Eyeworm (Thelazia californiensis) in Dogs

The Thelazia californiensis species of dog eyeworms are whitish worms found in the eyes of domestic dogs and cats. Humans have been known to become infected, but it is very rare. The eyeworms visibly move across the dog’s eyes in a rapid, serpentine motion. As many as a 100 worms may be present in the tear ducts, on the conjunctiva (the mucous membrane that covers the front of the eyes) and under the eyelids. This species of eyeworm is most commonly found in western North America and is more prevalent during fly season (late spring through early fall).

Eyeworms are parasites of the orbits of the eye, found on the conjunctiva and under the lids and nictitating membrane. This disease is commonly known as thelaziasis.

One or more of the following symptoms are usually noticed during an infection of dog eyeworms: 

• Excessive tearing of the eyes
• Inflamed conjunctiva (conjunctivitis)
• Opaque corneas of the dog’s eyes
• Slow healing sores or ulcers on the corneas of the eye
• Blindness (rare)
• Itching of the dog’s eyes
• Excess of blood vessels on the whites of the eyes
• Small, white to cream colored, active worms on the dog’s eyes, looking similar to nylon fishing line
• Light sensitivity to the eyes
• Swollen eyes 
• Pus in the eyes

Types

Thelazia californiensis is a species of parasitic roundworm that affects dogs, cats, and sometimes humans. It was first discovered in California during the 1930s and was recognized as a new species of eyeworm at that time. There are approximately a dozen species of eyeworms, but Thelazia californiensis is the most prevalent type that occurs in domestic animals such as dogs. Three to four year old, large dogs are more likely to become infected than younger, smaller dogs.

Transmission of face fly, housefly, or filth fly larvae is the cause of infection of the Thelazia californiensis species of dog eyeworm. The adult female eyeworm lays eggs in the tears of the eyes. Once in the tears, the eggs develop into larvae, which a face fly will then ingest. Inside the fly, the larvae develop for approximately 30 days at which time they will move to the mouth of the fly. When the adult flies are feeding near a dog’s eyes, the larvae move out of the fly’s mouth and move to the eyes of the dog. It takes about 3-9 weeks for the larvae to develop into adults and repeat the cycle.

Your veterinarian will examine and evaluate the dog’s eyes and tissues near the eyes. The eyes may be flushed (rinsed) to clean the infection and help to prevent secondary bacterial infection. The matter collected during the rinsing of the eyes may also be examined for presence of the worm. The dog eyeworms are usually very active and easy to see, making the diagnosis obvious. Your veterinarian may choose to remove the eyeworms. Most likely your dog will just be treated with medication. The medication will be sent home with instructions for continued treatment.

The method of treatment to eliminate the Thelazia californiensis eyeworm must be determined by your veterinarian. Correct use of these medications is necessary to achieve complete elimination of the eyeworm infection. The following are a few methods that may be recommended or administered.

Local anesthetic may be administered to the dog’s eyes by your veterinarian. Then forceps will be used to carefully remove the worms from the eyes.

Anti-parasitic topical eye solutions and ointments such as selamectin, moxidectin and levamisole solutions, or levamisole and morantel ointments may be prescribed. Sometimes administering eye treatments to your dog can be difficult. It requires your dog to be still for a brief time, which may require some assistance, especially if your dog is experiencing a lot of pain and discomfort from the infection.

Injected anti-parasitic medication such as moxidectin is sometimes administered by the veterinarian. This will be injected into the muscle of the dog.

Anti-parasitic oral medication such as ivermectin tablets, moxidectin tablets, or milbemycin oxime tablets are often times prescribed. This is an easy way to treat the disease.

In order to prevent eyeworms of the dogs, the fly population needs to be controlled. Controlling or eliminating the fly population and the opportunity for the face flies to feed on the dog’s eyes will significantly reduce the spread of the eyeworms. Waste management is necessary by removing accumulations of garbage, scraps, manure, and other matter and filth where flies like to live will help dramatically. While taking your dog for a walk in an area with a large fly population, using a fly repellent may be advisable.