Thelazia is a genus of parasitic roundwormsAnterior part of an adult Thelazia worm with characteristic cuticular striation. © J. Kaufmann / Birkhäuser Verlag that affects the eyes of numerous domestic animals, including dogs, cats, horses, cattle, sheep, goats and other livestock. Many wild mammals and birds are affected as well.

Prevalence in endemic regions can be rather high. Several studies found that up to 30-50% of cattle can be affected by Thelazia spp in the UK, Italy, Canada and parts of the USA. Dogs showed similar infection rates in several Southern European countries.

So far a dozen species have been described. Occurrence and prevalence depend on each species.

The most significant species that parasitize domestic animals are the following:

  • Thelazia californiensis affects dogs and cats, very occasionally humans as well. Found in Western North America.
  • Thelazia callipaeda affects dogs and cats, very occasionally humans as well. Found in the Far East, Russia and other parts of Europe.
  • Thelazia gulosa affects mainly cattle, less often sheep and goats. Found in Asia, Europe and North America.
  • Thelazia lacrymalis affects horses. Found worldwide.
  • Thelazia rhodesi affects mainly cattle, less often sheep, goats and horses, very occasionally humans as well . Found in Africa, Asia and Europe.
  • Thelazia skrjabini affects mainly cattle, less often sheep and goats. Found in Europe and North America.

The disease caused by Thelazia eyeworms is called thelaziasis (occasionally thelaziosis).

Are animals infected with Thelazia eyeworms contagious for humans?

  • NO. The reason is that most species are not human parasites. And those species that can infect humans are not transmitted directly from animals to humans, but need to spend part of their life cycle inside an intermediate host, mainly flies. These flies are responsible for transmission to humans. For additional information read the chapter below on the life cycle of eyeworms.

You can find additional information in this site on the general biology of parasitic worms and/or roundworms.

Final location of Thelazia eyeworms

Predilection site of adult Thelazia eyeworms are the eyes and the tissues around the eyes (eyelids, lacrimal ducts and glands, etc.).

Anatomy of Thelazia eyeworms

Adult Thelazia worms are up to 20 mm long, have a whitish color and the typical slender tubular form of roundworms. The worm's body is covered with a cuticle, which is flexible but rather tough. Characteristic for these worms is a transversal striation of the cuticle.

They have a tubular digestive system with two openings. They also have a nervous system but no excretory organs and no circulatory system, i.e. neither a heart nor blood vessels. Males have two dissimilar chitinous spicules for attaching to the female during copulation.

Thelazia worms are viviparous, i.e. the females do not lay eggs, but release already developed L1-larvae still surrounded by the eggshell. These larvae reach the tears and are ingested by flies that feed on the tears (e.g. Musca domesticaMusca autumnalis, Fannia spp, etc.).

Life cycle of Thelazia eyeworms

Musca domestic, the common housefly, a potential vector fo Thelazia eyeworms.

Thelazia eyeworms have an indirect life cycle with dogs, cats, livestock and several other mammals and birds as final hosts, and various Dipteran flies (e.g. Musca autumnalis, Musca domestica, Fannia spp, etc.) as intermediate hosts.

The adult females in the final host do not lay eggs but release sheathed L1-larvae. These larvae reach the tears of the infected host and are ingested by the flies that feed on these tears. Inside the flies these L1-larvae hatch, cross the gut's wall and migrate to various tissues (fat bodies, egg follicles, etc.). There they develop to infective L3 larvae in 2 to 4 weeks. These infective larvae migrate to the mouthparts of the flies.

When the infected fly visits a new host for tear-feeding, it transmits the infective larvae to the visited host. Once in their final host these infective larvae get into the eyes or surrounding tissues and complete development to adult worms in 3 to 9 weeks. Adult worms may live for one year or longer on their final host.

Harm caused by Thelaziainfections, symptoms and diagnosis

Thelazia spp in the eye of a cow. Picture from

Infections with a few worms may remain without clinical signs in livestock and pets. However infections with 10 to 15 worms may cause conjunctivitis, keratitis, excessive lacrimation and watery eyes, swollen eyes, purulent exudation, excessive light sensitivity, etc.

Chronic infections can cause corneal damage and ulceration, which can lead to blindness. Other parts of the eyes can also be damaged. In addition, eyeworm infections can favor secondary infections with bacteria, e.g. with Moraxella bovis, the causative agent of bovine keratoconjunctivitis, also called "pinkeye".

As a general rule, eyeworm infections are more frequent during the fly season, typically from late spring to early autumn in regions with moderate climate. Cattle aged 3 to 4 years are more often affected than older animals. Large dogs 3 to 4 years old are more likely to be infected than younger ones.

Diagnosis is done through visual examination of the eyes and surrounding tissues, or in the sediment of centrifuged wash obtained after eye or lacrimal duct rinsing. These worms are quite active and are seen moving quickly in their environment.

Prevention and control of Thelazia

The key element to avoid eyeworm infections on livestock and pets is to ensure adequate protection against houseflies and other nuisance flies, especially during the fly season. Control of houseflies and nuisance flies strongly depends on manure and waste removal in farms, and general hygiene measures in gardens, recreational areas, etc., especially eliminating humid habitats and any accumulation of organic refuse where the flies may breed. For more specific fly control measures read the articles on houseflies, face flies and filth flies in this site.

In endemic regions with high eyeworm incidence it can make sense to keep livestock indoors during the peaks of the fly season. To protect pets from flies during a walk it may be advisable to use fly repellents. Such repellents are normally useless for livestock, because they last only a few hours.

Among the anthelmintics available for worm control several macrocyclic lactones have shown systemic efficacy against established Thelazia infections, but not always at the usual recommended treatment regimes, which has to be determined by a veterinary doctor. Efficacy has also been reported for fenbendazole and levamisole.

For dogs and cats macrocyclic lactones are available mostly as tablets (e.g. ivermectin, milbemycin oxime, moxidectin), injectables (e.g. moxidectin) or spot-ons (e.g. selamectin). Ocular drops (e.g. moxidectin + levamisole) are marketed in some countries as well.

For cattle, sheep, goats and other livestock macrocyclic lactones are available mainly as injectablespour-ons or drenches (e.g. abamectin, doramectin, ivermectin, moxidectin). Slow-release boluses may be also available in some countries.

For horses most anthelmintics are available as pastes or gels for oral adimistration.

Directly treating livestock, horses or pets with insecticides to keep the flies under control and thus avoid eyeworm infections can show some effect if face flies are the major vectors involved. Where houseflies and other filth flies are the main vectors, on-animal treatments are not capable of reducing fly populations. The previously mentioned manure and waste management measures may be complemented with manure treatments using larvicides (so called larviciding), which are mainly insect development inhibitors such as cyromazine, diflubenzuron, methoprene, etc. Click here to learn more about premise and environmental treatments.

There are so far no true vaccines against Thelazia eyeworms, neither for livestock, nor for pets. To learn more about vaccines against parasites of livestock and pets click here.

Biological control of Thelazia eyeworms (i.e. using its natural enemies) is so far not feasible.

You may be interested in an article in this site on medicinal plants against external and internal parasites.

Resistance of Thelazia to anthelmintics

So far there are no reports on resistance of Thelazia eyeworms to anthelmintics.

This means that if an anthelmintic fails to achieve the expected efficacy, chance is very high that it was not due to resistance but to incorrect use, or the product was unsuited for the control of these parasites. Incorrect use is the most frequent cause of failure of antiparasitic drugs.

Ask your veterinary doctor! If available, follow more specific national or regional recommendations for Thelazia control.