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[Thelazia spp.]

Causal Agents

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).

Life Cycle


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 The number 5 where they shed first-stage larvae The number 1. 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 The number 2. 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 The number 3 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 The number 4. Humans may also serve as a final host The number 6, after infected flies feed on tears or other lacrimal secretions.

Geographic Distribution

Presumed worldwide; human infections have been recorded from the United States, Nepal, China, Thailand, Taiwan, Russia, Italy, France, India, and Japan.

Clinical Presentation

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 2.1 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.

Figure A: Anterior end of a female Thelazia callipaeda. Note the small buccal cavity (arrow) and prominent cuticular striations.

Figure B: Mid-section of a gravid female Thelazia callipaeda.,showing many typical spirurid-type eggs.

Figure C: Mid-section of a female Thelazia callipaeda. Note the prominent serrate cuticular striations.

Figure D: Posterior end of a female Thelazia callipaeda with protruding anal opening.

Thelazia gulosa

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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.

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Figure C: Deep, cup-like buccal cavity of Thelazia gulosa.

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Figure D: Esophageal-intestinal junction of Thelazia gulosa.

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Figure E: Non-protruding vulval opening (circle) of Thelazia gulosa almost at the same level as the esophageal-intestinal junction.

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Figure F: Mid-body of Thelazia gulosa with prominent cuticular striations, intestinal tube, and ovaries containing spirurid eggs.

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Figure G: Tail of female Thelazia gulosa with non-protruding anal opening and post-anal papilla.

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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.

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Figure I: Thelazia gulosa in situ on the surface of a patient’s conjunctiva (circle).

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Figure J: Adult Thelazia gulosa removed from the eye of a human on a person’s finger.

Intermediate hosts of Thelazia spp.

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.

Fannia canicularis

Figure K: Fannia canicularis, the lesser house fly. This species, as well as Fannia benjamini, is considered the primary vector of T. californiensis in the United States. Image courtesy of Joyce Gross.

Phortica variegata, Thelazia callipaeda

Figure L: Male (left) and female (right) Phortica variegata, the variegated fruit fly. This species is the main vector of Thelazia callipaeda in Europe and Asia. Image courtesy of Dr. Thomas Werner, Ph.D., Department of Biological Sciences, Michigan Technological University.

Musca autumnalis, T. gulosa

Figure M: Female Musca autumnalis, the autumn house fly. This species is the primary vector of T. gulosa in the United States and in Europe. Image courtesy of Joyce Gross.

Laboratory Diagnosis

Identification is made by the finding of adult worms in the conjunctival sac.

Treatment Information

For information about treatment please contact CDC-INFO.

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  • Page last reviewed: May 10, 2018
  • Page last updated: May 10, 2018
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