Occurrence of Cruzia tentaculata Larvae in the Land Snail Megalobulimus abbreviatus-Influence of Natural Parasite Load on Snail Physiology

The nematode Cruzia tentaculata is reported here for the first time in the land snail Megalobulimus abbreviatus in southern Brazil. The snails were infected with a high prevalence of larvae up to the L3 stage (68.6%). Cysts were located mainly in the mantle (pulmonary cavity) and the nerve ring. No other helminths were identified in the collected snails. Some physiological aspects were compared between snails infected with L3 larvae and non-infected snails and among infected animals with different parasite loads. No differences were found in hemolymph protein, glucose, or urea content between L3-infected and non-infected snails, nor among animals with different parasite loads. Discontinuous lesions in the rectum associated with the presence of encapsulated larvae were visible in animals with high parasite loads and were more frequent in adult animals. All analyses were carried out during the breeding season; however, the albumen glands of mature snails had a smaller volume in those with higher parasite loads. Ovotestis weight was also significantly negatively correlated with parasite load. Snail reproductive capacity could, therefore, be partially impaired but only for individuals with higher parasite loads. Considering only the measured parameters, natural infection by C. tentaculata does not appear to affect intermediary metabolism of M. abbreviatus. A greater number of larvae and greater severity of tissue injuries are more frequently observed in older snails.

Increased temperature has no consequence for behavioral manipulation despite effects on both partners in the interaction between a crustacean host and a manipulative parasite

Parasites alter many traits of their hosts. In particular, parasites known as "manipulative" may increase their probability of transmission by inducing phenotypic alterations in their intermediate hosts. Although parasitic-induced alterations can modify species' ecological roles, the proximate factors modulating this phenomenon remain poorly known. As temperature is known to affect host-parasite associations, understanding its precise impact has become a major challenge in a context of global warming. Gammarids are ecologically important freshwater crustaceans and serve as intermediate hosts for several acanthocephalan species. These parasites induce multiple effects on gammarids, including alterations of their behavior, ultimately leading to modifications in their functional role. Here, experimental infections were used to assess the effect of two temperatures on several traits of the association between Gammarus pulex and its acanthocephalan parasite Pomphorhynchus laevis. Elevated temperature affected hosts and parasites in multiple ways (decreased host survival, increased gammarids activity, faster parasites development and proboscis eversion). However, behavioral manipulation was unaffected by temperature. These results suggest that predicted change in temperature may have little consequences on the trophic transmission of parasites through changes in manipulation, although it may modify it through increased infection success and faster parasites development.

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.

Starving the Enemy? Feeding Behavior Shapes Host-Parasite Interactions

The loss of appetite that typically accompanies infection or mere exposure to parasites is traditionally considered a negative byproduct of infection, benefitting neither the host nor the parasite. Numerous medical and veterinary practices directly or indirectly subvert this 'illness-mediated anorexia'. However, the ecological factors that influence it, its effects on disease outcomes, and why it evolved remain poorly resolved. We explore how hosts use anorexia to defend against infection and how parasites manipulate anorexia to enhance transmission. Then, we use a coevolutionary model to illustrate how shifts in the magnitude of anorexia (e.g., via drugs) affect disease dynamics and virulence evolution. Anorexia could be exploited to improve disease management; we propose an interdisciplinary approach to minimize unintended consequences.

Parasitic manipulation of hosts' phenotype, or how to make a zombie--an introduction to the symposium

Nearly all animals in nature are infected by at least one parasite, and many of those parasites can significantly change the phenotype of their hosts, often in ways that increase the parasite's likelihood of transmission. Hosts' phenotypic changes are multidimensional, and manipulated traits include behavior, neurotransmission, coloration, morphology, and hormone levels. The field of parasitic manipulation of hosts' phenotype has now accrued many examples of systems where parasites manipulate the phenotypes of their hosts and focus has shifted to answering three main questions. First, through what mechanisms do parasites manipulate the hosts' phenotype? Parasites often induce changes in the hosts' phenotypes that neuroscientists are unable to recreate under laboratory conditions, suggesting that parasites may have much to teach us about links between the brain, immune system, and the expression of phenotype. Second, what are the ecological implications of phenotypic manipulation? Manipulated hosts are often abundant, and changes in their phenotype may have important population, community, and ecosystem-level implications. Finally, how did parasitic manipulation of hosts' phenotype evolve? The selective pressures faced by parasites are extremely complex, often with multiple hosts that are actively resisting infection, both in physiological and evolutionary time-scales. Here, we provide an overview of how the work presented in this special issue contributes to tackling these three main questions. Studies on parasites' manipulation of their hosts' phenotype are undertaken largely by parasitologists, and a major goal of this symposium is to recruit researchers from other fields to the study of these phenomena. Our ability to answer the three questions outlined above would be greatly enhanced by participation from individuals trained in the fields of, for example, neurobiology, physiology, immunology, ecology, evolutionary biology, and invertebrate biology. Conversely, because parasites that alter their hosts' phenotype are widespread, these fields will benefit from such study.