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.
The incidence of these worms may be decreasing due to the common use of the broad-spectrum anthelmintics.
Figure 1. Cow eye with eye worm, Thelazia spp. Image created by Jess Morgan
Further ecological information on worms and their control:
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.
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.