Human ocular thelaziasis with genetic analysis in Niigata Prefecture, Japan: A case report on an emerging zoonosis

We report the clinical findings and molecular identification of ocular Thelazia callipaeda from Niigata Prefecture in the Hokuriku area of Japan during winter.

A 77-year-old male visited an ophthalmology clinic in Niigata Prefecture in January 2022 after a 2-week-duration of a conjunctival injection in the left eye and foreign body sensation. Slit-lamp microscopy revealed 11 active nematodes in the left conjunctival sac. Morphological characteristics included longer female body length than male, buccal cavity lacking teeth and lips, and serrated striations along the body surface. The specimens were determined to be T. callipaeda. Genetic analysis of the mitochondrial cytochrome c oxidase subunit 1 (cox1) gene revealed an h9 haplotype.

T. callipaeda infection, especially the h9 haplotype, commonly occurs in western Japan owing to its higher incidence in warmer climates, suggesting the origin of the case. Here, we report a human case of Thelaziasis diagnosed in a cold region of Japan (the Hokuriku area) during winter. This human case of T. callipaeda infection from a cold, previously unassociated region, raises concerns about the potential geographical widening of its distribution, and further investigation may be warranted to prevent its spread.