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Eyeworm Disease (Thelaziasis) in Cats

By Kirk N. Gelatt, VMD, DACVO, Emeritus Distinguished Professor, Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Florida
David G. Baker, DVM, MS, PhD, DACLAM, Director and Professor, Division of Laboratory Animal Medicine, School of Veterinary Medicine, Louisiana State University
A. K. Eugster, DVM, PhD, Director and Head, Diagnostic Services, Texas Veterinary Medical Diagnostic Laboratory

Also see professional content regarding eyeworms in small animals.

Eyeworms (Thelazia californiensis and T. callipaeda) are parasites found in cats, dogs, and other animals, including humans, in the western United States and Asia. They are whitish, 0.5 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 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. Injectable or topical treatments may be effective, but physical removal is often the first choice for treatment.