Spirurid nematodes in the genus, Thelazia. Three species that have been implicated in human infection include T. callipaeda (the Oriental eye worm), T. gulosa (the cattle eyeworm), and T. californiensis (the California eye worm).
Dogs and other canids, cattle, and horses are the usual definitive hosts for Thelazia spp., although other mammals, including cats, lagomorphs, cervids and humans, can also become infected. Adults reside in the conjunctival sac of the definitive host where they shed first-stage larvae . These larvae are sheathed. The first-stage larvae are ingested by the intermediate host (usually flies, including muscid flies in the genera, Musca and Fannia), when they feed on tears and other lacrimal secretions . In the digestive tract of the intermediate host, the larvae shed the sheath and invade various host tissues, including the hemocoel, fat body, testis and egg follicles where they develop in capsules. The encapsulated larvae become infective L3 larvae after two molts. Afterwards, the L3 larvae break out of the capsules and migrate to the fly’s mouthparts, where they remain until the fly feeds on the tears of the definitive host. The larvae invade the conjunctival sac of the definitive host and become adults after about a month and two additional molts . Humans may also serve as a final host , after infected flies feed on tears or other lacrimal secretions.
Human infections have been recorded from the United States, Asia (including South Asia), Russia and Europe.
Adults in the eye cause varying degrees of inflammation and lacrimation. In heavier infections, photophobia, edema, conjunctivitis, and blindness may occur.
Adults of Thelazia spp. reside in the conjunctival sac of their definitive hosts, which are usually dogs or other canids, cattle, and horses; humans are usually only incidental hosts. Adults measure up to 2.1 cm in length. Thelazia species may be differentiated by the appearance of the cuticular striations, the depth and width of the buccal cavity, the placement of the vulval opening relative to the esophago-intestinal junction and the morphology of the tail and anal opening.
Thelazia spp. require an insect intermediate host for completion of their life cycles. The muscid fly genera Fannia and Musca are intermediate vector hosts of human thelaziasis in the United States. Fannia canicularis and Fannia benjamini are vectors of Thelazia californiensis, while Musca autumnalis is the vector of Thelazia gulosa in the United States and Europe. Other likely vectors of T. gulosa worldwide include M. vitripennis (Crimea), M. larvipara (Ukraine) and M. amica (Russian Far East). The primary vector host of Thelazia callipaeda in Asia and Europe is Phortica variegata, though in China, Amiota okadai is also a vector.
Identification is made by the finding of adult worms in the conjunctival sac.
DPDx is an educational resource designed for health professionals and laboratory scientists. For an overview including prevention, control, and treatment visit www.cdc.gov/parasites/.