Thelazia spp: Eyeworm

General Description: White roundworms, 7 to 18mm in length.

Life Cycle: Indirect; Female worms in the eye deposit larvae, which are picked up from the tears by muscid flies. Third-stage larvae infect cattle via the fly mouth parts.

Location: Tissues surrounding the eye; eyelids, anterior chamber.

Geographical Distribution: Widespread.

Significance: Infection may bother animals; thelaziasis may resemble pinkeye.

Effect on Host: Tissues of inner eyelids are irritated, causing tearing. 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.

Diagnostic Information: Worms can be seen around the eye and in eyelids. Thelaziasis should be considered when animals do not respond to pinkeye treatment.

Control: Reduce fly populations; remove adult worms. Ivermectin has been shown to be effective against these parasites.

Adult worm in eye		Anterior adult worm