Sickness and behaviour in animals: a motivational perspective

Proinflammatory cytokines, produced by the activated immune system, induce a whole set of non-specific symptoms in the infected individual (i.e. hypop hagia, adipsia, reduced social interest). However, evidence summarised in t his review shows that behavioural changes induced by cytokines are not mere ly the consequence of a degraded state but reflect motivational reorganisat ion. If the set-up of these new priorities is expressed by a general decrea se in behavioural activities (e.g. immobility, sleepiness), the sick indivi dual remains nevertheless an open system still able to respond to environme ntal stimuli. If these cues are evaluated as relevant to the new priorities (e.g. cues from scattered pups or cues from nest material when ambient tem perature is low), the sick individual interrupts sickness behaviour in orde r to respond specifically to the cues (e.g. retrieving of the pups or nest building). Once this is done, there is a return to recuperative behaviour. These findings represent a primary characterisation of biobehavioural actio n of immune stimuli, and they open new perspectives to facilitate further p rogress in our understanding of cytokine effects on behaviour. (C) 1999 Els evier Science Ltd. All rights reserved.