Thelaziasis

[Thelazia spp.]

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Causal Agents

Spirurid nematodes in the genus Thelazia are primarily veterinary parasites, but may occasionally infect humans. The majority of zoonotic infections involve T. callipaeda (the Oriental eye worm). T. californiensis (the California eye worm) and T. gulosa (the cattle eyeworm) are less common causative agents.

Life Cycle

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Adults reside in the conjunctival sac of the definitive host where the ovoviviparous females release first-stage (L1) larvae ensheathed in a shell membrane . L1 larvae are ingested by the face fly intermediate host during feeding on tears and lacrimal secretions . In the digestive tract of the intermediate host, L1 larvae become exsheathed and invade various host tissues, including the hemocoel, fat body, testis, and egg follicles where they develop in capsules. The encapsulated larvae molt twice to become infective L3 larvae . The fully developed 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 upon the fly intermediate host’s feeding and become adults after about a month and two additional molts . Humans may also serve as aberrant definitive hosts following exposure to an infected fly intermediate host in the same manner .

Hosts and Vectors

Both wild and domestic canids are considered the primary definitive hosts for Thelazia callipaeda. Natural infections have also been detected in felids, mustelids, and lagomorphs. T. californiensis infections have been reported in numerous mammals, mostly from wild and domestic canids, but also others (e.g. felids, bears, cervids, sheep, jackrabbits (Lepus californicus)). Some of these could possibly represent misidentifications of species. T. gulosa is a parasite of cattle, and occasionally other large ruminants.

Intermediate hosts or vectors for Thelazia spp. are drosophilid flies that feed on lacrimal secretions (lacrimophagous), and associations are parasite-specific. For example, Phortica (=Amiota) variegata and P. (=A.) okadai are the primary intermediate hosts for T. callipaeda. Fannia spp., such as Fannia benjamini (canyon fly) and F. canicularis (lesser house fly), are vectors for T. californiensis. Musca autumnalis (face fly) appears to be the most important vector for T. gulosa.

Geographic Distribution

Thelazia callipaeda is widespread across Asia and has now become established in continental Europe. T. californiensis has been reported from the western United States, although characterization of its range is incomplete. T. gulosa has a wide distribution across Asia, Europe, North America, and Australia.

Clinical Presentation

Adults in the eye cause varying degrees of inflammation and lacrimmation accompanied by a foreign body sensation. . In heavier infections, photophobia, epiphora, edema, corneal ulceration, and conjunctivitis 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.

Thelazia californiensis

Figure A: Anterior end of a female Thelazia californiensis showing small, shallow, buccal cavity (arrow) and prominent widely spaced cuticular striations.

Figure B: Mid-section of a female Thelazia californiensis. Note the prominent, widely spaced, and serrate cuticular striations.

Figure C: Posterior end of a female Thelazia californiensis with protruding anal opening and phasmids.

Thelazia callipaeda

Figure A: Adult male and female Thelazia callipaeda.

Figure B: Anterior of Thelazia callipaeda showing wide, moderate depth, buccal cavity and prominent tightly spaced cuticular striations.

Figure C: Anterior of Thelazia callipaeda female showing vulval opening (arrow), which is protruded, anterior to esophageal-intestinal junction (dart).

Figure D: Mid-section of adult female Thelazia callipaeda showing live larvae in utero

Figure E: Posterior of adult male Thelazia callipaeda showing spicules (Nomarsky phase contrast).

Figure F: Posterior of adult female Thelazia callipaeda showing anal opening (Nomarsky phase contrast)

Thelazia gulosa

Figure A: 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 B: Adult female Thelazia gulosa immediately after removal from the eye. Image courtesy of Oregon Health & Science University, Department of Pathology.

Figure C: Anterior of Thelazia gulosa showing buccal cavity, cuticular ridges, and esophageal-intestinal junction.

Figure D: Deep, cup-like buccal cavity and widely spaced, moderately distinct cuticular striations of Thelazia gulosa.

Figure E: Esophageal-intestinal junction of Thelazia gulosa.

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

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

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

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

Larvae of Thelazia spp

Figure A: L1 larva of Thelazia sp., likely T. callipaeda based on the patient’s travel history. L1 larvae measure up to 400 µm long and 13.5 µm wide. While not visible on this specimen, larvae are normally passed encased in a shell membrane.

Figure B: Anterior end of the larva. Note the lack of buccal cavity, and the developing cuticular striations.

Figure B: Anterior end of the larva in Figure B. Note the lack of buccal cavity, and the developing cuticular striations.

Intermediate hosts of Thelazia spp.

Figure A: 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.

Figure B: 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.

Figure C: 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

Diagnosis depends on the identification of adult worms in the conjunctival sac coupled with morphological examination for species identification. Molecular methods may be employed on adults or larvae for additional confirmation of species and genotypes.

Laboratory Safety

Standard laboratory precautions apply; only fixed adult worm specimens are usually examined.