Study of a Novel Host-Parasite Relationship: Mycoplasma gallisepticum in House Finches (Carpodacus Mexicanus)

Abstract

Although host-parasite interactions are well recognized as dynamic interactions leading to the occurrence of emerging infectious disease, we know little about how host- parasite systems evolve. To better understand how these systems may evolve, I studied how the bacterium Mycoplasma gallisepticum (MG) and its new host, the house finch (Carpodacus mexicanus), have changed since the infection was first observed in the eastern population of the house finch in the mid-1990s. To assess the evolution of MG in the house finch, the virulence of two isolates collected six years apart was examined. One isolate was collected in 1995 during the height of the epidemic, and the second isolate was collected in 2001, when the disease was less prevalent in the wild host population. For three of the four parameters of virulence that were tested (Initial infective dose, severity of clinical disease one week post-exposure, and rate of recovery), the 2001 isolate was more virulent than the 1995 isolate. The results of my study indicate that the house finch MG strain has become significantly more adapted to survival in its newly colonized host. The evolution of the host in response to infection with MG was also tested by comparing the response to MG infection using birds collected from an exposed population (Alabama) or from one of two naïve populations (California and Hawaii). While all three populations exhibited a range of responses to experimental infection with MG, in general, the results show that Alabama finches developed less severe clinical disease, and had a higher rate of recovery from infection than finches in the two naïve populations. These results support the theory that house finches in the eastern range have increased their resistance to MG. To better understand the impact the house finch MG strain on other wild songbirds, birds from nine species were exposed to the house finch MG strain by ocular inoculation. All four species in the family Fringillidae and one non-fringillid were susceptible to infection and developed clinical disease. Three additional species were susceptible to infection with the house finch MG strain although they did not develop clinical disease. These results indicate that while Fringillids are more susceptible to the house finch MG strain, additional wild bird species are susceptible and may play a role in transmission.