Thelazia		Contents		Rev: 10/25/2025
Eyeworms		Classification		Biology and Ecology
		Morphology and Anatomy		Life Cycle
Return to Thelazia Menu				Ecosystem Functions and Services
		Distribution		Management
Return to Thelaziidae Menu		Feeding		References
		Go to Nemaplex Main Menu		Go to Dictionary of Terminology

Classification:

Chromadorea
Chromadoria

           Rhabditida 

Spirurina

              Thelazioidea

      Thelaziidae

                      

            Thelazia Bosc, 1819

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Morphology and Anatomy:

• Adult small, thin, and white
• Lengths 1-2 cm.
• Mouth without lips.

Males:

•  Tail blunt, curved
• Spicules equal or unequal spicules depending on the species
• No gubernaculum, except maybe in  T. californiensis

Females:

• Tail bluntly rounded.
• Vulva in the region of the esophagus
• Viviparous

Ref: Marchiondo and Cruthers, 2020

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Distribution:

Representative species worldwide. Thelazia species are parasites principally found on or around the eyes in the conjunctival and lacrimal ducts of cattle and domestic animals, occasionally humans, causing keratitis.

Species include:

 Thelazia lacrymalis  in horses in Europe and North America;

Thelazia californiensis in dogs, cats, and occasionally sheep in North America;

Thelazia rhodesi, T. gulosa  and T. skrjabini in cattle

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Feeding:

Food Sources and Feeding strategies for the genus Thelazia

Nematode parasites on or around the eyes in the conjunctival and lacrimal ducts of cattle and domestic animals, someimes humans

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Biology and Ecology:

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Life Cycle:

Flies are intermedeiate hosts

• Females nematodes are viviparous; they pass L1 into the lachrymal secretions.
• The L1 are ingested by face flies (Musca autumnalis and Musca hervei) or the lesser housefly (Fannia canicularis), depending on the species of Thelazia,
• The larvae develop to the L3 in the ovarian follicles of the fly in 15-30 days during the summer.
• The L3 migrate to the mouthparts of the fly with transmission to the definitive host during fly feeding around the eyes.
• The L3 develops to the adult stages in or around the eye of the definitive host.
• The prepatent period is 3-11 weeks depending on the host species with infections occurring seasonally linked to periods of maximum fly activity.

Ref: Marchiondo and Cruthers, 2020

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Ecosystem Functions and Services:

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Management:

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References:

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Copyright © 1999 by Howard Ferris.
Revised: October 25, 2025.