Paying Upfront: Successful Initial Infections Protect Against Severe Future Infections.

Understanding the consistency with which individual hosts respond to repeated pathogen exposures is crucial for accurately modeling pathogen transmission and eco-evolutionary dynamics. When vertebrate hosts face repeated pathogen exposures, immune memory typically reduces the probability and/or severity of subsequent infections, yet it remains unclear whether individual hosts remain consistent in their level of response relative to other individuals. We investigated this question in house finches (Haemorhous mexicanus) from two populations varying in their history of endemism of the bacterial pathogen Mycoplasma gallisepticum (MG). MG-naïve individuals were experimentally inoculated twice with MG, allowing recovery between inoculations. We then asked if individual host's responses (i.e., susceptibility, resistance, and tolerance) to the second inoculation were predicted by their responses to initial inoculation, population of origin, or sex. Our results suggest that individuals were not consistent in their relative response to repeated exposure, although individuals that were relatively tolerant to initial MG infection had reduced probability of subsequent infection. Compared to finches successfully infected following first MG exposure, those that were uninfected following their first MG exposure were more likely to be infected upon subsequent exposure. Furthermore, these infections were more severe, with higher pathogen loads and reduced tolerance in finches that were unsusceptible to their first MG exposure. Demographic factors were important predictors of susceptibility, but not tolerance or resistance, to a second MG exposure. Finches from the MG-endemic population and males were less susceptible to second MG exposure than finches from the MG-naïve population and females, respectively. Incorporating individual variation in response to subsequent exposures can shed light on transmission dynamics and the evolution of host defense strategies in systems characterized by reinfections.