Interpreting immunological indices: The importance of taking parasite community into account. An example in blackbirds Turdus merula

Despite the intensive use of immune indices in immunoecology, whether to interpret the results of immune indices in terms of actual immune competence (i.e. ability to control and clear parasite infections as indicated by high values of immune indices associated with low parasite loads) or current immune activation (pathogenic infection being associated with high parasite load and high values of immune indices) is still an open question. Most studies to date have produced contrasting results focused on the effect of a single parasite species despite the fact that hosts usually harbour a community of parasites that influences one another's impact on host immune response. We simultaneously assessed blood parasites, intestinal parasites and ectoparasite loads in male blackbirds and compared these measures to several immune indices to investigate how parasites explain the variation around the mean of these immune indices. Parasite loads covaried within hosts. Immune indices better reflected the interacting effects of these parasites than their independent effect. Immune indices may therefore be better indicators of ongoing pathogenic infections than immunocompetence. Furthermore, intestinal parasites explained a significant part of the variance in most immune indices through their interactions with other parasites, suggesting that they have a strong influence in modulating immune function. Taking the parasite community into account in immunoecology studies will certainly help increase our understanding of immune indices.