MSD Manual

Please confirm that you are a health care professional

honeypot link
Pet Owner Version

Eyeworm Disease (Thelaziasis) in Cats


David G. Baker

, DVM, MS, PhD, DACLAM, Division of Laboratory Animal Medicine, School of Veterinary Medicine, Louisiana State University

Reviewed/Revised Jul 2018 | Modified Oct 2022

Eyeworms (Thelazia californiensis and Thelazia callipaeda) are parasites found in cats, dogs, and other animals, including humans, in the western United States and Asia. They are whitish, 0.25 to 0.75 inches (7 to 19 millimeters) long, and move in a rapid snake-like motion across the eye. Your veterinarian may find up to 100 eyeworms in the conjunctival sac, tear ducts, and the conjunctiva under the eyelids. Filth flies (including the common house fly) serve as intermediate hosts and deposit the infective eyeworm larvae on the eye while feeding on secretions from the eyes.

Signs include excessive watering and itching of the eyes, inflammation of the conjunctiva, cloudy corneas with slow-healing sores, and rarely, blindness. After a local anesthetic is applied, diagnosis can be confirmed by observing the worms. The veterinarian can remove the parasites with forceps. Oral treatments may also be effective, but physical removal is often the first choice for treatment.

quiz link

Test your knowledge

Take a Quiz!