Tony Rinaud, Oliver Krüger, Meinolf Ottensmann, Nayden Chakarov

Parasites trigger reactions in hosts, leading to suppressive resistance and/or tolerance, aiming to limit the parasitic costs. Both colonization by parasites and defense activation can induce varying amount of costs for the host. Understanding parasite-induced effects on host fitness crucially depends on assessing parasitic costs in specific wild host-parasite systems. To evaluate potential effects of blood parasite infection, we compared physiological traits of Common Buzzard nestlings Buteo buteo i) at different stages of infection and ii) with increasing levels of parasitemia (frequency of infected host cells) with Leucocytozoon toddi, a pathogen causing malaria-like conditions. We expected infection costs to appear from the onset of infection and during peak parasitemia, rather than when parasitemia was decreasing or in uninfected hosts. We found no difference in physiological traits among infection stages, indicating low immediate parasitic costs and pointing to infection tolerance in this ontogenic stage. Surprisingly, infection only led to decrease of host health during rare and particularly acute expressions of infection. Thus, Leucocytozoon toddi appears to typically display low virulence towards their hosts, during all stages of infection. Juveniles, as a relatively parasite-tolerant life-history stage, may enable parasite transmission in the population while avoiding infection-driven host mortality during this critical period.