Thelazia in Dogs: Symptoms and Treatment

Thelazia is an eye parasite that's becoming more and more common in dogs, though it's still largely unknown to the people who live with them. Even though it originated in Asia, it's spreading, and has already established itself in countries like Spain or Mexico.

Knowing its life cycle and how it's transmitted is essential to take the necessary precautions and prevent infection, since thelaziosis doesn't just affect dogs — it can also affect cats, horses, and even humans.

At SoyUnPerro we'll cover the symptoms of thelazia, what treatment options exist, and finally, give you some tips to prevent it.

What Is Thelazia and What Is Its Life Cycle?

Thelazia is a roundworm that lives inside the dog's eye. The most common variant of this parasite is usually Thelazia callipaeda, transmitted through fruit flies.

The warmer months, from May to November, carry the highest risk, since that's when these flies proliferate and seek out dogs' eye secretions to feed on. If they carry the parasite, that's when transmission occurs.

Within thelazia's life cycle, the dog is the definitive host, and the fruit fly is the intermediate host. When the fly ingests infected secretions, the larvae mature and, within about three weeks, move to the fly's mouthparts.

So, when it goes to feed on the eye secretions of another healthy dog or a person, it will transmit the parasite. And that's how thelazia's life cycle starts over again, time after time.

What Are the Symptoms of Canine Thelaziosis?

Thelazia can be diagnosed with the naked eye, by spotting the parasite in the dog's eye. The worms are thread-shaped and whitish in color. They can measure between 7 and 17 millimeters long, with a diameter of between 0.3 and 0.8 millimeters.

If they can't be seen directly, since they're hidden beneath the membrane of the third eyelid, there are a number of symptoms that can make us suspect the dog has thelaziosis. The most common are:

• Conjunctivitis
• Excessive tearing and discharge in the eyes.
• Involuntary blinking or eyelid twitching.
• Intense itching, which can cause hair loss around the eyes from scratching, or injuries such as a corneal ulcer.

If the dog shows these signs and tends to live outdoors, near fruit trees, it's a good idea to see a vet for an exam and diagnosis. These symptoms can appear in one or both eyes, between 7 and 15 days after the parasite arrives.

Treatment for Thelazia in Dogs

How successful treatment is depends on how quickly we see a specialist — remember, the longer the parasite has been established in our dog's eye, the more collateral damage it will cause.

Treatment for thelazia will vary depending on whether the worms can be spotted with the naked eye. If they can, the vet will usually remove them manually. Removal usually isn't painful, since the vet will first instill an anesthetic eye drop, to be able to manipulate the eye and look beneath the nictitating membrane, dogs' third eyelid.

This local anesthetic approach poses no risk to your pet's health and wellbeing — on the contrary, it makes it easier for the parasite to come out of the eye (the anesthetic paralyzes the parasite), causing some of them to move across the cornea.

After removing the parasites, your vet will apply spot-on and oral anthelmintics, since in many cases there can be other larval stages hidden in the eye's structures, which will later develop into adults.

That's why it's important to repeat the deworming treatment a week later. If there are ulcers or other lesions, these will need specific treatment: steroids, antibiotics, etc.

It's very important never to act on our own and to follow the instructions given by a vet, so as not to put the dog at risk, and to give the correct treatment, doses, etc.

What Is the Prognosis for Thelaziosis?

The prognosis will be excellent as long as all the worms are eliminated and there are no complications such as deep ulcers.

How to Prevent Canine Thelazia

To avoid reaching this point, the best remedy is good prevention. It's true that it's not easy to avoid these flies during the warm months, but following a few recommendations can reduce the odds of a dog getting infected with this parasite.

Some tips worth keeping in mind include:

• Try to keep dogs indoors more during the months fruit flies are active, so they don't come into contact with them.
• Avoid leaving fruit scraps around that could attract flies.
• Try to avoid walking through areas where you notice the presence of this type of fly.
• Try to keep fallen fruit, leaves, or other organic matter away, so flies don't proliferate.
• Use repellents and screens for better environmental control. Dog sunglasses are also highly recommended, since they make it harder for flies to reach the eyes.

During the fly's active season, we recommend keeping your pet on a preventive deworming schedule. In addition, during the more problematic months, your dog's eyes should be checked more often, along with more frequent routine vet check-ups, so that thelazia, if it does appear, doesn't catch us by surprise.

Since it's a zoonotic disease (humans can get sick too), it's worth keeping in mind the hygiene measures we should take to avoid catching it from our dog. This way, it can be treated as soon as possible to prevent bigger complications for our furry friend, especially if this isn't the first time it's happened.