Factors driving rapid changes in virulence of the bacterial pathogen Mycoplasma gallisepticum in house finches Haemorhous mexicanus

Interactions between emerging pathogens and their hosts cause changes in both host and pathogen. A novel strain of the bacterium Mycoplasma gallisepticum (MG), a widespread, economically important poultry pathogen, emerged in... [ view full abstract ]

Interactions between emerging pathogens and their hosts cause changes in both host and pathogen. A novel strain of the bacterium Mycoplasma gallisepticum (MG), a widespread, economically important poultry pathogen, emerged in 1994 in wild passerines in Eastern North America. In house finches – its primary wild bird host –the pathogen causes severe conjunctivitis resulting in reduced survival in the wild. As the epidemic spread rapidly in the eastern (introduced) part of the finch’s range it caused massive declines in host abundance. In 2002 MG successfully spread to the western (native) range of the host. Since its emergence we have collected MG samples, primarily from house finches. This made it possible to determine that all house finch MG isolates across North America are derived from a single successful host jump. Nevertheless, different isolates vary strongly in virulence: MG virulence, as measured by severity and duration of eye lesions, is rapidly and independently increasing both in eastern and western house finches. This provides us with a tractable system to experimentally test causes of genetic changes in virulence. We will report on experiments that test the hypothesis that increases in virulence are the result of imperfect host immunity selecting for pathogen isolates that are sufficiently virulent to successfully infect and transmit between non-naïve house finches. The ability of MG to evolve in virulence and persist in house finch populations could affect population dynamics of house finches for decades to come.