Dislodged abundance of two flea species from the arboreal squirrel indicates species-dependent response to urbanization

Abstract Urbanization influences not only free-living organisms, but also various pathogens, including those with limited visibility to humans. Although previous research on urban parasites has primarily focused on human-specific and zoonotic species, a comprehensive understanding of parasite ecology in urban settings requires attention to inconspicuous enzootic species. In this study, we investigated two flea species, Aenigmopsylla grodekovi and Ceratophyllus (Monopsyllus) indages indages, parasitizing Eurasian red squirrels Sciurus vulgaris orientis in and around Obihiro City, Hokkaido, Japan. These fleas, dislodged during the routine handling of host squirrels, were collected and quantified during spring and autumn surveys to assess seasonal and spatial patterns of infestation across urban and rural sites. Over the course of a four-year survey (2017–2020), 317 squirrels (168 males and 149 females) were examined, with recovering 284 individuals of A. grodekovi and 214 of C. (M.) indages indages. The former was rarely observed in spring but showed a pronounced increase in autumn, with rural areas exhibiting 13.65 times more dislodged individuals than urban areas, on average. In contrast, the latter was more prevalent in spring (2.44 times higher than in autumn) and displayed no consistent difference between urban and rural habitats. Both species possess simple life cycles that are conducive to urban adaptation, but our findings suggest divergent ecological responses, potentially driven by differences in seasonal phenology and host availability. While direct quantification of flea abundance remains ideal, the number of dislodged individuals may serve as a useful indicator for assessing parasitic responses to urbanization and infrapopulation dynamics.