Background: Mycoplasma gallisepticum is a widespread bacterial pathogen that is economically important in poultry. A novel strain, causing severe conjunctivitis in wild passerines, emerged in 1994, and spread rapidly across eastern North America. In house finches the pathogen causes severe conjunctivitis. As the epidemic spread through the eastern (introduced) part of the finch’s range it caused massive declines in host abundance. In 2002 it successfully spread to the western (native) range of the host. There it spread much more slowly and disease prevalence and effects on host abundance are much lower than in the east. Mycoplasmal conjunctivitis in house finches is a system with strong seasonal variation: conjunctivitis prevalence is minimal (often zero) during the breeding season (April July); In late summer and fall, prevalence increases gradually reaching a maximum in October to November. In December, prevalence reaches a new low, followed by a second smaller peak in late February and early March, after which prevalence returns to the breeding season minimum. The objective was to 1. determine the factors driving seasonal variation in disease prevalence, and test the hypothesis that relapse of recovered individuals can be the origin of fall epidemics; and 2. determine factors driving changes in virulence once the disease is established. In the first study, birds not previously exposed to Mycoplasma gallisepticum were held in large aviaries. After one individual had been inoculated and horizontal transmission had caused all birds in the group to be exposed, birds recovered and were allowed to breed. In September naïve juveniles were added to the group and to test if the recovered adults would infect them. In March, when disease prevalence was declining naturally, Mycoplasma gallisepticum was reintroduced in the flock. In the second study, birds were sequentially exposed with two Mycoplasma gallisepticum strains that differed in virulence to test for cross‐immunity between strains. The first result showed n naive juveniles added to a flock of asymptomatic, fully recovered adults became infected with Mycoplasma gallisepticum showing that previously‐exposed, recovered and asymptomatic individuals can be the source of a new epidemic. The introduction of Mycoplasma gallisepticum in March, when in the wild disease prevalence is low to zero, caused a new outbreak whereby previously exposed and recovered individuals relapsed. The second result showed that after recovery individuals reinfected with a more virulent strain became very sick, while individuals reinfected with a less virulent strain were resistant to reinfection. We drew two conclusions from the study. First, that asymptomatic individuals can be the source of a new outbreak. The introduction of the pathogen in recovered groups is sufficient to cause a new disease outbreak. Seasonality in outbreaks is most likely tightly linked to seasonal variation in bird movements and behavior, rather than with external seasonal drivers. Secondly, in this system there exists partial cross‐immunity, whereby more virulent strains are able to cause disease in individuals recovered from a previous infection with a less virulent one. This result has two implications. First partial cross‐immunity selects for more virulent strains in nature. Second, incomplete vaccination would also cause an increase in pathogen virulence, which might have implications for vaccination strategies in humans.


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  • Received: 05 March 2012
  • Accepted: 05 March 2012
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