By Jennifer Coates, DVM
Many different health problems can make a dog’s eyes red, swollen, and runny. Wounds, infections, allergies, anatomical abnormalities, and foreign material trapped on the surface of the eye are some of the most common, but did you know that a type of parasite called an eyeworm might also be to blame? Read on to learn all about eyeworms in dogs and what can be done to treat and prevent them.
Causes of Eyeworms in Dogs
Eyeworms (Thelazia californiensis is the most common species) are transmitted to dogs through contact with certain kinds of flies. Research points to canyon flies (Fannia benjamini complex) as the primary vector of eyeworms in the western part of the United States, but it is possible that different flies are involved in other locations. Thelazia californiensis and other types of eyeworms have been diagnosed in most parts of the world.
The eyeworm lifecycle is fairly straightforward. Adults that are living on the eyes an infected animal breed and lay their eggs. When a fly comes by to feed on the tears of the animal, it picks up the larvae that have hatched from the eggs. These larvae mature within the fly and then migrate to the fly’s mouthparts at which point they are deposited on the eyes of another animal when the fly feeds again. Eyeworms can infect dogs, cats, wildlife, livestock, and even people.
Symptoms of Eyeworms in Dogs
Unsurprisingly, the presence of a worm living on a dog’s eye is very irritating, particularly because Thelazia have sharp serrations on their outer surface that can damage the sensitive structures of a dog’s eye. The body reacts to this with a lot of inflammation, which can lead to redness of the eye and surrounding tissues, swelling of the conjunctiva (the mucous membranes around the eye), excessive tearing, itching, and squinting. Sometimes the cornea (the clear outer surface of the eye) can become ulcerated or scarred. When dogs have only a few eyeworms, their eyes may look almost normal. Heavily infected dogs typically have more severe symptoms.
The worms themselves are often visible upon close examination of a dog with eyeworms. They are white to cream in color, thin, and can range from 10 to 15 millimeters (around one-half of an inch) in length. Thelazia may be visible on the surface of the eye but can also be found under the eyelids (including the third eyelid) and within the ducts that carry tears to and from the eyes.
Diagnosing Eyeworm in Dogs
Veterinarians can usually diagnose eyeworms based on a physical examination alone. It may be necessary to numb the dog’s eyes with a topical anesthetic or to sedate the dog to get a good look under the eyelids.
Treating Eyeworm in Dogs
The most common way to treat eyeworms in dogs is to physically remove the worms. A veterinarian will apply a topical anesthetic to the surface of the eye and/or sedate the dog. The worms can then be gently plucked away using forceps or flushed out using a sterile saline rinse. Veterinarians may also prescribe medications that will kill the parasites. Options include ivermectin, moxidectin, imidacloprid, and selamectin. A veterinarian will determine which form of treatment is best based on the situation.
Dogs with severe eye inflammation caused by eyeworms may also need topical or systemic anti-inflammatory medications to bring down swelling and reduce redness and irritation. Antibiotics may also be prescribed to treat or prevent secondary bacterial infections.
Living and Management
Dogs that are recovering from an eyeworm infection should be monitored closely. If the symptoms worsen at any point or fail to improve over the course of a week or so, contact your veterinarian.
Preventing Eyeworm in Dogs
Keeping pets indoors during times when flies are active would prevent most cases of eyeworms in dogs. Research has shown that flies that can carry eyeworms are present year-round in Southern California, but fly activity could certainly differ in other parts of the country.
If fly avoidance is not possible, the routine use of a fly repellent labeled for dogs or a medication that kills parasites (e.g., oral milbemycin) may be advisable. Speak to your veterinarian about what type of preventive measures would be safe and effective for your dog.