Immunity, resistance and tolerance in bird-parasite interactions

Interacting pathogens and hosts have evolved reciprocal adaptations whose function is to allow host exploitation (from the pathogen stand point) or minimize the cost of infection (from the host stand point). Once infected, two strategies are offered to the host: parasite clearing (resistance) and withstanding the infection while paying a low fitness cost (tolerance). In both cases, the immune system plays a central role. Interestingly, whatever the defence strategy adopted by the host, this is likely to have an effect on parasite evolution. Given their short generation time and large population size, parasites are expected to rapidly adapt to the environmental conditions provided by their hosts. The immune system can therefore represent a powerful engine of parasite evolution, with the direction of such evolutionary trajectory depending on, among other factors, (i) the type of mechanism involved (resistance or tolerance) and (ii) the damage induced by overreacting immune defences. In this article, I will discuss these different issues focusing on selected examples of recent work conducted on two bird pathogens, the protozoa responsible for avian malaria (Plasmodium sp.) and the bacterium Mycoplasma gallisepticum.