Shelley A. Adamo

Parasitic manipulation: where are we and where should we go?

How a parasite (or its offspring) moves from onehost to the next remains a central topic in parasitol-ogy. Understanding such strategies is at the heart ofapplied aspects of parasitology, but it is also im-portant for solving more basic biological questions.One strategy of transmission that is especially in-triguing is that of host manipulation, which occurswhen a parasite enhances its own transmission byaltering host behaviour. We begin this paper witha brief historical overview of the ‘manipulation hy-pothesis,’ in order to illuminate past and present re-search on this transmission strategy, as well as currentchallenges.Scientists were beginning to suspect that parasitescould manipulate their hosts early in the 20th century(e.g. Cram, 1931). In 1952, van Dobben reported thatfish retrieved from cormorants (definitive hosts) werefar more likely to be intermediate hosts of the cestodeLigula intestinalis than were fish captured by fisher-men. Bethel and Holmes (1973, 1977) used labora-tory experiments to show that the cystacanths of theacanthocephalan Polymorphus paradoxus provoke ab-normal behaviours in the amphipod (Gammarus la-custris ; intermediate host), and then verified the re-sulting increased predation risk from ducks (definitivehosts).Since that time, there has been increasing enthusi-asm among parasitologists for the study of phenotypicchanges in parasitised animals. The idea that parasitescould manipulate the phenotype of their host and thusenhance their own transmission became rapidly popu-larnotonlybecauseitwasinherentlyafascinatingphe-nomenon, but also because it offered parasitologists anopportunity to demonstrate the ubiquitous importanceof parasites to a broader community of scientists. Dueto an impressive number of studies performed duringthelastthreedecadesonthistopic,parasite-inducedal-terations of host phenotypes are now documented for awiderangeofparasites(seeBarnardandBehnke,1990;Combes, 1991, 1998; Poulin, 1998; Moore, 2002 forreviews). These studies have demonstrated that a largerange of host phenotypic traits can be altered by para-sites (e.g. behaviour, morphology and/or physiology),and that the alterations can vary greatly in their magni-tude, from slight shifts in the percentage of time spentinperformingagivenactivitytotheproductionofcom-plex and spectacular behaviours (Poulin and Thomas,1999; Moore, 2002).The most popular example of parasitic manipula-tion in ecological textbooks seems to be the trema-tode“brainworm” Dicrocoeliumdendriticum .Ants(in-termediate hosts) infected with this trematode ascendblades of grass, a behaviour that probably enhancestransmission to grazing sheep. However, this is not thebest exemple of parasitic manipulation. As one mightimagine, it is difficult to study ant predation by sheep,and the relative numbers of infected and uninfectedants that are eaten by these herbivores remain a mys-tery. There are however many impressive examples ofapparent host manipulation that are more amenableto quantification. For instance, numerous trophicallytransmitted parasites have been shown to alter the be-haviour of their intermediate hosts in a way that in-creases their vulnerability to predatory definitive hosts(Lafferty,1999;Berdoyetal.,2000;Moore,2002).Par-asites also manipulate host habitat choice; arthropodsharbouring mature nematomorphs or mermithids seekwater and jump into it, thereby allowing the parasiticworm to reach the aquatic environment needed for itsreproduction (Thomas et al., 2002a). Mermithid nema-todes can also feminize male insect behaviour whenparasitetransmissionisdependentonafemale-specificbehaviour (Vance, 1996). Parasitic wasps can maketheir spider host weave a special cocoon-like structuretoprotectthewasppupaeagainstheavyrain(Eberhard,2000, see also Brodeur and Vet, 1994), or can evencause the host to seek protection within curled leavesto protect pupae from hyperparasitoids (Brodeur andMcNeil, 1989). Viruses may stimulate superparasitismbehaviour in solitary parasitoids and thus achieve hor-izontal transmission (Varaldi et al., 2003). Some dige-neans drive their molluscan intermediate hosts towardideal sites for the release of cercariae (Curtis, 1987).‘Enslaver’ fungi make their insect hosts die perchedin a position that favors the dispersal of spores by thewind (Maitland, 1994). Vector-borne parasites can ren-der their vertebrate hosts more attractive to vectors,and/orcanmanipulatethefeedingbehaviourofvectorsto enhance transmission (Hamilton and Hurd, 2002).Allthesespectacularphenotypicchangeshavebeenin-terpreted as the sophisticated products of natural selec-tionthathasfavoredhostmanipulation,thusincreasingthe likelihood that parasite propagules will encounterthe next host or a suitable habitat. From an evolution-ary point of view, these changes are classically seen ascompelling illustrations of the ‘extended phenotype’

How should behavioural ecologists interpret measurements of immunity

S everal important issues in behavioural ecology require an estimate of an individual’s disease resistance (e.g. Møller et al. 1999). Unfortunately, estimating disease resistance is not straightforward, as many researchers have pointed out (e.g. Lochmiller 1995; Siva-Jothy 1995; Sheldon & Verhulst 1996; Zuk 1996; Apanius 1998; Westneat & Birkhead 1998; Norris & Evans 2000; Zuk & Stoehr 2002). This challenge is not unique to behavioural ecology. Ecological toxicologists testing whether chemicals in our environment are immunosuppressive, immunologists and nutritionists studying how diet can influence the immune system, physiologists examining the effect of stress on immune systems and agriculturists and aquaculturists attempting to produce disease-resistant stock all require the ability to assess the relative strength of an individual’s immune system. The most common method of estimating immunocompetence (i.e. the magnitude and effectiveness of an animal’s immune response) in all these fields is to measure one or more components of the immune system. The implicit assumption is that these measures correlate with the ability to resist disease, or are at least an indicator of the relative ‘strength’ of the immune system. Unfortunately, recent research in these fields has shown that the connection between assays of immunity and disease resistance is complex. In this paper I will focus on the problems these complexities create for the interpretation of immune assay results. Most studies assume that a low value on an immune assay (e.g. the ability to form antibodies to a novel antigen) corresponds to lower disease resistance, or, at least an immune system that shows a less robust response to pathogens even if lower disease resistance cannot be demonstrated. Unfortunately, immunological studies demonstrate that neither assumption is necessarily true (e.g. Luster et al. 1993; Berczi & Nagy 1998; S. Wilson et al. 2001; Smith 2003). The major problems in interpreting the results of immune assays are: (1) correlations between assays of immunity and disease resistance are typically pathogen specific, (2) correlations between assays of immunity and disease resistance are sometimes weak or nonexistent, (3) research suggests that some immune components have a threshold value such that changes above that threshold value may have no biological significance, and (4) the immune system can change its response characteristics in order to optimize its defence against different kinds of intruders. Below I discuss these issues and their implications for behavioural ecologists.

Modulating the modulators: parasites, neuromodulators and host behavioral change.

Neuromodulators can resculpt neural circuits, giving an animal the behavioral flexibility it needs to survive in a complex changing world. This ability, however, provides parasites with a potential mechanism for manipulating host behavior. This paper reviews three invertebrate host-parasite systems to examine whether parasites can change host behavior by secreting neuromodulators. The parasitic wasp, Cotesia congregata, suppresses host feeding partly by inducing the host (Manduca sexta) to increase the octopamine concentration in its hemolymph. The increased octopamine concentration disrupts the motor pattern produced by the frontal ganglion, preventing the ingestion of food. Polymorphus paradoxus (Acanthocephalan) alters the escape behavior of its host, Gammarus lacustris (Crustacea), possibly through an effect on the host’s serotonergic system. The trematode Trichobilharzia ocellata inhibits egg-laying in its snail host (Lymnaea stagnalis), partly by inducing the host to secrete schistosomin. Schistosomin decreases electrical excitability of the caudodorsal cells. The parasite also alters gene expression for some neuromodulators within the host’s central nervous system. In at least two of these three examples, it appears that the host, not the parasite, produces the neuromodulators that alter host behavior. Producing physiologically potent concentrations of neuromodulators may be energetically expensive for many parasites. Parasites may exploit indirect less energetically expensive methods of altering host behavior. For example, parasites may induce the host’s immune system to produce the appropriate neuromodulators. In many parasites, the ability to manipulate host behavior may have evolved from adaptations designed to circumvent the host’s immune system. Immune-neural-behavioral connections may be pre-adapted for parasitic manipulation.

Defining and assessing animal pain

The detection and assessment of pain in animals is crucial to improving their welfare in a variety of contexts in which humans are ethically or legally bound to do so. Thus clear standards to judge whether pain is likely to occur in any animal species is vital to inform whether to alleviate pain or to drive the refinement of procedures to reduce invasiveness, thereby minimizing pain. We define two key concepts that can be used to evaluate the potential for pain in both invertebrate and vertebrate taxa. First, responses to noxious, potentially painful events should affect neurobiology, physiology and behaviour in a different manner to innocuous stimuli and subsequent behaviour should be modified including avoidance learning and protective responses. Second, animals should show a change in motivational state after experiencing a painful event such that future behavioural decision making is altered and can be measured as a change in conditioned place preference, self-administration of analgesia, paying a cost to access analgesia or avoidance of painful stimuli and reduced performance in concurrent events. The extent to which vertebrate and selected invertebrate groups fulfil these criteria is discussed in light of the empirical evidence and where there are gaps in our knowledge we propose future studies are vital to improve our assessment of pain. This review highlights arguments regarding animal pain and defines criteria that demonstrate, beyond a reasonable doubt, whether animals of a given species experience pain.

Parasites: evolution's neurobiologists.

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

Some like it hot: the effects of climate change on reproduction, immune function and disease resistance in the cricket Gryllus texensis

SUMMARY In many parts of the world, climate change is increasing the frequency and severity of heat waves. How do heat waves impact short-lived poikilotherms such as insects? In the cricket, Gryllus texensis, 6 days of elevated temperatures (i.e. 7°C above the average field temperature and 5°C above their preferred temperature) resulted in increased egg laying, faster egg development and greater mass gain. The increased temperature also increased activity of phenoloxidase and lysozyme-like enzymes, two immune-related enzymes, and enhanced resistance to the Gram-negative bacterium Serratia marcescens. When given a sublethal S. marcescens infection, G. texensis maintained increased reproductive output at the elevated temperature (33°C). These data suggest that heat waves could result in more numerous, disease resistant, crickets. However, resistance to the Gram-positive bacterium, Bacillus cereus was lower at temperatures above or below the average field temperature (26°C). A sublethal infection with B. cereus reduced egg laying at all temperatures and suppressed the increase in egg laying induced by higher temperatures. These results suggest that for some species–pathogen interactions, increased temperatures can induce trade-offs between reproduction and disease resistance. This result may partly explain why G. texensis prefers temperatures lower than those that produce maximal reproductive output and enhanced immune function.

Do cuttlefish (Cephalopoda) signal their intentions to conspecifics during agonistic encounters

Abstract Abstract. Male cuttlefish adopt a specific body pattern during agonistic behaviour called the Intense Zebra Display. Some components of the Display were variable, especially the chromatic component termed ‘dark face’, which could vary in the degree of darkness. Facial darkness was measured using a video analysis system. Males that eventually withdrew from conspecifics without fighting maintained a lighter face during the initial stage of agonistic encounters. When both males maintained dark faces, physical contact and fighting ensued. Therefore facial darkness could be used to predict which male–male encounters would escalate to physical contact. The strong correlation between facial darkness and subsequent behaviour suggested that males were signalling their agonistic motivation at the early stages of the encounter, which is contrary to what would be predicted from a traditional game theory analysis. It is proposed that males signal intent because the Intense Zebra Display simultaneously serves two functions: (1) it identifies the signaller as male, thus preventing unwanted copulations from other males, and (2) it functions as part of the agonistic behavioural repertoire. By using a modified (i.e. lighter-faced) version of the Display, males may be able to signal their sex, but without inducing another male to attack. In cases in which agonistic displays perform more than one function, signalling intent (i.e. signalling its likely subsequent behaviour) can be an evolutionarily stable strategy.

Why should an immune response activate the stress response? Insights from the insects (the cricket Gryllus texensis)

Mediators of the stress response (e.g. glucocorticoids and norepinephrine) can be immunosuppressive. Nevertheless, immune challenge leads to the release of these compounds in vertebrates. To resolve this paradox, it has been suggested that stress hormones help restore immune homeostasis, preventing self-damage. A comparative approach may provide additional hypotheses as to why an immune challenge induces the release of stress hormones/neurohormones. Octopamine, a neurohormonal mediator of the stress response in the cricket Gryllus texensis, increased in concentration in the hemolymph during an immune challenge. Therefore, the release of stress hormones during an immune response occurs in animals across phyla. Octopamine induced an increase in lipid concentration in the hemolymph. After an acute stress (flying or running) the total number of hemocytes in the hemolymph increased. Injections of octopamine had the same effect, suggesting that it may enhance hemocyte-dependent immune functions. On the other hand, octopamine decreased lysozyme-like activity in vitro, suggesting that it inhibits some immune functions. However, lysozyme-like activity was increased by the presence of heat-killed bacteria in vitro and this increase was significantly augmented by the presence of octopamine. Therefore, the effect of octopamine on immune function differed depending on the presence of pathogens. Stress hormones may help shift immune function into the most optimal configuration depending on the physiological context.

Illness-induced anorexia may reduce trade-offs between digestion and immune function

Animals from across the animal kingdom decrease feeding during an infection. Superficially this response seems maladaptive because the decline in food intake occurs at the same time as immune activation increases energy expenditure. However, illness-induced anorexia could be beneficial by decreasing trade-offs between the immune system and digestion. For example, in insects (i.e. crickets) there is a trade-off between lipid transport and immune function. We predicted that increasing the need for lipid transport (e.g. when digesting a high fat meal) would reduce immune function. After consuming a high fat meal, crickets (Gryllus texensis) showed an increase in haemolymph lipid concentration. Crickets also showed a decrease in resistance to bacterial infection (Serratia marcescens). After an immune challenge, crickets not only ate less, they also preferred foods containing less fat. This occurred whether the target food was an ecologically valid food item (dead cricket), natural foods (e.g. lettuce and ground meat) or an artificial diet containing different amounts of lipid. Therefore, the change in feeding behaviour after an immune challenge is consistent with the need to reduce lipid transport in order to maximize immune function. Illness-induced anorexia may be one method by which animals can bias physiological pathways towards enhanced immune function. Some behaviours may be adaptive because they can bias the direction of physiological trade-offs.

The effects of the stress response on immune function in invertebrates: An evolutionary perspective on an ancient connection

Stress-induced changes in immune function occur in animals across phyla, and these effects are usually immunosuppressive. The function of this immunomodulation remains elusive; however, the existence of specialized receptors on immune cells suggests that it is adaptive. A comparative approach may provide a useful perspective. Although invertebrates have simpler endocrine/neuroendocrine systems and immune systems than vertebrates, they have robust stress responses that include the release of stress hormones/neurohormones. Stress hormones modify immune function in mollusks, insects, and crustaceans. As in vertebrates, the effects of stress hormones/neurohormones on invertebrate immune function are complex, and are not always immunosuppressive. They are context-, stressor-, time- and concentration-dependent. Stress hormone effects on invertebrate immune function may help to re-align resources during fight-or-flight behavior. The data are consistent with the hypothesis that stress hormones induce a reconfiguration of networks at molecular, cellular and physiological levels that allow the animal to maintain optimal immunity as the internal environment changes. This reconfiguration enhances some immune functions while suppressing others. Knowing the molecular details of these shifts will be critical for understanding the adaptive function of stress hormones on immune function.