Anorexia during infection in mammals: Variation and its sources

A reduction in the voluntary food intake of animals, henceforth called anorexia, accompanies their exposure to pathogens and antigenic challenges. Whilst the phenomenon is well established, its timing, magnitude, duration and recovery from exposure to pathogens are poorly understood. In this paper we consider the sources that contribute to variation in these characteristics of anorexia, and their significance. (1) Pathogen: It is now well established that the characteristics of anorexia are very much dependent on pathogen type and dose. For example, anorexia appears within a few hours from infection with a micro parasite and has a relatively short duration, whereas it appears a few weeks post infection and lasts for several weeks in macro parasitic infections. We suggest that this is due to pathogen life cycle and its consequences on the host. (2) Host nutrition: Since anorexia is linked to the immune response it is reasonable to expect that host nutrition may affect the characteristics of anorexia through its effect on the immune response. Some of these characteristics appear to be more sensitive to host nutrition than others, such as anorexia duration and rate of recovery; an explanation to account for this is offered. It is thus possible to exploit host nutrition to manipulate the impact of pathogen exposure on the infected host. (3) Host genotype: There is significant less information regarding host genotype effects on anorexia. Here we suggest that the characteristics of anorexia depend on the ability of the host to cope with the pathogen and hence its immune response. In the absence of literature information we hypothesise how these host characteristics might affect the phenomenon of anorexia and suggest appropriately designed experiments to address this issue. Finally the timescales of anorexia are not always defined and it is common that the period of reduction in food intake and its recovery are lumped together. We explore how this practice might affect the conclusions one might draw about anorexia and how one should deal with it. Although pathogen-induced anorexia is an unavoidable consequence of infection, some of its features might be subject to manipulation and, therefore, its impact upon the infected animals may also be minimised. © Wageningen Academic Publishers The Netherlands, 2009. All rights are reserved