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Toxocariasis

(Visceral or Ocular Larva Migrans)

By Richard D. Pearson, MD, Emeritus Professor of Medicine, University of Virginia School of Medicine

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Toxocariasis is human infection with nematode ascarid larvae that ordinarily infect animals. Symptoms are fever, anorexia, hepatosplenomegaly, rash, pneumonitis, asthma, or visual impairment. Diagnosis is by enzyme immunoassay. Treatment is with albendazole or mebendazole. Corticosteroids may be added for severe symptoms or eye involvement.

Pathophysiology

The eggs of Toxocara canis, T. cati, and other animal ascarid helminths mature in soil and infect dogs, cats, and other animals. Humans may accidentally ingest eggs in soil contaminated by stool from infected animals or may ingest undercooked infected transfer hosts (eg, rabbits). The eggs hatch in the human intestine. Larvae penetrate the bowel wall and may migrate through the liver, lungs, CNS, eyes, or other tissues. Tissue damage is caused by focal eosinophilic granulomatous reactions to the migrating larvae.

The larvae usually do not complete their development in the human body but can remain alive for many months.

Symptoms and Signs

Visceral larva migrans (VLM)

VLM consists of fever, anorexia, hepatosplenomegaly, rash, pneumonitis, and asthmatic symptoms, depending on the affected organs. Larvae of other helminths including Baylisascaris procyonis, Strongyloides spp, and Paragonimus spp can cause similar symptoms and signs when they migrate through tissue.

VLM occurs mostly in 2- to 5-yr-old children with a history of geophagia or in adults who ingest clay.

The syndrome is self-limiting in 6 to 18 mo if egg intake ceases. Deaths due to invasion of the brain or heart occur rarely.

Ocular larva migrans (OLM)

OLM, also called ocular toxocariasis, is usually unilateral and has no or very mild systemic manifestations. OLM lesions consist mostly of granulomatous inflammatory reactions to a larva, resulting in uveitis and/or chorioretinitis. As a result, vision can be impaired or lost.

OLM occurs in older children and less commonly in young adults. The lesion may be confused with retinoblastoma or other intraocular tumors.

Diagnosis

  • Enzyme immunoassay plus clinical findings

Diagnosis of toxocariasis is based on clinical, epidemiologic, and serologic findings.

Enzyme immunoassay (EIA) for Toxocara antigens is recommended to confirm the diagnosis. However, serum antibody titers may be low or undetectable in patients with OLM. Isoagglutinins may be elevated, but the finding is nonspecific. CT or MRI can show multiple, ill-defined, 1.0- to 1.5-cm oval lesions scattered in the liver or poorly defined subpleural nodules in the chest.

Hyperglobulinemia, leukocytosis, and marked eosinophilia are common in VLM.

Biopsies of the liver or other affected organs may show eosinophilic granulomatous reactions, but larvae are difficult to find in tissue sections and biopsies are low yield. Stool examinations are worthless.

OLM should be distinguished from retinoblastoma to prevent unnecessary surgical enucleation of the eye.

Treatment

  • Albendazole or mebendazole

  • Symptomatic treatment

Asymptomatic patients and patients with mild symptoms do not require anthelmintic therapy because infection is usually self-limited.

For patients with moderate to severe symptoms, albendazole 400 mg po bid for 5 days or mebendazole 100 to 200 mg po bid for 5 days is used, but the optimal duration of therapy has not been determined.

Antihistamines may suffice for mild symptoms. Corticosteroids (prednisone 20 to 40 mg po once/day) are indicated for patients with severe symptoms. Corticosteroids, both local and oral, are also indicated for acute OLM to reduce inflammation within the eye.

Laser photocoagulation has been used to kill larvae in the retina.

Prevention

Infection with T. canis in puppies is common in the US; infection with T. cati in cats is less common. Both animals should be dewormed regularly. Contact with dirt or sand contaminated with animal feces should be minimized. Sandboxes should be covered.

Key Points

  • The Toxocara canis life cycle normally involves dogs; humans are infected only accidentally, when they ingest eggs in soil contaminated by stool from infected animals or ingest undercooked infected transfer hosts (eg, rabbits).

  • In humans, toxocariasis causes 2 main syndromes: visceral larva migrans (which causes various symptoms depending on the organ infected) and ocular larva migrans (which usually causes no or mild symptoms but can result in impaired or lost vision).

  • Diagnose based on clinical evaluation and enzyme immunoassay for Toxocara antigens.

  • Most cases of toxocariasis are self-limited and do not require treatment, but if needed, the following can be used: albendazole or mebendazole for moderate to severe symptoms, possibly antihistamines for mild symptoms, and corticosteroids for severe symptoms.

  • Deworming dogs and cats can help prevent toxocariasis.

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