Thelazia californiensis is found in dogs, cats, and deer in western USA; T callipaeda is found in dogs, cats, foxes, wolves, martens, and rabbits in Europe and Asia. T callipaeda appears to be spreading in some regions of Europe. Both species are zoonotic. The worms are whitish, 7–19 mm long, and move in a rapid serpentine motion across the eye. Up to 100 eyeworms may be seen in the conjunctival sac, tear ducts, and on the conjunctiva under the nictitating membrane and eyelids. Filth flies (Musca spp, Fannia spp) serve as intermediate hosts and deposit infective larvae on the eye while feeding on ocular secretions. Zoophilic fruit flies have emerged as potential intermediate hosts of T callipaeda. These include Phortica variegata in Europe and Amiota spp in Asia.
Clinical signs include excessive lacrimation and epiphora, ocular pruritus, conjunctivitis, keratitis with corneal opacity and ulceration, hyperemia, and rarely, blindness. After local anesthetic, diagnosis and treatment are readily accomplished by observing and removing the parasites with forceps. Thelazia spp infections have been successfully eliminated from dogs with ivermectin SC at 0.2 mg/kg, milbemycin oxime PO at a minimum dosage of 0.5 mg/kg (two treatments 1 wk apart improved efficacy), or spot-on treatment with moxidectin 2.5%. Ocular solutions (1% moxidectin or 2% levamisole) or ointments (1% levamisole or 4% morantel) also may be effective. Infection with T callipaeda has been prevented for the full season in dogs with sustained-release moxidectin at 0.17 mg/kg, SC, with milbemycin oxime PO at the dosage recommended for heartworm prevention, and with ivermectin PO at 0.2 mg/kg. Milbemycin oxime at the minimum oral dosage of 2 mg/kg showed high therapeutic efficacy in cats infected with T callipaeda.