Thelaziosis Dog Eyeworms - What and Why?

Thelaziosis: Keep your dog free from eyeworms

Have you heard of thelaziosis? In Europe, some fruit flies host an emerging parasite known as “eyeworm”, an infestation which causes more or less severe conjunctivitis, sometimes leading to corneal ulcers and even blindness in dogs.

Most pet owners probably do not associate the scientific name “thelaziosis” to eyeworm infestations experienced by their dogs. However, the so-called eyeworm Thelazia callipaeda is an emerging parasite spreading across Europe. Effective prevention is essential to keep pets – and humans – healthy, and being aware of the risks is the very first step towards safety!

What is thelaziosis?

It is a disease caused by a worm. Actually, an infestation by a small white worm called Thelazia callipaeda that lives on the surface of the eyes - usually under the third eyelid - of domestic and wild carnivores such as dogs, cats, foxes, as well as other mammals, like rabbits. This eyeworm causes conjunctivitis and keratitis (inflammation of the cornea). Corneal ulcers, perforation and blindness may also occur in distinct, severe forms.
The most frequent ocular signs observed in infested dogs include conjunctivitis, epiphora (excessive lacrymation), pruritus, purulent exudation, and blepharospasm (involuntary eye closure).

Male Phortica variegata
Credit: Courtesy of Prof Domenico Otranto, Bari University

Why is it on the rise?

For decades, the distribution of such eyeworms has been limited to Asia and the former Soviet Union. However, the parasite is spreading across the European continent due to infested dogs travelling to/from endemic regions, and to the worm’s intermediate host, a fruit fly we may find in our gardens. These flies carry and transmit the parasite, making re-infestations common.

Does it pose any risk for humans?

This parasite does not only infest companion animals, but it has also a zoonotic potential. Humans may be infested through the same intermediate host and in the same way. Thus, dogs may act as a reservoir for human infection!

How is it transmitted?

The intermediate host, the male fruit fly Phortica variegata transmits the parasite while feeding on tears of pets, depositing the worm infective larva on the eye surface.

How do I know if my pet is affected?

The main clinical signs in both animals and humans are excessive lachrymation, conjunctivitis, and inflammation of the cornea. Eye pruritus may also be a warning sign. In severe forms, corneal ulcers, perforation, and blindness may also occur. Any signs of eye irritation should drive a clinical suspicion, while the diagnosis relies on the detection of the parasites in the conjunctival area by the veterinarian.

What to do if my pet is infested?

The treatment has to be performed by your veterinarian. It consists on the mechanical removal of the worms with a swab and/or by the flushing of the conjunctival sac of the eyes. A pharmaceutical treatment may be added.