Animal hosts can adapt to emerging infectious disease through both disease resistance, which decreases pathogen numbers, and disease tolerance, which limits damage during infection without limiting pathogen replication. Both resistance and tolerance mechanisms can drive pathogen transmission dynamics. However, it is not well understood how quickly host tolerance evolves in response to novel pathogens or what physiological mechanisms underlie this defense. Using natural populations of house finches (Haemorhous mexicanus) across the temporal invasion gradient of a recently emerged bacterial pathogen (Mycoplasma gallisepticum), we find rapid evolution of tolerance (<25 years). In particular, populations with a longer history of MG endemism have less pathology but similar pathogen loads compared with populations with a shorter history of MG endemism. Further, gene expression data reveal that more-targeted immune responses early in infection are associated with tolerance. These results suggest an important role for tolerance in host adaptation to emerging infectious diseases, a phenomenon with broad implications for pathogen spread and evolution.

Department of Biological Sciences University of Memphis; Memphis Tennessee United States of America

Department of Biological Sciences Virginia Tech; Blacksburg Virginia United States of America

Department of Natural Resource Ecology and Management Iowa State University; Ames Iowa United States of America

Department of Poultry Science University of Georgia; Athens Georgia United States of America

Department of Zoology Charles University; Prague Czech Republic

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Varying conjunctival immune response adaptations of house finch populations to a rapidly evolving bacterial pathogen

Rapid adaptation to a novel pathogen through disease tolerance in a wild songbird