Thelaziasis in Dogs

By Mariana Castanheira, Licensed vet. Updated: August 6, 2018
Thelaziasis in Dogs

See files for Dogs

Perhaps the main and constant concern of people who live with dogs is deworming, both internally and externally. And we say main and constant because the parasites will affect all dogs, at all stages of their life and throughout the year. To fight them, it is essential to know them and apply the necessary preventive measures. In this AnimalWised article we will focus on a parasite that is becoming more and more common but still unknown to many animal keepers: Thelaziasis in dogs, also known as eyeworm or Thelazia. Read on to find out what it is, how it manifests itself, and what treatment it requires.

You may also be interested in: Ectropion in Dogs

What is Theliaziasis?

Thelazia in dogs is an ocular parasite, a nematode worm that lodges inside the eye and can cause discomfort. It is originally from Asian countries, in fact, it was formerly called "Eastern worm" or "eyeworm", but has already reached the whole Iberian Peninsula and several European countries. This worm, which is usually Thelazia callipaeda, is transmitted by the small flies characteristic of fruit trees, present during the warm months (depending on areas but usually from May to November, while the activity of these flies lasts). Flies look for the eye secretions of our dogs and it is by accessing them that the parasite arrives from the fly to the eye. Once installed in it, the worm causes irritation that can cause symptoms, although sometimes its presence is completely unnoticed. It is a zoonosis, i. e. an expanding disease transmissible to humans.

Thelaziasis life cycle

The Thelazia's biological cycle includes a definitive host, in this case, the dog, and an intermediary that is the fruit fly; it is known that transmission is carried out through the male. In the dog, adults of the parasite are located in different eye structures, such as under the third eyelid or in the nasolacrimal ducts. Females will put larvae in the dog's eyes and, in this way, when the flies feed on the eye secretions, they will ingest them. Inside, and within three weeks, these larvae will mature to their infectious form, at which time they are transferred to the mouth apparatus of the fly (proboscis) and thus restart the cycle when it feeds on the eye secretions of another dog, where it reaches maturity within a month.

Symptoms of thelaziasis in dogs and diagnosis

The diagnosis of Thelazia in dogs is made by direct observation of the parasite inside the eye. Its appearance is filiform and whitish, approximately 6-17 mm long and 0.3-0.8 mm in diameter, the females being the largest. Even so, it is not always possible to see it and the diagnosis will be made taking into account symptoms or environmental factors such as the time of year (probably in warm months) or the dog's habitat (life or access to an exterior with the presence of fruit trees). In other words, if the treatment we will see in the following section works, this means it was Thelazia even if no specimen was observed in the dog's eyes in the first place. Among the symptoms of Thelaziasis in dogs that we can detect are the following, in one or both eyes:

  • Conjunctivitis.
  • Tears.
  • Serous discharge that may become purulent if a secondary bacterial infection occurs.
  • Itching, the dog scratches, which can lead to injuries such as corneal ulcer.
  • Periocular alopecia, as a result of scratching there is a loss of hair around the eye (s).

A visit to the veterinarian is appropriate for any of these symptoms. Although they do not appear to be serious, not treating them can cause more serious problems, such as the ulcer we have mentioned. Symptomatology usually manifests itself 7-15 days after the parasites enter the eye.

Thelaziasis in Dogs - Symptoms of thelaziasis in dogs and diagnosis

How to treat Thelaziasis in dogs

As we have seen, Thelaziasis in dogs occurs inside the eye. The first step will be to manually get rid of the parasites observed, an operation to be carried out by the veterinarian. It is not always permissible, because sometimes, even though these worms are present, they won't be able to locate them.

The most currently recommended treatment of choice is milbemicin, a safe product that is also used for internal deworming. Watch out! We should never medicate our dog on our own. Even the most commonly used compounds can be life-threatening for some dogs. For this reason, it is always the veterinarian who has to tell us the right treatment and dosage.

How to prevent Thelaziasis in dogs

As with other parasites, prevention is complicated. In this case, it is impossible to control the presence of flies in trees during the hottest months, and it will not be possible to prevent flies from entering our dogs. So, while being aware that not all dogs will be able to follow them, these would be some recommendations aimed basically at avoiding contact with flies:

  • Keep dogs indoors during months when flies proliferate, usually the warmest, although the length of this period will depend on the geographic location.
  • Avoid or reduce walks during daylight hours, when flies are active.
  • Get rid of as much organic matter as possible, such as cut grass, fallen fruit from trees, leaves, etc., so as not to favor an increase in fly populations.
  • Try using sunglasses for dogs or any device that prevents or hinders the access of flies to the eyes, since, remember, it is this contact that transmits Thelazia to dogs.
  • Check eye condition frequently and see your vet for any symptoms.
  • According to the latest studies, a monthly dose of milbemicin can be maintained during troubled months[1]. However, the veterinarian should prescribe this method of prevention.
Thelaziasis in Dogs - How to prevent Thelaziasis in dogs

This article is purely informative. AnimalWised does not have the authority to prescribe any veterinary treatment or create a diagnosis. We invite you to take your pet to the veterinarian if they are suffering from any condition or pain.

If you want to read similar articles to Thelaziasis in Dogs, we recommend you visit our Eye problems category.

  1. Ferroglio E, Rossi L, Tomio E et al. Therapeutic and prophylactic efficacy or milbemycin oxime (interceptor) against Thelazia callipaeda in naturally exposed dogs. Veterinary Parasitology 2008, 154: 351-353.

Write a comment

Add an image
Click to attach a photo related to your comment
What did you think of this article?
1 comment
Melissa Havens
Very informative thank you so far best website for pet questions.
Administrador AnimalWised
Thanks a lot Melissa, we appreciate it as always!
Thelaziasis in Dogs
1 of 3
Thelaziasis in Dogs

Back to top