Thelazia species

The nematode Thelazia (eye worm) occurs in cattle and rarely sheep and goats around the world, including in Canada.


The nematode Thelazia (eye worm) occurs in cattle and rarely sheep and goats around the world, including in Canada.  Adult parasites live in the conjunctival sac and lachrymal duct.  The life cycle is indirect and requires a muscid fly (primarily Musca) intermediate host in which the infective third-stage larvae develop.  Pathology and clinical signs are associated with the activities of the pre-adult and adult nematodes and include excessive tearing and conjunctivitis.  These changes make affected eyes attractive to the flies.  Thelazia is not known to be zoonotic.


Phylum: Nematoda
Class: Rhabditea
Order: Spirurida
Suborder: Spirurina
Superfamily: Thelazoidea
Family: Thelaziidae

Among the closest relatives of Thelazia species of importance in veterinary medicine is Spirocerca lupi, the oesophageal parasite of dogs and other carnivores. Spirocerca lupi does not occur in Canada other than in dogs imported from endemic areas.

Note: Our understanding of the taxonomy of helminth, arthropod, and particularly protozoan parasites is constantly evolving. The taxonomy described in wcvmlearnaboutparasites is based on that in the seventh edition of Foundations of Parasitology by Larry S Roberts and John Janovy Jr., McGraw Hill Higher Education, Boston, 2005.


Adult Thelazia species recovered from cattle in Canada are up to approximately 12 mm (males) and 20 mm (females) in length and visible to the naked eye. They are usually identified on the basis of their location in the host.

Host range and geographic distribution

The various species of Thelazia occur primarily in cattle, but also in other hosts including horses and dogs, in many parts of the world. The species in cattle in Canada are T. gulosa and T. skrjabini, both of which are fairly common.

Life cycle - indirect

Adult Thelazia species live in the conjunctival sac and lachrymal duct. First-stage larvae hatched fom eggs laid by the females are ingested by fly intermediate hosts (primarily Musca species), where they develop to third-stage infective larvae which migrate to the mouthparts of the fly. Cattle are infected when these larvae are introduced into the conjunctival sacs during feeding by the flies. The pre-patent period is very short – approximately seven days for some species.


In Canada, transmission of Thelazia species occurs in the summer when the arthropod vectors are available. Sometimes the parasite is a herd problem.

Pathology and clinical signs

Adult Thelazia in cattle in Canada are usually considered to be non-pathogenic. In some cases, however, the parasites can cause conjunctivitis, which can progress to keratitis with exudate and pus covering the eyes. Affected animals may be particularly attractive to the flies, thus enhancing parasite transmission. The presence of Thelazia may also predispose cattle to "pink eye" – infection with the bacterium Moraxella bovis.


Diagnosis is usually made by recovery of adult Thelazia from the conjunctival sacs or of eggs and/or larvae from lacrimal secretions. Local anaesthetic may be required to facilitate specimen collection.

Treatment and control

Doramectin (DECTOMAX) and ivermectin (VARIOUS) are approved in Canada for the treatment of Thelazia species in cattle.

Detailed information on these products is available from the Compendium of Veterinary Products (CVP) (Twelfth Edition, 2011), or from the manufacturers.

Control of Thelazia species in cattle is difficult, other than by attempting to control the horn fly intermediate hosts and maintaining good general health.

Public health significance

The species of Thelazia in cattle in Canada are not known to be zoonotic, but T. californiensis (from sheep, deer, cats and dogs) in the US and T. callipaeda (from dogs) in Asia are known to infect people and to cause conjunctivitis.
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