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Spirurid nematodes in the genus, Thelazia. Two species that have been implicated in human infection include T. callipaeda (the Oriental eye worm) 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 drosophilid flies in the genus, Amioto, and 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.
Presumed worldwide; human infections have been recorded from the United States, China, Russia, India, Japan, and Thailand.
Adults in the eye cause varying degrees of inflammation and lacrimation. In heavier infections, photophobia, edema, conjunctivitis, and blindness may occur.