence of parasite-induced changes in behaviour, such as increased promiscuity, which would increase transmission opportunities for the parasite. In summary, contrary to a body of speculation, there was no evidence of any connection between infection and mating rate, in either sex. We discuss possible explanations for these findings.