Thelazia callipaeda: the eyeworm and the fruit fly
4-year-old cat with access to a farm with fruit trees in La Vera (Cáceres). In August he has epiphora and bilateral conjunctivitis. Lifting the third eyelid reveals several thin, white, motile worms.
How did he become infected?
chevron_right Show answer and explanation
A. Through Phortica flies that feed on lacrimal secretions and deposit larvae in the eye
Correct. Male Phortica variegata, a fruit fly (drosophilid), ingest larvae while feeding on tears and, after development, deposit L3 in another host's eye. That is why cases cluster in summer and in wooded rural areas.
B. Through direct contact with another infected cat's eye
Thelazia needs a vector: larvae must develop inside the fly to become infective. Direct contact between animals does not transmit infection.
C. Through mosquito bites, like Dirofilaria
Mosquitoes transmit Dirofilaria, but Thelazia callipaeda is transmitted by non-biting flies feeding on ocular secretions.
D. By eating snails or slugs carrying larvae
Gastropods are intermediate hosts of Angiostrongylus or Aelurostrongylus, not of Thelazia.
lightbulb Key takeaway
Thelazia callipaeda affects dogs, cats, foxes, hares and rabbits, and also people (zoonosis). In Spain it was first described in La Vera and is now diagnosed in many provinces. With summer conjunctivitis that does not respond, examine beneath the third eyelid.
Test yourself with 10 random questions
Parasitic diseases · 100 questions, a different test every time
Related questions
Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.