Prior pathogen exposure augments inter-individual heterogeneity in antibody levels and reinfection loads in a songbird-pathogen system.

Host responses to infection are often heterogeneous, with key consequences for pathogen transmission and evolution. Given the ubiquity of host re-exposure to the same pathogen, understanding how prior exposure alters inter-individual heterogeneity in disease responses is critical. Work in house finches (Haemorhous mexicanus) found that priming exposures to low or high doses of the bacterium Mycoplasma gallisepticum augmented host heterogeneity in susceptibility. Here we quantified whether priming similarly affects inter-individual heterogeneity in antibody responses, pathogen loads, and disease responses upon reinfection. Primed birds had more heterogeneous antibody responses prior to rechallenge relative to unprimed birds, with individual variation in antibody levels predictive of reinfection susceptibility. During rechallenge, pathogen loads, but not disease severity scores, were more heterogeneous in primed birds. However, when considering only birds successfully infected following re-challenge, priming did not significantly alter either mean pathogen loads or heterogeneity in loads. This suggests that priming effects on loads are largely explained by susceptibility differences. Nonetheless, because protective effects of priming limited sample sizes of successfully reinfected birds, further study is needed of whether priming influences load variability within successfully reinfected birds. Overall, prior exposure to pathogens alters inter-individual heterogeneity in several host traits, with implications for downstream infection dynamics.