Yak thelaziasis in the Orenburg Oblast: Musca autumnalis (De Geer, 1776) as a vector and Thelazia rhodesi (Desmarest, 1827) as the causative agent of infestation

Introduction. Thelaziasis remains a widespread vector-borne parasitic zoonosis both within the Russian Federation and globally. Thelaziasis in yaks remains insufficiently studied, with the available data being fragmentary. It is the first time a thelaziasis clinical case in yaks from the Orenburg Oblast is described and it is of significant interest to a broad range of specialists. Objective. Analysis and clinical case description of thelaziasis in yaks from the Orenburg Oblast, including the study of its causative agent and vector. Materials and methods. Studies conducted from 2021 to 2023 at the steppe field station of the Institute of Steppe of the Ural Branch of the Russian Academy of Sciences in the Belyayevsky Raion of the Orenburg Oblast included clinical examinations and assessment of pathological lesions and severity of inflammation in the eyes and conjunctiva of yaks. Parasitic secretophagous dipterans (flies) from the ocular region were collected and counted, and their abundance, species and sex ratios were determined. Helminthoscopy was performed, and the nematode species was determined morphologically. Results. Clinically, thelaziasis in yaks manifested as profuse lacrimation and recurrent keratoconjunctivitis. The extent of invasion (EI) was 100%, and the intensity of invasion (II) was 5. The detected helminths belonged to Thelazia rhodesi species. The intermediate hosts and vectors of Thelazia were facultative hematophages, specifically Musca autumnalis, a synbovine fly species ubiquitous in the steppe landscapes of the Orenburg Oblast. The ratio of females to males collected from the head region of yaks was 83 and 17%, respectively, confirming the leading role of female Musca autumnalis as vectors of nematodes of the genus Thelazia. Conclusion. Domestic yaks in the natural and climatic conditions of the Orenburg Oblast are susceptible to thelaziasis. The disease progress, its clinical manifestations, as well as extent and intensity of invasion are likely influenced by acclimatization of yaks, who are not indigenous to this region.