Toxocariasis (Ocular Larva Migrans)

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Elevated granuloma in toxocariasis.

Signs and symptoms: Toxocariasis is caused by a parasitic nematode. It may develop at any age, although the initial infection typically occurs in childhood. Some 10,000 infections occur in each year, and 700 of these people develop ocular conditions that result in permanent loss of vision.

When the macula or optic nerve is involved, the vision loss in these patients is devastating, typically hand motion or worse. The condition is usually unilateral, but affects both eyes in up to three percent of patients.

The organism induces an inflammatory reaction in the host eye. At the ophthalmoscope, you may see anterior uveitis, vitritis, neuroretinitis, papillitis, chronic endophthalmitis and RPE changes. Virtually any ocular condition associated with an inflammatory response may develop. The most common finding is a granuloma of either the retina or optic disc that is white, elevated and about one disc diameter in size.

Patients with a systemic infection will typically complain of abdominal pain, headache, weakness, lethargy, fever, coughing, asthma or pneumonia. Fortunately, most systemic infections are mild and produce no serious morbidity. Rarely are infections severe enough to result in death.

Pathophysiology: Human infection with Toxocara, the parasitic larvae of canine or feline roundworms, is known as visceral larval migrans (VLM) systemically and ocular larva migrans (OLM) when it affects the eye.

Two nematode larvae can cause toxocariasis. Both are round worms found in the intestines of dogs (T. canis) and cats (T. cati). About 20% of dogs release Toxocara canis in their stool. Patients can become infected after accidentally swallowing infective Toxocara eggs from larvae in soil or other contaminated surfaces. Puppies usually contract Toxocara canis from the mother before birth or from her milk. The larvae mature rapidly in the puppy's intestines and when the pup is three or four weeks old, they begin to produce large numbers of eggs that contaminate the environment through the animal's stool. The eggs soon develop into infective larvae. Contraction occurs from ingesting contaminated soil or feces.

Once ingested, the eggs hatch and larvae penetrate the intestinal wall and are then carried by the circulation to various tissues--liver, heart, lungs, brain, muscle, eyes--via blood and lymph. Ingested eggs may remain viable for years. While the larvae do not develop further in these sites, they can cause severe local inflammatory reactions.

With the ocular form, one or more larvae become trapped in the eye, causing a granuloma in the retina. The nematode may reside beneath or within the retina, or it may extend into the choroid or vitreous. It may remain viable for several years in the eye. The nematode may enter the eye via the central retinal artery, and will likely manifest as a peripheral granuloma. If the nematode enters via the short posterior ciliary arteries, the granuloma will likely be at the disc, macula or elsewhere in the posterior pole.

Management: If an isolated, elevated granuloma in the optic disc or retina presents with vision loss, the diagnosis can be rather straightforward. At this point, try to obtain additional history, looking for exposure to puppies, feces or contaminated soil. Order the ELISA test to check for Toxocara titers.

Treatment of the organism in the eye is unsuccessful. The damage is irreversible. Clinicians typically use oral steroids as the organism initiates inflammatory reactions in the eye. For systemic infections, practitioners use the antiparasitic drug mebendazole; it is not clear whether this is an effective ocular treatment. More aggressive treatments include laser photocoagulation and cryoretinopexy.

Clinical Pearls:

  • Most cases of isolated elevated granulomas of the disc or retina are due to Toxocara.
  • Virtually all cases of toxocariasis in the United States come from pets.
  • When suspecting ocular toxocariasis, attempt to elicit a history of exposure to puppies and soil.
  • While the systemic form is typically unnoticed and produces no permanent damage, a similar nematode found in raccoons can also infect humans and is much more virulent.

Other reports in this section

Eyelids & Eyelashes | Conjunctiva & Sclera | Cornea
Uvea | Vitreous & Retina | Optic Nerve & Brain | Oculosystemic Disease

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