Pathogen-induced anorexia: a herbivore strategy or an unavoidable consequence of infection?

A reduction in voluntary food intake is a common feature of infection with pathogens and is frequently referred to as pathogen-induced anorexia. Anorexia has been previously viewed either as an unavoidable consequence of infection or as an animal strategy that enables them to cope with the consequences of infection. Both approaches lead to certain expectations as far as the characteristics of anorexia are concerned. By linking anorexia to host immune response one should be able to make predictions about when and for how long anorexia would occur. By appreciating what an infected animal is trying to achieve through its feeding behaviour, one would be able to make predictions about the extent of anorexia on different quality foods. The thesis of the paper is that these approaches should no longer be viewed as mutually exclusive, but, by combining them, one should be able to make pathogen-induced anorexia more predictable. This is done in the development of a model that aims to predict the food intake of grazing sheep exposed to an abomasal parasite. The predictions of the model are consistent with the features and consequences of parasite-induced anorexia of sheep given access to moderate and high quality foods. However, there is a degree of uncertainty about the validity of predictions made by the model on anorexia seen on poor quality foods of low energy content. This is not a deficiency of the model developed but can be attributed to the lack of appropriate experiments against which model predictions can be tested.