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Home Roundworms of Cattle Eye Worm in Cattle

Eye Worm in Cattle

(Thelazia species)

Thelazia species (T. gulosa and T. skrjabini) are slender white roundworms, 7-19 mm in length and parasitic in the conjunctival (the pink membrane lining the eyelid) and lachrymal (tear) sacs of domestic animals. Infection can occur in cattle, sheep, horses, dogs and other domesticated animals and man.

Figure 1. Cow eye with eye worm, Thelazia spp. Image created by Jess Morgan

The incidence of these worms may be decreasing due to the common use of the broad-spectrum anthelmintics.

Further ecological information on worms and their control:

  • Roundworm life cycle and life stages
  • The life cycle is indirect with the adult worms in cattle and immature stages in flies. Female worms produce thin-walled eggs that release a first stage larvae into the secretions of the tear ducts of the eye. Flies pick-up these larvae during feeding. Larval development within the fly takes about 15 to 30 days. Reinfection or infection of new hosts occurs when larvae are liberated from the proboscis (mouthparts) of the flies when imbibing on the secretions from the tear ducts of cattle. Worms mature in about 6 weeks and can remain in the eyes of cattle for several months.
  • Climate factors contributing to paddock contamination with worms
  • Pasture management to reduce exposure to worms
  • Cost of roundworms

Location in cattle

These worms are found in the conjunctival sac, under the third eyelid and in the tear ducts of the eyes of cattle.


The early clinical signs are mild conjunctivitis with excessive tearing in one or both eyes. The cornea or outer part of the eye may become scarred if the infection is prolonged. This scarring may whiten the eye, causing blindness. With infection, eyes become more sensitive to light. Infection may be confused with pinkeye.

Signs of worms


Worms are difficult to detect on clinical examination. Immature and adult worms may be washed out of the eyes using normal saline in a plastic squeeze bottle with a long tapering nozzle.


Broad-spectrum anthelmintics have been shown to be effective against this parasite.

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