Thelazia californiensis

Kingdom: Animalia

Phylum: Nematoda

Class: Secernentea

Order: Spirurida

Family: Thelaziidae

Genus: Thelazia

Species: californiensis

Thelazia californiensis

Eye worm


Adult Parasite:

The adult worms measure 10 mm (male) to 15 mm (female) long.



  • Definitive - Dogs, cats, sheep, deer, and humans.
  • Intermediate - Face flies.


Life Cycle:

The eggs are laid and hatch in the conjunctival and lacrimal sacs. The first stage-larva is picked up by a face fly (Musca spp. or Fannia sp.) when it feeds on the secretions from the infected eye. The larva develops to the infective third-stage in the fly and then migrates to the mouthparts of the fly (this takes 2 to 4 weeks). When the fly feeds on the next host's eye the infective larva leaves the mouthparts. In the conjunctival and lacrimal sacs the larva develops to the adult stage.

Site where adult parasite is found in host:

Conjunctival and lacrimal sacs of the eye.


Diagnostic Stage:

Adult stage in the conjunctival and lacrimal sacs (see photograph of adult parasite above).


Common Diagnostic Test:

 Direct observation of the worm.


Clinical Signs:

Usually asymptomatic; in heavier infections a mild conjunctivitis may develop.



If possible, remove the adult worms with a fine forceps.

Drugs (Click on the name of the drug for more information):



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