Pathogen-Induced Stress in Wild House Finches (Haemorhous mexicanus): Leukocyte Dynamics as Health Indicators.

Birds are frequently stressed by their exposure to parasites and pathogens. The heterophil to lymphocyte (H:L) ratio has been proposed as a measure of physiological stress. This study investigates the prevalence of the bacterial pathogen Mycoplasma gallisepticum and Haemosporidia in Mexican House Finches (Haemorhous mexicanus). To test the extent to which pathogens stress their hosts we compared total and differential leucocyte counts in four groups of birds with different infection statuses: birds co-infected with M. gallisepticum and haemosporidia, birds infected with either M. gallisepticum or haemosporidia, and birds infected with neither. We hypothesized that co-infected birds would exhibit higher physiological stress, reflected in leukocyte parameters, plus, based on previous experiments, that both M. gallisepticum and haemosporidia would benefit from the presence of each other. We found that 73.6% of House Finches were infected with M. gallisepticum, and 43.1% with haemosporida. Both pathogens had a higher load in co-infected birds. Higher H:L ratios were found in co-infected birds compared to birds with a single infection. The study highlights the complexity of immune responses under co-infection. Our results contribute to the understanding of host-pathogen interactions and stress physiology in wild birds, highlighting the importance of hematological parameters as indicators of stress and providing insights into the ecological impacts of pathogen infections in avian populations. This emphasizes the significance of studying leukocyte profiles in the wild.