PHYSIOLOGICAL BASES FOR PARASITE-INDUCED ALTERATIONS OF HOST BEHAVIOR

Parasitism is defined in various ways as an intimate relationship in w hich one partner, the parasite, lives on or in another, the host, gene rally at the expense of the latter. Parasitism commonly results in a u nique array of host physiological responses and adaptations. Most stud ies of the physiological effects of parasitism have focused on the pat hological consequence of infection and disease. While many physiologic al changes contribute to pathogenesis, it is now recognized that paras itic infections at sub-clinical levels also produce physiological effe cts that either ameliorate or may not contribute to the disease proces s. Moreover, these physiological changes are often manifested by alter ed host behaviour. Behavioural studies have enabled an ecological- and evolutionary-oriented evaluation of host responses. In this fashion, physiological effects may be assessed as to whether they affect fitnes s and confer benefit or harm to one or both of the symbionts involved. We briefly examine how these physiological responses, specifically ne ural, endocrine, neuromodulatory, and immunomodulatory components, may interact to modify host behaviours. We consider the adaptiveness of t hese responses and how the behavioural patterns elicited may simultane ously appear adaptive for the parasite as well as the host. In additio n, we address how parasite-host physiological and behavioural interact ions may be altered during the course of parasitism.