Seasonal dynamics of mycoplasmal conjunctivitis in eastern North American house finches

1. Mycoplasma gallisepticum is an emerging eye disease that spread rapidly among wild house finches ( Carpodacus mexicanus ) in the eastern United States following initial reports of infected birds in 1994. The hallmark signs of infection have allowed systematic monitoring of disease at both local and continent-wide scales for more than 7 years since the onset of the epidemic. 2. Using data collected by a network of citizen science volunteers, we examined both long-term trends and seasonal dynamics of mycoplasmal conjunctivitis in three different climatic regions in eastern North America over a 77-month period (November 1994–March 2001). 3. Time-series prevalence data from all three regions suggest that following establishment, marked seasonal fluctuations in prevalence each year were characterized by autumn–winter epidemics (in October and February) and consistent summer declines (with prevalence falling close to zero from May to July). 4. The maximum peak, annual rates of increase and timing of epidemics varied among three geographical locations that were delineated by minimum winter temperatures. Annual autumn prevalence in the South increased more rapidly, and maximum prevalence was nearly three times greater in the South than in the colder North and Central regions. 5. Longer-term trends showed evidence for multiyear fluctuations in prevalence that were characterized by greater amplitude in the southern region. 6. Finally, monthly estimates of house finch flock sizes derived from a similar citizen science data set showed that winter flock sizes were associated positively with average monthly prevalence in the northern and central regions, although regional differences in flock sizes did not correspond to regional differences in maximum prevalence. 7. This study represents the first evidence of multiyear fluctuations, regional differences and highly predictable annual outbreaks of this recently emerged wildlife pathogen. Several factors associated with house finch life history and behaviour are likely to contribute to temporal and spatial variation in prevalence, including annual changes in host reproduction, social behaviour and environmental effects on host stress or immunocompetence.