ISSN 2398-2942      

Thelazia spp


Synonym(s): T. callipaeda, T. californiensis, Eye worm




  • Class: Secernentea
  • Order: Spirurida
  • Family: Thelazidae
  • Genus: Thelazia
  • Species: callipaeda, californiensis.

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Clinical Effects



  • L4, L5 and adult worms present on the surface of the eye. L1-L3 larvae within the fly vector.
  • No environmental stages.


  • Adult on the surface of eye.
  • L1 larvae deposited and subsequently ingested by fly vector.
  • Development to L3 takes place in fly vector.
  • L3 deposited back onto surface of the eye.
  • L3 develop into adult worms.


  • Feeding of fly vectors on lacrimal secretions.

Pathological effects

  • Larvae deposited by flies in conjunctival sacs.
  • Subsequent movement of larval and then adult worms on the conjunctival surface can lead to local inflammatory responses.
  • Subsequent epiphora, chemosis and conjunctivitis can develop.
  • Through progression of this inflammation or through secondary trauma due to the host rubbing the eye, subsequent ulceration and keratitis may develop.

Other Host Effects

  • Adult feeds on debris and lacrimal secretions.


Control via animal

  • Anthelmintic treatment of infected cats and dogs Worm control Therapeutics: parasiticide.
  • Mechanical removal of the adult worms with fine forceps or cotton swabs using local anesthetics.
  • Fly repellents. None are licensed for this purpose but it is likely that pyrethroids in dogs and flumethrin in cats may have some effect. In people, pyrethroid impregnated clothes and/or use of Deet or citronella is likely to have some effect in reducing transmission although the latter is short acting and requires frequent reapplication.
  • Use of fine mesh insecticide impregnated bed nets in endemic countries. Care must be taken these do not come into physical contact with cats.
  • Camping in windy/high altitude when visiting endemic countries as fruit and house flies are relatively poor fliers.
  • Frequent bathing of eyes to keep accumulation of lacrimal secretions to a minimum.

Control via chemotherapies

  • Milbemycin oxime Milbemcyin oxime should be given in 2 treatments, seven days apart.
  • Moxidectin and ivermectin Ivermectin have been demonstrated to have some efficacy. Treatment with ivermectin must not be used in collies because this breed has a genetic mutation known as MDR1 Multi-drug resistance gene (MDR1): overview, which reslts in ivermectin intolerance and toxicity at higher dosages. Ivermectin toxicity Ivermectin toxicity has been also reported in a Doberman Pinscher and two Australian Shepherds.

Control via environment

  • Remove feces to prevent contamination of soil and pens that may attract vector flies.
  • Good sanitation to reduce fly numbers.
  • Use of environmental insecticides such as pyrethroid sprays or fogging to reduce fly numbers. This is contraindicated if pet fish or reptiles are being kept in the environment and extreme care must be taken with cats living in treated areas.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Miró G, Montoya A, Hernández L et al (2011) Thelazia callipaeda: infection in dogs: a new parasite for Spain. Parasites & Vectors 4, 148 PubMed.
  • Otranto D & Eberhard M L (2011) Zoonotic helminths affecting the human eye. Parasites & Vectors 4, 41 PubMed.
  • Shen J, Gasser R B, Chu D et al (2006) Human thelaziosis--a neglected parasitic disease of the eye. Journal of Parasitology 92 (4), 872-875 PubMed.
  • Otranto D & Traversa D (2005) Thelazia eyeworm: an original endo- and ecto-parasitic nematode. Trends in Parasitology 21 (1), 1-4 PubMed.

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