Thelaziases are caused by spirurid nematodes (also known as eyeworms), which cause infestation of the orbital cavities and associated tissues of several animal species. These whitish nematodes are transmitted by nonbiting dipteran muscid flies or drosophila fruit flies (intermediate hosts) while feeding on the lacrimal secretions of infected animals. Adults of Thelazia spp localize under the eyelids and the nictitating membrane, as well as in nasolacrimal ducts, conjunctival sacs, and excretory ducts of lacrimal glands, depending on the species targeted. Thelaziases may be responsible for eye disease, with signs and symptoms of varying severity (eg, lacrimation, ocular discharge, epiphora, conjunctivitis, keratitis, and sometimes corneal opacity or ulcers). Levamisole and ivermectins (eg, doramectin) administered either subcutaneously or intramuscularly, are useful for the treatment of thelaziasis in cattle and horses. Some Thelazia spp may also infect humans.