The prevalence of avian haemosporidian parasites in an invasive bird is lower in urban than non-urban environments

Urbanization affects the ecology of wildlife diseases and while it has been suggested that there are lower risks of infection in urban areas, there have been no experiments to support this conclusion. We assessed haemosporidian prevalence and intensity in House Sparrows <i>Passer domesticus</i>using field and experimental data under contrasting conditions (i.e. urban vs. non-urban). For experimental data, we kept 32 male sparrows in captivity as a proxy of stress, and for field data we sampled 49 sparrows (17 females). We made use of microscopy to determine the relative intensity and used PCR to estimate infection prevalence. We obtained total leucocyte counts, leucocyte differentials, H/L ratios as measures of stress, and the Polychromatic Index as a measure of physiological condition (erythropoiesis). We identified a total of ten haemosporidian lineages. For field samples (both males and females), we found significantly higher prevalence in non-urban sparrows compared to urban ones. For experimental set up, non-urban sparrows showed higher prevalence than urban ones both before and after captivity, with a significant increase in parasite intensity. The number of infected birds increased after captivity for both urban (~32%) and non-urban sparrows(~19%), indicating either a recrudescence of chronic and relapses of latent infections or the appearance of infections that were acquired earlier. The H/L ratio was significantly higher for non-urban compared to urban male sparrows before captivity. No difference in H/Lwas found for urban sparrows before and after captivity, indicating tolerance to city stressors. Our results showed a significant decrease in H/L for non-urban birds after captivity, suggesting higher stress in the non-urban agricultural environment. Haemosporidian infections were not associated with the H/L ratio. Our study provides evidence that highly urbanized areas within cities represent lower haemosporidian infection risk compared to non-urban areas for House Sparrows.