Eyeworm Disease (Thelaziasis) Average Cost

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What is Eyeworm Disease (Thelaziasis) ?

Eyeworms are parasites that are transmitted by an insect called a face fly. The sexually developed worms mature in about 1 to 4 weeks in cattle and 10 to 11 weeks in horses and attain a length of approximately ¾ inch. The greatest activity level of this parasite tends to be in the warm season of the face fly who is the transmitter of the larvae, though they can be actively infecting horses and cattle all year long.

Eyeworms or thelazia cause extensive eye irritation and are a common parasite that seems to prefer horses and cattle. Documentation shows the eyeworm can also be found in dogs, cats, pigs, sheep, goats, deer, and hares.

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Symptoms of Eyeworm Disease (Thelaziasis) in Horses

The symptoms you might notice in your equine that would suggest the presence of eyeworms may be:

  • Chronic conjunctivitis which may range from mild to more severe
  • Red eyes, mucus discharge, excessive tearing
  • Sensitivity to light
  • Localized edema or swelling of the tissues of the eye surrounding the globe
  • Cloudy cornea
  • Subconjunctival cysts (located in the thin almost transparent membrane that covers the white of the eye)
  • Worsening conditions can cause blindness and eventual perforation and degradation of the eye globe


The types of thelaziasis that can be found in eyeworm disease are noted below:

  • T rhodesii - these are generally found in the conjunctival sac of the eye
  • T gulosa - these are generally found in cattle
  • T skrjabini - these are generally found in cattle
  • T lacrymalis - generally found in horses

T gulosa, T skrjabini and T lacrymalis are considered to be more aggressive and harder to see. Microscopic examination will be needed to observe these worms which are generally located in the conjunctival sac of the host.

Causes of Eyeworm Disease (Thelaziasis) in Horses

Eyeworm or thelaziasis is caused by the parasites noted above. The adult female deposits her larvae into the conjunctival sac of the host where the life cycle of the worm larvae progresses. At some point, the larvae are picked up by the face fly as it feeds on eye secretions and are deposited by the fly into another host as it goes from host to host to ingest what it needs to survive. Generally, in most parts of the world and in the United States, the heaviest infestation usually occurs in the warmer months of the year but the transmission of this disease is not dependent upon warm weather - it can be transmitted all year long. As it degrades the conjunctiva, cornea, and other eye tissue, it can cause a corneal perforation which will ultimately be the death of the eye globe.

Diagnosis of Eyeworm Disease (Thelaziasis) in Horses

An examination of the eye will be required to ascertain if the worms can be detected. The symptoms of chronic conjunctivitis, cloudy cornea, and swollen tissues around the eyes are the main clinical signs that your veterinarian will be addressing during his examination. Because some types are more aggressive and harder to see, microscopic examination of lacrimal fluids may be required to identify the eggs or larvae. The various tissues around and in the eye will likely need to be moved around and manipulated to see the worms and attempt to remove them. This can be done under topical anesthesia effectively.

Treatment of Eyeworm Disease (Thelaziasis) in Horses

Treatment for eyeworm disease will likely involve some manual manipulation of the tissues to attempt to remove the worms from the animal’s eyes. Topical anesthesia will be necessary to accomplish this task. An eyewash of 0.5% of iodine and 0.75% potassium iodide has been found to be effective in horses and cattle to clean the parasites and associated debris from the eyes. At the same time, your vet will likely want to use topical antibiotic with steroidal anti-inflammatory ointment for the inflammatory response. Your vet will likely recommend some type of fly control to be put in place to reduce the opportunity for repeat episodes.

Recovery of Eyeworm Disease (Thelaziasis) in Horses

It is important to note that these parasites aren’t the only reasons that your horse may have eye irritation and discharge as dust, pollen and other allergens can also create similar conditions. As noted above, initiation of fly control will protect against some of the repeat invasions of this parasite. If your horse is stabled and pastured in areas where there are shade and water, the fly population will be greater than if he was pastured in an open areas. Application of fly repellent to the horse’s face (avoiding the sensitive eye tissues) as well as utilizing a fly mask are other prevention options that may be helpful to reduce the opportunity for face flies to get into your horse’s eyes.