Epidemiological, morphological, and molecular characterization of Thelazia spp. in European bison (Bison bonasus) in Poland

Nematodes of the genus Thelazia (also referred to as eyeworms) pose a growing concern for wild and domestic ruminants. This study aimed to investigate the prevalence, species composition, and associated ocular pathology of Thelazia infection in several European bison populations in Poland. The eyes of 223 European bison were examined over five years (2020-2024). Nematodes of the genus Thelazia were identified based on morphometrical and molecular analysis of the cox1 sequences. Epidemiological analysis of Thelazia infection in relation to age, sex, genetic line of European bison, as well as severity of ocular changes, habitat, and season, was performed. Morphometrical and molecular analysis revealed the presence of two species - Thelazia gulosa and T. skrjabini. Nematodes were detected in 61.9% of examined animals, with T. skrjabini being significantly more prevalent than T. gulosa (p < 0.001). Infection with T. skrjabini was significantly associated with the severity of ocular lesions, which might suggest higher vulnerability of European bison to this eyeworm species. In the study period, prevalence of Thelazia infection significantly decreased in the Lowland line of European bison (p = 0.022) while it remained unchanged in the Lowland-Caucasian bison (p = 0.328). The study shows that Thelazia spp. is a common parasite of the European bison, equally spread across different studied populations, irrespective of habitat, geographical region, season or individual factors. It may indicate that thelaziosis occurs endemically in the Polish population of the European bison.