Thelaziasis is a parasitic ocular infection that affects the orbit and anterior surface of the eyes. It is primarily a zoonotic infection but can affect humans when the parasite is transmitted by flies which act as the vectors between animals and humans. Thelaziasis comes from the nematode species Thelazia callipaeda that is transmitted to animals and humans by Drosophilid flies. This disease is also known as the Oriental eye worm because it primarily manifests in Asian countries.

Thelaziasis was first described in a Chinese dog in 1910, and, since then, it has been reported in several animals such as cats, foxes, rabbits, and horses. The disease occurs mostly in lower socioeconomic areas and rural communities that live in close proximity to livestock. Newborns and children are most at risk of acquiring the disease due to their inability to fight off flies. Adult farmers are also at increased risk due to their interaction with livestock. Although most reported cases come from Asia, a few cases have been reported in Europe.

These nematodes present as white, translucent worms found in the conjunctiva sac, lacrimal duct, or lacrimal sac. Patients may complain of eye redness, foreign body sensation and tearing. Features may include ocular irritation, lacrimation, conjunctivitis, keratitis, corneal ulcer, and ectropion.

The life cycle of these parasites consists of 3 stages prior to their adult form. The first stage of larvae is found in lacrimal secretions of infected animals. The Drosophilid flies, which are the vectors, feed on these lacrimal secretions and ingest the larvae. In the gut of these flies, the larvae develop into the next stage within a few weeks. The third stage of larvae is the infective state, which can affect humans. The flies transmit the third stage of larvae to humans or other animals. It takes about one month for the third stage of larvae to develop into adult worms.