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.
Presumed worldwide; human infections have been recorded from the United States, Nepal, China, Thailand, Taiwan, Russia, Italy, France, India, and Japan.
Adults in the eye cause varying degrees of inflammation and lacrimation. In heavier infections, photophobia, edema, conjunctivitis, and blindness may occur.
Thelazia spp. adults.
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 2.0 cm in length. The cuticle has coarse striations, often giving the worms a serrate appearance in profile. The mouth is without lips and the esophagus is short. The tail of the male is recurved and without caudal alae; the tail of the female is bluntly rounded.
Figure A: Anterior end of a female Thelazia sp. Note the lack of lips (arrow) and prominent striations.
Figure B: Mid-section of a female Thelazia sp. Note the prominent striations.
Figure C: Mid-section of a gravid female Thelazia sp., showing many typical spirurid-type eggs.
Figure D: Posterior end of a female Thelazia sp.
Figure A: Adult female Thelazia gulosa immediately after removal from the eye. Image courtesy of Oregon Health & Science University, Department of Pathology.
Figure B: Anterior of Thelazia gulosa showing buccal cavity, cuticular ridges, and esophageal-intestinal junction.
Figure C: Deep, cup-like buccal cavity of Thelazia gulosa.
Figure D: Esophageal-intestinal junction of Thelazia gulosa.
Figure E: Non-protruding vulval opening (circle) of Thelazia gulosa almost at the same level as the esophageal-intestinal junction.
Figure F: Mid-body of Thelazia gulosa with prominent cuticular striations, intestinal tube, and ovaries containing spirurid eggs.
Figure G: Tail of female Thelazia gulosa with non-protruding anal opening and post-anal papilla.
Figure H: Adult female Thelazia gulosa removed from the surface of the eye of a human, showing intestine and egg-filled ovaries taking up the majority of the length of the body.
Figure I: Thelazia gulosa in situ on the surface of a patient’s conjunctiva (circle).
Figure J: Adult Thelazia gulosa removed from the eye of human on a person’s finger.
Intermediate hosts of Thelazia spp..
Thelazia spp. require an insect intermediate host for completion of their life cycles. These insects are usually flies. The muscid fly genera Fannia and Musca have been implicated in the transmission of thelaziasis in the United States and Asia; the drosophilid fly genus Amioto has been implicated in Japan.
Figure A: Fannia canicularis, the lesser house fly. This species has been implicated in the transmission of thelaziasis in the United States and Asia. Image courtesy of Parasite and Diseases Image Library, Australia.
Figure B: Musca domestica, the house fly. This species has been implicated in the transmission of thelaziasis in the United States and Asia. Image courtesy of Parasite and Diseases Image Library, Australia.
Identification is made by the finding of adult worms in the conjunctival sac.
For information about treatment please contact CDC-INFO.
DPDx is an education resource designed for health professionals and laboratory scientists. For an overview including prevention and control visit www.cdc.gov/parasites/.
- Page last reviewed: February 15, 2018
- Page last updated: February 15, 2018
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