Meeting Abstract
The host immune response can exert strong selective pressure on pathogen virulence, particularly when host immunity is incomplete. There have been few opportunities to test whether incomplete host immunity creates a within-host environment that favors higher pathogen virulence. In North American house finch populations, the bacterial pathogen Mycoplasma gallisepticum (MG) has increased in virulence since emerging in two geographically isolated populations. MG is an obligate parasite that is largely environmentally transmitted at bird feeders. Because MG is short-lived outside of the host, house finches are likely exposed to frequent low doses of pathogen while foraging at feeders. Previous work demonstrates that repeated exposure to low-doses of MG, a proxy for what birds likely experience while foraging, provides significant but incomplete protection against a high-dose homologous challenge. The aim of this experiment was to determine if this incomplete immunity produces a within-host environment that favors more virulent pathogen isolates. We manipulated prior exposure by giving MG-naïve house finches priming exposures that varied by dose and number of inoculations. After all animals were clinically healthy, individuals were given a challenge inoculation with one of three MG isolates ranging in virulence. The most virulent isolate had significantly higher pathogen fitness in hosts previously exposed to low levels of MG as compared to other isolates. A less virulent and a homologous isolate produced minimal infection loads with little or no disease in previously exposed hosts at all priming exposure levels. Our results indicate that previous low-level exposure to MG, which birds likely encounter at feeders, may produce a within-host environment that favors higher virulence.
