Characteristics of canine thelaziasis in the Republic of Korea: a retrospective study (2022–2024)

Importance: Previous reports on thelaziasis from the Republic of Korea focused on parasite identification and zoonotic infections in humans. This retrospective study aimed to characterize the clinical patterns of thelaziasis in pet dogs. Objective: To describe the characteristics of canine ocular thelaziasis in small animal clinical settings in the Republic of Korea. Methods: Medical records of 35 dogs (44 eyes), diagnosed with thelaziasis at Dr. Park’s Animal Eye Clinic between March 2022 and February 2024, were retrospectively reviewed. Results: Nine dogs had bilateral infections, while 26 had unilateral infections. Most cases (71.4%) were referred to by local clinics, but only 16% had been initially diagnosed with thelaziasis. Common clinical signs included conjunctivitis (25.7%), ocular discomfort (17.1%), corneal ulcers (14.3%), and parasite observation (14.3%). No breed, sex, or age predisposition was noted. Thelaziasis was more prevalent in fall (54.3%) and winter (28.6%), with fewer cases in spring (11.4%) and summer (5.7%). Dogs with thelaziasis had a higher incidence of follicular conjunctivitis of the third eyelid (42.9%) compared to non-infected dogs (9.7%). All affected dogs recovered well after manual parasite removal, followed by anti-inflammatory or antibiotic eye drops as needed. Conclusions and Relevance: Canine thelaziasis showed seasonal prevalence, peaking in fall and winter, and was linked to an increased risk of follicular conjunctivitis of the third eyelid. Veterinarians should consider thelaziasis in cases of recurrent ocular surface disease, especially in these seasons.