Parasites such as heartworms can cause significant health problems for your pet, and they also can affect their eyes, threatening their vision. Ophthalmomyiasis refers to the larval infestation of the eye, and our team at Veterinary Vision Center wants to provide information about this condition to help you protect your pet.

Heartworm ophthalmomyiasis in pets

Heartworms (i.e., Dirofilaria immitis) are transmitted when an infected mosquito bites your pet. Baby heartworms, called microfilariae, are deposited in the mosquito’s saliva near the bite wound and swim through the wound into your pet’s bloodstream. Once inside, they encyst in your pet’s skin to mature to a stage where they can migrate. Most commonly, the stage five larvae migrate to your pet’s heart and lungs, causing significant damage. Dogs are considered natural hosts for heartworms, and the parasites can mature to adulthood, mate, and produce offspring inside your pet’s heart. The worms can grow to 12 inches or more, causing inflammation in the lungs and decreasing the heart’s ability to pump effectively. Most dogs don’t show signs in the early stages, but as the disease progresses, your pet may develop a soft, persistent cough, exercise intolerance, and fluid accumulation in the abdomen. Sudden death also may occur. Cats are atypical hosts, and most worms do not mature to adulthood. However, cats can experience heartworm associated respiratory disease (HARD), which causes severe inflammation and mimics signs of feline asthma. Sudden death is also possible, occurring in 10% to 20% of cats. 

Heartworm larvae also have been known to migrate to the eye in pets. The larvae can infect the conjunctiva, causing conjunctivitis; the cornea, resulting in a cloudy appearance to the cornea; or the interior of the eye, causing uveitis (i.e., inflammation inside the eye). In addition, a rare case of thromboembolism of the optic vessels has been reported, which caused blindness. The larvae can be seen on an ophthalmic examination, migrating through the cornea or in the eye vitreous. The larvae must be removed surgically before heartworm medication is administered  to avoid causing further damage to the eye. The best way to prevent heartworm disease is to provide year-round preventive care and to have your pet tested once a year. 

Thelazia ophthalmomyiasis in pets

Thelazia callipaeda and Thelazia californiensis, also known as eyeworms, are transmitted by flies that drink tears and live on the surface of the eyes of domestic and wild carnivores such as dogs, cats, foxes, and other mammals like rabbits and livestock. Adult worms living in the eye of an infected animal breed and produce eggs. When a fly drinks the animal’s tears, they ingest the hatched larvae. The larvae mature inside the fly and then migrate to their mouth parts where they are deposited when the fly drinks from another animal’s tears. Thelazia often migrate underneath the eyelids and into the lacrimal duct, and sharp serrations on their outer surface cause severe irritation to the affected animal’s eyes. Signs include red eyes, conjunctival swelling, itching, squinting, and excessive tearing. In addition, the corneal surface can become ulcerated or scarred. Thelazia often can be seen, and range in length from 10 to 15 millimeters. Your pet’s eyes may need to be numbed to examine underneath their eyelids, and treatment typically involves sedating your pet and physically removing the parasites. Anti-parasitics also may be prescribed, and topical or systemic anti-inflammatories may be necessary to control the swelling and irritation caused by the worms. Keeping your pet indoors during times when flies are most active and providing year-round deworming medications can help prevent Thelazia.

Angiostrongylus ophthalmomyiasis in pets

Angiostrongylus vasorum is a parasitic nematode that infects dogs, foxes, wolves, and coyotes. Transmission occurs through intermediate hosts, primarily slugs and snails. Infection occurs when a susceptible animal eats an intermediate host or eats food contaminated by slime from the slugs or snails. Once inside a host, the larvae migrate to the lymph nodes to mature and then migrate to the heart and pulmonary arteries. Signs include a chronic cough, exercise intolerance, and difficulty breathing. Clotting abnormalities are also common. A. vasorum also can migrate through the central nervous system, causing hemorrhaging, which leads to lack of coordination, paralysis, behavioral changes, and seizures. In addition, the parasites have been known to infect the eyes, resulting in inflammation inside the eye and possibly blindness. Treatment involves surgical removal of the parasites from the eyes and addressing the systemic infection with appropriate anti-parasitics. 

Toxocariasis ophthalmomyiasis in pets

Toxocara canis is a common roundworm that affects dogs, and Toxocara cati affects about 25% of cats. Toxocara eggs are passed in the feces of infected dogs and cats. Other dogs and cats are infected when they ingest these eggs or food contaminated with the eggs. These parasites typically migrate through the infected pet’s tissues, such as their lungs, liver, and muscle. The larvae are coughed up from the lungs and swallowed. Once in the gastrointestinal tract, the parasites can attach to the intestine, irritating the intestinal wall and leaching nutrients. Signs include vomiting, diarrhea, and constipation. Toxocara sp. also have been reported to cause ocular problems, including retinal degeneration and optic nerve atrophy. The best way to manage toxocariasis is to prevent it with year-round anti-parasitics.

Thinking about parasites inside your pet’s eyes is disturbing, but providing year-round preventives typically can safeguard your pet. If you suspect that your pet’s eyes are affected by parasites, contact our team at Veterinary Vision Center so we can help save their sight.