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

Urbanization affects the ecology of wildlife diseases and although 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 Passer domesticus using field and experimental data under contrasting conditions (i.e. urban vs. non‐urban). For experimental data, we kept 32 male House Sparrows in captivity as a proxy of stress, and for field data we sampled 49 House Sparrows (17 females). We made use of microscopy to determine the relative intensity and used the polymerase chain reaction to estimate infection prevalence. We obtained total leucocyte counts, leucocyte differentials, heterophil/lymphocyte ratio (H/L) as a measure of stress, and the Polychromatic Index as a measure of physiological condition (erythropoiesis). We identified a total of 10 haemosporidian lineages. For field samples (both males and females), we found a significantly higher prevalence of infection in non‐urban House Sparrows than in urban ones. Under experimental conditions, non‐urban House Sparrows showed a 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 House Sparrows (~ 19%), indicating either a recrudescence of chronic and relapses of latent infections or the appearance of infections that had been acquired earlier. The H/L ratio was significantly higher for non‐urban than for urban male House Sparrows before captivity. No difference in H/L was found for urban House 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 risks than do non‐urban areas for House Sparrows.