Background: This study aimed at investigating the prevalence and risk factors of bovine thelaziosis, performed in the southeast of Iran, an endemic area for Iranian Sistani cattle. Methods: Between September 2012 to October 2014, 1924 cattle, Sistani breed (n= 1235) and Brahman breed (n=689) of all sex and age groups collected from Sistan and Baluchestan Province were examined using visual observation of the eyes by flushing the conjunctival sac and lachrymal duct with sterile saline solution. Results: The overall prevalence for thelaziosis was 50 of 1924 cows (2. 6%; 95% CI: 1. 9-3. 3%), with significant higher prevalence of infection in Sistani breed than in Brahman breed (3. 15% vs 1. 59%). Sixty adult worms (84. 5% of females and 15. 5% of males) were collected from the conjunctiva of the infected cattle: Thelazia gulosa (50/60, 83. 3%) was the most represented species followed by Thelazia. rhodesi (10/60, 16. 7%). The number of worms collected per cow ranged from one to seven (average ± standard deviation: 2. 08 ± 1. 49). Worms were gathered from cows throughout all months of the year. The difference in the seasonal variations of prevalence and the intensity of infection were significant, however, no significant correlation between prevalence, sex and age of cattle was noted. Conclusion: Bovine thelaziosis needs special attention by veterinarians in the differential diagnosis of ocular manifestations and considering its impact on cattle production.

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