Infectious keratoconjunctivitis in European bison (Bison bonasus) in Poland: risk factors, epidemiology and anatomopathological changes with analysis of potential role of Thelazia nematodes in the disease development.

Background: Infectious keratoconjunctivitis (IKC) is a common ocular disease of ruminants worldwide. Recently, an outbreak of infectious keratoconjunctivitis was observed in the European bison in Poland. Hundreds of animals show conjunctival congestion, corneal opacity, and ulceration, leading to total blindness. The present study aimed to examine the ocular changes of European bison and patterns of the disease occurrence with special emphasis on the role of Thelazia nematodes in the development of IKC.

Results: The eyes of 131 European bison, showing ocular signs and clinically healthy, were collected in Poland in 2021 - 2022 and subjected to parasitological and histopathological examination. Histopathology showed varying lesions, including corneal erosions or ulcers, diffuse purulent infiltrates to lymphocytic infiltration in the cornea, and lymphocytic or mixed conjunctivitis with CALT stimulation. The severity of ocular changes was higher in European bison from mountain areas and during the winter season. Two species of Thelazia nematodes - T. skrjabini and T. gulosa have been isolated from eyes. Prevalence of infection reached over 66.4%, and the infection intensity ranged from 1 to 16 nematodes per individual. Although nematodes of the genus Thelazia were prevalent in European bison, their occurrence did not correspond with the severity of ocular changes.

Conclusions: The results of our studies allowed to identify patterns related to the first outbreak of infectious keratoconjunctivitis in European bison. Living in mountain areas and winter season were the most predisposing factors for the development of ocular changes. Despite the high prevalence of Thelazia nematodes in the present study, their role in forming ocular lesions was not confirmed.