Reinfection with a Bacterial Pathogen Augments Heterogeneity in Host Disease Responses

Individual responses to infection are often highly heterogeneous within host populations, with key consequences for transmission dynamics and pathogen evolution. Because re-exposure to pathogens is ubiquitous, understanding how priming exposures to a given pathogen alter inter-individual heterogeneity in immune responses and transmission-relevant traits is critical. Recent work in house finches (Haemorhous mexicanus) found that priming exposures to the bacterial pathogen Mycoplasma gallisepticum augment population-level heterogeneity in susceptibility to infection. However, it remains unclear whether priming exposures exacerbate heterogeneity in both underlying immune responses and transmission-relevant traits during reinfection. Using wild-caught, pathogen-naïve house finches, we experimentally tested whether priming with a low or high dose of Mycoplasma gallisepticum affects population-level heterogeneity in antibody responses, pathogen loads, and disease responses upon reinfection. We find that any prior exposure results in more heterogeneous antibody responses, and more variable pathogen loads upon reinfection. Further, the detected patterns in antibody variability with prior exposure match previously documented patterns of population-level heterogeneity in susceptibility upon secondary challenge, suggesting that variability in antibody responses within a population can be a relevant proxy for heterogeneity in susceptibility. Overall, we show that prior exposure to pathogens contributes to subsequent heterogeneity in transmission-relevant traits, with implications for downstream infection dynamics.