Meeting Abstract
Few natural systems have provided the opportunity to empirically test the theory that incomplete host immunity selects for higher pathogen virulence. Since its emergence in North American house finch populations, the bacterial pathogen Mycoplasma gallisepticum (MG) has increased in virulence. Previous work has demonstrated that repeated low-dose exposure to MG, a proxy for what birds may experience at feeders, provides significant but incomplete protection against secondary infection and produces a within-host environment that favors more virulent strains. Here we sought to determine if previous exposure level mediates the transmission potential of a host, and if this favors the transmission of more virulent strains. To test this, we first created variation in prior exposure by exposing captive, wild-caught house finches to one of two priming exposures of MG, which varied by dose and total number, or to a media-only control. After recovering from priming exposures, individuals were inoculated with a single high dose of a strain of equal or higher virulence and paired with an MG-naïve cage-mate to assess pairwise transmission potential. Immunity from prior exposure significantly reduced the pairwise transmission potential of individuals, with the lowest transmission potential in house finches which were given high-dose priming exposures. The more virulent strain had higher overall transmission potential than the less virulent strain, but again this varied with host exposure history. Interestingly, individuals that received low-dose priming exposures, which prior work showed generate largely incomplete immunity in house finches, showed identical transmission potential between pathogen strains. Together these results demonstrate that host immunity, mediated by exposure history, can significantly alter the transmission potential of an individual.
