Data from: Differential host responses to parasitism shape divergent fitness costs of infection

Fitness costs of infection are fundamental to understanding the ecology
and evolution of host-parasite interactions. However, these costs, and
particularly their underlying mechanisms, are challenging to evaluate in
wild populations. Here, we quantified total and species-specific costs of
gastrointestinal worms on African buffalo, by combining the power of an
anthelmintic treatment experiment that perturbed the entire worm community
with a longitudinal study that tracked the two most dominant community
members. Reducing all worms improved buffalo body condition, which was
strongly associated with increases in survival and reproduction.
Species-specific analyses revealed that condition-mediated fitness costs
of infection differed between parasite species. Hosts that gained the
blood-sucking worm Haemonchus, lost condition, and this loss may have been
mediated by reductions in forage intake. Hosts that resisted Haemonchus by
elevating IL-4 and eosinophil immune defences were able to reduce their
parasite loads and gain back condition. Conversely, hosts that gained
Cooperia, a less pathogenic worm, gained condition and had higher survival
and reproductive success. Elevating immune defences had no effect on
Cooperia abundance. Coupled with the positive relationship observed
between Cooperia and host condition, our data suggest that hosts might
benefit from tolerating Cooperia rather than incurring the costs of
resistance. Overall, our study reveals that differential host responses to
parasites play a key role in mediating the costs of infection.