8.1 Mind games: acanthocephalan infection and behavioral modification

Author: Dina Yangirova

Introduction

Parasitism is a long-term, durable interaction between two species, in which one uses the other as a source of nutrients, a transport vehicle, or a habitat without providing anything in return (Combes 2001). The parasite’s activities lack the glory and drama of a predator eating its hard-won prey. Instead, the parasite works quietly and diligently behind the scenes, often hidden, but nevertheless profoundly influential in the life of its host. For instance, the parasite might literally, physically change the host, because some of its genes become expressed by the host. Moreover, the parasite influences the host by consuming the resources that it would otherwise use for its own growth, repair, or reproduction. It may also influence the dynamics of the host population by killing individuals susceptible to infection, and thus indirectly increasing the number of infection-resistant individuals. By doing so, the parasite is creating strong selective pressures for the host to develop defenses against exploitation, which in turn causes selection for parasites that can bypass these defenses. In this way, the host and the parasite find themselves in an evolutionary arms race, which has wide-ranging effects on both organisms (Combes 2001).

Effective transmission to another host or transmission of the eggs to another host is an important task for parasites, because otherwise when the host dies due to the parasite’s activities or otherwise – the parasite, its offspring, and most importantly its genes will perish as well. Transmission can occur through contact, consumption, or an active free-living stage (Combes 2001). This task becomes especially important if the parasite requires more than one host to complete its development and reproduce. In such cases, one of the most extreme cases of host manipulation by a parasite can be observed, in which the parasite turns its host into a veritable zombie by radically changing its behavior and appearance to increase the chance of transmission to the next host. An organism that vividly demonstrates this phenomenon is a thorn-headed worm of the phylum Acanthocephala, which causes radical changes in the life and behavior of its invertebrate intermediate host in order to improve the chances that it will be consumed by its next, vertebrate definitive host . A relationship between parasitic worms of the phylum Acanthocephala and their hosts provide great insights into the dynamics and complexity of parasite-induced behavioral modifications, and provide a great demonstration of the extended phenotype .