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This disease is an infestation with parasitic nematodes (roundworms) of the genus Thelazia. The adult worms inhabit the eyes and surrounding tissues (eyelids, tear ducts, etc.) of various mammal and bird hosts, including cattle and humans. Thelazia nematodes are often called eyeworms.

Infestation by these tiny worms may not always cause obvious signs, but frequently causes watery eyes, conjunctivitis, corneal opacity (cloudy eye), or corneal ulcers (ulcerative keratitis). Humans infested with these worms often feel something in the eye.

In the host animal, Thelazia worms have been found in various tissues of the eye socket, eyelids, tear glands, tear ducts, and third eyelid (nictitating membrane) and in the eyeball itself.

Several dozen species of Thelazia exist but only 7 are commonly seen in domestic animals. Other species are occasionally found in birds or wild mammals.

In humans, dogs and cats, eyes have been invaded by Thelazia callipaeda (primarily in Asia and Europe), and occasionally by T. californiensis and T.gulosa (in western North America). In cattle, T. gulosa is seen in Asia, Europe, and North America. T. rhodesii infects cattle in Africa, Asia, Europe, and T. skrjabini is seen in cattle in Europe and North America.

Various livestock and wildlife surveys have shown that thelaziasis is quite common in animals. For instance, a slaughterhouse survey in Canada found that about one-third (32%) of the cattle inspected over an 8-month period were infested with eyeworms.

Diagnosis involves examining the eyes and nearby tissues for worms. The adult worms are very active, and have been described as looking like short, lively pieces of nylon fishing line--about half an inch long.

Mild to severe conjunctivitis and inflammation of the eyelids (red, irritated and itchy) are common signs. Also, keratitis (inflammation of the cornea), including opacity, ulceration, perforation, and permanent fibrosis, may develop in severe cases, particularly with T. rhodesii infections in cattle.

Signs and severity vary somewhat with the different types of eyeworms.  For instance, the Thelazia infections in cattle in North America are often unnoticed, or may produce mild conjunctivitis, excessive tears, localized swelling, corneal clouding, and sometimes fluid-filled cysts beneath the conjunctiva (the membrane covering the eye and lining the eyelids). In Europe and Asia, however, thelaziasis in cattle is commonly associated with more severe clinical signs, including conjunctivitis, sensitivity to light, and severe inflammation of the cornea. These cases generally result in chronic conjunctivitis with an increase in white blood cells and a clear-to-thick discharge from the eyes.


  • Watering eyes
  • Inflammation of the conjunctiva
  • Cloudy cornera


The intermediate hosts of several Thelazia species are tear-feeding flies of the genera Musca (family Muscidae), which includes Musca domestica (the housefly), and Musca autumnalis (the face fly or autumn housefly); Phortica (family Drosophilidae—which include various fruit flies), or Fannia (family Fanniidae—the smaller housefly).

The face fly, Musca autumnalis, is the vector of several species of eyeworms in North America—T. lacrymalis, T. gulosa, and T. skrjabini. Feeding habits of the face fly include a preference for eye secretions, which allow for transmission of the worms, since these worms require both the fly and the animal host for their life cycle. The worm larvae can overwinter in face flies.

The female worms within the animal’s eye discharge larvae into the eye secretions, and these larvae are ingested by the fly. They continue their development within the fly and become infective in 2 to 4 weeks. Infective third-stage larvae then emerge from infected flies and are deposited in the host’s eye by the fly during feeding.

The worm embryos develop into first-stage larvae within the adult female worm (in the host eye) and remain within their “eggshell” (sheath). The female deposits these sheathed larvae into the tears of the mammal or bird host, and the larvae are ingested by tear-feeding flies. In the fly, the larvae hatch out of their outer sheath, penetrate the gut wall of the fly, and migrate to either the fat body, testes or egg follicles of the fly (depending on the species of worm).

There they develop into third-stage larvae and migrate to the head of the fly. The infective stage-three larvae wiggle out of the straw-like feeding apparatus (proboscis) of the fly when it feeds on tears of a mammal or bird. The larvae then enter the animal’s eye and develop into adults in the eye or surrounding tissues of the host, where they may live for more than a year.

Development of sexually mature worms takes 1 to 4 weeks in cattle, depending on the worm species, and 10 to 11 weeks for T. lacrymalis in horses. Eye infections may be found in cattle and horses year-round, but major disease outbreaks, particularly in cattle, are generally associated with warm season activities of flies. Infection rates tend to be lower in calves, and higher in adult cattle—especially in animals that are 2 to 3 years old.

The tear gland and ducts are common sites for T. lacrymalis and T. gulosa, but these worms can also be found sometimes within the third eyelid and the nasolacrimal ducts that carry tears from the eye into the nose.

T. skrjabini is normally found within the tear ducts of the third eyelid. Superficial locations on the cornea, in the conjunctival sac, and under the eyelids and third eyelid are more typical for T. rhodesii, but T.lacrymalis, T.skrjabini, and T.gulosa may be found in these sites, too.

Worms may also be found on the hair or skin around the eyes during anesthesia or following migration of the worms out of the eye tissues after death of the host. Localized irritation and inflammation is likely due to the rough, serrated cuticle of the worms, especially for T. rhodesii.

Careful inspection of the eyes may reveal the worms. T. rhodesii is often found in the conjunctival sac, but T. gulosa and T. skrjabini in cattle (and T. lacrymalis in horse), tend to be more invasive and are less apt to be seen. Topical anesthetics can be useful (to allow for tissue manipulation) when trying to detect and remove worms. Microscopic examination of tears (looking for worm eggs or larvae) is sometimes accomplished.


Because most, if not all, species of Thelazia are spread by flies, any practices that reduce fly numbers will also reduce the spread of thelaziasis.

Fly control measures, especially for face flies, can help prevent thelaziasis in cattle and horses. Cattle on large dry, pastures have fewer face flies than those on wet pastures where shade and water are present and provide better habitat for the flies.


Because they live so close to the outside of the body, Thelazia is one of the few nematode infections that can be treated topically with organophosphates (such as ecothiopate iodide or isofluorophate). Annual systemic treatment with certain deworming drugs can also be effective. Use of antibiotic-steroid ointment to control inflammation and secondary bacterial invaders is also recommended.

Mechanical removal of worms with forceps after deadening the eye with local anesthetic is useful for T. rhodesii in cattle and sometimes works for the more invasive T. gulosa or T. skrjabini in cattle or T.lacrymalis in horses. Flushing the eyes with 1.5 to 2.5 ounces of aqueous solution of 0.5% iodine and 0.75% potassium iodide may get rid of T. gulosa and T. krjabini.

Certain systemic anthelmintics (deworming drugs) are effective against eyeworms. In cattle, levamisole, ivermectin and doramectin have good activity against Thelazia worms. Pour-on formulations of ivermectin or doramectin, delivered at proper dosage, are also effective treatment of adult eyeworms in cattle.

The prevalence of eyeworms in livestock has declined in areas where macrocyclic lactones such as ivermectin and doramectin are in common use.

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EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.