Filarioid nematodes (commonly known as filarial worms) are known to impact human and domestic animal health, but studies examining their ecological relevance and impacts on wildlife are still underrepresented. In the case of birds, microfilariae are typically found at low prevalence, but they may negatively affect some fitness-related traits. Here, we study the prevalence and associations of microfilariae in a wild population of blue tits (Cyanistes caeruleus) inhabiting a woodland comprising different forestry formations. In addition, we characterize the filarioid lineages through the cytochrome c oxidase subunit I (COI) gene sequence. We found a moderate prevalence of microfilariae in the blue tit population (9.4%) and that the presence of such parasites was negatively associated with host body mass. Neither forest type nor host sex influenced microfilariae presence. Phylogenetic analyses revealed the presence of five filarioid lineages clustered in the Onchocercidae family-four out of five lineages clustered in the Splendidofilaria clade, while the remaining lineage could not be clearly assigned to a genus. In addition, this is the first study examining the filarioid lineages infecting the blue tit. Our results suggest that hosts in poorer body condition, in terms of lower body mass, are more susceptible to be parasitized by filarioid nematodes and call for further genetic studies of these parasites.

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