Abstract

Animals are beset by numerous parasites, whose effects range from mildly debilitating to fatal. As a consequence they have evolved a range of behavioural and physiological responses to deal with this threat, including the deployment of a suite of immune responses. However, an effective immune response depends upon the nutritional state of the host. The host needs its resources to service its immune response, whilst the parasite needs the host’s resources to grow and reproduce, and the ‘ideal’ nutritional state for a successful parasitic infection may not be the same as that of an effective immune response. Which leads to the question, which diets best suit the needs of the host and the parasite? Insects provide an excellent, ethically sound, model to address this question. They harbour a range of micro (bacteria/viruses/fungi/protozoa) and macroparasites and their immune response is evolutionarily and functionally equivalent to the mammalian innate system. Using insect models, we show that different arms of the immune system work most effectively in different regions of nutrient space, and that, when infected, hosts can modify their intake of nutrients to improve their immune response. We also show that parasite growth can be impeded directly by the host diet, and that hosts may in fact choose a diet that inhibits parasite replication, or mitigates its negative effects via tolerance. Thus the nutritional environment can have both bottom-up and top-down effects on host-parasite interactions, and nutrient intake plasticity is a likely target for selection in the battle against parasites.