Thelaziasis

Thelaziasis is the term for infestation with parasitic nematodes of the genus Thelazia. The adults of all Thelazia species discovered so far inhabit the eyes and associated tissues (such as eyelids, tear ducts, etc.) of various mammal and bird hosts, including humans. Thelazia nematodes are often referred to as 'eyeworms'. Thelaziasis is the term for infestation with parasitic nematodes of the genus Thelazia. The adults of all Thelazia species discovered so far inhabit the eyes and associated tissues (such as eyelids, tear ducts, etc.) of various mammal and bird hosts, including humans. Thelazia nematodes are often referred to as 'eyeworms'. In animal and human hosts, infestation by Thelazia may be asymptomatic, though it frequently causes watery eyes (epiphora), conjunctivitis, corneal opacity, or corneal ulcers (ulcerative keratitis). Infested humans have also reported 'foreign body sensation' – the feeling that something is in the eye. In the uterus of the adult female, the embryos develop into first-stage larvae (L1), which remain in the eggshell (sheath). The female deposits these sheathed larvae in the tears of the mammal or bird definitive host, and the larvae are ingested by tear-feeding flies. 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). There they develop into third-stage larvae (L3), which migrate to the head of the fly. The infective L3 larvae wiggle out of the straw-like feeding apparatus of the fly when it feeds on the tears of another mammal or bird host. The L3 larvae develop into adults in the eye or surrounding tissues of the host, where they may live for over one year. In the definitive host, Thelazia have been found in various tissues of the orbit (or socket) of the eye, including within the eyelids, in the tear glands, tear ducts, or the so-called 'third eyelid' (nictitating membrane) or in the eyeball itself. While a few dozen species of Thelazia have been described in the literature, only 3 have been reported to infest humans, and only 7 are commonly reported in veterinary contexts. The remaining species are occasionally found in birds or wild mammals. 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. Horses are infested by T. lacrymalis (worldwide) and, less frequently, by T. rhodesii (Africa, Asia, Europe). In cattle, T. gulosa (Asia, Europe, North America), T. rhodesii (Africa, Asia, Europe) and T. skrjabini (Europe, North America) are the primary species of concern. In camels, T. leesei infestations have been reported from the Post-Soviet states and India.