The adults of all Thelazia species discovered so far inhabit the eyes and associated tissues (such as eyelids, tear ducts, etc.)

In the fly, the larvae "hatch" (exsheath), penetrate the gut wall, and migrate to either the fat body, testes or egg follicles (depending on the species).

In humans, dogs and cats, thelaziasis cases due to Thelazia callipaeda (Asia, Europe), and occasionally T. californiensis and T. gulosa (western North America), have been reported.

Adult Thelazia are very active, one author described T. californiensis as a "short lively piece of nylon fishing line about 10 mm long.

Other sources have reported positive results treating dogs with moxidectin, imidacloprid, or milbemycin oxime.

[11] By one author's count, 257 human cases of Thelazia callipaeda had been reported worldwide by the year 2000,[12] though thelaziasis is still considered to be a rare disease.