Thelazia leesei Railliet & Henry, 1910 (Spirurida: Thelaziidae) of dromedary camel Camelus dromedarius: further morphological description, molecular characterization, and epidemiology in Iran.

Background: In camels, thelaziosis is mainly caused by Thelazia leesei Railliet & Henry, 1910, a little-known eyeworm species. Given the paucity of scientific data, this study aimed to provide new insights into the morphology, molecular characterization, and phylogenetic relationship of T. leesei and its occurrence in camels from Iran, where animals suffer from the high burden of eyeworms.

Methods: From December 2020 to November 2022, slaughtered camels (n = 400) of different sex and age groups were examined in Sistan-va-Baluchestan province in Southeast Iran's local abattoirs. Adult eyeworms were fixed and stored for morphological identification by light and scanning electron microscopy (SEM). Polymerase chain reaction (PCR) products corresponding to the partial sequences of the mitochondrial cytochrome c oxidase subunit I (cox1) of eyeworms were Sanger sequenced and analyzed phylogenetically.

Results: A total of 118 (29.5%) camels from all five counties examined were infected with eyeworms, with an abundance of 0.9 and a mean intensity of 3.2 (i.e., up to 18 worms from a single animal). The infection rate was higher in camels older than 4 years of age (P = 0.01901). Lachrymation was associated with infection in animals (P < 0.00001). The morphology of our specimens resembled that of T. leesei, with the exception of the position of the nerve ring and esophagus length. Genetic analysis showed that the cox1 partial sequences of our T. leesei specimens had genetic distances of 8.8% to 13.5% compared with other Thelazia species.

Conclusions: On the basis of the morphometrics and morphological characteristics, we identified our specimens as T. leesei. In the phylogenetic tree, T. leesei herein isolated formed a monophyletic group together with its congeners, and T. leesei formed a sister clade to T. lacrymalis. In addition, we demonstrated the epidemiology of the infestation of T. leesei in camels in the endemic areas of southeastern Iran. The data presented are crucial for better understanding the pathogenic role of T. leesei and developing effective treatment strategies. In particular, studies on the intermediate host(s) of T. leesei in these regions will support effective control strategies for this parasitosis.