Expanding the host range: Dirofilaria immitis and Crenosoma vulpis identified by morphological and molecular methods in brown bears (Ursus arctos) from Romania.

The brown bear (Ursus arctos) represents the largest carnivoran species in Europe, yet little data is available on respiratory parasitic fauna. While Romania hosts the largest population in Europe, the present study aimed to investigate the prevalence and diversity of lungworms in free-ranging brown bears from Romania using post-mortem examination followed by morphological and molecular identification. Between 2019 and 2025, 168 brown bear carcasses originating from 19 Romanian counties were examined by complete necropsy of the cardio-respiratory system. Detected parasites were collected, morphologically identified, and molecularly characterized using partial cox1 (for Crenosoma spp.) and ITS2 (for Dirofilaria immitis) gene sequencing. Overall, 5.4% (9/168; 95% CI: 2.5-9.9%) of the examined bears were infected with at least one lungworm species. Crenosoma vulpis was identified in 4.8% (8/168) of bears, representing the first molecular confirmation of this species in brown bears. Two adult male specimens of D. immitis were identified in two different individual hosts (1.2%), one of which was co-infected with C. vulpis. No microfilariae were detected by the modified Knott's test, and no gross lesions were observed during necropsy. These findings expand the known host range of C. vulpis and confirm incidental infection with D. immitis in brown bears from Romania. The low prevalence and absence of patent infections suggest that U. arctos is unlikely to act as a reservoir host, the results highlight the exposure of wildlife to endemic metastrongyloid and vector-borne filarioid parasites circulating in Romania.