Comparing mechanisms of host manipulation across host and parasite taxa
SUMMARY
Parasites affect host behavior in several ways. They can alter activity, microhabitats or both. For trophically transmitted parasites (the focus of our study), decreased activity might impair the ability of hosts to respond to final-host predators, and increased activity and altered microhabitat choice might increase contact rates between hosts and final-host predators. In an analysis of trophically transmitted parasites, more parasite groups altered activity than altered microhabitat choice. Parasites that infected vertebrates were more likely to impair the host’s reaction to predators, whereas parasites that infected invertebrates were more likely to increase the host’s contact with predators. {The parasites goal is to reproduce, and it does this by getting it's host eaten by predators - or perhaps at a higher level (such as humans) close proximity} The site of infection might affect how parasites manipulate their hosts. For instance, parasites in the central nervous system seem particularly suited to manipulating host behavior. Manipulative parasites commonly occupy the body cavity, muscles and central nervous systems of their hosts. Acanthocephalans in the data set differed from other taxa in that they occurred exclusively in the body cavity of invertebrates. In addition, they were more likely to alter microhabitat choice than activity. Parasites in the body cavity (across parasite types) were more likely to be associated with increased host contact with predators. Parasites can manipulate the host through energetic drain, but most parasites use more sophisticated means. For instance, parasites target four physiological systems that shape behavior in both invertebrates and vertebrates: neural, endocrine, neuromodulatory and immunomodulatory. The interconnections between these systems make it difficult to isolate specific mechanisms of host behavioral manipulation.
Introduction
Are parasites sophisticated puppet masters of their host’s behaviors? Many studies have demonstrated that differences in behavior between infected and uninfected hosts are beneficial to parasites. In other cases, the logical expectation for parasite adaptation is strong. In either case, proof is elusive and differences in behavior between infected and uninfected hosts do not, on their own, imply manipulation by a parasite. For instance, pathogenic parasites can affect host behavior in ways that do not benefit the parasite. For our Review, we assume that the examples we give are parasite adaptations, but readers should understand that support for this assumption varies.
Parasites target four physiological systems that shape behavior in both invertebrates and vertebrates: neural, endocrine, neuromodulatory and immunomodulatory (Adamo, 2002; Adamo, 2013; Beckage, 1993; Escobedo et al., 2009; Helluy, 2013; Moore, 2002; Thomas et al., 2005; Thompson and Kavaliers, 1994). These systems are connected and communicate via neurotransmitters, hormones, and neuromodulatory and immunomodulatory chemicals. As a result, identifying the precise mechanisms that underlie host behavior modification has proven a complex task. Moreover, chemically mediated modifications might be the result of parasite- and host-secreted substances acting in response to one another, further obscuring the mechanistic basis of the alterations (Adamo, 2013; Thomas et al., 2005). Less sophisticated parasites could manipulate host behavior by energetic drain or damaging key systems such as the central nervous system (CNS) (though targeting the CNS is arguably a sophisticated accomplishment).
