Age, sex, and season affect the risk of mycoplasmal conjunctivitis in a southeastern house finch population

Abstract

House finches (Carpodacus mexicanus (Muller, 1776)) in eastern North America have been affected by annual epidemics of an eye disease caused by the bacterium Mycoplasma gallisepticum since 1994. To identify factors associated with seasonal changes in prevalence and variation in host susceptibility, we monitored mycoplasmal conjunctivitis among wild house finches in a region of high prevalence in southeastern North America. We captured 888 birds between August 2001 and December 2003 and observed seasonal outbreaks characterized by rapid increases in prevalence from August to October each year. During periods of high prevalence, infection probability was significantly higher among juveniles than adults, and the severity of conjunctivitis among juvenile females was greater than for any other host category. We found no evidence linking moulting status to elevated infection risk among adult birds. Finally, house finches with conjunctivitis were in poorer condition than birds with no clinical signs of infection, particularly among those with severe infections. Results from this study are consistent with recent reports of seasonal and regional variation in mycoplasmal conjunctivitis and suggest that annual changes in host reproduction, behaviour, and age structure might be important determinants of the timing and magnitude of local epidemics.